In- and ex-vessel coupled analysis for in-vessel retention

In-vessel retention (IVR) in the manner of external reactor vessel cooling (ERVC) is an important severe accident mitigation strategy, which has been applied to some advanced light water reactors, e.g. AP1000. This main assessment method on the effectiveness of IVR is the lumped parameter (LP) model, which has been applied to assess the safety margin of IVR-ERVC for decades. However, this model deals with the safety margin analysis with several isolated processes, without considering the strong coupled relations about the heat transfer processes in the internal and external reactor pressure vessel (RPV). This paper addresses this coupled issue based on modified lumped parameter model using an iterative method, which focuses on the relations between the inner wall surface temperature of the reactor pressure vessel lower head and the heat flux through the RPV inner wall with a modified heat transfer method. The method has been applied to analyze the molten pool behaviour of LIVE-L4 tests. Comparison between the experimental data and the calculation results is performed to validate the accuracy of the coupled analysis method. High attention is paid to some important parameters, e.g. the heat flux through vessel, the inner and outer wall surface temperatures of the RVP lower head and the crust thickness. In addition, the paper analyzes the effectiveness of IVR for AP600 based on the modified method and compares with the UCSB assumed Final Bounding State (FIBS) benchmark calculation results to verify the modified coupled method. This method is applied to conduct the sensitivity analysis for some design parameters of AP600.