The Supercontainer (SC) is the reference concept for the post-conditioning of vitrified high-level nuclear waste and spent fuel in Belgium. It comprises a prefabricated concrete buffer that completely surrounds a carbon steel overpack. Welding is being considered as a final closure technique of the carbon steel overpack in order to ensure its water tightness. Welding is known to induce residual stresses near the weld zone, which may lead to an increased susceptibility to stress corrosion cracking (SCC). In this study, slow strain rate tests were conducted to study the SCC behavior of plain and welded UNS K02700 grade mild steel exposed to an artificial concrete pore water solution that is representative for the SC concrete buffer environment. The tests were performed under anoxic conditions at 140°C, a constant strain rate of 5 × 10-7 s-1 and open circuit potential. The effect of sulfide and thiosulfate on the SCC behavior was investigated up to levels of 15.6 mM S2- (500 mg/L) and 5.35 mM S2O32- (600 mg L-1), respectively.
In all nuclear power generating countries, the production of high-activity, long-lived radioactive waste is an unavoidable by-product of the contribution of this energy to the global electricity generation. Disposal in deep stable geological formations is, at present, the most promising option accepted at an international level for the long-term management of these wastes.1-3 Geological disposal relies on a combination of engineered (man-made) barriers and a natural barrier (the host rock), in order to prevent radionuclides and other contaminants ever reaching concentrations at which they could present an unacceptable risk for man and the environment. The engineered barriers should be designed to provide complete containment of the heat-generating waste at least throughout the thermal phase, which is assumed to last for at least several hundreds of years for vitrified high-level waste and possibly up to a few thousand years for spent fuel, to avoid coping with complex behaviors of radionuclide transport during this period.1,3