SR 97: Hydromechanical Evolution in a Defective Canister
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SR 97: Hydromechanical Evolution in a Defective Canister Patrik Sellin Swedish Nuclear Fuel and Waste Management Co. (SKB), Box 5864, S-102 40, Stockholm, Sweden ABSTRACT This paper describes the modelling of the hydromechanical evolution of a nuclear waste canister with an initial defect. The waste canister in the Swedish concept consists of a corrosionresistant copper shell that covers a cast iron insert that provides mechanical strength. If there is an initial penetrating defect in the copper shell, water will be able to contact the insert. Ingress of water is a prerequisite for corrosion of the iron insert, which in turn gives rise to production of hydrogen gas. Water is also a prerequisite for corrosion of the fuel assembly’s metal parts, fuel dissolution and radionuclide transport. Due to the fact that the corrosion processes both consume water and generate hydrogen, strong couplings exist between the chemical and the hydraulic evolution, which must therefore be described in a coupled context. Certain aspects of the mechanical evolution must also be dealt with in parallel with the hydrochemical processes. INTRODUCTION In preparation for upcoming site investigations for siting of a deep repository for spent nuclear fuel, SKB has carried out the long-term safety assessment (SR 97) [1] of a repository of the KBS-3 type. The KBS-3 repository for spent nuclear fuel is designed primarily to isolate the waste. If the isolation should for any reason, a secondary purpose of the repository is to retard the release of radionuclides. This objective is achieved with a system of barriers, (see Figure 1): • The fuel is placed in corrosion-resistant copper canisters. Inside the five-meter-long canisters is a cast iron insert that provides the necessary mechanical strength. • The canisters are surrounded by a layer of compacted bentonite clay. The bentonite protects the canister mechanically in the event of small rock movements and prevents groundwater and corrosive substances from reaching the canister. • The canisters with surrounding bentonite clay are emplaced at a depth of about 500 meters in the crystalline bedrock, where mechanical and chemical conditions are stable in a long-term perspective. • If any canister should be damaged, the chemical properties of the fuel and the radioactive species (for example their poor solubility in water) put severe limitations on the transport of radionuclides from the repository to the ground surface. The repository is thus built up of several barriers which support and complement each other. The safety of the repository must be adequate even if one barrier becomes defective or fail to perform as intended. This is the essence of the multiple barrier concept.
Figure 1. The KBS-3 Repository In the SR 97 safety assessment the future evolution of the repository system is analyzed in the form of five scenarios. The first is a base scenario where the repository is postulated to be built entirely according to specifications and where present-day conditions in the surroundings, including clim
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