As we saw in Section 8.1, a large variety of low-level wastes also arises from the nuclear program. Waste consisting of miscellaneous rubbish (such as rubber gloves and tissues contaminated with traces of radioactive material) is typically contained in steel drums, compacted to reduce bulk and then placed in steel containers and disposed of in a shallow trench area covered with at least 1 m of soil. Measurements on such an engineered disposal facility have indicated that the radiological significance of the disposal is negligible.

Wastes with medium levels of radioactivity from reprocessing, power reactor operations, and decommissioning, as well as plutonium-contaminated wastes, are usually contained within a concrete or bitumen matrix within stainless steel drums. There is widespread agreement that geological disposal is the best solu­tion for the management of such wastes. A geological repository of the type il­lustrated in Figure 8.6 would be suitable. Sweden already has an operational repository under the seabed for medium — and low-activity wastes at Forsmark; Finland has a similar repository at it Olkiluoto power station site. These facilities are about 100 m below the seabed-ground level. In Britain, UK Nirex Ltd. was set up in 1985 with the responsibility of providing radioactive waste disposal fa­cilities. Currently, a deep underground site near Sellafield, northwest England, is being investigated for a geological repository. An underground rock laboratory is planned as the first stage prior to the construction of the facility, expected to be brought into operation around 2010.

Liquid wastes at low activities arise from all nuclear sites, particularly from reprocessing plants, and are discharged under the regulations laid down by the licensing authority. Obviously, great care must be taken to avoid any public

danopr frnm such disrhar. es

Gaseous wastes, typically noble gas isotopes, are also produced from reac­tors and reprocessing plants. These are normally discharged to the atmosphere under carefully controlled conditions.

A final point on disposal concerns the decommissioning of a nuclear plant. Decommissioning is done in stages; stage 1 is concerned with the removal of spent fuel—defuelinrr—from the reactor. This starts at shutdown and can take up to 3 years for a large gas-cooled reactor. The spent fuel that is discharged is then managed in the same way as “operational” spent fuel. This reduces the total amount of radioactivity at the reactor site to less than one-seventh that at shutdown. The second stage involves all the dismantling of all nonradioactive plant and buildings other than the reactor and its concrete biological shield. This stage follows on from stage 1 and takes 5 to 10 years. The reactor building itself is then sealed for a period of surveillance. Finally, stage 3 involves the complete dismantling of the reactor and returning the site to a “greenfield” sta­tus. This stage occurs about 100 years after shutdown and takes about 10 years to complete.

A variant on this strategy involves the construction of a high-integrity in­truder-proof containment around the reactor building—Safestore-—that can be left for periods of up to 100 years before the final dismantling of the reactor. This strategy allows the maximum time for the radioactivity in the reactor build­ing to decay, thus minimizing the hazard when actual dismantling takes place. Modern P’^TC stations are designed for the replacement of all components with the exception of the reactor pressure vessel, and are therefore relatively straightfoiward to decommission.

So far about 80 nuclear reactors have been shut down worldwide and sev­eral sites have been cleared completely—the world’s first civil P’^TC station, Shippingport, for example. In the United Kingdom, decommissioning has started at three of the older Magnox station sites, Berkeley, Hunterston, and Trawsfynydd. Handling and disposal of radioactive waste from decommission­ing follow similar routes to reprocessing and reactor operational wastes. De­commissioning represents only a small fraction (approximately 5% maximum) of nuclear generating costs.

REFERENCES

Cooper, J. R., and J. W. Rose (1977). Technical Data on Fuel, p. 53. Scottish Academic Press.

Ealing, C. J. (1994). "Experience and Application of the GEC Alsthom Modular Vault Dry Store.” Nuclear Engineer 35 (March-April): 48-54.

Janbury, K. 0994). “Transport, Storage and Final Disposal of Spent Fuel in the Federal Republic of Germany.” Nuclear Engineer 35 (May-June): 78-83.

OECD 0988). Environmental Impacts of Renewable Energy. Report by the Organization for Economic Cooperation and Development.

Passant, F. H. 0994). “Waste Management and Decommissioning.” Nuclear Energy 33 (4): 223-229.

Stevens-Guille, P. D., and F. E. Pave 0994). “Development and Prospects of Canadian Technology for Dry Storage of Used Nuclear Fuel.” Nuclear Engineer 35 (March-April): 64-71.