Study On Heat Transfer Models Of Molten Pool In The Lower Head During A Severe Accident

In-Vessel Retention-External Reactor Vessel Cooling (IVR-ERVC) is one of the key strategies for severe accident management, and the current evaluation method of the effectiveness of IVR mainly derived from the lumped parameter (LP) model. Considering the characterization of the natural convection process of the molten pool, the thermal load to the lower head will be maximized when the molten pool reaches a thermal equilibrium state. Therefore, the steady-state thermal analysis of molten pool is the key factor to assess the likelihood of the occurrence for the reactor pressure vessel (RPV) lower head failure.In order to apply this management to the advanced large size PWRs, the influence of natural convection heat transfer performance on the effectiveness of IVR is investigated in this study. A lumped parameter analysis code MOPOL has been developed and improved for an accurate simulation of heat transfer behavior in the molten pool. According to the results of selected severe accident which has been calculated by MAAP4code, several key uncertainty input parameters of MOPOL are obtained.Considering the uncertainty of debris bounding configuration, a four-layered stratified molten pool is assumed to form in RPV lower head and the corresponding RPV wall heat flux distributions are calculated and discussed. In order to find out new approaches to remove the decay heat, a lumped model with premise that a water layer upon the light metal layer has been developed and analyzed.With the enlargement of core mass and the RPV size, the increase of Rayleigh number was found in the calculations of nature convection heat transfer for advanced large size PWRs. Considering the applicability of natural convection heat transfer correlations, a better natural convection heat transfer correlation under the high Rayleigh number was proposed and incorporated in the code. The heat flow distribution at the boundary of RPV wall was predicted, and a demonstration of good feasibility of the IVR-ERVC strategy under a selected severe accident for large size PWRs is shown by the code results. The effort is also devoted to propose the measures to improve the IVR-ERVC behavior by increasing the reactor core internals as well as the cold water injection above the metal layer.