Chernobyl: Chapter VII. Potential residual risks

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).

  • 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 every-where with 90Sr to levels exceeding 4 Bq/L. Caesium and plutonium are less mobile and contamination from these elements is confined to the immedi-ate vicinity of the disposal trenches.
  • The relative mobility of the 90Sr 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, 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.

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