CFD Simulations of the Effects of Thermal Stratification on the Start of Natural Circulation during SFR Transients

摘要

Thermal stratification is an important phenomenon to be considered when removing decay heat using natural circulation in Sodium Cooled Fast Reactors (SFR) during protected loss of flow accidents (PLOF). The computational fluid dynamics (CFD) model of the hot pool of a sodium fast reactor, the Advanced Burner Test Reactor (ABTR), has been developed using STAR-CCM+. ABTR is a 250 MWt SFR pre-conceptual design for the transmutation of transuranic waste recycled from Light Water Reactor (LWR) spent fuel. The purpose of this study is to evaluate the importance of 3-D thermal effects in CFD simulations when modeling transients that involve passive safety systems. The model features a volume of fluid approach to include the compression of the cover gas as the sodium level in the tank changes during the postulated transient. The reference model treats turbulence effects in the sodium with the SST k-ω model. Steady state simulations showed that the temperature field in the hot pool is fairly uniform, except in the reflector region where temperature is higher. The calculation was initialized by performing a relatively long unsteady simulation with boundary conditions fixed at the nominal operating conditions. Transient calculations were then performed with time-dependent boundary conditions from a safety system code analysis. During transient calculations, a thermal stratification phenomenon was observed as expected by formation of temperature layers where the top layer had the highest temperature and the bottom layer had the lowest, which impedes the start of natural circulation. This has a significant impact in performance and safety and it can determine the reactor's ability to remove decay heat during transient operations. The results from CFD standalone simulations confirmed that 3-D thermal effects must be accounted for when performing passive safety system analyses.