During the ex-vessel phase of a severe accident in a nuclear power plant, containment integrity might be jeopardized by three main mechanisms: concrete erosion (epsilon mode), over-pressurization (delta mode), combustible gas detonation (gamma mode). All of them are tightly related to molten core-concrete interaction. This study investigates the impact of uncertainties in corium properties on key variables describing the containment scenario (i.e., pressure, gas composition, and radial and axial ablation) during a station blackout accident in a 1000 MWe pressurized water reactor. The properties analyzed are viscosity, conductivity, specific heat, density and emissivity. A stand-alone containment Best Estimate Plus Uncertainty analysis has been conducted using the MELCOR v.2.2 2025.0 code and in-house Python scripts for sampling, propagation and results processing. The Order Statistics (95/95) was used and uniform probability density functions adopted for each property. The initial and boundary conditions were taken from a MELCOR full-scope plant analysis. Under the assumption of instantaneous melt-spreading within the cavity, the results indicate a small/moderate effect of corium properties uncertainties on concrete erosion and combustion risk. Containment pressure, though, shows an observable influence, which uncertainty band ranged from -17% to +33% with respect to the best estimate value. A complementary sensitivity analysis highlighted the specific heat uncertainties as the main driver of the band scatter noted in containment pressure.
EFFECT OF CORIUM PROPERTIES’ UNCERTAINTIES ON A PWR SBO SEVERE ACCIDENT
B. Mellace;M. Angelucci;S. Paci
2026-01-01
Abstract
During the ex-vessel phase of a severe accident in a nuclear power plant, containment integrity might be jeopardized by three main mechanisms: concrete erosion (epsilon mode), over-pressurization (delta mode), combustible gas detonation (gamma mode). All of them are tightly related to molten core-concrete interaction. This study investigates the impact of uncertainties in corium properties on key variables describing the containment scenario (i.e., pressure, gas composition, and radial and axial ablation) during a station blackout accident in a 1000 MWe pressurized water reactor. The properties analyzed are viscosity, conductivity, specific heat, density and emissivity. A stand-alone containment Best Estimate Plus Uncertainty analysis has been conducted using the MELCOR v.2.2 2025.0 code and in-house Python scripts for sampling, propagation and results processing. The Order Statistics (95/95) was used and uniform probability density functions adopted for each property. The initial and boundary conditions were taken from a MELCOR full-scope plant analysis. Under the assumption of instantaneous melt-spreading within the cavity, the results indicate a small/moderate effect of corium properties uncertainties on concrete erosion and combustion risk. Containment pressure, though, shows an observable influence, which uncertainty band ranged from -17% to +33% with respect to the best estimate value. A complementary sensitivity analysis highlighted the specific heat uncertainties as the main driver of the band scatter noted in containment pressure.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
