Analysis of DRACS and DRACS-RVACS Decay Heat Removal Experiments

Two large-scale decay heat removal experiments are analyzed to support the validation of the thermal-hydraulic code COMMIX and the design of advanced liquid metal reactors (ALMRs). The experiments were performed in the reactor vessel auxiliary cooling system (RVACS) test facility, which provides a scaled simulation of the passive decay heat removal paths of a pool ALMR with the core simulated by electrically heated rods. The first experiment simulates a transient where decay heat is removed by the direct reactor auxiliary cooling system (DRACS) only. In the second experiment, heat is removed by both the DRACS and RVACS. These experiments are characterized by (a) three-dimensional in-pool sodium flows of very low velocity, driven by sodium density differences, (b) a significant pool thermal stratification, and (c) a complex heat sink. In the DRACS test, the thermal stratification occurs in the hot pool while the cold pool is nearly isothermal. When both systems are in operation the thermal stratification of the hot pool is drastically reduced while the upper third of the cold pool is significantly stratified. The COMMIX predictions for the sodium pool temperatures and the air outlet temperature of the RVACS are in good agreement with measurements.