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.

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.

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

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.

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
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.

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
100 times the nominal power output.

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.

Chemical and physical forms

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

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.

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
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).

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).

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).

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.

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.

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.

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
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.

Outside the former Soviet Union, the radionuclides of importance are, again, the radioiodines and radiocaesiums, which, once deposited on the ground,
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.

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.

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.

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).

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 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.

Populations outside the former Soviet Union

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:

• Liquidators - Hundreds of thousands of workers, estimated to amount up to 800,000, were involved in clean-up operations. The most exposed, with doses of several grays, were the workers involved immediately after the beginning of the accident and the scientists who have performed special tasks in the sarcophagus. The average doses to liquidators are reported to have ranged between 170 mSv in 1986 and 15 mSv in 1989.

• Evacuees - More than 100,000 persons were evacuated during the first few weeks following the accident. The evacuees were exposed to internal irradiation arising from inhalation of radioiodines, especially iodine-131, and to external irradiation from radioactivity present in the cloud and deposited on the ground. Thyroid doses are estimated to have been, on average, about 1 Sv for small children under 3 years of age and about 70 mSv for adults. Whole-body doses received from external irradiation prior to evacuation from the Ukrainian part of the 30-km zone showed a large range of variation with an average value of 15 mSv.

• People living in contaminated areas of the former Soviet Union - About 270,000 people live in contaminated areas with caesium-137 deposition levels in excess of 555 kBq/m2. Thyroid doses, due mainly to the consumption of cow's milk contaminated with iodine-131, were delivered during the first few weeks after the accident; children in the Gomel region of Belarus appear to have received the highest thyroid doses with a range from negligible levels up to 40 Sv and an average close to 1 Sv for children aged 0 to 7. Because of the control of foodstuffs in those areas, most of the radiation exposure since the summer of 1986 is due to external irradiation from the caesium-137 activity deposited on the ground; the whole-body doses for the 1986-1989 time period are estimated to range from 5 to 250 mSv with an average of 40 mSv. In areas without food control, there are places, such as the Rovno region of Ukraine, where the transfer of caesium137 from soil to plant is very high, resulting in doses from internal exposure being greater than those from external exposure.

• Populations outside the former Soviet Union - The radioactive materials of a volatile nature (such as iodine and caesium) that were released during the accident spread throughout the entire northern hemisphere. The doses received by populations outside the former Soviet Union were relatively low, and showed large differences from one country to another depending mainly upon whether rainfall occurred during the passage of the radioactive cloud.

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).

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.

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.

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.

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:

• Thirty-one people died in the course of the accident or soon after and another 137 were treated for the acute radiation syndrome.

• Extensive psychological effects are apparent in the affected regions of the former Soviet Union, manifested as anxiety and stress. Severe forms induce a feeling of apathy and despair often leading to withdrawal. In the rest of the world these individual effects were minimal.

• In the last decade, there has been a real and significant increase in childhood and, to a certain extent, adult carcinoma of the thyroid in contaminated regions of the former Soviet Union (Wi940) which should be attributed to the Chernobyl accident until proven otherwise.

• In children, the thyroid cancers are:

  • largely papillary and particularly aggressive in nature often self presenting with local invasion and/or distant metastases,

  • more prevalent in children aged 0 to 5 years at the time of the accident, and in areas assessed to be the more heavily contaminated with iodine-131,

  • apparently characterised by a shorter latent period than expected and,

  • still increasing.

• There has been no increase in leukemia, congenital abnormalities, adverse pregnancy outcomes or any other radiation induced disease in the general population either in the contaminated regions or in Western Europe, which can be attributed to this exposure. It is unlikely that surveillance of the general population will reveal any significant increase in the incidence of cancer.

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.

In summary,

• Many countermeasures to control the contamination of agricultural products were applied with varying levels of efficacy. 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 farm animal activities are carried out, the food produced is subject to strict controls and restrictions of distribution and use;

• Similar problems, 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 controls and limitations for variable durations after the accident. Most of these restrictions have been lifted several years ago. However, there are still 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.

• Produce from forests may continue to be a radiological protection problem for a long time.

• At present drinking water is not a problem. Contamination of groundwater, especially with strontium-90, could be a problem for the future in the catchment basins downstream of the Chernobyl area.

• Contaminated fish from lakes may be a long-term problem in some countries.

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:

• the water table in the vicinity of Unit 4 has risen by 1 to 1.5 m in a few years to about 4 m from the ground level and may still be rising (apparently this is due mostly to the construction, in 1986, of a wall 3,5 km long and 35 m deep around the reactor to protect the Kiev reservoir from possible spread of contamination through the underground water, as well as to the ceasing of drainage activities formerly connected with the construction of new units on the site).

• in the most targeted study area 32 of 43 explored trenches are periodically or continually flooded;

• in that area the upper unconfined aquifer is contaminated everywhere with strontium-90 to levels exceeding 4 Bq/L. Caesium and plutonium are less mobile and contamination from these elements is confined to the immediate vicinity of the disposal trenches;

• the relative mobility of the strontium-90 is especially important in that, from the closest trenches, it might reach the Pripyat River in 10 to 20 years;

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.

The accident did not affect each country in the same way and a different emphasis was placed on various aspects of the accident with particular reference to the circumstances of that country. Thus, countries remote from the accident, with no domestic nuclear power programmes or neighbouring reactors, tended to emphasise food control and information exchange as their major thrust for improvement. Whereas those countries which were contaminated by the accident, and had their own nuclear power programmes and/or reactors in neighbouring countries, drew extensive lessons from the way the accident developed and was treated. For these reasons, not all the lessons learned were applied universally with the same emphasis.

Operational aspects

The Chernobyl accident was one of a kind, and, although it highlighted deficiencies in emergency preparedness and radiation protection, it should not be seen as the reference accident for future emergency planning purposes (Bu91).

It was very clear from the initial reactions of the competent national authorities that they were unprepared for an accident of such magnitude and they had to make decisions, as the accident evolved, on criteria that could have been established beforehand. This also meant that too many organisations were involved in the decision-making, as no clear-cut demarcations had been agreed and established. Areas of overlapping responsibility and jurisdiction needed to be clearly established prior to any accident. A permanent infrastructure needed also to be in place and maintained for any efficient implementation of protective measures. Such an infrastructure had to include rapid communications systems, intervention teams and monitoring networks. Mobile ground monitoring teams were required, as was aerial monitoring and tracking of the plume. Many countries responded to this need by establishing such monitoring networks and reorganising their emergency response.

Logistic problems associated with intervention plans, such as stable iodine distribution (Sc94, NE95a) and evacuation obviously needed to be in place and rehearsed long before the accident, as they are too complex and time-consuming to be implemented during the short time available during the evolution of the accident. Intervention actions and the levels at which they should be introduced needed to be agreed, preferably internationally, and incorporated into the emergency plans so that they could be immediately and efficiently implemented.

The accident also demonstrated the need to include the possibility of transboundary implications in the emergency plans, as it had been shown that the radionuclide release would be elevated and the dispersion of contamination more widespread. The concern, raised by the experience of Chernobyl, that any country could be affected not only by nuclear accidents occurring on its territory but also by the consequences of accidents happening abroad, stimulated the establishment of national emergency plans in several countries.

The transboundary nature of the contamination prompted the international organisations to promote international cooperation and communication, to harmonise actions (NE88, IA94, IC90, IC92, NE93, NE89, NE90, NE89b, WH88, WH87, IA89b, IA92, IA91a, IA89c, IA87a, IA94a, EC89a, EC89b) and to develop international emergency exercises such as those organised by the OECD/NEA in its INEX Programme (NE95). A major accomplishment of the international community were the agreements reached on early notification in the event of a radiological accident and on assistance in radiological emergencies through international Conventions in the frame of the IAEA and the EC (EC87, IA86b, IA86c).

Furthermore, in order to facilitate communication with the public on the severity of nuclear accidents, the International Nuclear Event Scale INES was developed by the IAEA and the NEA and is currently adopted by a large number of countries.

The accident provided the stimulus for international agreement on food contamination moving in trade, promoted by the WHO/FAO, as there is a need to import at least some food in most countries, and governments recognised the need to assure their citizens that the food that they eat is safe. Monitoring imported food was one of the first control measures instituted and continues to be performed (FA91, EC89c, EC93a).

This event also clearly showed that all national governments, even those without nuclear power programmes, needed to develop emergency plans to address the problem of transboundary dispersion of radionuclides. Of necessity these plans had to be international in nature, involving the free and rapid exchange of information between countries.

It is essential that emergency plans are flexible. It would be foolish to plan for another accident similar to Chernobyl without any flexibility, as the only fact that one can be sure of is that the next severe accident will be different. Emergency planners need to distil the general principles applicable to various accidents and incorporate these into a generic plan.

The accident emphasised the need for public information and public pressure at the time clearly demonstrated this need. A large number of persons who are knowledgeable about the technique of providing information, are needed to establish a credible source of information to the public before an accident, so that clear and simple reports can be disseminated continuously in a timely and accurate form (EC89).

Emergency plans also need to include a process by which large numbers of people could have their exposure assessed, and those with high exposures differentiated. The accident also highlighted the need for the prior identification of central specialised medical facilities with adequate transportation to treat the more highly exposed individuals.

Refinement and clarification of international advice was needed (Pa88). The recommendations for intervention in an accident contained in ICRP Publication 40 were not clearly understood when they came to be applied, and the Commission reviewed this advice in Publication 63 (IC92). This guide placed emphasis on the averted dose as the parameter against which an intervention measure should be assessed. It was also made clear that an intervention had to be "justified" in as far as it produced more good than harm, and that where a choice existed between different protection options, "optimisation" was the mechanism to determine the choice. Emphasis was also placed on the need to integrate all protective actions in an emergency plan, and not to assess each one in isolation, as one may well influence the efficacy of another.

Scientific and technical aspects

Prior to the accident, it was felt that the flora and fauna of the environment were relatively radioresistant and this was supported by the fact that no lethal radioecological injuries were noted after the accident except in pine forests (600 ha) and small areas of birch close to the reactor. A cumulative dose of less than 5 Gy has no gross effect even in the most sensitive flora of ecological systems, but there are still ecological lessons to be learned especially on the siting of nuclear power reactors (Al93).

Plant foliar and root uptake is being studied, as are resuspension and weathering. The transfer coefficients at all stages of the pathways to human exposure are being refined. Following the accident, an assessment of the models used at thirteen sites to predict the movement of iodine-131 and caesium-137 from the atmosphere to food chains (Ho91) indicated that models commonly used tended to overpredict by anything up to a factor of ten. The extensive whole-body monitoring of radioactivity in persons undertaken in conjunction with the measurement of ground and food contamination allowed refinement of the accuracy of the models for human dose assessment from the exposure through different pathways. The methods and techniques to handle contamination of food, equipment and soil have been improved.

Meteorological aspects, such as the relationship between deposition and precipitation and greater deposition over high ground and mountains, have been shown to be important especially in the development of more realistic models (NE96a). The importance of synoptic scale weather patterns used in predictions was established, and different models have been developed to predict deposition patterns under a wide variety of weather conditions. The chemico-physical changes in the radioactive gases and aerosols transported through the atmosphere are being studied to improve the accuracy of transport models.

Other impacts of the accident on model refinement include the improvement in understanding the movement of radionuclides in soil and biota, pathways and transfer factors; the effect of rainstorms and the influence of mountains and the alignment of valleys on deposition patterns; particulate re-suspension; long range pollution transport mechanisms; and the factors which influence deposition velocities (NE89a, NE96).

Uniform methods and standards were developed for the measurement of contaminating radionuclides in environmental samples.

In the case of high exposures the importance of symptomatic and prophylactic medical and nursing procedures, such as antibiotics, anti-fungal and anti-viral agents, parenteral feeding, air sterilisation and barrier nursing was demonstrated, as were the disappointing results of bone-marrow transplantation.

In addition, the accident led to an expansion of research in nuclear safety and the management of severe nuclear accidents.

On the other hand, there is a need to set up sound epidemiological studies to investigate potential health effects, both acute and chronic. In the Chernobyl case, the lack of routinely collected data, such as cancer registry data that are reliable enough, led to difficulty in organising appropriate epidemiological investigations in timely manner. There appears to be a need for developing and maintaining a routine health surveillance system within and around nuclear facilities.

In summary, besides providing new impetus to nuclear safety research, especially on the management of severe nuclear accidents, the Chernobyl accident stimulated national authorities and experts to a radical review of their understanding of, and attitude to radiation protection and nuclear emergency issues.

This led to expand knowledge on radiation effects and their treatment and to revitalise radioecological research and monitoring programmes. emergency procedures, and criteria and methods for the information of the public.

Moreover, a substantial role in these improvements was played by multiple international co-operation initiatives, including revision and rationalisation of radiation protection criteria for the management of accident consequences, as well as reinforcement or creation of international communication and assistance mechanisms to cope with the transboundary implications of potential nuclear accidents.