CFD Analysis of Thermal Stratification under PLOFA transient in CLEAR-S

Abstract

Lead-based reactor engineering validation facility CLEAR-S is a large scale integrated non-nuclear pool type facility which has been built to provide a unique experimental platform for full-scale prototype components test of lead based reactor. In this research three dimensional computational fluid dynamics (CFD) transient analysis of CLEAR-S was held by using FLUENT-15 in order to study the effects of protected loss of flow accident (PLOFA) on the temperature and flow profile of Lead Bismuth Eutectic (LBE) in different components of lead based engineering validation facility (CLEAR-S). Steady state forced circulation was simulated in order to validate the model by comparing simulation results with design parameters of CLEAR-S. It showed that the simulation had a good agreement with the design parameters of CLEAR-S. PLOFA was simulated by removing the momentum and energy sources from pump and heat exchanger (HX) zones respectively, Core simulator energy source was reduced to 7% of rated value and a cooling source represented by user defined function, was set in decay heat remover (DHR) zone in order to simulate DHR conditions at the startup of scram at t = 0 s. Results showed that after pump failure transient mass flow rate dropped completely within 2 s of occurrence of transient. There is a small increase in mass flow rate after an initial sharp dip due to the start of natural circulation. During natural circulation mass flow rate was 3% of rated mass flow rate. Thermal stratification layer became prominent in the hot pool at 200 s and moved upward as the time progressed. In the cold pool, thermal stratification was started from DHR outlet at 200 s and became prominent at 400 s in the bottom of cold pool. As the time progressed stratification layer moved up due to increase in the quantity of low temperature LBE in cold pool from the outlet of DHR. This study of thermal stratification in the pool type lead based facility is significant for the development of pool type lead based reactor technology because thermal stratification could cause thermal stresses on the walls and components inside the pools of reactor which is dangerous for the safety and integrity of the system.