Strategic choices in the Belgian supercontainer design and its treatment in a safety case

In Belgium, geological disposal in plastic clay is the reference solution to the safe long-term management of high-level and long-lived radioactive waste. Boom Clay is currently considered the reference formation for hosting a geological repository. Up to 2004, the reference engineered barrier design for vitrified high-level waste and spent fuel consisted of galleries in which the waste was placed in a central steel disposal tube and the remaining space backfilled by bentonite blocks. In recent years there has been a re-evaluation of the reference design, and the Belgian waste management agency ONDRAF/NIRAS now opts for a design based on a carbon steel overpack surrounded by a concrete buffer made of Ordinary Portland Cement. The use of massive amounts of cementitious materials in the repository imposes a high alkaline environment which ensures passivation of the carbon steel overpack. Moreover, the use of Supercontainers as separate shielded entities should facilitate underground operations for the workforce. This paper further explains the drivers of this design option and the implications of this so-called Supercontainer design in terms of long-term radiological safety.