Some Important Mechanisms and Processes in the Near Field of the Swedish Repository for Spent Nuclear Fuel
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SOME IMPORTANT MECHANISMS AND PROCESSES IN THE NEAR FIELD OF THE SWEDISH REPOSITORY FOR SPENT NUCLEAR FUEL IVARS NERETNIEKS Department of Chemical Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden.
ABSTRACT In repositories for nuclear waste there are many processes that will be instrumental in damaging the canisters and releasing the nuclides. Based on experiences from studies of the performance of repositories and of an actual design, the major mechanisms influencing the integrity and performance of a repository are described and discussed. The paper addresses only conditions in crystalline rock repositories. The low water flow rate in fractures and channels plays a dominant role in limiting the interaction between water and waste. Molecular diffusion in the backfill and rock matrix, as well as in the mobile water, is an important transport process, but actually limits the exchange rate because diffusive transport is slow. Solubility limits of both waste matrix and of individual nuclides are also important. Complicating processes include alpha-radiolysis, which may change the water chemistry in the near-field. The sizes and locations of water flowpaths and damages in the canisters considerably influence the release rates. Uncertainties in data are large. Nevertheless the system is very robust in the sense that practically no reasonably conceivable assumptions or data will lead to large nuclide releases. Several natural analogues have been found to exhibit similarities with a waste repository and help to validate concepts and to increase our confidence that all major issues have been considered. INTRODUCTION AND BACKGROUND High Level Nuclear Waste, HLW, or spent fuel, SF, is planned to be disposed of in deep repositories in crystalline rock in Sweden, Switzerland, and Canada. The highly radioactive waste will be encapsulated in canisters and placed in or adjacent to tunnels at depths below 500 m. The canisters are surrounded by a backfill consisting of bentonite clay, in some cases admixed with less reactive material, e.g., sand. In Sweden the canister is made of copper, possibly with an inner vessel of mild steel. Steel canisters are being studied in Switzerland, and in Canada stainless steel canisters are being contemplated. The crystalline rock has a very low hydraulic conductivity with the water flowing in channels in the fractures. Fracture zones with more densely spaced fractures and with enhanced hydraulic conductivity are also present in the rock. The canisters will fail sooner or later and the waste will come in contact with water that will dissolve nuclides released from the waste matrix. Part of the nuclides will diffuse or possibly be carried by slowly seeping water through the backfill and out to the mobile water in the channels in the fractures in the rock. The water chemistry will influence the rate of corrosion of the canisters and also the solubility of the nuclides. The release rate of the nuclides from the fuel is influenced by the chemistry as well as by sorption, by transp
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