Periodic Reporting for period 3 - NARSIS (New Approach to Reactor Safety ImprovementS) Reporting period: 2020-09-01 to 2022-02-28 Summary of the context and overall objectives of the project The PSA procedure allows practitioners to better understand the most causes prone to initiate nuclear accidents and to identify the most critical elements of the systems. However, despite the remarkable reliability of the PSA methodology, lessons learnt from the Fukushima Daiichi nuclear disaster point out the necessity of upgrading the current methodological framework related to areas such as cascading and/or conjunct events characterization, structural and equipment responses and uncertainties treatment. New developments in those areas would even enable the extension of their use in accident management.The NARSIS project aimed at proposing progress in Probabilistic Safety Assessment (PSA) Fundamentals for NPP Safety exposed to multiple external natural hazards events, relying on a multidisciplinary approach and a threefold methodology:1. Theoretical improvements in natural hazards assessment and their impacts, including the evaluation of the uncertainties and the reduction of subjectivity in expert judgments;2. Verification of the findings in the frame of the safety assessment through adequate model reduction strategies for less time-demanding simulations and finally,3. Application of the outcomes at the demonstration level by providing supporting decision tools for severe accident management.Five key objectives were to be addressed:1) Improving the characterization of natural external hazards, focusing on concomitant events (either simultaneous-yet-independent or cascading).2) Improving the physical and functional vulnerability assessment of main critical SSCs of a NPP subjected to complex agressions, by introducing some vector-based fragility curves/surfaces, correlation effects and consequent damage scenarios.3) Implementing an integrated multi-risk and safety assessment framework (e.g. using Bayesian Networks, BNs), for consequences assessment (large early release frequencies, core damage, ...) including sensitivity analyses. 4) Improving the evaluation and treatment of uncertainties, including uncertainties related to the integration of the expert judgment in the PSA.5) Verifying and testing the proposed methodologies on simplified (generic) as well as on real NPPs representative of the European generation II & III fleet. Comparison of different safety assessment strategies (e.g. fully deterministic or probabilistic, and mixed deterministic-probabilistic). Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far The main technical results obtained during the project are the following:- Development of the NARSIS Multi-Hazard (MH) Explorer (open-source, open-access), to account for single, cascade and combination of natural external events and scenarios at different time scales, including secondary effects and uncertainty quantification. - Improvement of existing Probabilistic Hazard Assessment methodologies (tsunami, extreme weather, flooding). - Development of refined fragility derivation methods in order to increase the accuracy of the estimation of SSC failure rates, thanks to current advances in quantitative hazard modelling and computational capacities- Investigation of various numerical models and approaches to account for cumulative effects: ageing mechanisms (creep, corrosion), thermal fatigue & extreme earthquake loading, soil-structure interactions.- Investigation of the benefits of using vector-valued Intensity Measures for fragility assessment of components against single (earthquake) and multi-hazard natural events (earthquake & flood). - Investigating the use of BNs for multi-risk integration, comparing with MH PSA (based on fault trees); - Development of: 1) a novel BN-based method for human error probability; 2) a new approach to the analysis of common cause failures; 3) a detailed Extended-BEPU methodology combining deterministic and probabilistic approaches for NPP safety assessment in transient conditions; 4) methods to constrain/reduce uncertainties (identification of the most influential sources of uncertainty) in a high reliability context (little data available on failures); 5) a support Decision-Making (DM) tool SEVERA, for Severe Accident Management, relying on the PSA techniques and current status of SAM Guidelines for extensive damage and severe accident management.- Treatment of expert-based information using the tools of new uncertainty theories.- Investigation of metamodelling (Artificial Neural Networks, Support Vector Machines coupled with an Active Learning algorithm) and model-order reduction techniques (LATIN/PGD) for seismic safety analyses and uncertainty assessment.- Testing the applicability of the BN-based multi-risk integration for the safety analysis of a simplified gen III PWR NPP.- Application of the E-BEPU method in the development and V&V of SAMG involving the LB LOCA reclassification. - 56 peer-reviewed publications (23 journal papers)- 59 deliverables & 32 Milestones- 3 international workshops (2 trainings) Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The project provides new tools and methodologies for PSA of NPPs, in order to fill the following gaps and shortcomings:• Need for an extension of the PSA procedure to all types of external events (not only seismic or flood hazards);• Accounting for cascading or conjunct events in PSA, which have been proven to lead to unforeseen dramatic consequences.• Exploring extreme events in a probabilistic context, as extensive Monte-Carlo simulations only provide a partial solution.• Demonstrating the applicability of improvements related to the quantification and propagation of uncertainty sources (e.g. related to expert-based information) in other fields, for PSA of NPPs.• Putting emphasis on the functionality loss of equipment, contrary to the standard practice (fragility models focused on the physical damage of components only).• Investigating the applicability of a multi-hazard harmonization recently addressed by the FP7 MATRIX and INFRARISK projects, to complex systems systems such as NPPs, to bring new perspectives in nuclear safety analyses.Manifold impacts are expected from the NARSIS project:• Scientific outcomes linked to the integrated framework useful for combined hazard scenarios analysis as well as uncertainty quantification and comparison. Theoretical improvements obtained also in risk assessment and PSA methodologies.• Operational outcomes achieved through the demonstration of the SEVERA tool for SAMG implementation, useful for actual NPPs and potentially contributing to reducing the residual risk from NPPs operation. • At European level:- Knowledge basis for national and EU policies: some key scientific knowledge should be made available to the decision and policy makers. - Better safety culture: A main foreseen achievement of NARSIS is related to the SAM DM tool (demonstration purposes) and guidelines planned in WP5. Moreover, the project contribution is not only focused on technical aspects (i.e. design of safer systems) or on the direct influence of human behaviour (i.e. operator error) but also on the impact of social and organizational factors (i.e. policies and procedures, see WP3) on the outcome of safety performance.- Pedagogic (e.g. lectures) materials, to be produced for Master students and young professionals. - Foster European cooperation: As NARSIS consortium is composed of a large number of European key actors in the nuclear fields, having different safety cultures and practices, it will contribute to enhance the mutual understanding at European level. Logo of NARSIS project
