Development of the numerical simulation method for molten core behavior in lower head based on MPS method

Nagatake, Taku ; Yoshida, Hiroyuki  

A core melt accident occurs, and a molten core accumulates in the bottom of the lower head of the RPV in some severe accident scenarios of LWRs. The decay heat heats a molten core pool and puts a heat load on the wall of RPV. As a result, RPV is damaged, and the molten core flows out. Then, an understanding a molten core behavior in a lower head of RPV is important to predict the progress of severe accidents and optimize a safety management method such as the IVR. In this study, we have been developing a numerical simulation method for simulating a molten core behavior in the lower head of LWR based on POPCORN code, which has been developed in JAEA based on the MPS method. As for the validation experiment, we chose the SIMECO experiment. The SIMECO experiment has been performed in KTH to obtain validation data of natural convection behavior in the lower head of LWR. In the SIMECO experiment, several different types of fluids are used as working fluids in the SIMECO experiments. In this study, we selected a natural convection behavior inside the simulated lower head in SIMECO water-test. And the temperature distributions on the center line and heat flux between fluid and wall were compared as the first step of validation. The numerical results of the temperature distribution in the upper region were in good agreement with the experimental results. On the other hands, the numerical results of temperature in the lower region were higher than the experimental results. And the numerical results of heat flux were different from experimental results (lower in the upper region and higher in the lower region). Then it is thought that one of the reasons for the temperature difference in the lower region is the misevaluation of heat flux between fluid and wall. Then, in future work, the development of POPCORN code is continued, including improvement of a heat transfer model between fluid and solid wall, to perform more accurate simulation.