Main outcomes from the IVR code benchmark performed in the IVMR project

Laure Carénini (1) , Florian Fichot (1) , Nikolai Bakouta (2) , Aleksandr Filippov (3) , Romain Le Tellier (4) , Louis Viot (4) , Ivan Melnikov (5) , Peter Pandazis (6)

1 IRSN/PSN-RES/SAM/LEPC - Laboratoire d'Etude de la Physique du Corium
2 EDF Labs
3 IBRAE
4 CEA Cadarache
5 NRC KI - National Research Center "Kurchatov Institute"
6 GRS - Gesellschaft für Anlagen- und Reaktorsicherheit

In-Vessel Retention (IVR) of corium is one of the possible strategies for Severe Accident (SA) mitigation. Its main advantage lies in the fact that, by maintaining the corium within the vessel, the integrity of the last containment barrier against corium aggression is preserved. One of the issues for the demonstration of the success of this strategy is the evaluation of the behaviour of the corium relocated in the lower head and how it stabilizes and affects the integrity of the vessel wall. The first modelling was developed in the nineties and assessed the heat transfers in a stratified corium pool with a top metal layer made of steel and Zirconium only. About 10 years later, the results of the MASCA program highlighted the possibility of having more complex stratified configurations, including a dense metal layer. Addressing thermochemical effects on the stratification makes the modelling of the corium pool in the lower head more difficult and, in addition, knowledge of the associated kinetics is still limited. As a consequence, available SA codes, either integral or dedicated to the lower head, can differ significantly in their models, which leads to discrepancies in the results when evaluating the IVR strategy. In order to identify the main modelling issues and to assess the capabilities of the codes, a benchmark exercise for code validation was made in the scope of the European H2020 project IVMR (In-Vessel Melt Retention). It is based on the definition of different IVR configurations at reactor scale with an increase in the complexity of the phenomena involved: starting from a steady-state stratified pool with metal on the top, up to consideration of corium phase separation at thermochemical equilibrium and progressive ablation of metallic structures and vessel wall. In this paper, the main results and outcomes obtained are presented and discussed. Six organisations took part in this benchmark (CEA, EDF, GRS, IBRAE, IRSN, NRC-KI) and 6 different codes were used (ASTEC, ATHLET-CD, MAAP_EDF, PROCOR, HEFEST_URAN and HEFEST – stand-alone version of the corresponding module of the SOCRAT code). Finally, sensitivity studies are performed and allow obtaining a more consolidated range of results. Thanks to this benchmark exercise and to the approach followed with a progressive increase of complexity, the capabilities of codes to evaluate the heat flux profile applied to the vessel wall in steady-state are demonstrated. Then, the larger differences between code results obtained in transient situations have been identified and associated modelling assumptions discussed.

Mots clés: IVR strategy; SA code benchmark; stratified corium pool evolution

Domaines: Sciences de l'ingénieur [physics]; Physique [physics]

Laure Carénini, Florian Fichot, Nikolai Bakouta, Aleksandr Filippov, Romain Le Tellier, et al.. Main outcomes from the IVR code benchmark performed in the IVMR project. 9th Conference on Severe Accident Research (ERMSAR 2019), UJV REZ, Mar 2019, Prague, Czech Republic. pp.107612, ⟨10.1016/j.anucene.2020.107612⟩. ⟨irsn-04115899⟩