Subcriticality Guidelines for Liquid-Metal Fast Breeder Reactor Spent Fuel Shipping Cask Designs

A series of three-dimensional Monte Carlo criticality calculations has been performed to examine the criticality problems that exist in liquid-metal fast breeder reactor spent fuel shipping casks. The reactivity effects of subassembly loading sequences, total fuel payloads, subassembly pitches, neutron poisons, and choices of cask coolants and structural materials were considered. The analyses showed that, for a given type of fuel, the system k_eff can be displayed in a manner that permits determination of the maximum number of subassemblies that can be safely loaded in a particular cask. For a given cask coolant, this maximum number can be fixed independent of

1. the sequence or positions in which the sub-assemblies are loaded
2. the detailed design of the basket supporting the subassemblies, including small changes in the distance between the subassemblies
3. changes in the cask design outside the fuel region.

Thus, the shielding, structural and thermal problems inherent in the design of the outer cask structure can be addressed without reference to the criticality problem.