Report 2008-6

Radionuclide Release and Transport – RNT-2008

Nykyri. M., Nordman, H., Marcos, N., Löfman, J., Poteri, A. & Hautojärvi. A.

English

164

The Finnish nuclear waste disposal company, Posiva Oy, is planning an underground repository for spent nuclear fuel to be constructed on the island of Olkiluoto on the south-west coast of Finland. The repository design is based on the KBS-3V concept, where spent fuel elements in copper-iron canisters are emplaced in vertical holes in the bedrock and surrounded by bentonite clay. This report presents the radionuclide release and transport analysis RNT-2008 that forms a part of Posiva's safety case. The analysis covers the subject from the release of radionuclides from spent nuclear fuel to their arrival in the biosphere.

RNT-2008 is a deterministic analysis. Its assumptions are purposely conservative, meaning that they shall ensure that the results, with high degree of certainty, overestimate radioactive releases and radiation exposures. The calculation cases are composed from several scenarios, the main assumptions being that 1) a disposal canister will have initially a penetrating hole in its wall or corrode at a high rate to form one, 2) the bentonite buffer will be partly missing its function as a barrier, 3) rock shear movements will damage the near field barriers, 4) groundwater flow rates will be higher than the flow analysis suggest, 5) groundwater chemistry will be less favourable than predicted, and 6) gas generation will enhance the transport of radionuclides.

The main quantitative results are expressed as indicative dose rates from the use of a stylised drinking-water well and as activity release rates from the geosphere to the biosphere. The results suggest that the assessed repository system well complies with the regulatory criteria. Based on the WELL-2008 dose rates, the most significant radionuclides in most cases are I-129, C-14, Cs-135, Pa-231 and Cl-36, in this order. The results indicate that the engineered barriers together with the sparsely fractured host rock around the near field dominate the capacity of the repository to retain the radionuclides and retard their migration. The central role of the bedrock farther from the canisters is to provide stable and favourable chemical and physical conditions for the engineered barrier system, and to hinder inadvertent human intrusion into the repository.

Safety case, safety assessment, long-term safety, spent nuclear fuel, nuclear waste, crystalline bedrock, KBS-3

Radionuclide Release and Transport – RNT-2008 (pdf) (6.9 MB)