Comparisons between passive RCCSs on degree of passive safety features against accidental conditions and methodology to determine structural thickness of scaled-down heat removal test facilities

•A comparison between two RCCSs based on radiation and natural circulation was made.•The comparison under the same conditions to understand their advantages was made.•This study covered all natural phenomena and accidents that need to be considered.•The RCCS based on radiation has advantage against disturbance of outside air.•The RCCS based on radiation can control amount of heat removal by heat transfer area. Reactor Cavity Cooling Systems (RCCSs) with passive safety features use the atmosphere as a coolant, which cannot be lost; however, the drawback is that they are easily affected by atmospheric disturbances. To realize the commercial application of two types of passive RCCSs, their safety must be evaluated, i.e., they must be able to remove heat from the reactor at all times and under any conditions, including both expected and unexpected natural phenomena and accidents. The objectives of this study are to understand the characteristics and the degree of passive safety features for heat removal by comparing RCCSs based on atmospheric radiation and on atmospheric natural circulation under the same conditions. Simulations of accidental conditions, such as increasing average heat transfer coefficient via natural convection due to natural disasters, were performed using STAR-CCM+, and a methodology to control the amount of heat removal was discussed. A new RCCS based on atmospheric radiation is recommended because of the excellent degree of passive safety features/conditions, and the ability to control amount of heat removal by heat transfer surfaces. Finally, a methodology to determine the structural thickness of RCCS of scaled-down heat removal test facilities for reproducing natural convection and radiation was developed, and experimental methods using pressurized and decompressed chambers were proposed. The authors are convinced that the proposed RCCS based on atmospheric radiation has advantage that the temperature of the RPV can be maintained stably against the disturbance of outside air.