Methodology to make a robust estimation of the carbon steel overpack lifetime with respect to the Belgian Supercontainer design

The Supercontainer (SC) design is the preferred Belgian option for the final disposal of vitrified high-level waste (VHLW) and spent fuel (SF) in deep underground clay layers. The SC consists of a carbon steel overpack, containing VHLW canisters or SF assemblies, surrounded by a thick concrete buffer, which in turn, is entirely encased in a stainless steel envelope. An integrated R&D strategy is developed to demonstrate and defend that the integrity of the carbon steel overpack can be ensured at least during the thermal phase. This integrated approach, proposed to estimate the lifetime of the carbon steel overpack, consists of three steps: lifetime prediction, validation, and confidence building. Under the predicted conditions within the SC (highly alkaline concrete buffer), the carbon steel overpack is expected to undergo uniform corrosion (passive dissolution). The methodology exists in demonstrating that corrosion forms other than uniform corrosion (e.g. localised corrosion such as pitting corrosion, crevice corrosion and stress corrosion cracking) cannot occur (‘exclusion principle’). This paper elaborates on how this methodology is implemented.