Abstract

Nordic Boiling Water Reactor (BWR) design employs ex-vessel debris coolability in a deep pool of water as a severe accident management strategy. Depending on melt release conditions from the vessel and core-melt coolant interactions, containment integrity can be threatened by formation of non-coolable debris bed, or energetic steam explosion. Melt release from the vessel affect ex-vessel phenomena and is recognized as the major source of uncertainty. The Risk Oriented Accident Analysis Methodology (ROAAM+) is used for quantification of the risk of containment failure in Nordic BWR where Melt Ejection Surrogate Model (MEM SM) provides initial conditions for the analysis of debris agglomeration and ex-vessel steam explosion which determine the respective loads on the containment. MEM SM is based on the system analysis code MELCOR. Modelling of vessel failure and melt release from the vessel in MELCOR is based on parametric models, allowing a user to select different assumptions that effectively control lower head behavior and melt release. The work addresses the effect of epistemic uncertain parameters and modelling assumptions in MEM SM on the containment loads due to ex-vessel steam explosion in Nordic BWR. Sensitivity and uncertainty analysis performed to identify the most influential parameters and uncertainty in the risk of containment failure due to ex-vessel steam explosion. The results of the analysis provide valuable insights regarding the effect of MELCOR models, modelling parameters and sensitivity coefficients on melt release conditions and predictions of ex-vessel steam explosion loads on the containment structures.

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