In this paper, the impact of the uncertainties associated with core degradation on the state of the corium at vessel rupture is studied using the severe accident code ASTEC for selected severe accident scenarios and reactor designs. For this analysis, a specific methodology is proposed, including the development of a simplified input deck based on a complete reactor calculation, allowing for the direct introduction of uncertainties in the initial phase of degradation.

In addition, uncertainty propagation studies are carried out, with an approach using a progressive increase in the complexity, the aim of which is to understanding the interactions between the various physical phenomena influencing the state of corium at vessel rupture and the successive relocations down to the reactor cavity. On the basis of 2400 calculations, the ranges of variation in the outputs of interest (time of first vessel rupture, characteristics of the corium flowing out of the vessel, associated kinetics, etc.) are defined. The main correlations between the uncertainties in the degradation calculation and the state of the corium at vessel failure and its kinetics of relocation out of the vessel are identified. Such outcomes are of high interest for severe accident strategies, with the aim to stabilize the corium outside the vessel, especially regarding the assessment of its coolability.