Chapter VII
Potential residual risks
Conclusions
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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% 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 137Cs, 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 137Cs 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. 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 137Cs, 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 90Sr, 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).
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, but are being studied.
Although it was previously felt that, radioelements could spread to
the Pripyat river and down stream to the Black see, this has not occurred.
Radionuclides have been effectively held up in soils and river sediments
near the accident site (see Chapter 2).
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 radio-logical consequences would be limited to a relatively
small area around the site. 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.
Radionuclides have not, however, migrated as far from the site as was
once expected.
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. This will be developed in the following
chapter.
