INVESTIGATION OF CATALYTIC FCS PERFORMANCE CONSIDERING PLANT SYSTEM BEHAVIOR

摘要

A severe accident in a light water reactor has the potential to generate hydrogen resulting from metal-water reaction and radiolysis. In order to prevent hydrogen combustion, the flammability gas control system (FCS) is installed in the reactor containment. While most of the current FCS combine hydrogen with oxygen by heating, recently the catalytic recombiner has been developed. The catalytic FCS performs its function passively and has the advantages of the robustness during an accident, easy maintenance and low cost compared with the current FCS.The accident conditions (total pressure, hydrogen concentration and aerosol concentration) affect the catalytic FCS performance. Although the capability of the catalytic FCS has been examined varying the test conditions, no analysis with relation to the plant system behavior during a severe accident has been performed. Therefore it is important to analyze catalytic FCS capability considering overall system behavior of an actual plant.In order to analyze the generation of hydrogen and the catalytic FCS capability to remove hydrogen during an accident, the simplified models to predict the generation of hydrogen and oxygen due to radiolysis of water, and the catalytic FCS performance have been incorporated into the severe accident analysis code MAAP. Generation of hydrogen and oxygen due to radiolysis of water is calculated based on radioactive Fission Product (FP) distribution and effective G value, considering both recombination caused by excessive hydrogen and the interruption by iodide as a poison. The catalytic FCS model evaluates the hydrogen removal rate based on experimental data, and also considers the effect of total pressure, hydrogen concentration and aerosol deposition on the catalyst surface.The analysis results for a BWR plant by using the modified MAAP indicate that for hydrogen and oxygen generated by radiolysis it takes a substantial time to reach flammable condition during a severe accident, and the catalytic FCS can effectively maintain the concentration of hydrogen and oxygen well below the flammable limit.However, the test data on which the MAAP-FCS model is based was obtained under limited experimental conditions, especially regarding the information for catalyst performance degradation caused by aerosol deposition. Additional experimental investigation is expected to reduce the uncertainty on the catalytic performance degradation effect.