Filtration performance and hydraulic characteristic analysis on the bottom nozzle in a fuel assembly of a nuclear energy system

In the nuclear reactor system, the bottom nozzle of fuel assemblies is designed to filter debris and homogenize the flow field to avoid fuel failure due to debris wear. This study aims to determine the flow characteristics around the bottom nozzle and verify the filtering ability. Thus, relevant research is necessary to decide how debris type and bottom nozzle construction affect the flow and filtration behavior of the bottom nozzle. This study built a filtration analysis technique by establishing a visualization test facility and developing a numerical method by coupling computational fluid dynamics and the discrete element method. Three different bottom nozzles' filtration and flow characteristics were analyzed for various debris sizes and shapes. The results show that the increased geometric complexity of the bottom nozzle and debris improves filtration efficiency. According to the analysis, strip debris tends to travel parallel to the flow direction in the flow field, which weakens the filtration capability. For each bottom nozzle, debris blockage and stuckness in a small range have no significant impact on the pressure drop due to the flow distribution function of the bottom nozzle. The analysis technique and results of this study can provide a reference for the design of the filter.