Uncertainty Evaluation for a Postulated Severe Accident Scenario at a VVER1000 NPP Using ASTEC-SUNSET

This study presents the main results from an uncertainty evaluation performed for a postulated severe accident scenario - a Station Blackout (SBO) in a Nuclear Power Plant (NPP) equipped with a VVER1000 reactor. The primary aim of this investigation was to quantify uncertainties in code predictions and, additionally, to perform a sensitivity analysis of key output variables. The analysis was conducted using the ASTEC severe accident simulation code, integrated with the SUNSET uncertainty quantification tool, forming a comprehensive analytical platform. A total of eight input parameters were considered, with the focus placed on evaluating the uncertainty in ten fission products (FP) as output variables predicted by ASTEC. The statistical framework provided by SUNSET, which uses Wilks' formula, guided the determination of the required number of simulations and enabled the assessment of how input uncertainties influence output variability. The study details the analytical process, including the ASTEC-SUNSET coupling methodology, and presents key findings from the uncertainty analysis. The results demonstrate the effectiveness of the integrated approach in identifying the most influential input parameters and characterising the range of predicted fission products. These findings contribute to enhancing the robustness of uncertainty quantification practices in nuclear safety analysis and support ongoing safety evaluations for VVER1000 reactors.

Keywords: Uncertainty, fission product, severe accident, VVER1000, ASTEC-SUNSET.