A REASSESSMENT OF LOW PROBABILITY CONTAINMENT FAILURE MODES

摘要

The recent events at Fukushima have highlighted the risks associated with beyond design basis accidents and the need for increased regulatory oversight of severe accident management. Even though the evidence is that the radiological health risk to humans is extremely small, in the future it is expected that there will be increased emphasis on preventing an unmitigated release of radioactivity to the environment with the potential for land contamination. Historically, risk assessments have given little credit for recovery actions but have also paid little attention to the impacts on severe accident progression of these actions. This work focuses on station blackout in large, dry pressurized water reactor (PWR) designs. The risk-dominant containment failure modes of NUREG-1150 are reassessed using the current state of knowledge for the severe accident phenomena that contribute to these failure modes. Some of the 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 high pressure melt ejection, have subsequently undergone extensive review including additional experimentation and can be excluded from further consideration as viable failure modes for large, dry PWR containments. Other phenomena, such as combustible gas explosions, still require careful consideration when making severe accident management decisions. We have performed sensitivity studies using MELCOR to examine some of these phenomena. Particular attention has been paid to combustible gas generation, the effect of containment cooling on deflagration limits and the combustion load on containment within the context of possible mitigative actions. Combustion loads are compared to the NUREG-1150 containment fragility curve to assess the likelihood of containment failure.