Implementation of framework for assessment of severe accident management effectiveness in Nordic BWR

•Probabilistic framework for risk assessment has been developed and implemented.•The framework employs iterative processes of knowledge refinement.•Treatment of partial or incomplete probabilistic knowledge in risk analysis.•Framework has been applied to the analysis of steam explosion risk in nordic BWR. Nordic Boiling Water Reactor (BWR) design employs ex-vessel debris coolability in a deep pool of water as a severe accident management (SAM) strategy. Depending on melt release conditions from the vessel and core-melt coolant interactions, containment integrity can be threatened by (i) formation of non-coolable debris bed, or (ii) energetic steam explosion. In order to assess the effectiveness of SAM the Risk Oriented Accident Analysis Methodology framework (ROAAM+) has been developed. The framework is further extension of the approach originally developed and applied by Prof. Theofanous and co-workers. This paper presents the implementation of ROAAM+ probabilistic framework for uncertainty quantification and risk analysis. We further apply ROAAM+ to the analysis of steam explosion risk in Nordic BWR assuming different state-of-knowledge situations and different containment fragilities. We employ an iterative processes of knowledge refinement using risk analysis as a guiding tool in identification of the major sources of uncertainty. We estimate failure domains and discuss ROAAM+ results in terms of possibility vs necessity of containment failure.