Modelling issues related to molten pool behaviour in case of In-Vessel Retention strategy

Abstract

The In-Vessel Retention (IVR) strategy for Light Water Reactors (LWR) intends to stabilize and retain the core melt in the reactor pressure vessel. This type of Severe Accident Management (SAM) strategy has already been incorporated in the SAM guidance (SAMG) of several operating small size LWR (reactors below 500MWe, like VVER440) and is part of the SAMG strategies for some Gen III+ PWRs of higher power like the AP1000.The demonstration of IVR feasibility for high power reactors requires using less conservative models as the margins are reduced.In this paper modelling issues related to molten pool behaviour in case of IVR are highlighted based on sensitivity studies performed with the severe accident integral code ASTEC (Accident Source Term Evaluation Code) for a typical PWR reactor. In particular, the following physical processes are addressed:-Heat transfer coefficients in thin light metal layer and in heavy metal layer.-Kinetics of corium stratification and thermal exchanges between oxide and metal phases during inversion of stratification.-Relocation of steel coming from ablated vessel wall.-Characteristics of oxide crust.Analyses based on a simple configuration of corium in the lower head allow identifying potential impact of each modelling assumption on the heat flux distribution along the vessel wall, especially in transient situations. Based on this study, the most critical areas where the knowledge is too limited or where the current analytical methods should be improved are identified as being (i) the duration of situations with thin metal layer on the top of the oxide, associated to the knowledge of the kinetics of corium stratification and (ii) the possible super-heat of the metal layer at stratification inversion coupled with the risk of oxide crust failure.This work is performed in the frame of the European project IVMR (In-Vessel Melt Retention) coordinated by IRSN. This project has been launched in 2015 and gathers 27 organizations. Its main objective is the evaluation of feasibility of IVR strategy for LWR (PWR, VVER, BWR) of total power 1000 MWe or higher.