Study on the Influence of Ambient Temperature and RPV Temperature on Operation Performance of HTR-PM Reactor Cavity Cooling System

High Temperature Gas-cooled Reactor (HTGR) is the Generation IV advanced nuclear reactor, which can realize inherent safety and prevent core melt. Institute of Nuclear and New Energy Technology (INET) of Tsinghua University has developed a commercial scale 200 MWe High Temperature gas-cooled Reactor Pebble bed Module project (HTR-PM), which entered commercial operation on December 6, 2023. A passive Reactor Cavity Cooling System (RCCS) is designed for HTR-PM to export heat from the reactor cavity in normal operation and also in accident condition, keeping the safety of the reactor pressure vessel (RPV) and reactor cavity. RCCS of HTR-PM has been designed as three independent sets, normal operation of two sets of RCCS can guarantee the safety of the PRV and reactor activity. The heat can be transferred from the RPV to the final heat sink, atmosphere, through the thermal radiation and natural convection in the reactor cavity, and the natural circulation of water and air in the RCCS. The CAVCO code has been developed by INET to simulate the behavior of RCCS. In this paper, assuming different RPV temperatures and different ambient temperatures, as well assuming all or parts of the RCCS sets work, the performances of RCCS are studied by CAVCO to evaluate its operational reliability, so as to provide reference for further optimization. The analysis results indicate that even under hypothetically extremely RPV temperatures, two sets of RCCS could effectively remove heat without causing water boiling or system failure. However, during winter when ambient temperatures are low, particularly when the reactor operates at lower RPV temperature, additional attention must be given to the operational safety of the system. It is crucial to prevent system failure caused by the freezing of circulating water and the potential cracking of water-cooling pipes due to freezing. Depending on the reactor status and ambient conditions, one or all three sets of RCCS may need to be taken offline. Besides, the maximum heat removal capacity of the RCCS with only two sets operational exceeds the design requirement of 1.2 MW. When the ambient temperature fluctuates significantly, it may be advisable to increase the number of available RCCS sets to mitigate the effect of abrupt changes in cooling water temperature on pipeline thermal stress.