Treatment of ex-vessel debris coolability in IDPSA context
This study discusses ex-vessel debris bed coolability from safety analysis perspective, espe-cially regarding level 2 probabilistic risk assessment (PRA). The goal is to explore practical and risk-informed ways to deal with debris coolability issues in plant scale risk considerations. Analyses exploit deterministic accident progression simulations which are performed by using MELCOR, and thus this study builds upon IDPSA (integrated deterministic and probabilistic safety assessment) framework.
Literature part covers the most important debris bed parameters that can affect debris coola-bility. Also mechanisms that provide cooling are briefly presented. Empirical research on de-bris coolability has been performed at VTT and also elsewhere, and some results relevant to this study are referred to. Analytical capabilities play a crucial role in coolability assessment and some codes developed for that purpose are introduced in short.
Probability of failure to provide cooling for ex-vessel debris is evaluated by using load vs. ca-pacity concept. Debris bed heat flux is used as load variable and dryout heat flux serves as capacity variable. Two scenarios are employed: pressurized and gravity-driven melt ejection modes. Analyses indicate that when primary system is pressurized when vessel fails, the probability of having non-coolable debris is smaller than in depressurized cases due to e.g. less threatening debris shape and other properties that favour coolability. However, it must be kept in mind that primary system depressurization has other benefits to its name when it comes to severe accident management. All in all, this study provides useful insights to ex-vessel phase of severe reactor accidents with respect to PRA needs.