Ultimate strength and rupture modes of the metal containment of a BWR NPP in case of severe accident

Abstract The purpose of this paper is to present an assessment of the BWR MARK III steel containment under severe accident conditions. The accident follows a station black-out with the unavailability of the emergency cooling system, core melting and RPV rupture. The performance of the containment vessel has been evaluated with and without a possible flaw in the most stressed regions of the shell. The accident conditions assumed in the analysis derive from the rise of the March 3 model. The response of the containment to those conditions has been evaluated by means of the elastic-plastic finite-element computer code MENTAT-MARC with large displacements. The flow-stress is locally achieved under a differential pressure of about two times the calculated severe accident maximum pressure and about eight times the design LOCA pressure. It appears that the BWR steel-containment vessel of the MARK III type can withstand high loads deriving from a severe accident even if the presence of a flaw is assumed. Penetrations represent the most limiting areas which may lead to a leakage well before instability.