Numerical Analysis of Catalytic Recombiner Performance Considering a 3-Dimensional Gas Flow

Metal-water reaction and radiolysis of water generate hydrogen during a severe accident in a light water reactor. To prevent hydrogen combustion, a flammability gas control system (FCS) is installed in reactor containment. Most of the current FCS combine hydrogen with oxygen by heating and employ active devices to maintain the gas flow through the FCS. Recently a catalytic recombiner has been developed as passive FCS. The catalytic recombiner performs its function passively and has the advantages of the robustness during an accident, easy maintenance and low cost compared with the current active FCS. The hydrogen depletion rate of the catalytic recombiner is affected by the local thermal hydraulic conditions during an accident. To evaluate hydrogen depletion by the catalytic recombiner considering these phenomena in the containment, a 3-dimensional fluid dynamics analysis is useful. A theoretical catalytic recombiner model has been developed in which the flammable gas depletion rate is estimated accounting for the gas transfer rate in porous catalyst material. The model has been incorporated with a thermal hydraulic model for the fluid dynamics in a containment that has been developed using a 3-dimensional CFD code STRA-CD. This catalytic recombiner model has been confirmed using a catalytic recombiner performance test that was carried out in the Battelle Model Containment (BMC). Further verification of the analysis model has been conducted using the test data described in NUREG/CR-6580 which addressed the wall effect on the catalytic recombiner performance. The predicted performance of the catalytic recombiner shows a good agreement with the test data, and especially the parameters effects on the recombiner performance is well described, which include the effects of the containment wall, gas flow rate to the catalytic recombiner and gas concentration distribution in the containment. (authors)