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.