2010:09 Copper Thermodynamics in the Repository Environment up to 130˚C

The mechanisms of copper corrosion for a KBS-3 repository for spent nuclear fuel need to be known with a high level of confidence. This is because the overall rate of copper canister corrosion (accounting for corroding species concentrations, geochemical conditions and the mass transport through surrounding barriers) provides an essential performance indicator in safety assessment. A complicating boundary condition of the corrosion assessment is the repository thermal evolution, which is initially driven by the decay heat of the disposed spent nuclear fuel. The deviation in thermal conditions from the standard state of tabulated thermodynamic data (298 K) needs to be explicitly accounted for. The canister surface is expected to exhibit temperatures in excess of 70 ˚C for the first 100 years and be appreciably elevated for at least 1000 years. The thermal corrections of thermodynamic data introduce an uncertainty source which needs to be scrutinized.

There are reasonably reliable methods to extrapolate thermodynamic data, which should work well within the relatively restricted range of elevated temperature in the repository environment. The present work examines the revised Helgeson-Krikham-Flowers (HFK) method and applies this concept for copper corrosion scoping calculations. It is suggested that uncertainties originating from extrapolations to account for the expected temperature interval are not larger than those caused by the normal interpretation of the underlying experimental data. Relevant thermodynamic data for copper speciation have been compiled from various sources. Calculations with these data show that the temperature influence on equilibrium conditions is rather modest, but increased temperature will nonetheless reduce the immunity area of copper under relevant groundwater conditions.