The supercontainer (SC) is the current reference concept for the geological disposal 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 the final closure technique of the carbon steel overpack in order to ensure its water tightness. Welding is known to induce residual stresses in the weld zone and its vicinity, 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 carbon steel exposed to an artificial concrete pore water solution that is representative for the SC concrete buffer environment. The tests were performed at 140 °C, at a constant strain rate of 5·10-7 s-1 and at open circuit potential under anoxic conditions. The synergistic effect of chloride, sulfide and thiosulfate on the SCC behavior of carbon steel was investigated up to levels of 35,000 mg?L-1 Cl-, 500 mg?L-1 S2- and 600 mg?L-1 S2O32-. In all the tested electrolyte solutions, none of the studied steel configurations, including welded and heat-affected zone materials, were found to be susceptible to SCC.