Abstract: The analysis code related to molten corium in-vessel retention (IVR) was developed by taking steady state enveloping computational results in the world as references. The calculation results were verified by comparing with the results from ERI, DOE and INEEL. According to molten pool parameter analysis and uncertainty analysis, the probability of exceeding-CHF accident in large-scale advanced PWR is smaller than 7% when the flow path design of ULPU-2000 configuration Ⅳ is used, but the probability that the maximum of melt thickness exceeds 15 cm is very large. If there is no other mitigation method, the pressure vessel thickness of more than 20 cm is suggested to be increased to. Heat flux distribution is the main factor that influences the molten pool behavior. It is suggested that measures should be taken to adjust the heat flux distribution of the molten corium pool for mitigating heat transfer crisis at the region near the interface between oxide zone and metal zone.