Safety evaluation of the Megapie experimental facility: results and insights from the application of probabilistic safety assessment

A scoping-level probabilistic safety assessment (PSA) was applied to selected systems of the experimental facility hosting the Megawatt Pilot Target Experiment (MEGAPIE) at the Paul Scherrer Institut (PSI). MEGAPIE demonstrated the feasibility of a liquid lead-bismuth target for spallation facilities at a proton beam power level of 1 MW (web site: http://megapie.web.psi.ch/). In performing a PSA to this type of facility, a number of challenges arise, mainly due to: 1) the extensive use of electronic and programmable components and of one-of-a-kind components; 2) difficulties in estimating the probabilities of initiating and failure events. A previous work by the same authors emphasised the qualitative results obtainable from the application of PSA. No attempt was made to evaluate probabilities of initiating and failure events. Some level of importance/significance evaluation was nevertheless feasible, and practical and detailed recommendations concerning potential system improvements were derived. In this 'raper, a preliminary quantification of the facility risk profile is attempted. This provides more information on risk significance, which allows prioritizing the insights and recommendations obtainable from the PSA. At the present stage, the limited knowledge on initiating and failure events leads 10 uncertainties in their probabilities; the propagation of these uncertainties through the PSA model results in large uncertainties in the quantitative results. Consequently, the reported results should he considered as examples of how quantification can support risk-informed decisions, rather than realistic figures of the facility risk profile. (authors)