Water-jacket reactor cavity cooling system concept to mitigate severe accident consequence of high temperature gas-cooled reactor

Abstract

Abstract The High Temperature Gas-Cooled Reactor (HTGR), as a Gen-IV reactor, adopts a Reactor Cavity Cooling System (RCCS) to remove core decay heat after a reactor trip. During a postulated accident such as a pipe break in the reactor coolant system, the core decay heat can be properly removed by a RCCS to ensure the integrity of the fuel, reactor pressure vessel (RPV), and concrete silo. In this study we are more interested in the impact of a failure beyond that of the design basis of the RCCS functions, particularly the case in which natural convection of external air is not established. Such a case could happen when a chimney is collapsed by an external event. Against a severe disaster in which air convection is completely lost, a mixed air-water RCCS concept having a water-jacket system is suggested in order to mitigate the significant overheating of the RPV. In order to quantify the effectiveness of the water-jacket RCCS functions, the Depressurized Conduction Cooldown event (DCC) of a HTGR system with a thermal power of 350 MW is analyzed using the GAMMA+ system transient analysis code. As the RCCS air-cooling function is impaired, the water-jacket system increases its role to take part in the heat removal. From the analysis results of a DCC event with complete failure of the RCCS air-cooling, it is confirmed that, compared to the case without the water-jacket system, the maximum temperature of the reactor vessel can be reduced by about 400 °C thus ensuring its integrity. Therefore, the water-jacket system can be utilized as a backup system to mitigate the consequences of a severe accident such as the collapse of the chimney of the RCCS.