The Sarcophagus
In the aftermath of the accident several designs to encase the
damaged reactor were examined (Ku95). The option which
was chosen provided for the construction of a massive structure
in concrete and steel that used as a support what remained of
the walls of the reactor building (Ku95).
By August 1986 special sensors monitoring gamma radiation and
other parameters were installed in various points by using cranes
and helicopters. These sensors had primarily the function of assessing
the radiation exposure in the areas where the work for the construction
was to be carried out.
An outer protective wall was then erected around the perimeter
and other walls in the turbine building, connected to the reactor
Unit 3 building through an intermediate building, the so-called
"V" building, and a steel roof completed the structure.
The destroyed reactor was thus entombed in a 300,000-tonne concrete
and steel structure known as the "Envelope" or "Sarcophagus".
This mammoth task was completed in only seven months, in November
1986.
Multiple sensors were placed to monitor such parameters as gamma
radiation and neutron flux, temperature, heat flux, as well as
the concentrations of hydrogen, carbon monoxide and water vapour
in air. Other sensors monitor the mechanical stability of the
structure and the fuel mass so that any vibration or shifts of
major components can be detected. All these sensors are under
computer control. Systems designed to mitigate any changing adverse
conditions have also been put into place. These include the injection
of chemicals to prevent nuclear criticality excursions in the
fuel and pumping to remove excess water leaking into the Sarcophagus
(To95).
An enormous effort was required to mount the clean-up operation;
decontaminating ground and buildings, enclosing the damaged reactor
and building the Sarcophagus was a formidable task, and it is
impressive that so much was achieved so quickly. At that time
the emphasis was placed on confinement as rapidly as possible.
Consequently, a structure which would effectively be permanent
was not built and the Sarcophagus should rather be seen as a provisional
barrier pending the definition of a more radical solution for
the elimination of the destroyed reactor and the safe disposal
of highly radioactive materials. In these conditions, to maintain
the existing structure for the next several decades poses very
significant engineering problems. Consultations and studies by
an international consortium are currently taking place to provide
a permanent solution to this problem.
The fuel in the damaged reactor exists in three forms, (a) as
pellets of 2 per cent enriched uranium dioxide plus some fission
products essentially unchanged from the original forms in the
fuel rods, (b) as hot particles of uranium dioxide a few tens
of microns in diameter or smaller particles of a few microns,
made of fuel fused with the metal cladding of the fuel rods, and
(c) as three extensive lava-like flows of fuel mixed with sand
or concrete. The amount of dispersed fuel in the form of dust
is estimated to amount to several tons (Gl95).
The molten fuel mixture has solidified into a glass-like material
containing former fuel. The estimates of the quantity of this
fuel are very uncertain. It is this vitrified material that is
largely responsible for the very high dose rates in some areas
(Se95a). Inside the reactor envelope, external exposure
is largely from caesium-137, but the inhalation of fuel dust is
also a hazard. As was noted earlier, a small special group of
scientists who have worked periodically inside the Sarcophagus
for a number of years have accumulated doses in the estimated
range of 0.5 to 13 Gy (Se95a). Due to the fact that these
doses were fractionated over a long time period, no deterministic
effects have been noted in these scientists. Since the beginning
of 1987 the intensity of the gamma radiation inside the structure
fell by a factor 10. The temperature also fell significantly.
Outside the Sarcophagus, the radiation levels are not high, except
for the roof where dose rates up to 0.5 Gy/h have been measured
after the construction of the Sarcophagus. These radiation levels
on the roof have now decreased to less than 0.05 Gy/h.
Nine years after its erection, the Sarcophagus structure, although
still generally sound, raises concerns for its stability and long-term
resistance and represents a standing potential risk. Some supports
for the enclosure are the original Unit 4 building structures
which may be in poor condition following the explosions and fire,
and their failure could cause the roof to collapse. This situation
is aggravated by the corrosion of internal metallic structures
due to the high humidity of the Sarcophagus atmosphere provoked
by the penetration of large quantities of rain water through the
numerous cracks which were present on the roof and were only recently
repaired (La95). The existing structure is not designed
to withstand earthquakes or tornados. The upper concrete biological
shield of the reactor is lodged between walls, and may fall. There
is considerable uncertainty on the condition of the lower floor
slab, which was damaged by the penetration of molten material
during the accident. It this slab failed, it could result in the
destruction of most of the building.
A number of potential situations have been considered which could
lead to breaches in the Sarcophagus and the release of radionuclides
into the environment. These include the collapse of the roof and
internal structures, a possible criticality event, and the long-term
migration of radionuclides into groundwater.
Currently, the envelope is not leaktight even if its degree of
confinement has been recently improved. Although the current emissions
into the environment are small, not exceeding 10 Gbq/y for caesium-137
and 0.1 GBq/y for plutonium and other transuranic elements, disturbance
of the current conditions within the Sarcophagus, such as the
dislodgement of the biological shield could result in more significant
dispersion of radionuclides (To95). The dispersion in this
case would not be severe and would be confined to the site provided
that the roof did not collapse. However, collapse of the roof,
perhaps precipitated by an earthquake, a tornado or a plane crash,
combined with collapse of internal unstable structures could lead
to the release of the order of 0.1 PBq of fuel dust, contaminating
part of the 30-km exclusion zone (Be95).
More improbable worst case scenarios would result in higher contamination
of the exclusion zone, but no significant contamination is expected
beyond that area. Perhaps the situation causing most concern is
the effect that the collapse of the Sarcophagus might have on
the reactor Unit 3, which is still producing power and whose building
is connected to the Sarcophagus through the "V" Building,
which is not very stable.
Currently, criticality excursions are not thought to be likely
(IP95). Nevertheless, it is possible to theorise (Go95,
Bv95) on hypothetical accident scenarios, however remote,
which could lead to a criticality event. One such scenario would
involve a plane crash or earthquake with collapse of the Sarcophagus,
combined with flooding. An accident of this type could release
about 0.4 PBq of old fuel dust and new fission products to the
atmosphere to contaminate the ground mainly in the 30-km zone.
Leakage from the Sarcophagus can also be a mechanism by which
radionuclides are released into the environment. There are currently
over 3,000 m3 of water in various rooms in the Sarcophagus (To95).
Most of this has entered through defects in the roof. Its activity,
mainly caesium-137, ranges from 0.4 to 40 MBq/L. Studies on the
fuel containing masses indicate that they are not inert and are
changing in various ways. These changes include the pulverisation
of fuel particles, the surface breakdown of the lava-like material,
the formation of new uranium compounds, some of which are soluble
on the surface, and the leaching of radionuclides from the fuel
containing masses. Studies to date indicate that this migration
may become more significant as time passes.
Another possible mechanism of dispersion of radioactivity into
the environment may be the transport of contamination by animals,
particularly birds and insects, which penetrate and dwell in the
Sarcophagus (Pu92). Finally, the possibility of leaching
of radionuclides from the fuel masses by the water in the enclosure
and their migration into the groundwater has been considered.
This phenomenon, however, is expected to be very slow and it has
been estimated that, for example, it will take 45 to 90 years
for certain radionuclides, such as strontium-90, to migrate undergound
up to the Pripyat river catchment area. The expected radiological
significance of this phenomenon is not known with certainty and
a careful monitoring of the evolving situation of the groundwater
will need to be carried out for a long time.
Radioactive waste storage sites
The accident recovery and clean-up operations have resulted in
the production of very large quantities of radioactive wastes
and contaminated equipment. Some of these radioactive wastes are
buried in trenches or in containers isolated from the groundwater
by clay or concrete screens within the 30-km zone (Vo95).
A review of these engineered sites concluded that, provided the
clay layer remained intact, their contribution to groundwater
contamination would be negligible. On the other hand, 600 to 800
waste trenches were hastily dug in the immediate vicinity of the
Unit 4 in the aftermath of the accident. These unlined trenches
contain the radioactive fallout that had accumulated on trees,
grass, and in the ground to a depth of 10-15 cm and which was
bulldozed from over an area of roughly 8 km2. The estimated activity
amount is now of the order of 1 PBq, which is comparable to the
total inventory stored in specially constructed facilities next
to Unit 4. Moreover, a large number of contaminated equipment,
engines and vehicles are also stored in the open air.
The original clean-up activities are poorly documented, and much
of the information on the present status of the unlined trenches
near Unit 4 and the spread of radioelements has been obtained
in a one-time survey. Some of the findings of the study (Dz95)
are that:
It is clear that large uncertainties remain which require a correspondingly
large characterisation effort. For instance, at present, most
disposal sites are unexplored, and a few are uncharted; monitoring
for groundwater movement is insufficient and the interpretation
of the hydrologic regime is complicated by artificial factors
(pumping, mitigative measures, etc.); the mechanisms of radionuclide
leaching from the variety of small buried particles are not well
understood.
The problem of the potential spread of radioelements to the Pripyat
river is especially important in that the latter may act as a
shortcut for the dispersion of additional radioactive elements
outside the 30-km exclusion zone.
In summary, the sarcophagus was never intended to be a permanent
solution to entomb the stricken reactor. The result is that this
temporary solution may well be unstable in the long term. This
means that there is the potential for collapse which needs to
be corrected by a permanent technical solution.
The accident recovery and clean-up operations have also resulted
in the production of very large quantities of radioactive wastes
and contaminated equipment which are currently stored in about
800 sites within and outside the 30-km exclusion zone around the
reactor. These wastes are partly conserved in containers and partly
buried in trenches or stored in the open air.
In general, it has been assessed that the Sarcophagus and the
proliferation of waste storage sites in the area constitute a
series of potential sources of release of radioactivity that threatens
the surrounding area. However, any accidental releases from the
sarcophagus are expected to be very small in comparison with those
from the Chernobyl accident in 1986 and their radiological consequences
would be limited to a relatively small area around the site. On
the other hand, concerns have been expressed by some experts that
a more important release might occur if the collapse of the Sarcophagus
should induce damage in the Unit 3 of the Chernobyl power plant.
As far as the radioactive wastes stored in the area around
the site are concerned, they are a potential source of contamination
of the groundwater which will require close monitoring until a
safe disposal into an appropriate repository is implemented.
Initiatives have been taken internationally, and are currently
underway, to study a technical solution leading to the elimination
of these sources of residual risk on the site.
Chapter VII
POTENTIAL RESIDUAL RISKS
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