Development of relocation model for debris particles in core disruptive accident analysis of sodium-cooled fast reactor

During a core disruptive accident in a sodium-cooled fast reactor, the molten fuel or steel is solidified into debris particles that form a debris bed in the lower plenum. When the debris bed starts boiling, the flow of the coolant and vapor together relocates and flattens the debris particles. This process is called debris relocation. The thickness of the debris bed and its porosity have significant influence on the cooling ability of the fuel debris in the lower plenum. Therefore, it is necessary to evaluate the transient changes in the shape and thickness of the debris bed in the relocation behavior for accident analysis. To simulate the relocation behavior, a debris-relocation model based on the COMMEN code was developed in this study. The debris-relocation model is based on the shear strength mechanism, which is widely applied in soil mechanics. Shear strength is a function of the density, position, and shape characteristics of particles. The debris bed is relocated only when the shear stress of the global debris particles is greater than the shear strength. By incorporating the relocation model into the COMMEN code, the relocation transient processes of different particle beds in a nitrogen-injection experiment were simulated, and the results were compared with experimental results. The analysis indicates that the proposed debris-relocation model can effectively predict the occurrence of debris-bed relocation and reasonably simulate its transient process, although future improvement is necessary.