Third-generation advanced pressurized water reactors adopt the external reactor vessel cooling (ERVC) strategy to ensure the pressure vessel is not at risk of melt-through in severe accidents, thereby completely controlling radioactive materials in the pile. However, due to its long-term service, vessel aging, steel corrosion, and oxidation may lead to deformation at different locations on its outer surface, forming various shapes of sawtooth structures, thus affecting the heat transfer behavior of the high-temperature walls during ERVC. In this paper, the Fluent code, coupled with boiling heat and mass transfer equations based on user-defined functions (UDFs) was used to simulate the thermal-hydraulic processes on the lower head with three typical sawtooth structures. The distribution of the stagnation zone for vapor buildup was the main focus. By varying the heat flux installed on the lower head and the inlet velocity of the flow channel, the onset time of critical boiling and the development of the location of critical heat flux over time were further investigated. It was found that a long period of unstable bubble generation and detachment before the onset of critical boiling may occur on the lower head. These findings can provide technical support for the safety design of advanced nuclear reactors.
