During a severe accident, a nuclear power plant faces extreme conditions that can threaten containment integrity, particularly during the ex-vessel phase. In this phase, molten corium slumps into the cavity beneath the vessel, resulting in a Molten Core Concrete Interaction (MCCI), which causes both radial and axial ablation of the cavity. In addition, gases (water vapor, carbon dioxide) coming from concrete thermal decomposition go through molten corium and oxidize remaining metals producing additional combustible gases (hydrogen and carbon monoxide). As a consequence, containment integrity might be jeopardized from both pressurization and energetic combustion events. In all these processes, corium properties do have the potential to affect any of the mechanisms contributing to the containment threats described. This investigation explores the effect of uncertainties in corium properties on variables describing the containment scenario (i.e., pressure, gas composition, radial and axial ablation) during a Station Blackout (SBO) accident. The properties studied are density, emissivity, conductivity, viscosity and specific heat. After having a broad characterization of the suitable properties range to explore, a Best Estimate Plus Uncertainty (BEPU) analysis has been conducted. The MELCOR v.2.2 2025.0 code has been used along with in-house Python scripts to articulate sampling, propagation and results processing of this research. Preliminary results indicate a moderate impact on containment status from the properties uncertainties. The variable showing a higher effect is pressure, which has been found to range between -15% and +15% of the BE. Further studies are planned to be conducted, to extend the methodology to other scenarios and reactor designs.
EFFECT OF CORIUM PROPERTIES’ UNCERTAINTIES ON A PWR SBO SEVERE ACCIDENT
B. Mellace
;M. Angelucci;S. Paci;
2025-01-01
Abstract
During a severe accident, a nuclear power plant faces extreme conditions that can threaten containment integrity, particularly during the ex-vessel phase. In this phase, molten corium slumps into the cavity beneath the vessel, resulting in a Molten Core Concrete Interaction (MCCI), which causes both radial and axial ablation of the cavity. In addition, gases (water vapor, carbon dioxide) coming from concrete thermal decomposition go through molten corium and oxidize remaining metals producing additional combustible gases (hydrogen and carbon monoxide). As a consequence, containment integrity might be jeopardized from both pressurization and energetic combustion events. In all these processes, corium properties do have the potential to affect any of the mechanisms contributing to the containment threats described. This investigation explores the effect of uncertainties in corium properties on variables describing the containment scenario (i.e., pressure, gas composition, radial and axial ablation) during a Station Blackout (SBO) accident. The properties studied are density, emissivity, conductivity, viscosity and specific heat. After having a broad characterization of the suitable properties range to explore, a Best Estimate Plus Uncertainty (BEPU) analysis has been conducted. The MELCOR v.2.2 2025.0 code has been used along with in-house Python scripts to articulate sampling, propagation and results processing of this research. Preliminary results indicate a moderate impact on containment status from the properties uncertainties. The variable showing a higher effect is pressure, which has been found to range between -15% and +15% of the BE. Further studies are planned to be conducted, to extend the methodology to other scenarios and reactor designs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
