Quantitative safety assessment models for the Canadian LLRW disposal facility (IRUS)

AECL is currently seeking a license to construct a prototype near-surface repository for Low Level Radioactive Waste (LLRW) disposal. The repository, called IRUS (for Intrusion Resistant Underground Structure), consists of a concrete vault located above the water table in an unsaturated zone of existing overburden. The horizontal dimensions of the vault are about 20 m by 30 m, and the depth of the vault is about 8 m. The repository is filled with waste packages that are backfilled with sand, in about an one to one volumetric ratio. Waste packages consist of a waste form, such as bitumen or concrete, sealed in a container. The bottom of the vault is a permeable buffer layer (sand-clay mix), which permits drainage in the event of water infiltration (to prevent flooding, or 'bathtubbing'). The top of the vault is covered with a concrete cap and additional protective layers, designed to act as barriers to intrusion and flow. AECL has submitted a preliminary safety assessment report to Canadian regulatory authorities requesting permission to construct the IRUS repository. The report was reviewed and the regulator's comments are being addressed. To support the licensing application, AECL has developed a performance assessment model for near-surface LLRW disposal; it has been implemented in the SYVAC-NSURE code (1). This paper will present a description of the model, with emphasis on the source term for the groundwater pathway. (authors). 4 figs., 6 refs