Direct Containment Heating: A Solved Risk Issue

In case of a High Pressure Melt Ejection (HPME) heated gas and corium may be expelled from the bottom head of a reactor vessel reaching the containment atmosphere, leading to a Direct Containment Heating (DCH). In addition, released gases might burn (e.g. hydrogen) causing a high load of the reactor containment building. Corium dispersal phenomena also strongly affect consequences of Molten Core-Concrete Interaction (MCCI) since the corium mass, which remains within the cavity, may remarkably diminish. Several computer codes are able to simulate the response of nuclear plants during hypothetical severe accidents: MELCOR, MAAP, ESCADRE and ASTEC have the capability to describe corium slump into the reactor cavity, vessel gases blow-down and possible corium entrainment. In this paper the various steps of model development, validation, plant-specific applications, etc., are described in the attempt of establishing a risk-oriented methodology with the target of solving this particular risk-issue. ENEA mature expertise in level-2 PSA analyses shows that the DCH phenomenology can be considered a solved risk issue. The applied methodology gives also hints and/or guidelines for solving similar risk issues in current PSA (level 2) analysis.