ANALYSIS AND DEVELOPMENT OF UNCERTAINTY QUANTIFICATION METHODOLOGIES FOR THE DETERMINISTIC SAFETY ANALYSIS OF SEVERE ACCIDENT PROGRESSIONS IN FISSION NUCLEAR POWER PLANTS

Deterministic safety analyses (DSAs) are conducted to simulate the response of a nuclear system during normal and transient conditions to evaluate its safety in terms of fulfilment of acceptance criteria. Among the accidental scenarios evaluated for the performance of DSAs, particular attention is placed by the international scientific community on the study of Severe Accidents (SAs), which are characterized by specific conditions (e.g. postulated failure of part or all the safety systems) that lead to reactor core meltdown. In those countries where nuclear energy is included in the national energy mix, such scenarios are crucial in the development of SA management programmes.To simulate the broad spectrum of phenomena characterizing such scenarios, specific computational tools, called SA integral codes, are used. Considering the level of maturity reached by these codes, the international scientific community involved in SAs has been focused on assessing the state-of-the-art definition and consolidation for the quantification of the codes’ uncertainties employing the Best Estimate Plus Uncertainty (BEPU) approach.Given the knowledge gathered from uncertainty quantification in the thermal-hydraulics field, several uncertainty methodologies have been established. Among them, the “probabilistic method to propagate input uncertainty” is commonly used by the international scientific nuclear community and the present work aims to highlight research needs related to the application of this method on the development of DSAs in the SA domain following a BEPU approach.After presenting the development of the main computational environments by using the SA integral code MELCOR, developed by SNL for the USNRC, the uncertainty methodology has been first tested against experimental data of the internationally recognized test Phebus FPT1, to investigate the code uncertainty on aerosol physics, and QUENCH-06, to study the code uncertainties on hydrogen generation. Then, the uncertainty methodology has been applied and tested to full plant application, in particular on a generic Pressurized Water Reactor (PWR) and a generic integral PWR (iPWR).The activities have been carried out during the three years of the PhD program in “Energy”, curriculum “Low carbon energetics and innovative nuclear systems”, XXXVI cycle, in close collaboration with the ENEA FSN-SICNUC, in the framework of the projects HORIZON 2020 MUSA, the IAEA CRP and the Horizon Euratom SASPAM-SA.