Low Concentration Hydrogen Combustion and Its Influence Analysis outside PCS Internal Heat Exchanger

In severe accidents, the zirconium water reaction produces a large amount of hydrogen and inevitably releases a large amount of heat. Passive containment cooling system (PCS) internal heat exchanger is a nuclear safety class equipment and an important component of PCS, which could export heat from the containment. Vapor condenses near PCS internal heat exchangers when heat is removed in response to severe incidents. This process leads to an increase in the concentration of hydrogen and air, which increases the possibility of hydrogen combustion near the internal heat exchanger of PCS. Therefore, the commercial fluid calculation software ANSYS was used to conduct hydrogen combustion outside PCS internal heat exchanger and its influence analysis. The hydrogen combustion model was verified by experiment data before use. Liu Yinhe’s cylindrical experiment in Xi’an Jiaotong University was choosed to verify the numerical model of hydrogen combustion. The Fluent module in ANSYS was used to simulate the flame propagation velocity and peak combustion pressure under the initial conditions of 10% initial hydrogen concentration, 0.1 MPa initial pressure and 323 K initial temperature. The calculated results are in good agreement with experimental data. Therefore, the hydrogen combustion model selected in this paper can effectively simulate the hydrogen combustion outside PCS internal heat exchanger. The influence of initial hydrogen concentration, convective heat transfer coefficient in heat exchanger tube and tube bundle on thermal characteristics of the heat exchanger was studied by using Fluent module in ANSYS. The initial hydrogen concentration was 8%, 10% and 12%. The range of convective heat transfer coefficient in heat exchange tubes was 200-2 000 W/(m²·K). The tube bundle consisted of three heat exchange tubes. It is found that the most important factors affecting the thermal characteristics of the heat exchanger are the initial hydrogen concentration and the convective heat transfer coefficient in the heat exchange tube. The static structural module of ANSYS was used to simulate the stress of the heat exchanger under external hydrogen combustion condition. Primary membrane stress, local membrane stress and local membrane stress plus bending stress were obtained after stress linearization on the path including upper and lower header shell along wall thickness direction and the maximum stress point along the heat exchanger tube wall thickness direction. Then, compared them with ASME Code Section Ⅲ Division 1, Division NC, and Division 1 Appendices. It is found that the stress level of the evaluation path meets the requirement of the evaluation standard under the condition analyzed in this paper. The research results indicate that under severe accidents, the hydrogen combustion with a concentration of 12% inside the containment does not affect the safe use of the internal heat exchanger of the PCS.