A Reassessment of Low Probability Containment Failure Modes and Phenomena in a Long-Term Station Blackout

The risk-dominant containment failure modes of a pressurized water reactor are reassessed using the current state of knowledge for the phenomena that contribute to these failure modes. Our review concludes that some mechanisms that were considered as having the potential to result in containment failure at the time of NUREG-1150, such as in-vessel steam explosions and vessel launch (i.e., the alpha-mode containment failure), have subsequently undergone sufficient review and can be excluded from further consideration. For other phenomena, such as high-pressure melt ejection (HPME) and combustible gas explosions, our review concludes that substantial uncertainties still exist with regard to modeling in system level codes; for combustion events, careful consideration is still required when making severe accident management decisions. With regard to HPME, sensitivity studies have been performed with the MELCOR computer code to address the effects of modeling uncertainties on containment loading. Sensitivity studies using MELCOR have also been performed with regard to combustion events to examine gas generation, the effect of containment cooling on the potential for deflagrations, and the combustion load on containment. Combustion loads are compared to the NUREG-1150 containment fragility curve to assess the likelihood of containment failure. Our MELCOR analyses agree with the NUREG-1150 assumption that insufficient hydrogen is generated in-vessel to result in containment failure. Sensitivity studies regarding the rate and timing of reflooding a degraded core do not indicate a significant effect on hydrogen production in-vessel or a significant challenge to containment integrity regarding HPME. However, it is observed that recovery actions resulting in cooling of the containment atmosphere could result in deinerting the containment and lead to a sufficiently energetic combustion event that can fail the containment.