The role of safety functions, scoping calculations and process models in supporting the choice of a reference design for Belgian high-level waste and spent fuel disposal

ONDRAF/NIRAS has selected a Super-container with a Portland Cement (PC) buffer as the new Engineered Barrier System (EBS) reference design for disposal of vitrified NLW and spent fuel. The design satisfies several long-term functional requirements and has been developed as part of a step-wise approach to repository design. The selection process involved a multi-criteria analysis of different design options, which were evaluated against a range of long-term safety and feasibility criteria. The containment function is considered essential during the first phase in the evolution of the disposal system after repository closure (the thermal phase) and is primarily provided by the carbon steel overpack, which will surround the HLW canisters or spent fuel assemblies. A PC concrete has been chosen for the buffer because it will provide a highly alkaline chemical environment lasting thousands of years. In this environment, corrosion of the carbon steel overpack will be limited because the external surface will be passivated. ONDRAF/NIRAS has made a preliminary evaluation of the viability of the reference Super-container design. Scoping studies of gas generation due to radiolysis and corrosion, thermo-hydraulic (TH) behaviour of the concrete buffer, metal corrosion, the chemical and mineralogical evolution of the concrete buffer, and criticality were made. This paper describes how the modelling studies are used to support design choice (work in progress), and to identify remaining design and performance uncertainties. Prioritization and recommendations for future work are also given. (authors)