Development of fissile package contents limits for intermediate level waste packages

Radioactive Waste Management Ltd (RWM) is responsible for developing a geological disposal facility for the UK's higher activity waste. In support of this RWM develops and maintains a generic transport system design to demonstrate that radioactive waste packaged now will be safe to transport to a geological disposal facility in the future.Some Intermediate Level Wastes planned for disposal at a geological disposal facility will require transport in a Type B(U/M)F package design owing to their fissile material content. Accordingly RWM has historically defined safe fissile masses for a range of package designs. Diversity in waste composition has been allowed for by grouping wastes into bands and defining a safe fissile mass for each band. For instance bands covering a range of isotopic composition e.g. irradiated natural/slightly enriched uranium, low enriched uranium, etc. On the other hand uncertainty in the disposition of material within a package has been allowed for by pessimistically assuming the material forms an optimal configuration. This pessimistic assumption has led to the definition of very restrictive safe fissile masses.RWM working together with International Nuclear Services and Sellafield Ltd has developed an approach to grouping wastes by material distribution in order to reduce the pessimism in the safe fissile masses. Material distribution bands have been defined for a worst-case of unlimited non-uniformity (corresponding to the historical limits for an optimal configuration) and for material distributions that allow credit to be taken for a known degree of mixing. The approach is supported by compliance rules to enable wastes to be assigned to an appropriate distribution band. This paper describes the approach to defining less restrictive safe fissile masses.