SOURCE TERM UNCERTAINTIES IN UNMITIGATED SBO SEQUENCES IN A PWR-1000: INSIGHTS FROM THE EU-MUSA PROJECT

The Management and Uncertainties of Severe Accidents (MUSA) project focused on the testing, adapting, and employing Uncertainty Quantification (UQ) and sensitivity methods for Severe Accident (SA) analysis. In collaboration with the University of Pisa, CIEMAT conducted an Uncertainty and Sensitivity Analysis (UaSA) of Source Term (ST) estimates in an unmitigated Station Black-Out (SBO) scenario involving a 3-loop Pressurized Water Reactor with Westinghouse design (PWR-W). The study employed the MELCOR 2.2 code (version 2.2.21402) in conjunction with DAKOTA 6.7 as the UQ tool. To propagate uncertainties, the Wilks' approach was employed, with random sampling of 24 selected uncertain parameters. The Figures Of Merit (FOMs) chosen for the analysis were iodine, cesium, and noble gases releases to the environment. A sensitivity analysis, based on Pearson’s and Spearman’s Correlation Coefficients (CCs), was also conducted. In relative terms, the magnitude of the ST displayed a narrow uncertainty band. The early containment failure led to most iodine released from fuel to reach the environment, with a narrow variability. Conversely, a visible uncertainty surrounded the onset time of release to the environment (approximately 5 hours), potentially influencing the implementation of Accident Management (AM) measures that might be adopted in the scenario. Although sensitivity analyses did not identify dominant variables, emphasis was placed on the relationship between the release onset time and fuel failure time (temperature controlled), as well as the subsequent diffusional release of radionuclides.