Numerical modelling of cover gas thermal hydraulics in Sodium-cooled Fast Reactors

Abstract

An argon gas cover layer is usually used in Sodium-cooled Fast Reactors (SFR) above the pool of sodium to prevent leakage of air into the vessel getting in contact with the liquid metal coolant. Heat transfer across the cover gas layer is of significant interest in terms of understanding the overall behaviour of the reactor and determining the thermal load of the roof slab, meanwhile, deposits of aerosol on to the structure surfaces across the cover gas region may hamper the operation of the penetrations and influence the coolant leakage detection system. In order to develop a comprehensive understanding of the thermal hydraulics in the cover gas region, a multi-physics model simulating the aerosol growth by condensation, aerosol transport and deposition, along with the natural convection of the gas mixture of argon and sodium vapour with thermal radiation across the semi-transparent mist has been developed. The model has been used for the simulation of a test facility for the study of cover gas dynamics and aerosol characteristics. The simulation results of the temperature profiles and aerosol concentrations under various conditions agree well with the experimental results. Under a normal operating condition of a sodium pool temperature of 550 °C, a roll-like structure is observed over the domain with a large circulation in one plane and several small vortices normal to it. Thermal radiation is found to be approximately 70% of the total heat flux through the roof. Gravitational settling is a significant factor in terms of aerosol dynamics which leads to an accumulation of droplets near the pool surface and the down flow region. An empirical correlation has been established based on the numerical and experimental data which shows that aerosol concentration increases with the sodium pool temperature.