Near-Field Modelling in the Safety Assessment SR-Can
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Near-Field Modelling in the Safety Assessment SR-Can C. Fredrik Vahlund Swedish Nuclear Fuel and Waste Management Co, SKB Box 250 SE-101 24 Stockholm, Sweden ABSTRACT Spent nuclear fuel from the Swedish energy programme will be stored in an underground repository situated in saturated fractured rock at a depth of approximately 500 m. This paper describes numerical simulations of radionuclide migration in the near-field (consisting of a canister filled with spent fuel and an engineered system backfilled with swelling clays) for the recently completed safety assessment SR-Can [1] using a Matlab / Simulink code. Handling of input data for the models from the site descriptive programme from on-going investigations at two candidate sites and the numerical modelling concept are discussed. INTRODUCTION The Swedish Nuclear Fuel and Waste Management Co. (SKB) is jointly owned by the operators of the Swedish nuclear power plants and is responsible for interim storage and final disposal of the spent radioactive fuel produced within the Swedish nuclear energy programme. A research programme for developing a repository system has been ongoing since the 1970s and has resulted in SKB suggesting a KBS-3 type of repository for final storage of the spent fuel. The KBS-3 method is based on storing the spent fuel in corrosion resistant copper canisters with a cast iron insert to provide mechanical stability. The canisters are to be placed in an underground repository constructed in saturated granitic rock at a depth of around 500 m. Deposition holes where the canisters are placed and tunnels connecting the holes will be backfilled with swelling clay in order to provide a suitable environment for the canisters. For the deposition holes, a bentonite clay is planned to be used, while the deposition tunnels, based on the present design, will be backfilled with some other sort of swelling clay, for instance Friedland clay with rather different properties to the bentonite buffer clay. At emplacement, the clay will be comparatively dry, but as water from the fractured rock surrounding the repository enters the clay system, the clay becomes more saturated and swells to ideally seal off fractures and limit the inflow of water. The low hydraulic conductivity of the clay in the deposition hole (the buffer) ensures diffusion to be the dominating mechanism for radionuclide migration and that the transport time is relatively long. In the deposition tunnel a larger conductivity is allowed and advective transport must be considered in those parts of the system. Modelled point releases (particles) that are assumed to be transported through the system are either corrodants migrating towards the canister and corrosion products and possibly radionuclides in the case of a damaged canister (either initially or later in time, through corrosion). Based on knowledge gained in the current development program, the likelihood for having an initially damaged canister is so low that it was ruled out in SKB’s most recent safety assessment for the KBS-3 system [1]. P
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