Criticality analyses of the used and spent fuel storage facility of the 400 MWth PBMR plant
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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. This study covers a variety of tasks, from the modelling of a single fuel sphere to modelling of the entire Sphere Storage System for the normal and various off-normal conditions, and for the determination of keff values for the system under these conditions. Additional models were also created to investigate the phenomena of clustering of low burnup fuel spheres and the effects of graphite spheres being mixed with the fuel spheres in the storage containers. The entire study was done using the SCALE 5.1 computer code package. SCALE 5.1 is licensed by the United States Nuclear Regulatory Commission (US NRC) and is a package that is widely used in the US and around the world to perform criticality safety analyses as well as other nuclear-related calculations. For this study the control module CSAS6 was specifically used to develop the appropriate models because of its suitability for the modelling of pebble fuel and its advanced geometric modelling capabilities. It also automatically invokes the specific functional modules using the sequence CSAS26 in order to obtain the appropriate information as required by another functional module KENO-VI, which calculates keff for the specified input models. 3 The results from the models for the various scenarios representing normal and off-normal conditions show that the design of the proposed current design of the Sphere Storage System remains critical safe (keff < 0.95) for all the plausible scenarios considered. Any change to the current design requires new Criticality Safety Analyses to be performed. However, the methodology developed in this study can be used as a guide for future studies.
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