A review of corrosion considerations of container materials relevant to underground disposal of HLW in Belgium

The underground formation that is currently being considered in Belgium for the permanent disposal of vitrified HLW and spent fuel is an argillaceous sediment (viz. Boom Clay layer), which is located in the northeast of Belgium and extending under the Mol-Dessel nuclear site at a depth between 180 and 280 meter. The corrosion resistance of the metallic container is an important aspect within the disposal concept as it has to provide a high integrity, i.e. no through-the-wall corrosion should occur, at least for the duration of the thermal phase (500 years for HLW and 2000 years for spent fuel). An extensive corrosion evaluation programme was started in the mid 1980's. The main objective is to evaluate the long-term corrosion performance of a broad range of candidate container materials. In addition, the influence of several parameters, such as temperature, oxygen content, groundwater composition (chloride, sulphate and thiosulphate), γ-radiation, ... are investigated. The experimental approach consisted of in situ experiments (performed in the underground research facility, HADES), electrochemical experiments, immersion experiments and a large scale demonstration test (OPHELIE). Degradation modes considered include general corrosion, localised corrosion (pitting) and stress corrosion cracking.