RADIOLOGICAL AND HEALTH IMPACT
An Assessment by
the NEA Committee on Radiation Protection and Public Health
November 1995
OECD NUCLEAR ENERGY AGENCY
FOREWORD
Several years after the Three Mile Island accident, in the United States, the Chernobyl accident completely changed the public's perception of nuclear risk. While the first accident provided the impetus to develop new research programmes on nuclear safety, the second, with its human death toll and the dispersion of a large part of the reactor core into the environment, raised a large number of problems of "management" not only for the treatment of severely exposed persons, but also for the decisions that had to be taken affecting the population. Clearly, the national authorities were not ready to manage an accident whose consequences were not confined to their territory.
The way the accident was managed and the lack of information provoked a feeling of distrust in the minds of the public that was reinforced by the fact that radiation cannot be perceived by humans and also that it is easily detected even at a very low level. The prospect of contaminated food, aggravated by ambiguous, even contradictory recommendations by national authorities, gave rise to a variety of reactions, and sometimes overreactions, in the management of the accident consequences in several European countries.
In the country of the accident itself, where political, social and economic conditions were worsening, the association of the Soviet regime with nuclear activities contributed to raise feelings of mistrust towards the public authorities.
Ten years later, many improvements in radiation protection and emergency preparedness have been made possible by the Chernobyl experience and we are also able to arrive at a more accurate assessment of the impact of this accident. The fact remains that the future consequences in terms of health effects remain imprecise for simple technical reasons, and, because of this, lend themselves to a competition between those who want to minimise the consequences of the accident and those who wish to promote a catastrophic assessment.
In these circumstances, having discussed in 1994 the question of the future of radiation protection with the dawning of the next millenium (NE94), the NEA Committee on Radiation Protection and Public Health (CRPPH) wished to make an honest assessment, ten years on of the accident, on the state of the contaminated territories and the state of health of the populations and, on this basis, to attempt an appreciation of the risks to be expected not only for man but also for his environment.
This review does not end there. The CRPPH also details the lessons that have been learned by Member countries and the international organisations such as the ICRP, the IAEA, the EC, the WHO, etc. It has also organised international emergency exercises, the INEX Programme. Information between States and the public has been considerably expanded.
The accident was followed by numerous assistance and research programmes supported by international organisations and bilateral agreements. All these organisations are or will be publishing their results. This report differs from those in that it is a synthetic consensus view aimed at those persons who wish to know the salient points without having to go into the technical details which one can find elsewhere.
We thank all those organisations (UNSCEAR, FAO, WHO, EC) which have put information at our disposal so that this report could be as up to date as possible. However, those Agencies are still generating a large amount of information to be submitted to the forthcoming international Conference "One Decade After Chernobyl" to be held in April 1996, some of which could not be made available in time for incorporation into this report.
The report was drafted by Dr. Peter Waight (Canada) under the direction of an editing committee chaired by Dr. Henri Métivier (France). The members of the Editing Committee were:
Dr. H. Métivier IPSN, France Dr. P. Jacob GSF, Germany Dr. G. Souchkevitch WHO, Geneva Mr. H. Brunner NAZ, Switzerland Mr. C. Viktorsson SKI, Sweden Dr. B. Bennett UNSCEAR, Vienna Dr. R. Hance FAO/IAEA Division of Nuclear Techniques, Vienna Mr. S. Kumazawa JAERI, Japan Dr. S. Kusumi Institute of Radiation Epidemiology, Japan Dr. A. Bouville National Cancer Institute, United States Dr. J. Sinnaeve EC, Brussels Dr. O. Ilari OECD/NEA, Paris Dr. E. Lazo OECD/NEA, Paris
Introduction
On 26 April, 1986, the Chernobyl nuclear power station, in Ukraine, suffered a major accident which was followed by a prolonged release to the atmosphere of large quantities of radioactive substances. The specific features of the release favoured a widespread distribution of radioactivity throughout the northern hemisphere, mainly across Europe. A contributing factor was the variation of meteorological conditions and wind regimes during the period of release. Activity transported by the multiple plumes from Chernobyl was measured not only in Northern and in Southern Europe, but also in Canada, Japan and the United States. Only the Southern hemisphere remained free of contamination.
This had serious radiological, health and socio-economic consequences for the populations of Belarus, Ukraine and Russia, and to some extent they are still suffering from these consequences. Although the radiological impact of the accident in other countries was generally very low, and even insignificant outside Europe, this event had, however, the effect of enhancing public apprehension all over the world on the risks associated with the use of nuclear energy.
This is one of the reasons explaining the renewed attention and effort devoted during the last decade to the reactor safety studies and to emergency preparedness by public authorities and the nuclear industry. This also underlies the continuing attention of the public opinion to the situation at Chernobyl.
The forthcoming tenth anniversary of the accident appears, therefore, the right moment to review the status of our knowledge of the serious aspects of the accident impact, to take stock of the information accumulated and the scientific studies underway, as well as to assess the degree to which national authorities and experts have implemented the numerous lessons that the Chernobyl accident taught us.
This brief report, prepared under the aegis of the Committee on Radiation Protection and Public Health (CRPPH) of the OECD Nuclear Energy Agency, presents a collective view by OECD radiation protection experts on this matter.
The accident
The Unit 4 of the Chernobyl nuclear power plant was to be shutdown for routine maintenance on 25 April 1986. On that occasion, it was decided to carry out a test of the capability of the plant equipment to provide enough electrical power to operate the reactor core cooling system and emergency equipment during the transition period between a loss of main station electrical power supply and the start up of the emergency power supply provided by diesel engines.
Unfortunately, this test, which was considered essentially to concern the non-nuclear part of the power plant, was carried out without a proper exchange of information and co-ordination between the team in charge of the test and the personnel in charge of the operation and safety of the nuclear reactor. Therefore, inadequate safety precautions were included in the test programme and the operating personnel were not alerted to the nuclear safety implications and potential danger of the electrical test.
This lack of co-ordination and awareness, resulting from an insufficient level of "safety culture" within the plant staff, led the operators to take a number of actions which deviated from established safety procedures and led to a potentially dangerous situation. This course of actions was compounded by the existence of significant drawbacks in the reactor design which made the plant potentially unstable and easily susceptible to loss of control in case of operational errors.
The combination of these factors provoked a sudden and uncontrollable power surge which resulted in violent explosions and almost total destruction of the reactor. The consequences of this catastrophic event were further worsened by the graphite moderator and other material fires that broke out in the building and contributed to a widespread and prolonged release of radioactive materials to the environment.
Dispersion and deposition of radionuclides
The release of radioactive materials to the atmosphere consisted of gases, aerosols and finely fragmented nuclear fuel particles. This release was extremely high in quantity, involving a large fraction of the radioactive product inventory existing in the reactor, and its duration was unexpectedly long, lasting for more than a week. This duration and the high altitude (about 1 km) reached by the release were largely due to the graphite fire which was very difficult to extinguish.
For these reasons and the concomitant frequent changes of wind direction during the release period, the area affected by the radioactive plume and the consequent deposition of radioactive substances on the ground was extremely large, encompassing the whole Northern hemisphere, although significant contamination outside the former Soviet Union was only experienced in part of Europe.
The pattern of contamination on the ground and in foodchains was, however, very uneven in some areas due to the influence of rainfall during the passage of the plume. This irregularity in the pattern of deposition was particularly pronounced at larger distances from the reactor site.
Reactions of national authorities
The scale and severity of the Chernobyl accident had not been foreseen and took most national authorities responsible for public health and emergency preparedness by surprise. The intervention criteria and procedures existing in most countries were not adequate for dealing with an accident of such scale and provided little help in decision-making concerning the choice and adoption of protective measures. In addition, early in the course of the accident there was little information available and considerable political pressure, partially based on the public perception of the radiation danger, was being exerted on the decision-makers.
In these circumstances, cautious immediate actions were felt necessary and in many cases measures were introduced that tended to err, sometimes excessively so, on the side of prudence rather than being driven by informed scientific and expert judgement.
Within the territory of the former Soviet Union, short-term countermeasures were massive and, in general, reasonably timely and effective. However, difficulties emerged when the authorities tried to establish criteria for the management of the contaminated areas on the long term and the associated relocation of large groups of population. Various approaches were proposed and criteria were applied over the years. Eventually, criteria for population resettlement or relocation from contaminated areas were adopted in which radiation protection requirements and economic compensation considerations were intermingled. This was and continues to be a source of confusion and possible abuse.
The progressive spread of contamination at large distances from the accident site caused considerable concern in many countries outside the former Soviet Union and the reactions of the national authorities to this situation were extremely varied, ranging from a simple intensification of the normal environmental monitoring programmes, without adoption of specific countermeasures, to compulsory restrictions concerning the marketing and consumption of foodstuffs.
Apart from the objective differences of contamination levels and regulatory and public health systems between countries, one of the principal reasons for the variety of situations observed in the different countries stems from the different criteria adopted for the choice and application of intervention levels for the implementation of protective actions. These discrepancies were in some cases due to misinterpretation and misuse of international radiation protection guidelines, especially in the case of food contamination, and were further enhanced by the overwhelming role played in many cases by non-radiological factors, such as socio-economic, political and psychological, in determining the countermeasures.
This situation caused concern and confusion among the public, perplexities among the experts and difficulties to national authorities, including problems of public credibility, as well as a waste of efforts and unnecessary economic losses. These problems were particularly felt in areas close to international borders due to different reactions of the authorities and media in bordering countries. However, all these issues were soon identified as an area where several lessons should be learned and international efforts were undertaken to harmonise criteria and approaches to emergency management.
Radiation dose estimates
Most of the population of the Northern hemisphere was exposed, to various degrees, to radiation from the Chernobyl accident. After several years of accumulation of dosimetric data from all available sources and dose reconstruction calculations based on environmental contamination data and mathematical models, it is now possible to arrive at a reasonable, although not highly accurate, assessment of the ranges of doses received by the various groups of population affected by the accident.
The main doses of concern are those to the thyroid due to external irradiation and inhalation and ingestion of radioactive iodine isotopes, and those to the whole body due to external irradiation from and ingestion of radioactive caesium isotopes. According to current estimates, the situation for the different exposed groups is the following:
Doses to the thyroid ranging from 70 millisieverts to adults up to about 1,000 millisieverts (i.e., 1 sievert) to young children and an average individual dose of 15 millisieverts [mSv] to the whole body were estimated to have been absorbed by this population prior to their evacuation. Many of these people continued to be exposed, although to a lesser extent depending on the sites of their relocation, after their evacuation from the 30-km zone.
A restricted number, of the order of 400, including plant staff, firemen and medical aid personnel, were on the site during the accident and its immediate aftermath and received very high doses from a variety of sources and exposure pathways. Among them were all those who developed acute radiation syndrome and required emergency medical treatment. The doses to these people ranged from a few grays to well above 10 grays to the whole body from external irradiation and comparable or even higher internal doses, in particular to the thyroid, from incorporation of radionuclides. A number of scientists, who periodically performed technical actions inside the destroyed reactor area during several years, accumulated over time doses of similar magnitude.
The largest group of liquidators participated in clean-up operations for variable durations over a number of years after the accident. Although they were not operating anymore in emergency conditions and were submitted to controls and dose limitations, they received significant doses ranging from tens to hundreds of millisieverts.
Health impact
The health impact of the Chernobyl accident can be described in terms of acute health effects (death, severe health impairment), late health effects (cancers) and psychological effects liable to affect health.
The acute health effects occurred among the plant personnel and the persons who intervened in the emergency phase to fight fires, provide medical aid and immediate clean-up operations. A total of 31 persons died as a consequence of the accident, and about 140 persons suffered various degrees of radiation sickness and health impairment. No members of the general public suffered these kinds of effects.
As far as the late health effects are concerned, namely the possible increase of cancer incidence, in the decade following the accident there has been a real and significant increase of carcinomas of the thyroid among the children living in the contaminated regions of the former Soviet Union, which should be attributed to the accident until proved otherwise. There might also be some increase of thyroid cancers among the adults living in those regions. From the observed trend of this increase of thyroid cancers it is expected that the peak has not yet been reached and that this kind of cancer will still continue for some time to show an excess above its natural rate in the area.
On the other hand, the scientific and medical observation of the population has not revealed any increase in other cancers, as well as in leukaemia, congenital abnormalities, adverse pregnancy outcomes or any other radiation induced disease that could be attributed to the Chernobyl accident. This observation applies to the whole general population, both within and outside the former Soviet Union. Large scientific and epidemiological research programmes, some of them sponsored by international organisations such as the WHO and the EC, are being conducted to provide further insight into possible future health effects. However, the population dose estimates generally accepted tend to indicate that, with the exception of thyroid disease, it is unlikely that the exposure would lead to discernible radiation effects in the general population above the background of natural incidence of the same diseases. In the case of the liquidators this forecast should be taken with some caution.
An important effect of the accident, which has a bearing on health, is the appearance of a widespread status of psychological stress in the populations affected. The severity of this phenomenon, which is mostly observed in the contaminated regions of the former Soviet Union, appears to reflect the public fears about the unknowns of radiation and its effects, as well as its mistrust towards public authorities and official experts, and is certainly made worse by the disruption of the social networks and traditional ways of life provoked by the accident and its long-term consequences.
Agricultural and environmental impacts
The impact of the accident on agricultural practices, food production and use and other aspects of the environment has been and continues to be much more widespread than the direct health impact on humans.
Several techniques of soil treatment and decontamination to reduce the accumulation of radioactivity in agricultural produce and cow's milk and meat have been experimented with positive results in some cases. Nevertheless, within the former Soviet Union large areas of agricultural land are still excluded from use and are expected to continue to be so for a long time. In a much larger area, although agricultural and dairy production activities are carried out, the food produced is subjected to strict controls and restrictions of distribution and use.
Similar problems of control and limitation of use, although of a much lower severity, were experienced in some countries of Europe outside the former Soviet Union, where agricultural and farm animal production were subjected to restrictions for variable durations after the accident. Most of these restrictions have been lifted several years ago. However, there are still today some areas in Europe where restrictions on slaughter and distribution of animals are in force. This concerns, for example, several hundreds of thousands of sheep in the United Kingdom and large numbers of sheep and reindeer in some Nordic countries.
A kind of environment where special problems were and continue to be experienced is the forest environment. Because of the high filtering characteristics of trees, deposition was often higher in forests than in other areas. An extreme case was the so-called "red forest" near to the Chernobyl site where the irradiation was so high as to kill the trees which had to be destroyed as radioactive waste. In more general terms, forests, being a source of timber, wild game, berries and mushrooms as well as a place for work and recreation, continue to be of concern in some areas and are expected to constitute a radiological problem for a long time.
Water bodies, such as rivers, lakes and reservoirs can be, if contaminated, an important source of human radiation exposure because of their uses for recreation, drinking and fishing. In the case of the Chernobyl accident this segment of the environment did not contribute significantly to the total radiation exposure of the population. It was estimated that the component of the individual and collective doses that can be attributed to the water bodies and their products did not exceed 1 or 2 percent of the total exposure resulting from the accident. The contamination of the water system has not posed a public health problem during the last decade; nevertheless, in view of the large quantities of radioactivity deposited in the catchment area of the system of water bodies in the contaminated regions around Chernobyl, there will continue to be for a long time a need for careful monitoring to ensure that washout from the catchment area will not contaminate drinking-water supplies.
Outside the former Soviet Union, no concerns were ever warranted for the levels of radioactivity in drinking water. On the other hand, there are lakes, particularly in Switzerland and the Nordic countries, where restrictions were necessary for the consumption of fish. These restrictions still exist in Sweden, for example, where thousands of lakes contain fish with a radioactivity content which is still higher than the limits established by the authorities for sale on the market.
Potential residual risks
Within seven months of the accident, the destroyed reactor was encased in a massive concrete structure, known as the "sarcophagus", to provide some form of confinement of the damaged nuclear fuel and destroyed equipment and reduce the likelihood of further releases of radioactivity to the environment. This structure was, however, not conceived as a permanent containment but rather as a provisional barrier pending the definition of a more radical solution for the elimination of the destroyed reactor and the safe disposal of the highly radioactive materials.
Nine years after its erection, the sarcophagus structure, although still generally sound, raises concerns for its long-term resistance and represents a standing potential risk. In particular, the roof of the structure presented for a long time numerous cracks with consequent impairment of leaktightness and penetration of large quantities of rain water which is now highly radioactive. This also creates conditions of high humidity producing corrosion of metallic structures which contribute to the support of the sarcophagus. Moreover, some massive concrete structures, damaged or dislodged by the reactor explosion, are unstable and their failure, due to further degradation or to external events, could provoke a collapse of the roof and part of the building.
According to various analyses, a number of potential accidental scenarios could be envisaged. They include a criticality excursion due to change of configuration of the melted nuclear fuel masses in the presence of water leaked from the roof, a resuspension of radioactive dusts provoked by the collapse of the enclosure and the long-term migration of radionuclides from the enclosure into the groundwater. The first two accident scenarios would result in the release of radionuclides into the atmosphere which would produce a new contamination of the surrounding area within a radius of several tens of kilometres. It is not expected, however, that such accidents could have serious radiological consequences at longer distances.
As far as the leaching of radionuclides from the fuel masses by the water in the enclosure and their migration into the groundwater are concerned, this phenomenon is expected to be very slow and it has been estimated that, for example, it will take 45 to 90 years for certain radionuclides such as strontium90 to migrate underground up to the Pripyat River catchment area. The expected radiological significance of this phenomenon is not known with certainty and a careful monitoring of the evolving situation of the groundwater will need to be carried out for a long time.
The accident recovery and clean-up operations have resulted in the production of very large quantities of radioactive wastes and contaminated equipment which are currently stored in about 800 sites within and outside the 30-km exclusion zone around the reactor. These wastes and equipment are partly buried in trenches and partly conserved in containers isolated from groundwater by clay or concrete screens. A large number of contaminated equipment, engines and vehicles are also stored in the open air.
All these wastes are a potential source of contamination of the groundwater which will require close monitoring until a safe disposal into an appropriate repository is implemented.
In general, it can be concluded that the sarcophagus and the proliferation of waste storage sites in the area constitute a series of potential sources of release of radioactivity that threatens the surrounding area. However, any such releases are expected to be very small in comparison with those from the Chernobyl accident in 1986 and their consequences would be limited to a relatively small area around the site. On the other hand, concerns have been expressed by some experts that a much more important release might occur if the collapse of the sarcophagus should induce damage in the Unit 3 of the Chernobyl power plant, which currently is still in operation.
In any event, initiatives have been taken internationally, and are currently underway, to study a technical solution leading to the elimination of these sources of residual risk on the site.
Lessons learned
The Chernobyl accident was very specific in nature and it should not be seen as a reference accident for future emergency planning purposes. However, it was very clear from the reactions of the public authorities in the various countries that they were not prepared to deal with an accident of this magnitude and that technical and/or organisational deficiencies existed in emergency planning and preparedness in almost all countries.
The lessons that could be learned from the Chernobyl accident were, therefore, numerous and encompassed all areas, including reactor safety and severe accident management, intervention criteria, emergency procedures, communication, medical treatment of irradiated persons, monitoring methods, radioecological processes, land and agricultural management, public information, etc.
However, the most important lesson learned was probably the understanding that a major nuclear accident has inevitable transboundary implications and its consequences could affect, directly or indirectly, many countries even at large distances from the accident site. This led to an extraordinary effort to expand and reinforce international co-operation in areas such as communication, harmonisation of emergency management criteria and co-ordination of protective actions. Major improvements were achieved in this decade and important international mechanisms of co-operation and information were established, such as the international conventions on early notification and assistance in case of a radiological accident, by the IAEA and the EC, the international nuclear emergency exercises (INEX) programme, by the NEA, the international accident severity scale (INES), by the IAEA and NEA and the international agreement on food contamination, by the FAO and WHO.
At the national level, the Chernobyl accident also stimulated authorities and experts to a radical review of their understanding of and attitude to radiation protection and nuclear emergency issues. This prompted many countries to establish nationwide emergency plans in addition to the existing structure of local emergency plans for individual nuclear facilities. In the scientific and technical area, besides providing new impetus to nuclear safety research, especially on the management of severe nuclear accidents, this new climate led to renewed efforts to expand knowledge on the harmful effects of radiation and their medical treatment and to revitalise radioecological research and environmental monitoring programmes. Substantial improvements were also achieved in the definition of criteria and methods for the information of the public, an aspect whose importance was particularly evident during the accident and its aftermath.
Conclusion
The history of the modern industrial world has been affected on many occasions by catastrophes comparable or even more severe than the Chernobyl accident. Nevertheless, this accident, due not only to its severity but especially to the presence of ionising radiation, had a significant impact on human society.
Not only it produced severe health consequences and physical, industrial and economic damage in the short term, but, also, its long-term consequences in terms of socio-economic disruption, psychological stress and damaged image of nuclear energy, are expected to be long standing.
However, the international community has demonstrated a remarkable ability to apprehend and treasure the lessons to be drawn from this event, so that it will be better prepared to cope with a challenge of this kind, if ever a severe nuclear accident should happen again.
The Site
At the time of the Chernobyl accident, on 26 April 1986, the
Soviet Nuclear Power Programme was based mainly upon two types
of reactors, the WWER, a pressurised light-water reactor, and
the RBMK, a graphite moderated light-water reactor. While the
WWER type of reactor was exported to other countries, the RBMK
design was restricted to republics within the Soviet Union.
The Chernobyl Power Complex, lying about 130 km north of Kiev,
Ukraine (Figure 1), consisted of four nuclear reactors of the
RBMK-1000 design, Units 1 and 2 being constructed between 1970
and 1977, while Units 3 and 4 of the same design were completed
in 1983 (IA86). Two more RBMK reactors were under construction
at the site at the time of the accident.
This area of Ukraine is described as Belarussian-type woodland
with a low population density. About 3 km away from the reactor,
in Pripyat, there were 49,000 inhabitants. The town of Chernobyl,
which had a population of 12,500, is about 15 km to the South-east
of the complex. Within a 30-km radius of the power plant, the
total population was between 115,000 and 135,000.
The RBMK-1000 reactor
The RBMK-1000 (Figure 2) is a Soviet designed and built graphite
moderated pressure tube type reactor, using slightly enriched
(2 per cent uranium-235) uranium dioxide fuel. It is a boiling
light water reactor, with
The moderator, whose function is to slow down neutrons to make
them more efficient in producing fission in the fuel, is constructed
of graphite. A mixture of nitrogen and helium is circulated between
the graphite blocks largely to prevent oxidation of the graphite
and to improve the transmission of the heat produced by neutron
interactions in the graphite, from the moderator to the fuel channel.
The core itself is about 7 m high and about 12 m in diameter.
There are four main coolant circulating pumps, one of which is
always on standby. The reactivity or power of the reactor is controlled
by raising or lowering 211 control rods, which, when lowered,
absorb neutrons and reduce the fission rate. The power output
of this reactor is 3,200 MW(t) [megawatt thermal] or 1,000 MW(e),
although there is a larger version producing 1,500 MW(e). Various
safety systems, such as an emergency core cooling system and the
requirement for an absolute minimal insertion of 30 control rods,
were incorporated into the reactor design and operation.
The most important characteristic of the RBMK reactor is that
it possesses a "positive void coefficient". This means
that if the power increases or the flow of water decreases, there
is increased steam production in the fuel channels, so that the
neutrons that would have been absorbed by the denser water will
now produce increased fission in the fuel. However, as the power
increases, so does the temperature of the fuel, and this has the
effect of reducing the neutron flux (negative fuel coefficient).
The net effect of these two opposing characteristics varies with
the power level. At the high power level of normal operation,
the temperature effect predominates, so that power excursions
leading to excessive overheating of the fuel do not occur. However,
at a lower power output of less than 20 per cent of the maximum,
the positive void coefficient effect is dominant and the reactor
becomes unstable and prone to sudden power surges. This was a
major factor in the development of the accident.
Events leading to the accident (IA86, IA86a)
The Unit 4 reactor was to be shutdown for routine maintenance
on 25 April 1986. It was decided to take advantage of this shutdown
to determine whether, in the event of a loss of station power,
the slowing turbine could provide enough electrical power to operate
the emergency equipment and the core cooling water circulating
pumps, until the diesel emergency power supply became operative.
The aim of this test was to determine whether cooling of the core
could continue to be ensured in the event of a loss of power.
This type of test had been run during a previous shut-down period,
but the results had been inconclusive, so it was decided to repeat
it. Unfortunately, this test, which was considered essentially
to concern the non-nuclear part of the power plant, was carried
out without a proper exchange of information and co-ordination
between the team in charge of the test and the personnel in charge
of the operation and safety of the nuclear reactor. Therefore,
inadequate safety precautions were included in the test programme
and the operating personnel were not alerted to the nuclear safety
implications of the electrical test and its potential danger.
The planned programme called for shutting off the reactor's emergency
core cooling system (ECCS), which provides water for cooling the
core in an emergency. Although subsequent events were not greatly
affected by this, the exclusion of this system for the whole duration
of the test reflected a lax attitude towards the implementation
of safety procedures.
As the shutdown proceeded, the reactor was operating at about
half power when the electrical load dispatcher refused to allow
further shutdown, as the power was needed for the grid. In accordance
with the planned test programme, about an hour later the ECCS
was switched off while the reactor continued to operate at half
power. It was not until about 23:00 hr on 25 April that the grid
controller agreed to a further reduction in power.
For this test, the reactor should have been stabilised at about
1,000 MW(t) prior to shut down, but due to operational error the
power fell to about 30 MW(t), where the positive void coefficient
became dominant. The operators then tried to raise the power to
700-1,000 MW(t) by switching off the automatic regulators and
freeing all the control rods manually. It was only at about 01:00
hr on 26 April that the reactor was stabilised at about 200 MW(t).
Although there was a standard operating order that a minimum of
There was an increase in coolant flow and a resulting drop in
steam pressure. The automatic trip which would have shut down
the reactor when the steam pressure was low, had been circumvented.
In order to maintain power the operators had to withdraw nearly
all the remaining control rods. The reactor became very unstable
and the operators had to make adjustments every few seconds trying
to maintain constant power.
At about this time, the operators reduced the flow of feedwater,
presumably to maintain the steam pressure. Simultaneously, the
pumps that were powered by the slowing turbine were providing
less cooling water to the reactor. The loss of cooling water exaggerated
the unstable condition of the reactor by increasing steam production
in the cooling channels (positive void coefficient), and the operators
could not prevent an overwhelming power surge, estimated to be
The sudden increase in heat production ruptured part of the fuel
and small hot fuel particles, reacting with water, caused a steam
explosion, which destroyed the reactor core. A second explosion
added to the destruction two to three seconds later. While it
is not known for certain what caused the explosions, it is postulated
that the first was a steam/hot fuel explosion, and that hydrogen
may have played a role in the second.
The accident
The accident occurred at 01:23 hr on Saturday, 26 April 1986,
when the two explosions destroyed the core of Unit 4 and the roof
of the reactor building.
In the IAEA Post-Accident Assessment Meeting in August 1986 (IA86),
much was made of the operators' responsibility for the accident,
and not much emphasis was placed on the design faults of the reactor.
Later assessments (IA86a) suggest that the event was due
to a combination of the two, with a little more emphasis on the
design deficiencies and a little less on the operator actions.
The two explosions sent a shower of hot and highly radioactive
debris and graphite into the air and exposed the destroyed core
to the atmosphere. The plume of smoke, radioactive fission products
and debris from the core and the building rose up to about 1 km
into the air. The heavier debris in the plume was deposited close
to the site, but lighter components, including fission products
and virtually all of the noble gas inventory were blown by the
prevailing wind to the North-west of the plant.
Fires started in what remained of the Unit 4 building, giving
rise to clouds of steam and dust, and fires also broke out on
the adjacent turbine hall roof and in various stores of diesel
fuel and inflammable materials. Over 100 fire-fighters from the
site and called in from Pripyat were needed, and it was this group
that received the highest radiation exposures and suffered the
greatest losses in personnel. These fires were put out by 05:00
hr of the same day, but by then the graphite fire had started.
Many firemen added to their considerable doses by staying on call
on site. The intense graphite fire was responsible for the dispersion
of radionuclides and fission fragments high into the atmosphere.
The emissions continued for about twenty days , but were much
lower after the tenth day when the graphite fire was finally extinguished.
The graphite fire
While the conventional fires at the site posed no special firefighting
problems, very high radiation doses were incurred by the firemen.
However, the graphite moderator fire was a special problem. Very
little national or international expertise on fighting graphite
fires existed, and there was a very real fear that any attempt
to put it out might well result in further dispersion of radionuclides,
perhaps by steam production, or it might even provoke a criticality
excursion in the nuclear fuel.
A decision was made to layer the graphite fire with large amounts
of different materials, each one designed to combat a different
feature of the fire and the radioactive release. Boron carbide
was dumped in large quantities from helicopters to act as a neutron
absorber and prevent any renewed chain reaction. Dolomite was
also added to act as heat sink and a source of carbon dioxide
to smother the fire. Lead was included as a radiation absorber,
as well as sand and clay which it was hoped would prevent the
release of particulates. While it was later discovered that many
of these compounds were not actually dropped on the target, they
may have acted as thermal insulators and precipitated an increase
in the temperature of the damaged core leading to a further release
of radionuclides a week later.
By May 9, the graphite fire had been extinguished, and work began
on a massive reinforced concrete slab with a built-in cooling
system beneath the reactor. This involved digging a tunnel from
underneath Unit 3. About four hundred people worked on this tunnel
which was completed in 15 days, allowing the installation of the
concrete slab. This slab would not only be of use to cool the
core if necessary, it would also act as a barrier to prevent penetration
of melted radioactive material into the groundwater.
In summary, the Chernobyl accident was the product of a lack
of "safety culture". The reactor design was poor from
the point of view of safety and unforgiving for the operators,
both of which provoked a dangerous operating state. The operators
were not informed of this and were not aware that the test performed
could have brought the reactor into explosive conditions. In addition,
they did not comply with established operational procedures. The
combination of these factors provoked a nuclear accident of maximum
severity in which the reactor was totally destroyed within a few
seconds.
The source term
The "source term" is a technical expression used to
describe the accidental release of radioactive material from a
nuclear facility to the environment. Not only are the levels of
radioactivity released important, but also their distribution
in time as well as their chemical and physical forms. The initial
estimation of the Source Term was based on air sampling and the
integration of the assessed ground deposition within the then
Soviet Union. This was clear at the IAEA Post-Accident Review
Meeting in August 1986 (IA86), when the Soviet scientists
made their presentation, but during the discussions it was suggested
that the total release estimate would be significantly higher
if the deposition outside the Soviet Union territory were included.
Subsequent assessments support this view, certainly for the caesium
radionuclides (Wa87, Ca87, Gu89). The initial estimates
were presented as a fraction of the core inventory for the important
radionuclides and also as total activity released.
Atmospheric releases
In the initial assessment of releases made by the Soviet scientists
and presented at the IAEA Post-Accident Assessment Meeting in
Vienna (IA86), it was estimated that 100 per cent of the
core inventory of the noble gases (xenon and krypton) was released,
and between 10 and 20 per cent of the more volatile elements of
iodine, tellurium and caesium. The early estimate for fuel material
released to the environment was 3 ± 1.5 per cent (IA86).
This estimate was later revised to 3.5 ± 0.5 per cent (Be91).
This corresponds to the emission of 6 t of fragmented fuel.
The IAEA International Nuclear Safety Advisory Group (INSAG) issued
in 1986 its summary report (IA86a) based on the information
presented by the Soviet scientists to the Post-Accident Review
Meeting. At that time, it was estimated that 1 to 2 exabecquerels
(EBq) were released. This did not include the noble gases, and
had an estimated error of ±50 per cent. These estimates of
the source term were based solely on the estimated deposition
of radionuclides on the territory of the Soviet Union, and could
not take into account deposition in Europe and elsewhere, as the
data were not then available.
However, more deposition data (Be90) were available when,
in their 1988 Report (UN88), the United Nations Scientific
Committee on the Effects of Atomic Radiation (UNSCEAR) gave release
figures based not only on the Soviet data, but also on worldwide
deposition. The total caesium-137 release was estimated to be
70 petabecquerels (PBq) of which 31 PBq were deposited in the
Soviet Union.
Later analyses carried out on the core debris and the deposited
material within the reactor building have provided an independent
assessment of the environmental release. These studies estimate
that the release fraction of caesium-137 was 20 to 40 per cent
(85 ± 26 PBq) based on an average release fraction from fuel
of 47 per cent with subsequent retention of the remainder within
the reactor building (Be91). After an extensive review
of the many reports (IA86, Bu93), this was confirmed.
For iodine-131, the most accurate estimate was felt to be 50 to
60 per cent of the core inventory of 3,200 PBq. The current estimate
of the source term (De95) is summarised in Table 1.
The release pattern over time is well illustrated in Figure 3
(Bu93). The initial large release was principally due to
the mechanical fragmentation of the fuel during the explosion.
It contained mainly the more volatile radionuclides such as noble
gases, iodines and some caesium. The second large release between
day 7 and day 10 was associated with the high temperatures reached
in the core melt. The sharp drop in releases after ten days may
have been due to a rapid cooling of the fuel as the core debris
melted through the lower shield and interacted with other material
in the reactor. Although further releases probably occurred after
6 May, these are not thought to have been large.
The release of radioactive material to the atmosphere consisted
of gases, aerosols and finely fragmented fuel. Gaseous elements,
such as krypton and xenon escaped more or less completely from
the fuel material. In addition to its gaseous and particulate
form, organically bound iodine was also detected. The ratios between
the various iodine compounds varied with time. As mentioned
Unexpected features of the source term, due largely to the graphite
fire, were the extensive releases of fuel material and the long
duration of the release. Elements of low volatility, such as cerium,
zirconium, the actinides and to a large extent barium, lanthanium
and strontium also, were embedded in fuel
particles. Larger fuel particles were deposited close to the accident
site, whereas smaller particles were more widely dispersed. Other
condensates from the vaporised fuel, such as radioactive ruthenium,
formed metallic particles. These, as well as the small fuel particles,
were often referred to as "hot particles", and were
found at large distances from the accident site (De95).
Dispersion and deposition
Within the former Soviet Union
During the first 10 days of the accident when important releases
of radioactivity occurred, meteorological conditions changed frequently,
causing significant variations in release direction and dispersion
parameters. Deposition patterns of radioactive particles depended
highly on the dispersion parameters, the particle sizes, and the
occurrence of rainfall. The largest particles, which were primarily
fuel particles, were deposited essentially by sedimentation within
100 km of the reactor. Small particles were carried by the wind
to large distances and were deposited primarily with rainfall.
The radionuclide composition of the release and of the subsequent
deposition on the ground also varied considerably during the accident
due to variations in temperature and other parameters during the
release. Caesium-137 was selected to characterise the magnitude
of the ground deposition because (1) it is easily measurable,
and (2) it was the main contributor to the radiation doses received
by the population once the short-lived iodine-131 had decayed.
The three main spots of contamination resulting from the Chernobyl
accident have been called the Central, Bryansk-Belarus, and Kaluga-Tula-Orel
spots (Figure 4). The Central spot was formed during the initial,
active stage of the release
The Bryansk-Belarus spot, centered 200 km to the North-northeast
of the reactor, was formed on 28-29 April as a result of rainfall
on the interface of the Bryansk region of Russia and the Gomel
and Mogilev regions of Belarus. The ground depositions of caesium-137
in the most highly contaminated areas in this spot were comparable
to the levels in the Central spot and reached 5,000 kBq/m2 in
some villages (Ba93).
In addition, outside the three main hot spots in the greater part
of the European territory of the former Soviet Union, there were
many areas of radioactive contamination with caesium-137 levels
in the range 40 to 200 kBq/m2. Overall, the territory of the former
Soviet Union initially contained approximately 3,100 km2 contaminated
by caesium-137 with deposition levels exceeding 1,500 kBq/m2;
7,200 km2 with levels of 600 to 1,500 kBq/m2; and 103,000 km2
with levels of 40 to 200 kBq/m2 (US91).
Outside the former Soviet Union
Radioactivity was first detected outside the Soviet Union at a
Nuclear Power station in Sweden, where monitored workers were
noted to be contaminated. It was at first believed that the contamination
was from a Swedish reactor. When it became apparent that the Chernobyl
reactor was the source, monitoring stations all over the world
began intensive sampling programmes.
The radioactive plume was tracked as it moved over the European
part of the Soviet Union and Europe (Figure 6). Initially the
wind was blowing in a Northwesterly direction and was responsible
for much of the deposition in Scandinavia, the Netherlands and
Belgium and Great Britain. Later the plume shifted
The radioactive cloud initially contained a large number of different
fission products and actinides, but only trace quantities of actinides
were detected in most European countries, and a very small number
were found in quantities that were considered radiologically significant.
This was largely due to the fact that these radionuclides were
contained in the larger and heavier particulates, which tended
to be deposited closer to the accident site rather than further
away. The most radiologically important radionuclides detected
outside the Soviet Union were iodine-131, tellurium/iodine-132,
caesium-137 and caesium-134.
While the plume was detectable in the Northern hemisphere as far
away as Japan and North America, countries outside Europe received
very little deposition of radionuclides from the accident. No
deposition was detected in the Southern hemisphere (Un88).
In summary it can be stated that there is now a fairly accurate
estimate of the total release. The duration of the release was
unexpectedly long, lasting more than a week with two periods of
intense release. Another peculiar feature was the significant
emission (about 4 per cent) of fuel material which also contained
embedded radionuclides of low volatility such as cerium, zirconium
and the actinides. The composition and characteristics of the
radioactive material in the plume changed during its passage due
to wet and dry deposition, decay, chemical transformations and
alterations in particle size. The area affected was particularly
large due to the high altitude and long duration of the release
as well as the change of wind direction. However, the pattern
of deposition was very irregular, and significant deposition of
radionuclides occurred where the passage of the plume coincided
with rainfall. Although all the Northern hemisphere was affected,
only territories of the former Soviet Union and part of Europe
experienced contamination to a significant degree.
The scale and severity of the Chernobyl accident with its widespread
radioactive contamination had not been foreseen and took by surprise
most national authorities responsible for emergency preparedness.
No provisions had been made for an accident of such scale and,
though some radiation protection authorities had made criteria
available for intervention in an accident, these were often incomplete
and provided little practical help in the circumstances, so that
very few workable national guidelines or principles were actually
in place. Those responsible for making national decisions were
suddenly faced with an accident for which there were no precedents
upon which to base their decisions. In addition, early in the
course of the accident there was little information available,
and considerable political pressure, partially based on the public
perception of the radiation danger, was being exerted on the decision-makers.
In these circumstances, cautious immediate action was felt necessary,
and measures were introduced that tended to err, sometimes excessively
so, on the side of prudence rather than being driven by informed
scientific and expert judgement.
Within the former Soviet Union
The town of Pripyat was not severely contaminated by the initial
release of radionuclides, but, once the graphite fire started,
it soon became obvious that contamination would make the town
uninhabitable. Late on 26 April it was decided to evacuate the
town, and arrangements for transport and accommodation of the
evacuees were made. The announcement of evacuation was made at
11:00 hr the following day. Evacuation began at 14:00 hr, and
Pripyat was evacuated in about two and one half hours. As measurements
disclosed the extensive pattern of deposition of radionuclides,
and it was possible to make dose assessments, the remainder of
the people in a 30-km zone around the reactor complex were gradually
evacuated, bringing the total evacuees to about 135,000.
Other countermeasures to reduce dose were widely adopted (Ko90).
Decontamination procedures performed by military personnel included
the washing of buildings, cleaning residential areas, removing
contaminated soil, cleaning roads and decontaminating water supplies.
Special attention was paid to schools, hospitals and other buildings
used by large numbers of people. Streets were watered in towns
to suppress dust. However, the effectiveness of these countermeasures
outside the 30-km zone was small. An attempt to reduce thyroid
doses by the administration of stable iodine to block radioactive
uptake by the thyroid was made (Me92), but its success
was doubtful.
The Soviet National Committee on Radiation Protection (NCRP) proposed
a 350-mSv lifetime dose intervention level for the relocation
of population groups (Il87). This value was lower by a
factor of 2 to 3 than that recommended by the International Commission
on Radiological Protection (ICRP) for the same countermeasure.
Nevertheless, this value proposed by the NCRP was strongly criticised
as being a very high level. The situation was further complicated
by the political and social tension in the Soviet Union at that
time. As a result, the NCRP proposal was not adopted by the Supreme
Soviet. Later, a special Commission was established which developed
new recommendations for intervention levels. These recommendations
were based on the levels of ground contamination by the radionuclides
caesium-137, strontium-90 and plutonium239. As has been mentioned
above, large areas were contaminated mainly by caesium-137 and
a ground contamination level by this radionuclide of 1,480 kBq/m2
was used as the intervention criterion for permanent resettlement
of population, and of 555 to 1,480 kBq/m2 for temporary relocation.
People who continued to live in the heavily contaminated areas
were given compensation and offered annual medical examinations
by the government. Residents of less contaminated areas are provided
with medical monitoring. Current decisions on medical actions
are based on annual doses. Compensation is provided for residents
whose annual dose is greater than 1mSv. The use of locally produced
milk and mushrooms is restricted in some of these areas. Relocation
is considered in Russia for annual doses above 5 mSv.
As is mentioned in the section on psychological effects, in Chapter
V, the Soviet authorities did not foresee that their attempts
to compensate those affected by the accident would be misinterpreted
by the recipients and increase their stress, and that the label
of "radiophobia" attributed to these phenomena was not
only incorrect, but was one that alienated the public even more.
Some of these initial approaches have been recognised as being
inappropriate and the authorities are endeavouring to rectify
their attitude to the exposed population.
Outside the former Soviet Union
The progressive spread of contamination at large distances from
the accident site has caused considerable concern in Member countries,
and the reactions of national authorities to this situation have
been extremely varied, ranging from a simple intensification of
the normal environmental monitoring programmes, without adoption
of any specific countermeasures, to compulsory restrictions concerning
the marketing and consumption of foodstuffs. This variety of responses
has been accompanied by significant differences in the timing
and duration of the countermeasures.
In general, the most widespread countermeasures were those which
were not expected to impose, in the short time for which they
were in effect, a significant burden on lifestyles or the economy.
These included advice to wash fresh vegetables and fruit before
consumption, advice not to use rainwater for drinking or cooking,
and programmes of monitoring citizens returning from potentially
contaminated areas. In reality, experience has shown that even
these types of measures had, in some cases, a negative impact
which was not insignificant.
Protective actions having a more significant impact on dietary
habits and imposing a relatively important economic and regulatory
burden included restrictions or prohibitions on the marketing
and consumption of milk, dairy products, fresh leafy vegetables
and some types of meat, as well as the control of the outdoor
grazing of dairy cattle. In some areas, prohibitions were placed
on travel to areas affected by the accident and on the import
of foodstuffs from the Soviet Union and Eastern European countries.
In most Member countries, restrictions were imposed on the import
of foodstuffs from Member as well as non-Member countries.
The range of these reactions can be explained primarily by the
diversity of local situations both in terms of uneven levels of
contamination and in terms of national differences in administrative,
regulatory and public health systems. However, one of the principal
reasons for the variety of situations observed in Member countries
stems from the criteria adopted for the choice and application
of intervention levels for the implementation of protective actions.
In this respect, while the general radiation protection principles
underlying the actions taken in most Member countries following
the accident have been very similar, discrepancies arose in the
assessment of the situation and the adoption and application of
operational protection criteria. These discrepancies were further
enhanced by the overwhelming role played in many cases by non-radiological
factors, such as socio-economic, political and psychological,
in determining the countermeasures.
This situation caused concern and confusion among the public,
perplexities among the experts and difficulties to national authorities,
especially in maintaining their public credibility. This was,
therefore, identified as an area where several lessons should
be learned from the accident and efforts directed towards better
international harmonisation of the scientific bases and co-ordination
of concepts and measures for the protection of the public in case
of emergency.
Nowhere was this problem better illustrated than by the way that
contaminated food was handled. In some countries outside the Soviet
Union the main source of exposure to the general population was
the consumption of contaminated food. Mechanisms to handle locally
produced as well as imported contaminated food had to be put in
place within a few weeks of the accident. National authorities
were in an unenviable position. They had to act quickly and cautiously
to introduce measures to protect the "purity" of the
public food supply and, what is more, they had to be seen to be
effective in so doing. This inevitably led to some decisions which
even at the time appeared to be over-reactions and not scientifically
justified. In addition, dissenting opinions among experts added
to the difficulties of the decision-makers.
Some countries without nuclear power programmes and whose own
food was not contaminated, argued that they did not need to import
any "tainted" food and refused any food containing any
radionuclides whatsoever. This extreme and impracticable measure
might well have been regarded as an example of how well the authorities
of those countries were protecting the health of their population.
Sometimes this attitude appeared to promote a neighbourly rivalry
between countries to see which could set the more stringent standards
for food contamination, as though, by so doing, their own citizens
were more protected. The result was that often slightly contaminated
food was destroyed or refused importation to avoid only trivial
doses.
In 1986, the EC imposed a ban on the importation of food containing
more than 370 Bq/kg of radiocaesium for milk products and 600
Bq/kg for any other food, regardless of the quantity consumed
in the average European diet. Thus, food items with a trivial
consumption (and dose), such as spices, were treated the same
as items of high consumption such as vegetables. However, these
values were later relaxed for some food items in order to remove
inconsistent treatment of food groups.
In some special circumstances, decisions had to be made based
on the local situation. For example, in some Northern European
communities, reindeer meat is a major component of the diet; due
to the ecological circumstances, these animals tend to concentrate
radiocaesium, which will then expose the populations which depend
on them. Special countermeasures, such as pasturing reindeer in
areas of lower contamination, were introduced in some countries
to avoid this exposure.
The variety of solutions led to confusion and made any international
consensus on Derived Intervention Levels for food extremely difficult
to achieve, and it was only with the WHO/FAO Codex Alimentarius
Meeting in Geneva in 1989 that any agreement was reached on guideline
values for the radioactivity of food moving in international trade (Table 2).
Often the national authorities were not able accurately to predict
the public response to some of their advice and pronouncements.
For example, in some European countries, soon after the accident
the public were advised to wash leafy vegetables. The national
authority felt that this was innocuous advice as most people washed
their vegetables anyway, and they were unprepared for the public
response which was to stop buying these vegetables. This resulted
in significant economic loss to local producers which far outweighed
any potential benefit in terms of radiological health.
In some countries, the public was told that the risks were very
small but, at the same time, were given advice on how to reduce
these low risks. It was very difficult to explain this apparently
contradictory advice, and the national authority came under criticism
from the media (Sj87). Outside the Soviet Union, the initial
confusion led to inconsistent and precipitate actions which, although
understandable, were sometimes ill-advised and unjustified.
However, it should be emphasised that great progress has been
made since this early confusion. As a result of the actions of
the international organisations to harmonise intervention criteria
and the willingness of countries to cooperate in this endeavour,
a firm groundwork for uniform criteria based on accepted radiation
protection principles has been established, so that relative consistency
can hopefully be expected in their implementation in the event
of a possible future nuclear accident.
In summary, the Chernobyl accident took authorities by surprise
as regards extent, duration and contamination at long distance.
As no guidelines were available for such an accident, little information
was available and great political and public pressure to do something
were experienced, overprecautious decisions were often taken in
and outside the Soviet Union. The psychological impact of some
official decisions on the public were not predicted and variable
interpretations or even misinterpretations of ICRP recommendations,
especially for intervention levels for food, led to inconsistent
decisions and advice. These added to public confusion and provoked
mistrust and unnecessary economic losses. However, there were
exceptions and very soon international efforts started to harmonise
criteria and approaches to emergency management.
The exposure of the population as a result of the accident resulted
in two main pathways of exposure. The first is the radiation dose
to the thyroid as a result of the concentration of radioiodine
and similar radionuclides in the gland. The second is the whole-body
dose caused largely by external irradiation mainly from radiocesium.
The absorbed dose to the whole body is thought to be about 20
times more deleterious, in terms of late health effects incidence,
than the same dose to the thyroid (IC90).
The population exposed to radiation following the Chernobyl accident
can be divided into four categories: (1) the staff of the nuclear
power plant and workers who participated in clean-up operations
(referred to as "liquidators"); (2) the nearby residents
who were evacuated from the 30-km zone during the first two weeks
after the accident; (3) the population of the former Soviet Union,
including especially the residents of contaminated areas; and
(4) the population in countries outside the former Soviet Union.
A number of liquidators estimated to amount up to 800,000 took
part in mitigation activities at the reactor and within the 30-km
zone surrounding the reactor. The most exposed workers were the
firemen and the power plant personnel during the first days of
the accident. Most of the dose received by the workers resulted
from external irradiation from the fuel fragments and radioactive
particles deposited on various surfaces.
About 135,000 people were evacuated during the first days following
the accident, mainly from the 30-km zone surrounding the reactor.
Prior to evacuation, those individuals were exposed to external
irradiation from radioactive materials transported by the cloud
and deposited on the ground, as well as to internal irradiation
essentially due to the inhalation of radioactive materials in
the cloud.
The relative contributions to the external whole-body dose from
the main radionuclides of concern for that pathway of exposure
and during the first few months after the accident are shown in
Figure 7. It is clear that tellurium-132 played a major role in
the first week after the accident, and that, after one month,
the radiocaesiums (caesium-134 and caesium-137) became predominant.
Subsequently, however, caesium-134 decayed to levels much lower
than those of caesium-137, which became after a few years the
only radionuclide of importance for practical purposes. It is
usual to refer to caesium-137 only, even when the mix of caesium-134
and caesium-137 is meant, because the values for the constituents
can be easily derived from those for caesium-137.
Among the population of the former Soviet Union, it is usual to
single out the residents of the contaminated areas, defined as
those with caesium-137 deposition levels greater than 37 kBq/m2.
About 4 million people live in those areas. Of special interest
are the inhabitants of the spots with caesium-137 deposition levels
greater than 555 kBq/m2. In those areas, called "strict control
zones", protection measures are applied, especially as far
as control of consumption of contaminated food is concerned.
Early after the accident, the All-Union Dose Registry (AUDR) was
set up by the Soviet Government in 1986 to record medical and
dosimetric data on the
Outside the former Soviet Union, the radionuclides of importance
are, again, the radioiodines and radiocaesiums, which, once deposited
on the ground,
The liquidators
Most of the liquidators can be divided into two groups: (1) the
people who were working at the Chernobyl power station at the
time of the accident viz. the staff of the station and the firemen
and people who went to the aid of the victims. They number a few
hundred persons; and (2) the workers, estimated to amount up to
800,000, who were active in 1986-1990 at the power station or
in the zone surrounding it for the decontamination, sarcophagus
construction and other recovery operations.
On the night of 26 April 1986, about 400 workers were on the site
of the Chernobyl power plant. As a consequence of the accident,
they were subjected to the combined effect of radiation from several
sources: (1) external gamma/beta radiation from the radioactive
cloud, the fragments of the damaged reactor core scattered over
the site and the radioactive particles deposited on the skin,
and (2) inhalation of radioactive particles (UN88).
All of the dosimeters worn by the workers were over-exposed and
did not allow an estimate of the doses received. However, information
is available on the doses received by the 237 persons who were
placed in hospitals and diagnosed as suffering from acute radiation
syndrome. Using biological dosimetry, it was estimated that 140
of these patients received whole-body doses from external irradiation
in the range 1-2 Gy, that 55 received doses between 2 and 4 Gy
, that 21 received between 4 and 6 Gy, and that the remaining
21 received doses between 6 and 16 Gy. In addition, it was estimated
from thyroid measurements that the thyroid dose from inhalation
would range up to about 20 Sv, with 173 individuals in the 0-1.2
Sv range and five workers with thyroid doses greater than 11 Sv
(UN88).
The second category of liquidators consists of the large number
of adults who were recruited to assist in the clean-up operations.
They worked at the site, in towns, forests and agricultural areas
to make them fit to work and live in. Several hundreds of thousands
of individuals participated in this work. Initially, 50 per cent
of those workers came from the Soviet armed forces, the other
half including personnel of civil organisations, the security
service, the Ministry of Internal Affairs, and other organisations.
The total number of liquidators has yet to be determined accurately,
since only some of the data from some of those organisations have
been collected so far in the national registries of Belarus, Russia,
Ukraine and other republics of the former Soviet Union (So95).
Also, it has been suggested that, because of the social and economic
advantages associated with being designated a liquidator, many
persons have contrived latterly to have their names added to the
list.
There are only fragmented data on the doses received by the liquidators.
Attempts to establish a dosimetric service were inadequate until
the middle of June of 1986; until then, doses were estimated from
area radiation measurements. The liquidators were initially subjected
to a radiation dose limit for one year of 250 mSv. In 1987 this
limit was reduced to 100 mSv and in 1988 to 50 mSv (Ba93).
The registry data show that the average recorded doses decreased
from year to year, being about 170 mSv in 1986, 130 mSv in 1987,
30 mSv in 1988 and 15 mSv in 1989 (Se95a). It is, however,
difficult to assess the validity of the results as they have been
reported.
It is interesting to note that a small special group of 15 scientists
who have worked periodically inside the sarcophagus for a number
of years have estimated accumulated whole-body doses in the range
0.5 to 13 Gy (Se95a). While no deterministic effects have
been noted to date, this group may well show radiation health
effects in the future.
The evacuees from the 30-km zone
Immediately after the accident monitoring of the environment was
started by gamma dose rate measurements. About 20 hours after
the accident the wind turned in the direction of Pripyat, gamma
dose rates increased significantly in the town, and it was decided
to evacuate the inhabitants. About 20 hours later the 49,000 inhabitants
of Pripyat had left the town in nearly 1,200 buses. About a further
80,000 people were evacuated in the following days and weeks from
the contaminated areas.
Information relevant for the assessment of the doses received
by these people have been obtained by responses of the evacuees
to questionnaires about the location where they stayed, the types
of houses in which they lived, the consumption of stable iodine,
and other activities (Li94).
Doses to the thyroid gland
The iodine activity in thyroid glands of evacuees was measured.
More than 2,000 measurements of former inhabitants of Pripyat
had sufficient quality to be useful for dose reconstruction (Go95a).
A comparative analysis with the questionnaire responses showed
that thyroid doses were mainly due to inhalation of iodine-131.
Average individual doses and collective doses to the thyroid are
shown in Table 3 for three age groups. Individual doses in the
age classes were distributed over two orders of magnitude. The
main factor influencing the individual doses was found to be the
distance of the residence from the reactor.
Whole-body doses
The whole-body doses to the evacuees were mainly due to external
exposure from deposited tellurium-132/iodine-132, caesium-134
and caesium137 and short lived radionuclides in the air. Measurements
of the gamma dose rate in air were performed every hour at about
thirty sites in Pripyat and daily at about eighty sites in the
30-km zone. Based on these measurements and using the responses
to the questionnaires, whole-body doses were reconstructed for
the 90,000 persons evacuated from the Ukrainian part of the 30-km
zone (Li94). There was a wide range of estimated doses
with an average value of 15 mSv. The collective dose was assessed
to be 1,300 person-Sv. The 24,000 persons evacuated in Belarus
might have received slightly higher doses, since the prevailing
wind was initially towards the north.
People living in the contaminated areas
Doses to the thyroid gland
The main information source for the dose reconstruction is the
vast amount of iodine activity measurements in thyroid glands.
In Ukraine 150,000 measurements, in Belarus several hundreds of
thousands of measurements and in the Russian Federation more than
60,000 measurements were performed in May/June 1986. Some of the
measurements were performed with inadequate instrumentation and
measurement conditions and are not useful for dose assessment
purposes.
The large variability of individual doses makes estimates of dose
distributions difficult and current dose estimates are still subject
to considerable uncertainties, especially in areas where only
a few activity measurements in the thyroid were performed. Children
in the Gomel oblast (region) in Belarus received the highest doses.
An estimate (Ba94) of the dose distribution among these
children is shown in Table 4. For the whole Belarus the collective
thyroid dose to children (0 to 14 years) at the time of the accident
was assessed to be about 170,000 person-Sv (Ri94). In the
eight most contaminated districts of Ukraine where thyroid measurements
were performed, the collective dose to this age group was about
60,000 person-Sv and for the whole population about 200,000 person-Sv
(Li93). In the Russian Federation the collective dose to
the whole population was about 100,000 person-Sv (Zv93).
Whole-body doses
Two major pathways contributed to the whole-body doses of the
population in contaminated areas, the exposure to external irradiation
from deposited radionuclides (Iv95) and the incorporation
into the body of radio-caesium in food.
The external exposure is directly related to the radionuclide
activity per unit area and it is influenced by the gamma dose
rates in air at the locations of occupancy. Forestry workers and
other workers living in woodframe houses received the highest
doses.
Most of the higher contaminated areas are rural and a large part
of the diet is locally produced. Therefore, the uptake of caesium
by the plants from the soil is a deciding factor in the internal
exposure. These are regions with extraordinarily high transfer
factors, as the Rovno region in Ukraine, where even moderate soil
contaminations led to high doses. In order of decreasing magnitude
of transfer factors these regions are followed by regions with
peaty soil, sandy podzol (acidic infertile forest soil), loamy
podzol, and chernozem which is rich black soil.
In the first years after the accident the caesium uptake was dominated
practically everywhere by the consumption of locally produced
milk (Ho94). However, later mushrooms began to contribute
significantly in many settlements to the caesium incorporation
for two reasons. First, the milk contamination decreased with
time, whereas the mushroom contamination remained relatively constant.
Second, due to changes in the economic conditions in the three
republics, people are again collecting more mushrooms than they
were in the first years after the accident .
Table 5 summarises a recent estimate of whole-body doses to people
living in the higher contaminated areas. On average, external
irradiation was by far the highest contributor to the total population
exposure (Er94). However, the highest doses to individuals
were produced by the consumption of food from areas with high
transfers of radionuclides.
Even though the releases of radioactive materials during the Chernobyl
accident mainly affected the populations of Belarus, Russia and
Ukraine, the released materials became further dispersed throughout
the atmosphere and the volatile radionuclides of primary importance
(iodine-131 and caesium-137) were
detected in most countries of the Northern hemisphere. However,
the doses to the population were in most places much lower than
in the contaminated areas of the former Soviet Union; they reflected
the deposition levels of caesium-137 and were higher in areas
where the passage of the radioactive cloud coincided with rainfall.
Generally speaking, however, and with a few notable exceptions,
the doses decreased as a function of distance from the reactor
(Ne87).
During the first few weeks, iodine-131 was the main contributor
to the dose, via ingestion of milk (Ma91). Infant thyroid
doses generally ranged from 1 to 20 mSv in Europe, from 0.1 to
5 mSv in Asia, and were about 0.1 mSv in North America. Adult
thyroid doses were lower by a factor of about 5 (UN88).
Later on, caesium-134 and caesium-137 were responsible for most
of the dose, through external and internal irradiation (Ma89).
The whole-body doses received during the first year following
the accident generally ranged from 0.05 to 0.5 mSv in Europe,
from 0.005 to 0.1 mSv in Asia, and of the order of 0.001 mSv in
North America. The total whole-body doses expected to be accumulated
during the lifetimes of the individuals are estimated to be a
factor of 3 greater than the doses received during the first year
(UN88).
In summary, a large number of people received substantial doses
as a result of the Chernobyl accident:
As ionising radiation passes through the body, it interacts with
the tissues transfering energy to cellular and other constituents
by ionisation of their atoms. This phenomenon has been extensively
studied in the critical genetic material, DNA, which controls
the functions of the cells. If the damage to DNA is slight and
the rate of damage production is not rapid, i.e. at low dose rate,
the cell may be able to repair most of the damage. If the damage
is irreparable and severe enough to interfere with cellular function,
the cell may die either immediately or after several divisions.
At low doses, cell death can be accommodated by the normal mechanisms
that regulate cellular regeneration. However, at high doses and
dose rates, repair and regeneration may be inadequate, so that
a large number of cells may be destroyed leading to impaired organ
function. This rapid, uncompensatable cell death at high doses
leads to early deleterious radiation effects which become evident
within days or weeks of exposure, and are known as "deterministic
effects". These deterministic effects can be life-threatening
in the short term if the dose is high enough, and were responsible
for most of the early deaths in the Chernobyl accident.
Lower doses and dose rates do not produce these acute early effects,
because the available cellular repair mechanisms are able to compensate
for the damage. However, this repair may be incomplete or defective,
in which case the cell may be altered so that it may develop into
a cancerous cell, perhaps many years into the future, or its transformation
may lead to hereditable defects in the long term. These late effects,
cancer induction and hereditary defects, are known as "stochastic
effects" and are those effects whose frequency, not severity,
is dose dependent. Moreover, they are not radiation-specific and,
therefore, cannot be directly attributed to a given radiation
exposure.
For this reason, low dose health effects in humans cannot be measured
and, therefore, risk projections of the future health impact of
low-dose ionising radiation exposure have to be extrapolated from
measured high-dose effects. The assumption was made that no dose
of ionising radiation was without potential harm and that the
frequency of stochastic effects at low doses would be proportional
to that occurring at high doses. This prudent assumption was adopted
to assist in the planning of radiation protection provisions when
considering the introduction of practices involving ionising radiations.
The ICRP has estimated the risk of fatal cancer to the general
population from whole-body exposure to be 5 per cent per sievert
(IC90).
The health impact of the Chernobyl accident can be classified
in terms of acute health effects ("deterministic effects")
and of late health effects ("stochastic effects"); moreover
there are also psychological effects which can influence health.
Acute health effects
All the acute deterministic health effects occurred among the
personnel of the plant, or in those persons brought in for fire
fighting and immediate clean-up operations.
Two deaths were immediately associated with the accident: one
person killed by the explosion and another who suffered a coronary
thrombosis. A third person died early the morning of the accident
from thermal burns. Twenty-eight other persons died later in the
treatment centres, bringing the total to 31 deaths in the first
weeks after the accident (UN88).
All symptomatic exposed persons from the site were placed in hospitals.
Of the total of 499 persons admitted for observation, 237 of these
were initially diagnosed as suffering from acute radiation syndrome
and most of these were hospitalised in the first 24 hours. The
severity and rapidity of onset of their symptoms depended on their
dose. The initial early signs and symptoms of radiation sickness
from high doses included diarrhoea, vomiting, fever and erythema.
Over 200 patients were placed in regional hospitals and specialised
centres in the first 24 hours. Patients were allocated to four
categories of radiation sickness severity according to their symptoms,
signs and dose estimates. The differential white blood cell count
showed reduced circulating lymphocytes (lymphocytopenia) which
was the initial indicator of the severity of the exposure and
became evident in the first 24-36 hours for those most severely
irradiated.
No members of the general public received such high whole-body
doses as to induce Acute Radiation Syndrome (IA86). This
was confirmed in Belarus, where, between May and June 1986, 11,600
people were investigated without the discovery of any cases of
acute radiation sickness.
In the highest exposure group (6-16 Gy), the first reaction was
usually vomiting, occurring within 15-30 minutes of exposure.
These patients were desperately ill; fever and intoxication as
well as diarrhoea and vomiting, were prominent features. Mucous
membranes were severely affected, becoming swollen, dry and ulcerated,
making breathing and swallowing extremely painful and difficult.
Extensive burns both thermal and due to beta radiation often complicated
the illness. Within the first two weeks white blood cells and
platelets fell dramatically, indicating a very high dose which
had compromised the production of blood cells in the bone marrow,
making it virtually impossible for the patients to fight infection
or to retain the natural clotting activity of the blood. Almost
all the patients with such high doses died (20 of 21), in spite
of the intensive specialised medical treatment provided.
At lower exposures, the symptoms, signs and laboratory findings
improved. Vomiting began later, platelet and white cell counts
did not drop so precipitously and the fever and toxaemia were
less pronounced. Beta radiation burns to the skin were a major
complicating factor and mucous membrane damage was difficult to
treat, but survival improved markedly at lower doses, so that
no early deaths were noted in the less than 1-2 Gy exposure group
(Table 6).
The treatment of the depression of bone-marrow function encountered
after the accident was largely supportive. Special hygienic measures
were taken; patients' clothes were changed at least twice a day
and aseptic techniques used. Those patients who received doses
above 2 Gy were given anti-fungal agents after the second week.
Antibiotics and gamma globulin were also administered.
Bone-marrow transplantation was undertaken in 13 patients who
were judged to have irreversible bone marrow damage after doses
greater than 4 Gy. All but two of these patients died, some before
the transfused marrow had had a chance to "take", but
others had short-term takes. It was concluded that, even after
very high radiation doses, the bone marrow may well not be completely
destroyed and may recover at least some function at a later stage.
It is this recovery which may lead to later rejection of the transplanted
marrow through a "Host versus Graft" reaction. The physicians
responsible for treating the victims of the accident concluded
that bone marrow transplantation should play a very limited role
in treatment.
Burns, both thermal and from beta radiation, were treated with
surgical excision of tissue that was not viable, and any fluid
and electrolyte loss was compensated for by the parenteral feeding
set up to treat the gastro-intestinal syndrome which is a prominent
feature of acute radiation sickness. The oro-pharyngeal syndrome
of mucosal destruction, oedema and the absence of lubrication
caused by radiation damage to the mucosa of the mouth and pharynx
was extremely difficult to treat, and severely impaired swallowing
and breathing.
The organisational aspects of treating large numbers of very ill
patients also presented significant problems. Intensive nursing
care and monitoring had to be provided 24 hours a day in small
units. Personnel had to be taught new techniques of care and patient
handling, and a large number of diagnostic samples had to be examined.
The logistic requirements of medical handling needed to be well-established
before any therapeutic programme could be run efficiently.
Late health effects
There have been many reports of an increase in the incidence of
some diseases as a result of the Chernobyl accident. In fact,
the accident has, according to present knowledge, given rise to
an increase in the incidence of thyroid cancers. Also, it has
had negative psychological consequences. As far as other diseases
are concerned, the scientific community has not been able to relate
those to the effects of ionising radiation. However, large research
projects have been conducted and are under way to further study
the matter. For example, the WHO (WH95) established the
International Programme on the Health Effects of the Chernobyl
Accident (IPHECA). This programme initially concentrated on pilot
projects involving leukaemia, thyroid diseases, oral health in
Belarus, mental health in children irradiated before birth and
the development of epidemiological registries. The pilot phase
came to an end in 1994 and, as a result of the findings, efforts
are underway to develop long-term permanent programmes involving
thyroid diseases, the accident recovery workers, dose reconstruction
and guidance to the public in the event of an accident. It is
expected that these new projects will provide further insight
into any future health effects.
An estimate (An88) of the total lifetime cancers which
could be expected in Europe as a result of the accident suggested
an increase of about 0.01 per cent above their natural incidence.
Another assessment placed the increase in cancer incidence at
0.004 per cent in the Northern hemisphere, a lower percentage
increase due probably to including the large population of the
whole hemisphere (Pa89). These predictions are remarkably
similar and support the view that the average doses to the general
population of the Northern hemisphere were so low that only fractions
of a percent increases in cancer incidence could be expected in
this population (Pe88, Re87). Large parts of the
Northern hemisphere, such as North America (Hu88, Br88),
Asia and Siberia, were not significantly contaminated and doses
were inconsequential. Therefore, the following sections focus
on the late health effects in the population of the contaminated
regions of the former Soviet Union.
In the International Chernobyl Project organised by the IAEA
(IA91), field studies were undertaken in the latter half
of 1990 on the permanent residents of the rural settlements with
a surface caesium contamination of greater than 555 kBq/m2, and
on control settlements of 2,000 to 50,000 persons, using an age
matched study design. Seven contaminated and six control settlements
were chosen by the medical team of the Chernobyl Project. Since
all persons could not be examined, representative samples were
taken from various age groups. In all, 1,356 people were examined,
and the aim was to examine about 250 from each of the larger settlements.
Three medical teams each spent two weeks conducting medical examinations
to provide the data for these assessments.
The medical examinations were quite comprehensive, and the general
conclusions reached were that there were no health abnormalities
which could be attributed to radiation exposure, but that there
were significant non-radiation related health disorders which
were similar in both contaminated and control settlements. The
accident had had substantial negative psychological consequences
which were compounded by the socio-economic and political changes
occurring in the former Soviet Union. The official data provided
to the medical teams was incomplete and difficult to evaluate,
and were not detailed enough to exclude or confirm the possibility
of an increase in the incidence of some tumour types. On this
subject, it was suggested in 1991 that the incidence of cancer
in Ukraine showed no large increase even in the most contaminated
areas (Pr91).
The International Chernobyl Project Report (IA91) suggested
that the reported high thyroid doses in some children were such
that there could be a statistically detectable increase in the
incidence of future thyroid tumours. The Chernobyl Project team
finally concluded that, on the basis of the doses estimated by
the team and the currently accepted radiation risk estimates,
future increases over the natural incidence of cancer or hereditary
defects would be difficult if not impossible to discern, even
with very large and well-designed long-term epidemiological studies.
However, it should be remembered that this health survey took
place four years after the accident, before any increase in cancer
incidence might be expected and reflects the status of the people
examined in a few months of 1990. The sample size was also criticised
as being too small.
Nevertheless, the dose estimates generally accepted indicate that,
with the exception of thyroid disease, it is unlikely that the
exposure would lead to discernible radiation effects in the general
population. Many predictions of the future impact of the accident
on the health of populations have been made, all of which, apart
from thyroid disease, indicate that the overall effect will be
small when compared with the natural incidence and therefore not
expected to be discernible (An88, Be87, Hu87, Mo87,
De87, Be87).
Thyroid cancer
Early in the development of the Chernobyl accident, it became
obvious that the radioiodines were contributing significant thyroid
doses (Il90), especially to children, and the then Soviet
authorities made every effort not only to minimise doses, but
also to record the thyroid doses as accurately as possible. The
results of these measurements and dose reconstruction assessments
indicated that some groups in the population received high doses
to their thyroids, and that an increase in thyroid abnormalities,
including cancer, was a very real possibility in the future. This
was particularly true for children in the contaminated regions
in Belarus, northern Ukraine and the Bryansk and Kaluga regions
of the Russian Federation. These were not inconsequential thyroid
doses and, as early as 1986, it was predicted by experts from
the Soviet Union that the thyroid would be the target organ most
likely to show evidence of radiation effects, especially an increased
incidence of benign and malignant tumours.
It was known from previous studies of largely external irradiation
of the thyroid that an increase in thyroid tumours tended to appear
six to eight years following irradiation, and continue for more
than twenty years after exposure, particularly in children. What
was not expected was that thyroid abnormalities would already
become detectable about four years after the accident. At the
same time, the current conventional wisdom was that internal radioiodine
exposure was less carcinogenic than external irradiation of the
thyroid. It was estimated that the incidence of thyroid cancers
in children, defined as those diagnosed between the ages of 0
and 14, might increase by about 5 per cent, and in adults by about
0.9 per cent over the next 30 years. As will be seen, a substantial
increase has already been detected in the more contaminated regions.
A determined effort was made to estimate doses, record the data,
initiate medical examinations and follow the cohorts already identified
as being most at risk.
In Ukraine, more than 150,000 examinations were conducted by special
dosimetric teams, and a realistic estimate of the collective thyroid
dose of 64,000 person-Sv has been made, leading to a projection
of 300 additional thyroid cancers (Li93a). In the contaminated
regions of Russia, namely Bryansk, Tula and Orel, it was predicted
that an excess thyroid cancer total of 349 would appear in a population
of 4.3 million (Zv93). This represents an increase of 3
to 6 per cent above the spontaneous rate.
A programme to monitor the thyroid status of exposed children
in Belarus was set up in Minsk in May/June 1986. The highest doses
were received by the evacuated inhabitants of the Hoiniki rayon
(district) in the Gomel oblast. In the course of this study, it
was noted that the numbers of thyroid cancers in children were
increasing in some areas. For Belarus as a whole (WH90, Ka92,
Wi94), there has been a significantly increasing trend in
childhood thyroid cancer incidence since 1990 (Pa94). It
was also noted that this increase is confined to regions in the
Gomel and Brest oblasts, and no significant increase has been
noted in the Mogilev, Minsk or Vitebsk areas where the radioactive
iodine contamination is assessed to have been lower. Over 50 per
cent of all the cases are from the Gomel oblast.
For the eight years prior to 1986, only five cases of childhood
thyroid cancer were seen in Minsk, which is the main Belarussian
centre for thyroid cancer diagnosis and treatment on children
(De94). From 1986 to 1989, 2 to 6 cases of thyroid cancer
in children were seen annually in Minsk. In 1990, the number jumped
to 29, to 55 in 1991, then to 67 in 1992. By the end of 1994 the
total had reached over 300 in Belarus. Nearly 50 per cent of the
early (1992) thyroid cancers appeared in children who were aged
between one and four years at the time of the accident.
The histology of the cancers has shown that nearly all were papillary
carcinomata (Ni94) and that they were particularly aggressive,
often with prominent local invasion and distant metastases, usually
to the lungs. This has made the treatment of these children less
successful than expected, whether undertaken in Minsk or in specialised
centres in Europe. In all, about 150,000 children in Belarus had
thyroid uptake measurements following the accident. Other data
from Ukraine and Russia show a similar, but not as pronounced,
increase in the incidence of childhood thyroid cancer since 1987.
The increase in Belarus was confirmed by the final report of an
EC Expert Panel (EC93) convened in 1992 to investigate
the reported increase. In 1992 the incidence of childhood thyroid
cancer in Belarus as a whole was estimated to be 2.77 per 100,000,
whereas in the Gomel and Brest oblasts it was 8.8 and 4.76 respectively.
This increased incidence was not confined to children, as a larger
number of adult cases was registered in Belarus and in Ukraine
(WH94).
There is some difficulty in comparing the numbers quoted by the
health authorities of the former Soviet Union with previous incidence
statistics, as previous data collection was not sufficiently rigorous.
However, in Belarus all cases of childhood thyroid cancer have
been confirmed since 1986 by international review of the histology,
and, because of more rigid criteria for data collection, reliance
can be placed on accuracy and completeness. An attempt to review
incidence estimates was made in the above-mentioned EC Report
(EC93). These experts confirmed that the incidence of childhood
thyroid cancer (0-14 y) prior to the accident in Belarus (between
0 and 0.14/100,000/y) was similar to that reported by other cancer
registries. This indicates that the data collection in Belarus
was adequate. They noted that it jumped to 2.25/100,000/y in 1991,
about a twenty-fold increase.
When this increase was first reported, it was very quickly pointed
out (Be92) that any medical surveillance programme introduced
would apparently increase the incidence by revealing occult disease
and rectifying misdiagnoses. While this may account for some of
the increase (Ro92), it cannot possibly be the sole cause,
as the increase is so large and many of the children presented
not with occult disease, but with clinical evidence of thyroid
and/or metastatic disease. In fact, only 12 per cent of the childhood
thyroid cancers were discovered by ultrasound screening alone
in Belarus (WH95). In addition, subsequent examination
by serial section of the thyroids of persons coming to autopsy
in Belarus have confirmed that the number of occult thyroid cancers
is similar to that found in other studies (Fu93) and showed
none of the aggressive characteristics found in the childhood
cancers presenting in life (Fu92).
The most recent published rates of childhood thyroid cancer (St95)
show unequivocal increases as seen in Table 7. At the time of
writing three children have died of their disease.
Ukraine
Russia 25 0.5 60 1.1 149 3.4
As far as other thyroid disorders are concerned, no difference
in Russia was detected by ultrasound examination, in the percentage
incidence of cysts, nodules or autoimmune thyroiditis in the contaminated
versus the uncontaminated areas (Ts94). Following the accident,
children in the Ukrainian contaminated regions exhibited a transient
dose-dependent increase in serum thyroxine level, without overt
clinical thyrotoxicosis, which returned to normal within 12 to
18 months (Ni94). This was most marked in the youngest
children. This finding cannot be regarded as an adverse health
effect, as no abnormality was permanent. However, it may be a
pointer to future thyroid disease, especially when it may be associated
with mild regional dietary iodine deficiency, and indicates the
need for continued monitoring.
The majority of the estimates indicate that the overall health
impact from these thyroid disorders will be extremely small and
not detectable when averaged over the population potentially at
risk. This viewpoint is widely held by the competent risk assessors
who have examined the potential effects of the accident.
Other late health effects
From data in the Russian National Medical Dosimetric Registry
(RNMDR), the reported incidence of all types of disease has risen
between 1989 and 1992 (Iv94). There has also been a reported
increase in malignant disease which might be due to better surveillance
and/or radiation exposure. The crude mortality rate of the liquidators
from all causes in the Russian Federation has increased from 5
per 1,000 in 1991 to 7 per 1,000 in 1992. The crude death rate
from respiratory cancer is reported to have increased significantly
between 1990 and 1991, and for all malignant neoplasms between
1991 and 1992. It is not clear what influence smoking has had
on these data, and the overall significance of these findings
will need to be established by further surveillance, especially
when there are distinct regional variations in the crude death
rate and the mortality rates from lung, breast and intestinal
cancer are rising in the general population of the Russian Federation.
From the dosimetric data in the RNMDR (Iv94), a predicted
excess 670 cancer deaths may occur in the exposed groups covered
by the Registry, peaking in about 25 years. This is about 3.4
per cent of the expected cancer deaths from other causes. Data
from the other national dose registries is not readily available
in the published literature.
In view of the difficulties associated with these Registry data,
such as the dose estimates, the influence of such confounding
factors as smoking, the difficulty in follow-up, the possible
increase in some diseases in the general population and also the
short time since the accident, it is not possible to draw any
firm conclusions from these data at this time. The only inference
that can be made is that these groups are the most exposed and
that, if any radiation effects are to be seen, they will occur
in selected cohorts within these registries, which will require
long-term future surveillance.
A predicted increase of genetic effects in the next two generations
was 0.015 per cent of the spontaneous rate, and the estimated
lifetime excess percentage of all cancers as a result of living
in the strict control zones was 0.5 per cent, provided that a
lifetime dose limit of 350 mSv was not exceeded (Il90).
Childhood leukemia incidence has not changed in the decade since
the accident. There is no significant change in the level of leukemia
and related diseases in the contaminated (more than 555 kBq/m2)
and noncontaminated territories of the three states (WH95).
Other attempts through epidemiological studies have failed to
establish a link between radiation exposure from the Chernobyl
accident and the incidence of leukemia and other abnormalities.
No epidemiological evidence of an increase in childhood leukemia
around Chernobyl (Iv93), in Sweden (Hj94) or the
rest of Europe (Pa92, Wi94) has been established. However,
it may be prudent to withold final judgement on this issue for
a few more years.
Other studies
Various reports (Pa93, Sc93, Se95, St93, Ve93) have been
published on the incidence of chromosome aberrations among people
exposed both in the contaminated regions and in Europe. Some of
these have shown little or no increase, while others have. This
may reflect the wide variation in dose. However, there is a trend
for the incidence of chromosome aberrations to return to normal
with the passage of time. Other studies have not shown evidence
of lymphocytic chromosome damage (Br92).
In East Germany one study found no rise in foetal chromosome aberrations
between May and December 1986. Chromosome aberrations are to be
expected in any exposed population, and should be regarded as
biological evidence of that exposure, rather than an adverse health
effect.
Another study in Germany suggesting a link between Down's syndrome
(Trisomy 21) and the Chernobyl accident has been severely criticised
and cannot be accepted at face value because of the absence of
control for confounding factors (Sp91), and it was not
confirmed by more extensive studies (Li93). Another study
in Finland (Ha92) showed no association of the incidence
of Trisomy 21 with radiation exposure from Chernobyl.
There are no clear trends in data for birth anomalies in Belarus
or Ukraine (Li93, Bo94). Two epidemiological studies in
Norway concluded that no serious gross changes as to pregnancy
outcome were observed (Ir91), and that no birth defects
known to be associated with radiation exposure were detected (Li92).
In Austria, no significant changes in the incidence of birth defects
or spontaneous abortion rates which could be attributed to the
Chernobyl accident were detected (Ha92a).
A review by the International Agency for Research on Cancer (IARC)
showed no consistent evidence of a detrimental physical effect
of the Chernobyl accident on congenital abnormalities or pregnancy
outcomes (Li93, EG88). No reliable data have shown any
significant association between adverse pregnancy outcome or birth
anomalies even in the most contaminated regions and the doses
indicate that none would be expected.
There have been reports that have suggested that radiation exposure
as a result of the accident resulted in altered immune reactions.
While immune suppression at high whole-body doses is known to
be inevitable and severe, at the low doses experienced by the
general population it is expected that any detected alterations
will be minor and corrected naturally without any medical consequences.
These minor changes may be indicative of radiation exposure, but
their mild transitory nature is unlikely to lead to permanent
damage to the immune system. All immunological tests of radiation
exposure are in their infancy, but tests such as stimulated immunoglobulin
production by lymphocytes hold promise for the future as a means
of assessing doses below one Gy (De90).
Psychological effects
The severity of the psychological effects of the Chernobyl accident
appears to be related to the public's growing mistrust of officialdom,
politicians and government, especially in the field of nuclear
power. Public scepticism towards authority is reinforced by its
difficulty in understanding radiation and its effects, as well
as the inability of the experts to present the issues in a way
that is comprehensible. The impression that an unseen, unknowable,
polluting hazard has been imposed upon them by the authorities
against their will, fosters a feeling of outrage.
Public outrage is magnified by the concept that their existing
or future descendants are also at risk from this radiation pollution.
This widespread public attitude was not confined to one country,
and largely determined the initial public response outside the
Soviet Union. The public distrust was increased by the fact that
the accident that they had been told could not happen, did happen,
and it induced anxiety and stress in people not only in the contaminated
areas but, to a lesser extent, all over the world.
While stress and anxiety cannot be regarded as direct physical
adverse health effects of irradiation, their influence on the
well-being of people who were exposed or thought that they might
have been, may well have a significant impact on the exposed population.
Several surveys have shown that the intensity of the anxiety and
stress are directly related to the presence of contamination.
It should also be remembered that the stress induced by the accident
was in addition to that produced in the general population by
the severe economic and social hardship caused by the break-up
of the Soviet Union.
Within the former Soviet Union
Within the Soviet Union additional factors came into play to influence
the public reaction. It should be remembered that this accident
occurred during the initial period of "glasnost" and
"perestroika". After nearly seventy years of repression,
the ordinary people in the Soviet Union were beginning openly
to express all the dissatisfaction and frustration that they had
been harbouring. Distrust and hatred of the central government
and the Communist system could be expressed for the first time
without too much fear of reprisal. In addition, nationalism was
not repressed. The Chernobyl accident appeared to epitomise everything
that was wrong with the old system, such as secrecy, witholding
information and a heavy-handed authoritarian approach. Opposition
to Chernobyl came to symbolise not only anti-nuclear and anti-communist
sentiment but also was associated with an upsurge in nationalism.
The distrust of officialdom was so great that even scientists
from the central government were not believed, and more reliance
was placed on local "experts" who often had very little
expertise in radiation and its effects. The then Soviet Government
recognised this problem quickly, and tried to counteract the trend
by inviting foreign experts to visit the contaminated areas, assess
the problems, meet with local specialists and publicise their
views in open meetings and on television. These visits appeared
to have a positive effect, at least initially, in allaying the
fears of the public. In the contaminated Republics, anxiety and
stress were much more prevalent and were not just confined to
the more heavily contaminated regions (WH90a). Several
surveys conducted by Soviet (Al89) and other researchers
(Du94) have shown that the anxiety induced by the accident
has spread far beyond the more heavily contaminated regions.
During this period there was severe economic hardship which added
to the social unrest and reinforced opposition to the official
system of government. Anti-nuclear demonstrations were commonplace
in the larger cities in Belarus (Gomel and Minsk), and Ukraine
(Kiev and Lvov) in the years following the accident (Co92).
The dismissive attitude of some Soviet scientists and government
officials in describing the public reaction as "radiophobic"
tended to alienate the public even further by implying some sort
of mental illness or reaction which was irrational and abnormal.
It also served as a convenient catch-all diagnosis which suggested
that the public was somehow at fault, and the authorities were
unable to do anything about its manifestations.
The concern of people for their own health is only overshadowed
by their concern for the health of their children and grandchildren.
Major and minor health problems are attributed to radiation exposure
no matter what their origin, and the impact that the accident
has had on their daily lives has added to the stress. Whole communities
are facing or have faced evacuation or relocation. There are still
widespread restrictions on daily life affecting schooling, work,
diet and recreation.
The accident has caused disruption of social networks and traditional
ways of life. As most inhabitants of the contaminated settlements
are native to the area and often have lived there all their lives,
relocation has in many cases, destroyed the existing family and
community social networks, transferring groups to new areas where
they may well be resented or even ostracised. In spite of these
drawbacks, about 70 per cent of the people living in contaminated
areas wished to be relocated (IA91). This may well be influenced
by the economic incentives and improved living standards that
result from relocation by the government.
There are two additional circumstances and events which have tended
to increase the psychological impact of the accident, the first
of which was an initiative specifically designed to alleviate
these effects in Ukraine. This was the introduction of the compensation
law in Ukraine in 1991. Some three million Ukrainians were affected
in some way by the post-accident management introduced, upon which
approximately one sixth of the total national budget was spent
(Du94). Different surveys have shown a general feeling
of anxiety in all sectors of the population, but it was particularly
acute among those who had been relocated. People were fearful
of what the future might bring for themselves and their offspring,
and were concerned about their lack of control over their own
destiny.
The problem is that the system of compensation may well have exaggerated
these fears by placing the recipients into the category of victims.
This tended to segregate them socially and increased the resentment
of the native population into whose social system these "victims"
had been injected without consultation. This had the effect on
the evacuees of increasing stress, often leading to withdrawal,
apathy and despair. Locally, this compensation was often referred
as a "coffin subsidy"! It is interesting to note that
the 800 or so mostly elderly people who have returned to their
contaminated homes in the evacuated zones, and hence receive no
compensation, appear to be less stressed and anxious, in spite
of worse living conditions, than those who were relocated. It
should be pointed out that compensation and assistance are not
harmful in themselves, provided that care is taken not to induce
an attitude of dependence and resignation in the recipients.
The second factor which served to augment the psychological impact
of the accident was the acceptance by physicians and the public
of the disease entity known as "vegetative dystonia".
This diagnosis is characterised by vague symptoms and no definitive
diagnostic tests. At any one time, up to 1,000 children were hospitalised
in Kiev, often for weeks, for treatment of this "disease"
(St92). The diagnosis of vegetative dystonia appears to
be tailor-made for the post-accident situation, assigned by parents
and doctors to account for childhood complaints and accepted by
adults as an explanation for vague symptoms.
There is great pressure on the physicians to respond to their
patients' needs in terms of arriving at an acceptable diagnosis,
and "Vegetative Dystonia" is very convenient as it will
fit any array of symptoms. Such a diagnosis not only justifies
the patients' complaints by placing the blame for this "disease"
on radiation exposure, it also exonerates the patient from any
responsibility, which is placed squarely on the shoulders of those
responsible for the exposure - the Government. When the need for
extended hospitalisation is added, the justification to accept
this as a real disease is enhanced. It can be understood why there
is an epidemic of this diagnosis in the contaminated areas.
Outside the former Soviet Union
Psychological effects in other countries were minimal compared
with those within the former Soviet Union, and were generally
exhibited more as concerned social reactions rather than psychosomatic
symptoms. In the contaminated regions of the former Soviet Union,
many people were convinced that they were suffering from radiation
induced disease, whereas in the rest of the world, where contamination
was much less, news of the accident appeared to reinforce anti-nuclear
perceptions in the general population. This was evidenced, for
example, by the demonstrations on 7 June 1986 demanding the decommissioning
of all nuclear power plants in the Federal Republic of Germany
(Ze86). While in France public support for nuclear power
expansion dropped since the accident, 63 per cent of the population
felt that French nuclear power reactors operated efficiently (Ch90).
The minimal impact of the Chernobyl accident on French public
opinion was probably due to the fact that about 75 per cent of
their electrical power is derived from nuclear stations, and in
addition, France was one of the least contaminated European countries.
The Swedish public response has been well-documented (Dr93,
Sj87). In the survey, the question was asked: "With
the experience that we now have, do you think it was good or bad
for the country to invest in Nuclear Energy?" Those that
responded "bad" jumped from 25 per cent before, to 47
per cent after Chernobyl. The accident probably doubled the number
of people who admitted negative attitudes towards nuclear power
(Sj87). This change was most marked among women, who, it
was felt, regarded nuclear power as an environmental problem,
whereas men regarded it as a technical problem which could be
solved. Media criticism of the radiation protection authorities
in that country became more common, with the charge that the official
pronouncements on the one hand said that the risk in Sweden was
negligible, and yet on the other, gave instructions on how it
could be reduced. The concept that a dose, however small, should
be avoided if it could be done easily and cheaply, was not understood.
This sort of reaction was common outside the former Soviet Union,
and while it did not give rise to significant psychosomatic effects,
it tended to enhance public apprehension about the dangers of
nuclear power and foster the public's growing mistrust of official
bodies.
In addition, public opinion in Europe was very sceptical of the
information released by the Soviet Union. This mistrust was reinforced
further by the fact that the traditional sources of information
to which the public tended to turn in a crisis, the physicians
and teachers, were no better informed and often only repeated
and reinforced the fears that had been expressed to them. Added
to this were the media, who tended to respond to the need to print
"newsworthy" items by publishing some of the more outlandish
claims of so-called radiation effects.
The general public was confused and cynical and responded in predictable
but extreme ways such as seeking induced abortions, postponing
travel and not buying food that might conceivably be contaminated.
Another global concern that was manifested, was the apprehension
over travel to the Soviet Union. Potential travellers sought advice
from national authorities on whether to travel, what precautions
to take and how they could check on their exposure. Many people,
in spite of being reassured that it was safe to travel, cancelled
their trip, just to be on the safe side, exhibiting their lack
of confidence in the advice they received.
As has been seen, governments themselves were not immune from
the influence of these fears and some responded by introducing
measures such as unnecessarily stringent intervention levels for
the control of radionuclides in imported food. Thus, in the world
as a whole, while the individual psychological effects due to
anxiety and stress were probably minimal, the collective perception
and response had a significant economic and social impact. It
became clear that there was a need to inform the public on radiation
effects, to provide clear instructions on the precautions to be
taken so that the public regains some level of personal control,
and for the authorities to recognise the public's need to be involved
in the decisions that affect them.
In summary, it can be stated that:
Agricultural impact
All soil used anywhere for agriculture contains radionuclides
to a greater or lesser extent. Typical soils (IA89a) contain
approximately 300 kBq/m3 of potassium-40 to a depth of 20 cm.
This radionuclide and others are then taken up by crops and transferred
to food, leading to a concentration in food and feed of between
50 and 150 Bq/kg. The ingestion of radionuclides in food is one
of the pathways leading to internal retention and contributes
to human exposure from natural and man-made sources. Excessive
contamination of agricultural land, such as may occur in a severe
accident, can lead to unacceptable levels of radionuclides in
food.
The radionuclide contaminants of most significance in agriculture
are those which are relatively highly taken up by crops, have
high rates of transfer to animal products such as milk and meat,
and have relatively long radiological half-lives. However, the
ecological pathways leading to crop contamination and the radioecological
behaviour of the radionuclides are complex and are affected not
only by the physical and chemical properties of the radionuclides
but also by factors which include soil type, cropping system (including
tillage),climate, season and, where relevant, biological half-life
within animals. The major radionuclides of concern in agriculture
following a large reactor accident are iodine-131, caesium-137,
caesium-134 and strontium-90 (IA89a). Direct deposition
on plants is the major source of contamination of agricultural
produce in temperate regions.
While the caesium isotopes and strontium-90 are relatively immobile
in soil, uptake of roots is of less importance compared with plant
deposition. However, soil type (particularly with regard to clay
mineral composition and organic matter content), tillage practice
and climate all affect propensity to move to groundwater. The
same factors affect availability to plants insofar as they control
concentrations in soil solution. In addition, because caesium
and strontium are taken up by plants by the same mechanism as
potassium and calcium respectively, the extent of their uptake
depends on the availability of these elements. Thus, high levels
of potassium fertilisation can reduce caesium uptake and liming
can reduce strontium uptake.
Within the former Soviet Union
The releases during the Chernobyl accident contaminated about
125,000 km2 of land in Belarus, Ukraine and Russia with radiocaesium
levels greater than 37 kBq/m2, and about 30,000 km2 with radiostrontium
greater than 10 kBq/m2. About 52,000 km2 of this total were in
agricultural use; the remainder was forest, water bodies and urban
centres (Ri95). While the migration downwards of caesium
in the soil is generally slow (Bo93), especially in forests
and peaty soil, it is extremely variable depending on many factors
such as the soil type, pH, rainfall and agricultural tilling.
The radionuclides are generally confined to particles with a matrix
of uranium dioxide, graphite, iron-ceramic alloys, silicate-rare
earth, and silicate combinations of these materials. The movement
of these radionuclides in the soil not only depends on the soil
characteristics but also on the chemical breakdown of these complexes
by oxidation to release more mobile forms. The bulk of the fission
products is distributed between organomineral and mineral parts
of the soil largely in humic complexes. The 30-km exclusion zone
has improved significantly partly due to natural processes and
partly due to decontamination measures introduced.
There were also large variations in the deposition levels. During
1991 the caesium-137 activity concentrations in the 0-5 cm soil
layer ranged from 25 to 1,000 kBq/m3 and were higher in natural
than ploughed pastures. For all soils, between 60 and 95 per cent
of all caesium-137 was found to be strongly bound to soil components
(Sa94). Ordinary ploughing disperses the radionuclides
more evenly through the soil profile, reducing the activity concentration
in the 0-5 cm layer and crop root uptake. However, it does spread
the contamination throughout the soil, and the removal and disposal
of the uppermost topsoil may well be a viable decontamination
strategy.
The problem in the early phase of an accident is that the countermeasures
designed to avoid human exposure are of a restrictive nature and
often have to be imposed immediately, even before the levels of
contamination are actually measured and known. These measures
include the cessation of field work, of the consumption of fresh
vegetables, of the pasturing of animals and poultry, and also
the introduction of uncontaminated forage. Unfortunately, these
measures were not introduced immediately and enhanced the doses
to humans in Ukraine (Pr95).
Furthermore, some initial extreme measures were introduced in
the first few days of the accident when 15,000 cows were slaughtered
in Ukraine irrespective of their level of contamination, when
the introduction of clean fodder could have minimised the incorporation
of radiocaesium. Other countermeasures, such as the use of potassium
fertilisers, decreased the uptake of radiocaesium by a factor
of 2 to 14, as well as increased crop yield.
In some podzolic soils, lime in combination with manure and mineral
fertilisers can reduce the accumulation of radiocaesium in some
cereals and legumes by a factor of thirty. In peaty soils, sand
and clay application can reduce the transfer of radiocaesium to
plants by fixing it more firmly in the soil. The radiocaesium
content of cattle for human consumption can be minimised by a
staged introduction of clean feed during about ten weeks prior
to slaughter. A policy of allocating critical food production
to the least contaminated areas may be an effective common sense
measure.
In 1993, the concentration of caesium-137 in the meat of cows
from the Kolkhoz in the Sarny region, where countermeasures could
be implemented effectively, tended to be much lower than that
in the meat from private farms in the Dubritsva region (Pr95).
The meat of wild animals which could not be subjected to the same
countermeasures had a generally high concentration of radiocaesium.
Decontamination of animals by the use of Prussian Blue boli was
found to be very effective where radiocaesium content of feed
is high and where it may be difficult to introduce clean fodder
(Al93). Depending on the local circumstances, many of the
above mentioned agricultural countermeasures were introduced to
reduce human exposure.
Since July 1986, the dose rate from external irradiation in some
areas has decreased by a factor of forty, and in some places,
it is less than 1 per cent of its original value. Nevertheless,
soil contamination with caesium-137, strontium-90 and plutonium-239
is still high and in Belarus, the most widely contaminated Republic,
eight years after the accident 2,640 km2 of agricultural land
have been excluded from use (Be94). Within a 40-km radius
of the power plant, 2,100 km2 of land in the Poles'e state nature
reserve have been excluded from use for an indefinite duration.
The uptake of plutonium from soil to plant parts lying above ground
generally constitutes a small health hazard to the population
from the ingestion of vegetables. It only becomes a problem in
areas of high contamination where root vegetables are consumed,
especially if they are not washed and peeled. The total content
of the major radioactive contaminants in the 30-km zone has been
estimated at 4.4 PBq for caesium-137, 4 PBq for strontium-90 and
32 TBq for plutonium-239 and plutonium-240.
However, it is not possible to predict the rate of reduction as
this is dependent on so many variable factors, so that restrictions
on the use of land are still necessary in the more contaminated
regions in Belarus, Ukraine and Russia. In these areas, no lifting
of restrictions is likely in the foreseeable future. It is not
clear whether return to the 30 km exclusion zone will ever be
possible, nor whether it would be feasible to utilise this land
in other ways such as grazing for stud animals or hydroponic farming
(Al93). It is however, to be recognised that a small number
of generally elderly residents have returned to that area with
the unofficial tolerance of the authorities.
Within Europe
In Europe, a similar variation in the downward migration of caesium-137
has been seen, from tightly bound for years in the near-surface
layer in meadows (Bo93), to a relatively rapid downward
migration in sandy or marshy areas (EC94). For example,
Caslano (TI) experienced the greatest deposition in Switzerland
and the soil there has fallen to 42 per cent of the initial caesium-137
content in the six years after the accident, demonstrating the
slow downward movement of caesium in soil (OF93). There,
the caesium-137 from the accident has not penetrated to a depth
of more than 10 cm, whereas the contribution from atmospheric
nuclear weapon tests has reached 30 cm of depth.
In the United Kingdom, restrictions were placed on the movement
and slaughter of 4.25 million sheep in areas in southwest Scotland,
northeast England, north Wales and northern Ireland. This was
due largely to root uptake of relatively mobile caesium from peaty
soil, but the area affected and the number of sheep rejected are
reducing, so that, by January 1994, some 438,000 sheep were still
restricted. In northeast Scotland (Ma89), where lambs grazed
on contaminated pasture, their activity decreased to about 13
per cent of the initial values after 115 days; where animals consumed
uncontaminated feed, it fell to about 3.5 per cent. Restrictions
on slaughter and distribution of sheep and reindeer, also, are
still in force in some Nordic countries.
The regional average levels of caesium-137 in the diet of European
Union citizens, which was the main source of exposure after the
early phase of the accident, have been falling so that, by the
end of 1990, they were approaching pre-accident levels (EC94).
In Belgium, the average body burden of caesium137 measured in
adult males increased after May 1986 and reached a peak in late
1987, more than a year after the accident. This reflected the
ingestion of contaminated food. The measured ecological half-life
was about 13 months. A similar trend was reported in Austria (Ha91).
In short, there is a continuous, if slow, reduction in the level
of mainly caesium-137 activity in agricultural soil.
Environmental impact
Forests
Forests are highly diverse ecosystems whose flora and fauna depend
on a complex relationship with each other as well as with climate,
soil characteristics and topography. They may be not only a site
of recreational activity, but also a place of work and a source
of food. Wild game, berries and mushrooms are a supplementary
source of food for many inhabitants of the contaminated regions.
Timber and timber products are a viable economic resource.
Because of the high filtering characteristics of trees, deposition
was often higher in forests than in agricultural areas. When contaminated,
the specific ecological pathways in forests often result in enhanced
retention of contaminating radionuclides. The high organic content
and stability of the forest floor soil increases the soil-to-plant
transfer of radionuclides with the result that lichens, mosses
and mushrooms often exhibit high concentrations of radionuclides.
The transfer of radionuclides to wild game in this environment
could pose an unacceptable exposure for some individuals heavily
dependent on game as a food source. This became evident in Scandinavia
where reindeer meat had to be controlled. In other areas, mushrooms
became severely contaminated with radiocaesium.
In 1990, forest workers in Russia were estimated to have received
a dose up to three times higher than others living in the same
area (IA94). In addition, some forest-based industries,
such as pulp production which often recycle chemicals, have been
shown to be a potential radiation protection problem due to enhancement
of radionuclides in liquors, sludges and ashes. However, harvesting
trees for pulp production may be a viable strategy for decontaminating
forests (Ho95).
Different strategies have been developed for combatting forest
contamination. Some of the more effective include restriction
of access and the prevention of forest fires.
One particularly affected site, known as the "Red Forest"
(Dz95), lies to the South and West close to the site. This
was a pine forest in which the trees received doses up to 100
Gy, killing them all. An area of about 375 ha was severely contaminated
and in 1987 remedial measures were undertaken to reduce the land
contamination and prevent the dispersion of radionuclides through
forest fires. The top 10-15 cm of soil were removed and dead trees
were cut down. This waste was placed in trenches and covered with
a layer of sand. A total volume of about 100,000 m3 was buried,
reducing the soil contamination by at least a factor of ten.
These measures, combined with other fire prevention strategies,
have significantly reduced the probability of dispersion of radionuclides
by forest fires (Ko90). The chemical treatment of soil
to minimise radionuclide uptake in plants may be a viable option
and, as has been seen, the processing of contaminated timber into
less contaminated products can be effective, provided that measures
are taken to monitor the by-products.
Changes in forest management and use can also be effective in
reducing dose. Prohibition or restriction of food collection and
control of hunting can protect those who habitually consume large
quantities. Dust suppression measures, such as re-forestation
and the sowing of grasses, have also been undertaken on a wide
scale to prevent the spread of existing soil contamination.
Water bodies
In an accident, radionuclides contaminate bodies of water not
only directly from deposition from the air and discharge as effluent,
but also indirectly by washout from the catchment basin. Radionuclides
contaminating large bodies of water are quickly redistributed
and tend to accumulate in bottom sediments, benthos, aquatic plants
and fish. The main pathways of potential human exposure may be
directly through contamination of drinking-water, or indirectly
from the use of water for irrigation and the consumption of contaminated
fish. As contaminating radionuclides tend to disappear from water
quickly, it is only in the initial fallout phase and in the very
late phase, when the contamination washed out from the catchment
area reaches drinking-water supplies, that human exposure is likely.
In the early phase of the Chernobyl accident, the aquaeous component
of the individual and collective doses from water bodies was estimated
not to exceed 1-2 per cent of the total exposure (Li89).
The Chernobyl Cooling Pond was the most heavily contaminated water
body in the exclusion zone.
Radioactive contamination of the river ecosystems (Figure 8) was
noted soon after the accident when the total activity of water
during April and early May 1986 was 10 kBq/L in the river Pripyat,
5 kBq/L in the Uzh river and 4 kBq/L in the Dniepr. At this time,
shortlived radionuclides such as iodine-131 were the main contributors.
As the river ecosystem drained into the Kiev, then the Kanev and
Kremenchug reservoirs, the contamination of water,sediments, algae,
molluscs and fish fell significantly.
In 1989, the content of caesium-137 in the water of the Kiev reservoir
was estimated to be 0.4 Bq/L, in the Kanev reservoir 0.2 Bq/L,
and in the Kremenchug reservoir 0.05 Bq/L. Similarly, the caesium-137
content of Bream fish fell by a factor of 10 between the Kiev
and Kanev reservoirs, and by a factor of two between the Kanev
and Kremenchug reservoirs to reach about 10 Bq/kg (Kr95).
In the last decade, contamination of the water system has not
posed a public health problem. However, monitoring will need to
be continued to ensure that washout from the catchment area which
contains a large quantity of stored radioactive waste will not
contaminate drinking-water.
A hydrogeological study of groundwater contamination in the 30-km
exclusion zone (Vo95) has estimated that strontium-90 is
the most critical radionuclide, which could contaminate drinking-water
above acceptable limits in 10 to 100 years from now.
Outside the former Soviet Union, direct and indirect contamination
of lakes has caused and is still causing many problems, because
the fish in the lakes are contaminated above the levels accepted
for sale in the open market. In Sweden, for instance, about 14,000
lakes (i.e., about 15 per cent of the Swedish total) had
fish with radiocaesium concentrations above 1,500 Bq/kg (the Swedish
guideline for selling lake fish) during 1987. The ecological half-life,
which depends on the kind of fish and types of lakes, ranges from
a few years up to some tens of years (Ha91).
In the countries of the European Union, the content of caesium-137
in drinking-water has been regularly sampled and reveals levels
at, or below, 0.1 Bq/L from 1987 to 1990 (EC94), which
are of no health concern. The activity concentration in the water
decreased substantially in the years following the accident due
largely to the fixation of radiocaesium in the sediments.
The Sarcophagus
In the aftermath of the accident several designs to encase the
damaged reactor were examined (Ku95). The option which
was chosen provided for the construction of a massive structure
in concrete and steel that used as a support what remained of
the walls of the reactor building (Ku95).
By August 1986 special sensors monitoring gamma radiation and
other parameters were installed in various points by using cranes
and helicopters. These sensors had primarily the function of assessing
the radiation exposure in the areas where the work for the construction
was to be carried out.
An outer protective wall was then erected around the perimeter
and other walls in the turbine building, connected to the reactor
Unit 3 building through an intermediate building, the so-called
"V" building, and a steel roof completed the structure.
The destroyed reactor was thus entombed in a 300,000-tonne concrete
and steel structure known as the "Envelope" or "Sarcophagus".
This mammoth task was completed in only seven months, in November
1986.
Multiple sensors were placed to monitor such parameters as gamma
radiation and neutron flux, temperature, heat flux, as well as
the concentrations of hydrogen, carbon monoxide and water vapour
in air. Other sensors monitor the mechanical stability of the
structure and the fuel mass so that any vibration or shifts of
major components can be detected. All these sensors are under
computer control. Systems designed to mitigate any changing adverse
conditions have also been put into place. These include the injection
of chemicals to prevent nuclear criticality excursions in the
fuel and pumping to remove excess water leaking into the Sarcophagus
(To95).
An enormous effort was required to mount the clean-up operation;
decontaminating ground and buildings, enclosing the damaged reactor
and building the Sarcophagus was a formidable task, and it is
impressive that so much was achieved so quickly. At that time
the emphasis was placed on confinement as rapidly as possible.
Consequently, a structure which would effectively be permanent
was not built and the Sarcophagus should rather be seen as a provisional
barrier pending the definition of a more radical solution for
the elimination of the destroyed reactor and the safe disposal
of highly radioactive materials. In these conditions, to maintain
the existing structure for the next several decades poses very
significant engineering problems. Consultations and studies by
an international consortium are currently taking place to provide
a permanent solution to this problem.
The fuel in the damaged reactor exists in three forms, (a) as
pellets of 2 per cent enriched uranium dioxide plus some fission
products essentially unchanged from the original forms in the
fuel rods, (b) as hot particles of uranium dioxide a few tens
of microns in diameter or smaller particles of a few microns,
made of fuel fused with the metal cladding of the fuel rods, and
(c) as three extensive lava-like flows of fuel mixed with sand
or concrete. The amount of dispersed fuel in the form of dust
is estimated to amount to several tons (Gl95).
The molten fuel mixture has solidified into a glass-like material
containing former fuel. The estimates of the quantity of this
fuel are very uncertain. It is this vitrified material that is
largely responsible for the very high dose rates in some areas
(Se95a). Inside the reactor envelope, external exposure
is largely from caesium-137, but the inhalation of fuel dust is
also a hazard. As was noted earlier, a small special group of
scientists who have worked periodically inside the Sarcophagus
for a number of years have accumulated doses in the estimated
range of 0.5 to 13 Gy (Se95a). Due to the fact that these
doses were fractionated over a long time period, no deterministic
effects have been noted in these scientists. Since the beginning
of 1987 the intensity of the gamma radiation inside the structure
fell by a factor 10. The temperature also fell significantly.
Outside the Sarcophagus, the radiation levels are not high, except
for the roof where dose rates up to 0.5 Gy/h have been measured
after the construction of the Sarcophagus. These radiation levels
on the roof have now decreased to less than 0.05 Gy/h.
Nine years after its erection, the Sarcophagus structure, although
still generally sound, raises concerns for its stability and long-term
resistance and represents a standing potential risk. Some supports
for the enclosure are the original Unit 4 building structures
which may be in poor condition following the explosions and fire,
and their failure could cause the roof to collapse. This situation
is aggravated by the corrosion of internal metallic structures
due to the high humidity of the Sarcophagus atmosphere provoked
by the penetration of large quantities of rain water through the
numerous cracks which were present on the roof and were only recently
repaired (La95). The existing structure is not designed
to withstand earthquakes or tornados. The upper concrete biological
shield of the reactor is lodged between walls, and may fall. There
is considerable uncertainty on the condition of the lower floor
slab, which was damaged by the penetration of molten material
during the accident. It this slab failed, it could result in the
destruction of most of the building.
A number of potential situations have been considered which could
lead to breaches in the Sarcophagus and the release of radionuclides
into the environment. These include the collapse of the roof and
internal structures, a possible criticality event, and the long-term
migration of radionuclides into groundwater.
Currently, the envelope is not leaktight even if its degree of
confinement has been recently improved. Although the current emissions
into the environment are small, not exceeding 10 Gbq/y for caesium-137
and 0.1 GBq/y for plutonium and other transuranic elements, disturbance
of the current conditions within the Sarcophagus, such as the
dislodgement of the biological shield could result in more significant
dispersion of radionuclides (To95). The dispersion in this
case would not be severe and would be confined to the site provided
that the roof did not collapse. However, collapse of the roof,
perhaps precipitated by an earthquake, a tornado or a plane crash,
combined with collapse of internal unstable structures could lead
to the release of the order of 0.1 PBq of fuel dust, contaminating
part of the 30-km exclusion zone (Be95).
More improbable worst case scenarios would result in higher contamination
of the exclusion zone, but no significant contamination is expected
beyond that area. Perhaps the situation causing most concern is
the effect that the collapse of the Sarcophagus might have on
the reactor Unit 3, which is still producing power and whose building
is connected to the Sarcophagus through the "V" Building,
which is not very stable.
Currently, criticality excursions are not thought to be likely
(IP95). Nevertheless, it is possible to theorise (Go95,
Bv95) on hypothetical accident scenarios, however remote,
which could lead to a criticality event. One such scenario would
involve a plane crash or earthquake with collapse of the Sarcophagus,
combined with flooding. An accident of this type could release
about 0.4 PBq of old fuel dust and new fission products to the
atmosphere to contaminate the ground mainly in the 30-km zone.
Leakage from the Sarcophagus can also be a mechanism by which
radionuclides are released into the environment. There are currently
over 3,000 m3 of water in various rooms in the Sarcophagus (To95).
Most of this has entered through defects in the roof. Its activity,
mainly caesium-137, ranges from 0.4 to 40 MBq/L. Studies on the
fuel containing masses indicate that they are not inert and are
changing in various ways. These changes include the pulverisation
of fuel particles, the surface breakdown of the lava-like material,
the formation of new uranium compounds, some of which are soluble
on the surface, and the leaching of radionuclides from the fuel
containing masses. Studies to date indicate that this migration
may become more significant as time passes.
Another possible mechanism of dispersion of radioactivity into
the environment may be the transport of contamination by animals,
particularly birds and insects, which penetrate and dwell in the
Sarcophagus (Pu92). Finally, the possibility of leaching
of radionuclides from the fuel masses by the water in the enclosure
and their migration into the groundwater has been considered.
This phenomenon, however, is expected to be very slow and it has
been estimated that, for example, it will take 45 to 90 years
for certain radionuclides, such as strontium-90, to migrate undergound
up to the Pripyat river catchment area. The expected radiological
significance of this phenomenon is not known with certainty and
a careful monitoring of the evolving situation of the groundwater
will need to be carried out for a long time.
Radioactive waste storage sites
The accident recovery and clean-up operations have resulted in
the production of very large quantities of radioactive wastes
and contaminated equipment. Some of these radioactive wastes are
buried in trenches or in containers isolated from the groundwater
by clay or concrete screens within the 30-km zone (Vo95).
A review of these engineered sites concluded that, provided the
clay layer remained intact, their contribution to groundwater
contamination would be negligible. On the other hand, 600 to 800
waste trenches were hastily dug in the immediate vicinity of the
Unit 4 in the aftermath of the accident. These unlined trenches
contain the radioactive fallout that had accumulated on trees,
grass, and in the ground to a depth of 10-15 cm and which was
bulldozed from over an area of roughly 8 km2. The estimated activity
amount is now of the order of 1 PBq, which is comparable to the
total inventory stored in specially constructed facilities next
to Unit 4. Moreover, a large number of contaminated equipment,
engines and vehicles are also stored in the open air.
The original clean-up activities are poorly documented, and much
of the information on the present status of the unlined trenches
near Unit 4 and the spread of radioelements has been obtained
in a one-time survey. Some of the findings of the study (Dz95)
are that:
It is clear that large uncertainties remain which require a correspondingly
large characterisation effort. For instance, at present, most
disposal sites are unexplored, and a few are uncharted; monitoring
for groundwater movement is insufficient and the interpretation
of the hydrologic regime is complicated by artificial factors
(pumping, mitigative measures, etc.); the mechanisms of radionuclide
leaching from the variety of small buried particles are not well
understood.
The problem of the potential spread of radioelements to the Pripyat
river is especially important in that the latter may act as a
shortcut for the dispersion of additional radioactive elements
outside the 30-km exclusion zone.
In summary, the sarcophagus was never intended to be a permanent
solution to entomb the stricken reactor. The result is that this
temporary solution may well be unstable in the long term. This
means that there is the potential for collapse which needs to
be corrected by a permanent technical solution.
The accident recovery and clean-up operations have also resulted
in the production of very large quantities of radioactive wastes
and contaminated equipment which are currently stored in about
800 sites within and outside the 30-km exclusion zone around the
reactor. These wastes are partly conserved in containers and partly
buried in trenches or stored in the open air.
In general, it has been assessed that the Sarcophagus and the
proliferation of waste storage sites in the area constitute a
series of potential sources of release of radioactivity that threatens
the surrounding area. However, any accidental releases from the
sarcophagus are expected to be very small in comparison with those
from the Chernobyl accident in 1986 and their radiological consequences
would be limited to a relatively small area around the site. On
the other hand, concerns have been expressed by some experts that
a more important release might occur if the collapse of the Sarcophagus
should induce damage in the Unit 3 of the Chernobyl power plant.
As far as the radioactive wastes stored in the area around
the site are concerned, they are a potential source of contamination
of the groundwater which will require close monitoring until a
safe disposal into an appropriate repository is implemented.
Initiatives have been taken internationally, and are currently
underway, to study a technical solution leading to the elimination
of these sources of residual risk on the site.
Chapter I
THE SITE AND ACCIDENT SEQUENCE
To the South-east of the plant, an artificial lake of some 22
km2, situated beside the river Pripyat, a tributary of the Dniepr,
was constructed to provide cooling water for the reactors.
direct steam feed to the turbines, without an intervening heat-exchanger.
Water pumped to the bottom of the fuel channels boils as it progresses
up the pressure tubes, producing steam which feeds two 500-MW(e)
[megawatt electrical] turbines. The water acts as a coolant and
also provides the steam used to drive the turbines. The vertical
pressure tubes contain the zirconium-alloy clad uranium-dioxide
fuel around which the cooling water flows. A specially designed
refuelling machine allows fuel bundles to be changed without shutting
down the reactor.
30 control rods was necessary to retain reactor control, in the
test only 6-8 control rods were actually used. Many of the control
rods were withdrawn to compensate for the build up of xenon which
acted as an absorber of neutrons and reduced power. This meant
that if there were a power surge, about
20 seconds would be required to lower the control rods and shut
the reactor down. In spite of this, it was decided to continue
the test programme.
100 times the nominal power output.
Chapter II
THE RELEASE, DISPERSION AND DEPOSITION OF RADIONUCLIDES
Chemical and physical forms
Table 1. Current estimate of radionuclide releases during the Chernobyl
accident (modif. from De95)
Core inventory Total release during
on 26 April 1986 the accident
Nuclide Half-life Activity Percent of Activity
(PBq) inventory (PBq)
33Xe 5.3 d 6 500 100 6500
131I 8.0 d 3 200 50 - 60 ~1760
134Cs 2.0 y 180 20 - 40 ~54
137Cs 30.0 y 280 20 - 40 ~85
132Te 78.0 h 2 700 25 - 60 ~1150
89Sr 52.0 d 2 300 4 - 6 ~115
90Sr 28.0 y 200 4 - 6 ~10
140Ba 12.8 d 4 800 4 - 6 ~240
95Zr 1.4 h 5 600 3.5 196
99Mo 67.0 h 4 800 >3.5 >168
103Ru 39.6 d 4 800 >3.5 >168
106Ru 1.0 y 2 100 >3.5 >73
141Ce 33.0 d 5 600 3.5 196
144Ce 285.0 d 3 300 3.5 ~116
239Np 2.4 d 27 000 3.5 ~95
238Pu 86.0 y 1 3.5 0.035
239Pu 24 400.0 y 0.85 3.5 0.03
240Pu 6 580.0 y 1.2 3.5 0.042
241Pu 13.2 y 170 3.5 ~6
242Cm 163.0 d 26 3.5 ~0.9
above, 50 to 60 per cent of the core inventory of iodine was thought
to have been released in one form or another. Other volatile elements
and compounds, such as those of caesium and tellurium, attached
to aerosols, were transported in the air separate from fuel particles.
Air sampling revealed particle sizes for these elements to be
0.5 to 1 mm.
predominantly to the West and North-west (Figure 5). Ground depositions
of caesium-137 of over 40 kilobecquerels per square metre [kBq/m2]
covered large areas of the Northern part of Ukraine and of the
Southern part of Belarus. The most highly contaminated area was
the 30-km zone surrounding the reactor, where caesium-137 ground
depositions generally exceeded 1,500 kBq/m2 (Ba93).
See also Figure 4 at end of file
See also Figure 5 at end of file
The Kaluga-Tula-Orel spot in Russia, centered approximately 500
km North-east of the reactor, was formed from the same radioactive
cloud that produced the Bryansk-Belarus spot, as a result of rainfall
on 28-29 April. However, the levels of deposition of caesium-137
were lower, usually less than 600 kBq/m2 (Ba93).
to the South and much of Central Europe, as well as the Northern
Mediterranean and the Balkans, received some deposition, the actual
severity of which depended on the height of the plume, wind speed
and direction, terrain features and the amount of rainfall that
occurred during the passage of the plume.
Most countries in Europe experienced some deposition of radionuclides,
mainly caesium-137 and caesium-134, as the plume passed over the
country. In
Austria, Eastern and Southern Switzerland, parts of Southern Germany
and Scandinavia, where the passage of the plume coincided with
rainfall, the total deposition from the Chernobyl release was
greater than that experienced by most other countries, whereas
Spain, France and Portugal experienced the least deposition. For
example, the estimated average depositions of caesium-137 in the
provinces of Upper Austria, Salzburg and Carinthia in Austria
were 59, 46 and 33 kBq/m2 respectively, whereas the average caesium-137
deposition in Portugal was 0.02 kBq/m2 (Un88). It was reported
that considerable secondary contamination occurred due to resuspension
of material from contaminated forest. This was not confirmed by
later studies.
Chapter III
REACTIONS OF NATIONAL AUTHORITIES
FOODS FOR GENERAL CONSUMPTION
Radionuclide Level (Bq/kg)
americium-241, plutonium-239
strontium-90
iodine-131, caesium-134, caesium-137
10
100
1,000
INFANT FOODS AND MILK
americium-241, plutonium-239
iodine-131, strontium-90
caesium-134, caesium-137
1
100
1,000
It should be remembered that these guideline values were developed
to facilitate international trade in food, and should be regarded
as levels "below regulatory concern". Levels above these
do not necessarily constitute a health hazard, and if found, the
competent national authority should review what action should
be taken.
Chapter IV
DOSE ESTIMATES
With regard to internal doses from inhalation and ingestion of
radionuclides, the situation is similar: radioiodine was important
during the first few weeks after the accident and gave rise to
thyroid doses via inhalation of contaminated air, and, more importantly,
via consumption of contaminated foodstuffs, mainly cow's milk.
After about one month, the radiocaesiums (caesium-134 and caesium-137)
again became predominant, and, after a few years, caesium-137
became the only radionuclide of importance for practical purposes,
even though strontium-90 may in the future play a significant
role at short distances from the reactor.
population groups expected to be the most exposed: (1) the liquidators,
(2) the evacuees from the 30-km zone, (3) the inhabitants of the
contaminated areas, and (4) the children of those people. In 1991,
the AUDR contained data on 659,292 persons. Starting from 1992,
national registries of Belarus, Russian Federation, and Ukraine
replaced the AUDR.
give rise to doses from ingestion through the consumption of foodstuffs.
Deposited radiocaesium is also a source of long-term exposure
from external irradiation from the contaminated ground and other
surfaces. Most of the population of the Northern hemisphere was
exposed, in varying degrees, to radiation from the Chernobyl accident.
The caesium-137 deposition outside the former Soviet Union ranged
from negligible levels to about 50 kBq/m2.
Year of birth
Number of
people
Average individual
dose (Sv)
Collective dose
(person-Sv)
1983 - 1986
2,400
1.4
3,300
1971 - 1982
8,100
0.3
2,400
< = 1970
38,900
0.07
2,600
Assessments of the doses to the thyroid gland of the evacuees
from the
30-km zone (Li93a) showed similar doses for young children
as those for the Pripyat evacuees. Exposures to adults were higher.
These high doses were due to a greater consumption of food contaminated
with iodine-131 among those evacuated later from the 30-km zone.
Thyroid dose (Sv)
Number of children
Collective dose
(person-Sv)
0 - 0.3
15,100
2,300
0.3 - 2
13,900
11,500
2 - 10
3,100
13,700
10 - 40
300
4,700
Evaluations of questionnaires on food consumption rates in the
period May/June 1986 and measurements of food contamination showed
iodine-131 in milk as the major source for the thyroid exposure
of the population living in the contaminated areas. However, in
individual cases the consumption of fresh vegetables contributed
significantly to the exposure.
Whole-body
dose (mSv)
External exposure
Total exposure
No. of
persons
Collective dose
(person-Sv)
No. of
persons
Collective dose
(person-Sv)
5 - 20
20 - 50
50 - 100
100 - 150
150 - 200
> 200
132,000
111,000
24,000
2,800
530
120
1,700
3 ,500
1,600
330
88
26
88,000
132,000
44,000
6,900
1,500
670
1,200
4,200
3,000
820
250
160
Total 270,000 7,300 273,000 9,700
Populations outside the former Soviet Union
Chapter V
HEALTH IMPACT
Number of patients
Estimated Dose
(Gy)
Deaths
21
21
55
140
6 - 16
4 - 6
2 - 4
less than 2
20
7
1
0
Total 237
28
There is a large range of medical treatments that can be attempted
to mitigate the acute radiation syndrome. All these procedures
were applied to the persons hospitalised with varying degrees
of success. The hospital treatment following the accident included
replacement therapy with blood constituents, fluids and electrolytes;
antibiotics; antifungal agents; barrier nursing and bone marrow
transplantation.
1981 - 85
1986 - 90
1991- 94
Area
No.
Rate
(per million)
No.
Rate
(per million)
No.
Rate
(per million)
Belarus
Gomel
Five
North
Regions
Bryansk
& Kaluga
Regions
3
1
1
0
0.3
0.5
0.1
0
47
21
21
3
4
10.5
2
1.2
286
143
97
20
30.6
96.4
11.5
10
It can be concluded that there is a real, and large, increase
in the incidence of childhood thyroid cancer in Belarus and Ukraine
which is likely to be related to the Chernobyl accident. This
is suggested by features of the disease, which differ somewhat
from the so-called natural occurrence, as well as by its temporal
and geographic distribution.
Chapter VI
AGRICULTURAL AND ENVIRONMENTAL IMPACTS
In summary,
Chapter VII
POTENTIAL RESIDUAL RISKS