Criticality safety for UK spent fuel disposal in the pre-closure phase of a Geological Disposal Facility

Nuclear Waste Services (NWS) specialises in the management and disposal of radioactive waste produced by nuclear technologies in the UK. NWS's goal is to ensure all radioactive waste is managed safely and securely through innovative and sustainable approaches, including siting and construction of a UK's Geological Disposal Facility to contain higher activity radioactive wastes (HAW). Some of these HAW can include significant quantities of fissile nuclides and, therefore, demonstration of the criticality safety under disposal conditions forms an important component of NWS's GDF disposal system safety case. This must include consideration of criticality safety during both the pre-closure (including transport of the waste to the disposal facility and during disposal operations up to the point of closure) and post-closure (i.e., after facility closure) phases. This paper presents the background and results for a range of scoping level analyses that have been performed to enhance understanding for the pre-closure criticality safety justification for HAW requiring geological disposal. It focuses on Spent Fuel (including sensitivities to low/unirradiated fuel) as one of a number of waste streams that are identified in the UK Inventory for Geological Disposal and demonstrates that a robust criticality safety demonstration for these materials can be made. (author)