Effect of Decay Heat Level on Reflood Phenomena during PWR-LOCA

The effect of the decay heat on the reflood phenomena in a PWR-LOCA were investigated under a large-scale facility, by using the Cylindrical Core Test Facility (CCTF). Two tests were performed at the initial average linear power of 1.06 and 1.4 kW/m. It was found that the core inlet boundary condition (the core flooding mass flow rate, the pressure and the subcooling) being important for core cooling was little influenced by the variation of the linear power for the entire reflood period. It was also found that the core cooling behavior (the heat transfer coefficient) was little Influenced for the early reflood period (until the turnaround time), and the heat transfer coefficient was lower in the higher power test for the later reflood period, indicating that assuming the higher power is conservative for safety evaluation. The reason of the nearly identical core flooding mass flow rate and the nearly identical heat transfer coefficient for the early period regardless the different linear power is that the in-core steam generation rate is contributed mostly by the release of the Initial stored energy rather than the supplied linear power and that the initial stored energy was identical between both tests. The insensitiveness of the core flooding mass flow rate and the heat transfer coefficient for the first period against the decay heat could be extrapolated to a PWR scale, since the insensitiveness was commonly observed in the present large scale reflood experiments and the small scale reflood experiments, FLECHT. In addition, the insensitiveness of the core flooding mass flow rate against the decay heat was analytically confirmed.