Criticality Analyses of the Used and Spent Fuel Storage Facility of the 400 MWth

The development of the Pebble Bed Modular Reactor entails the design of numerous systems for various purposes. One such system of significant importance is the Sphere Storage System (a subsystem of the Fuel Handling and Storage system) where fuel spheres that are unloaded from the core will be stored until approximately eighty years after the power plant has been decommissioned. Over and above the normal conventional safety analyses that one expects to be performed for any new system being designed, in the case of the Sphere Storage System a detailed Criticality Safety Analysis must be performed. The universally accepted Effective Neutron Multiplication Factor, keff, was used to indicate the margins of subcriticality for all the conditions modelled. Since this Used and Spent Fuel storage facility is a Critical Safety-relevant system that will store nuclear fuel for a long time, it is required by regulation that the Criticality Safety Analyses be performed to verify whether this system will always remain “critical safe ” (keff < 0.95) under all plausible conditions