Comparison Between EDF MAAP5.04 and ASTECv3 Codes on Hypothetical Severe Accidents in an Integral PWR

Abstract

This paper presents a comparison between EDF MAAP 5.04 and ASTECv3 codes in simulating postulated Severe Accident (SA) scenarios in a generic integral PWR characterized by a submerged containment and about 60 MWe. The code crosswalk was performed by EDF and Tractebel within the Horizon Euratom project SASPAM-SA (Safety Analysis of SMR with PAssive Mitigation strategies – Severe Accidemt). MAAP is a deterministic code licensed by EPRI whilst ASTEC is an integral code developed by IRSN. Both codes were validated through intense and extensive validation activities and recognized with MELCOR as reference tools for SA deterministic studies in nuclear reactors. The aim of this work is to investigate capabilities and limitations of SA codes, as well as identify causes leading to relevant deviations in results. This is because the phenomena that occur in SMRs may be different from those that occur in commercial large reactors and may not be captured correctly by models currently implemented. Comparisons performed between EDF MAAP5.04 (SMR code version developed by EPRI and improved by EDF) and ASTEC V3.1 include accident progression from initial events to long-term in-vessel retention of the corium. Particular attention was also paid to the solution adopted to reproduce the strong vessel-containment interaction typical of SMRs. The main differences observed in the results were related to core degradation evolution and corium slumping mechanism in the lower plenum. Discrepancies in core degradation kinetics depend partly on how debris formation is modeled and partly on the different amounts of power exchanged between vessel and containment predicted by the codes.