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Document Type:Technical Report
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The Fukushima Dai-ichi accident highlights the need to reduce the magnitude of radioactive fission product releases from BWR Mark I and II containments following beyond-design-basis events. There is no evidence that this accident has a long-term effect on public health and safety; however, the Fukushima Dai-ichi accident did result in widespread contamination of surrounding areas, both on-site and off-site. This report assesses various strategies that can be used to maintain BWR Mark I and II containment systems as effective barriers and to retain fission products in containment following a beyond-design-basis accident.
The best way to avoid radiological release and potential land contamination is to prevent an accident from occurring by improving and augmenting the strategies for preventing core damage. These improvements are treated in other assessments and are outside the scope of this report. This report focuses on severe accidents in which the containment vessel provides the chief barrier to fission product release. Various studies have shown that accidents involving long-term loss of electric power and subsequent loss of core cooling capability can significantly challenge the containment barrier.
This report evaluates strategies that maintain or enhance the containment function in scenarios involving long-term loss of electric power. The strategies evaluated include water injection (by flooding or spraying), alternative containment heat removal, venting, controlled venting, filtered venting, and combinations. Key insights from the analysis include the following:
No single strategy is effective. No single strategy is optimal in retaining radioactive fission products in the containment system. The most effective strategies involve combinations of active debris cooling strategies and containment venting. Even containment vent filters are rendered ineffective when active debris cooling strategies are not used.
Active core debris cooling is required. Core debris cooling is an important element of a robust strategy for mitigating releases. If debris cooling is not provided through water injection or spray into the drywell, containment failure or bypass is likely. Without core debris cooling, the containment can be challenged in several ways. Molten debris can come into direct contact with the containment wall, melting the liner and providing a release path to the environment. Elevated drywell temperatures in the containment atmosphere can cause seals and other containment penetrations to fail, leading to containment bypass. Finally, core–concrete interactions can generate large quantities of noncondensable gases that increase containment pressure and also can accelerate concrete erosion that could challenge containment integrity over time.
Existing severe accident management guidelines (SAMGs) strategies provide substantial benefit. The strategies defined in existing SAMGs provide benefit in reducing radiological releases. For example, active debris cooling with containment flooding or spraying is addressed in the SAMGs and represents an effective element of release reduction strategies for BWR Mark I and II containments. Containment venting under severe accident conditions is also addressed in the SAMGs as important in providing steam and heat removal in addition to release mitigation.
Spraying the containment atmosphere is beneficial. Spraying the drywell atmosphere reduces the airborne fission products in containment. Current and previous analyses confirm that the amount of fission products removed using a particular strategy (as measured by the decontamination factor [DF]) is higher when sprays are used. Sensitivity studies conducted for this analysis also confirm that an effective spray pattern can increase the overall containment DF by a factor of two, as compared to a containment flooding case.
Venting prevents uncontrolled release and manages hydrogen. The severe accident scenarios evaluated in this report assume that core debris is discharged into the containment. As previously noted, water is needed to cool the debris The quenching of the debris is beneficial; however, it produces a large amount of steam.Unless active heat removal systems are available to remove the steam, pressurization will continue beyond containment design pressure to the point of containment failure. Therefore, even if water is available to cool the core debris, containment venting is required to avoid containment failure. Venting also helps manage the buildup of hydrogen and other noncondensable gases generated during the core melting process. Up to 20% of the pressure inside containment can be the result of hydrogen and other noncondensable gases. Venting maintains the containment pressure below the design pressure and removes hydrogen and other gases from containment.
Control of the vent provides significant benefit. The key to controlling the amount of radioactive material released to the environment is minimizing the amount of contaminants that are airborne in containment during venting. Opening and closing the vent at the most appropriate times is essential. Such controlled venting strategies could be beneficial, but additional analysis is needed to more fully understand this option and ensure coordination with the plant’s emergency procedures.
Low-efficiency filters can further reduce radionuclide releases. The analyses conducted for this research indicate that several of the combined strategies could reduce radiological releases significantly, with DFs greater than 1000. These combined strategies could potentially be enhanced by adding a low-efficiency filter to the vent path to provide additional fission product capture. However, the aerosol remaining after using the strategies would be composed of much smaller particles, and the efficiency of the removal of these very small particles has not been demonstrated with current filter designs. Additional research is needed to assess the efficacy of current filter designs when used in combination with the combined strategies to evaluate whether new filter designs significantly change radiological releases.
The research shows that viable methods exist for reducing radiological releases and the potential for land contamination from hypothetical severe core damage accidents in BWR Mark I and II containments. Many of the strategies would likely be applicable to other nuclear reactor types, as well. No single strategy provides assurance of reduced releases across the potential spectrum of severe accident scenarios. In fact, combinations of debris cooling strategies requiring active systems and various containment venting strategies appear to provide the greatest benefit. Plant-specific analyses would be necessary to optimize implementation.
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