2011:09 Is Copper Immune to Corrosion When in Contact With Water and Aqueous Solutions?

The KBS-3concept implies that spent nuclear fuel is placed in copper canisters surrounded by clay and finally placed approximately 500 m down from surface into granitic bedrock, in order to isolate the spent nuclear fuel from humans and environment for very long time scales (i.e. millions of years). The concept is based on the multi-barrier principle, in this respect the barriers are the copper canister, clay material and finally the granitic bedrock. All barriers will work alone and together in order to retard the spent nuclear fuel to enter into the biosphere. In this report it is only degradation of the first barrier, the copper canister that is considered.

The mechanisms of copper corrosion in the planned repository for spent nuclear fuel according to the KBS-3 concept need to be fully understood in order to review the license application in an authoritative way. Copper as a canister material was chosen by SKB already 1978 in the KBS-2 report. The main reasons for selecting copper were 1) copper is thermodynamically immune in the presence of pure anoxic water, and 2) copper has sufficient mechanical strength to resist all plausible mechanical loads present in the repository. Thermodynamically, immunity of copper in pure anoxic water implies that copper will not corrode in the presence of pure anoxic water. The reason for why copper cannot corrode in the immune state is that this will cause a positive change in the Gibbs Energy. This contradicts the second law of thermodynamics, which states that for a spontaneous reaction to occur the change in Gibbs Energy has to be negative.