Safety analysis of disposal of spent nuclear fuel: Dissertation

The spent fuel from the Olkiluoto nuclear power plant (2 x 710 MWe BWR's) is planned to be disposed of in a repository to be constructed at a depth of about 500 metres in the crystalline bedrock. The canister design consists of an inner container of steel as a load-bearing element and an outer container of oxygen-free copper to provide a shield against corrosion. Preliminary site investigations were carried out between 1987 and 1992 in five areas consisting of different types of the Precambrian crystalline bedrock. In the safety analysis, all site-specific data has been selected in such a way that a more favourable environment can be found for the repository at each of the five candidate sites. The results of the safety analysis show that the planned disposal system fulfils the safety requirements and criteria proposed by the authorities. If the conditions in the geosphere in the vicinity of the repository do not change drastically and no major disruptive event hits the repository, the copper-steel canisters remain intact for millions of years and no significant amount of radioactive substances will ever escape from the repository. The spent fuel, the bentonite buffer between the canister and rock, and the geosphere efficiently restrict the release of radionuclides even if the canister is initially defective or is broken soon after the sealing of the repository. The safety analysis also includes the evaluation of consequences of the very unlikely disruptive event where a large postglacial rock displacement is assumed to intersect the repository. Even if the rock displacement occurred already after 1 000 years and damaged all 60 canisters in a deposition tunnel and, in addition, there were oxidizing conditions in the geosphere because of the glacial melt water (this kind of combination is actually possible only after 30 000 years or so), the resulting dose rate would be smaller than the dose rate caused by the natural background radiation. No extraordinary characteristics are required from a site in the crystalline bedrock to ensure the long-term safety of a deep repository for spent nuclear fuel. The main functions of the geosphere are to isolate the waste from the human habitat, to protect it against external impacts, to provide stable mechanical and chemical conditions for the repository, and to limit the amount of water coming into contact with the canisters. Positioning of the repository gallery within a site and properties of the rock in the vicinity of the repository (e.g. the disturbed rock zone around the excavated tunnels) are of greater importance than the regional geology or the specific type of the crystalline rock. Suitable places for the repository can be found at each of the five candidate sites. It is essential to characterize in detail the site where the repository will be excavated, so that fracture zones can be taken into consideration when constructing the repository.