Development of a 0D model for the investigation of in vessel melt retention in integral pressurized water reactors

The In Vessel Melt Retention (IVMR) strategy developed for Light Water Reactors (LWR) is an appealing solution for the mitigation of many Severe Accident (SA) scenarios. This approach is typical of innovative reactors like the AP1000 and the Hualong One, both GW-size Pressurized Water Reactors (PWR). In this paper, the IVMR application for integral PWRs (iPWR) is investigated. Indeed, many iPWRs rely on IVMR as a mitigation strategy. These kinds of reactors are characterized by an electrical power output generally lower than 300 MWe, an integral layout and the use of passive safety systems. During a postulated accidental sequence that leads to fuel melting, the molten corium relocates down to the lower plenum of the Reactor Pressure Vessel (RPV). By flooding the reactor cavity where the RPV is immersed, the external face of the vessel is cooled, and corium is maintained inside the lower head. The overall phenomenology occurring during IVMR in an iPWR is similar to the one in a high-power LWR, but some discrepancies are expected due to the different design features. This paper describes a 0D model developed to perform the steady-state analysis of idealized cases in an iPWR. The aim is to provide an initial exploration of the safety margins for several existing iPWR designs, considering available data and plausible assumptions for the unknown parameters, while recognizing the inherent uncertainties in the analysis. The 0D model has been written in Python language and conceived to be design independent. By adjusting a few geometrical or material characteristics, it may be readily adapted to the generic iPWR layout. The development activity has been carried out in the framework of the SASPAM-SA Horizon Euratom project. The main goal achieved is the identification of some specific features characterizing the phenomenon occurring in an iPWR. Among the variations, it is interesting to note that the molten pool is relatively shallow in height because of the vessel's large diameter (integral RPV) compared to the lower mass of relocated materials. This has an impact on the heat flux profile along the vessel wall. Another difference is the rather thick oxide crust associated with the low residual power in the pool, for which the modelling approach has been adapted with respect to what is usually done for the analysis of high-power reactors. The outcomes obtained with this work will also prepare the next steps of SASPAM-SA project, identifying the models in SA codes needing some improvements. Integral reactor calculations focused on the IVMR are also planned: they will be compared to the results of the current 0D approach.