Numerical study of thermohydraulics induced by highly under-expanded jet during in-vessel LOCA for the CFETR WCCB concept

In-vessel Loss of Coolant Accident (LOCA) is one of the fundamental design-basis accidents that must be considered in the design phase of the China Fusion Engineering Test Reactor (CFETR). If high-temperature coolant leaks into the Vacuum Vessel (VV), a flashing jet is formed, leading to localized pressure and temperature peaks. As the VV serves as a primary barrier for radioactive protection, these peaks may pose significant challenges to its integrity. In this study, a three-dimensional simulation model of the CFETR VV is developed to analyze the evolution of flow, pressure, and temperature fields during an in-vessel LOCA. The study investigates the structural evolution of the highly under-expanded jet and its impact on pressure and temperature dynamics. The results indicate that the pressure at the jet impact surface is significantly higher than at other locations within the VV. Comparisons of temperature and pressure variations at different monitoring points reveal that if the bursting valve is positioned at the furthest location, the pressure at the jet impingement surface may reach the VV’s pressure limit before the VV Pressure Suppression System is activated, as determined using the lumped-parameter analysis method. Additionally, temperature rises caused by gas compression at wave crossings pose challenges to the VV material’s thermal limits. Furthermore, a simulation with a larger breakage size of the First Wall coolant pipes is conducted. The findings show that with increased breakage size, the under-expanded jet evolves more rapidly, pressure inside the VV rises more quickly, and the VV reaches its pressure limit sooner. Moreover, localized temperature spikes exceeding material limits are triggered by the convergence of jet-induced compression waves.