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Technical Report NTB 20-01

Development of Copper Coated Canisters for the Disposal of SF and HLW in Switzerland

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Abstract

 

The current reference canister design for the disposal of high-level waste (HLW) and spent fuel (SF) in Switzerland is based on the use of carbon steel. However, Nagra is also exploring the feasibility of alternative designs and materials, including a copper-coated carbon steel canister. In this concept, a thin copper coating (in the range 3 – 10 mm) would provide resistance to corrosion and the carbon steel sub-structure would provide structural stability. One significant advantage of a copper-coated canister is that the rate of gas generation would be much lower than for an all steel canister. Nagra is co-operating with other international waste management organizations in the ongoing development of the copper-coated canister design and this report describes the current status of the concept.

Two techniques are currently under investigation for the application of the copper coating: electrodeposition and cold spray. Electrodeposition would be used to coat the main body and lid of the canister, except for a narrow strip for the final closure weld in the steel sub-structure. After the canister is loaded with HLW/SF and sealed, the weld region would be covered by a layer of cold spray copper. Alternatively, the body and two heads are coated by electrodeposition separately, leading to two welds which will then be coated by cold spray, one before and one after the loading of the canister. The current status of the development of the electrodeposition and cold spray processes for this application is reviewed, along with design concepts for the canister and a preliminary analysis of the structural performance under possible external loads in the repository.

The corrosion behaviour of copper for use as a HLW/SF canister material has been studied for the past 30 – 40 years, primarily in the context of the copper-cast iron KBS-3 canister design. Although copper exhibits excellent corrosion properties, the use of a reduced wall thickness (compared with the 50-mm-thick copper shell of the KBS‑3 canister design) necessarily requires a greater degree of certainty regarding the nature and extent of corrosion processes. A key question is whether the corrosion behaviour of electrodeposited and cold spray copper is the same as that for the wrought material that constitutes this existing knowledge base. The current state of knowledge of the corrosion of copper canisters is reviewed, including the general corrosion behaviour under both oxic and anoxic (i.e., sulfide-containing) conditions, the extent of localised forms of corrosion, and the likelihood of stress corrosion cracking and microbiologically influenced corrosion. Other corrosion processes, such as the effects of irradiation or radiation-induced processes, hydrogen-related degradation, and galvanic corrosion, are also considered. Compared with single-shell canister designs, a copper-coated steel canister would be relatively insensitive to interactions between mechanical- and corrosion-related degradation modes, but the case of the stress corrosion cracking of the copper barrier is quantitatively assessed. Finally, approaches for predicting the lifetimes of HLW/SF canisters are also reviewed and a preliminary assessment is provided of the corrosion allowance for periods of 100'000 years and 1 million years.

The development of the copper-coated canister concept is ongoing and areas requiring further study are identified in the context of the Swiss disposal concept for a repository in Opalinus Clay.

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