Evidence for internal diffusion of sulphide from groundwater into grain boundaries ahead of crack tip in Cu OFP copper

Copper canister is a central technical barrier for radioactive release from high level nuclear waste in the so called KBS-3 concept planned to be used in Finland and Sweden for disposal of spent nuclear fuel. Canisters will be placed in the granitic bedrock at about 400-500 m depth and surrounded by a layer of bentonite clay planned to protect the canister from any chemical and mechanical damage, and especially acting as a diffusion barrier. While researching for the possibility of stress corrosion cracking in phosphorus microalloyed copper in presence of sulphides in the groundwater, indications were found for a new potential degradation mechanism involving internal diffusion of sulphide. This paper describes the evidence for the new mechanism and discusses the scenarios involving diffusion of sulphide onto the copper canister surface.