Presentation of DEFI-PROSAFE Project: Probabilistic Methodology to Assess Margin in Deterministic RPV Integrity Assessment and Proposed Benchmark

In this paper the project DEFI-PROSAFE is presented. In the frame of Nugenia+ project, a work package was dedicated to the “DEFInition of reference case studies for harmonized PRObabilistic evaluation of SAFEty margins in integrity assessment for long-term operation of reactor pressure vessel” (acronym DEFI-PROSAFE). A methodology is proposed to assess safety margins in RPV integrity accounting for uncertainties propagation, because no commonly accepted European approach exists for probabilistic assessment of RPV fast fracture risk. The DEFI-PROSAFE methodology, which is based on the comparison between deterministic and probabilistic assessment, will be detailed. The experience gained from the US Screening Criteria (NUREG-1806) and past projects (ICAS, PROSIR), and guideline (IAEA TecDoc 1627) as well as aspects specific to European deterministic integrity approach have been considered. Usually for probabilistic fracture assessment of the RPV, the parameters (describing flaws, material and neutron fluence) are sampled and the RPV cylindrical region is assessed using deterministic thermal hydraulic loading for evaluation of initiation or failure conditional probability. Within DEFI-PROSAFE methodology the uncertainties in thermal-hydraulic input parameters are taken into account and their propagation in the structural assessment is considered. Comparison of RELAP5 thermal hydraulic results and mixing calculation results (KWU-MIX) with experiment relevant to PTS assessment was performed. Furthermore the DEFI-PROSAFE methodology considers RPV discontinuity regions (like RPV nozzle) and specific PIRT analysis has been performed for selection of the TH-parameters. A new benchmark for probabilistic assessment of RPV was defined within the DEFI-PROSAFE project. The benchmark definition and case studies based on previous R&D project (ICAS, PROSIR) will be presented. The performance of the new benchmark may be submitted as new European project.