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TASK CH5: SEVERE ACCIDENT PHENOMENA

The highly energetic reactivity excursion accident at Chernobyl mechanically disrupted the core, rapidly vaporized the water coolant with which the fragmented fuel came into contact, and generated combustible hydrogen by chemical reaction of core materials (notably zirconium) and water at the high temperatures reached in the accident. Because of basic design differences between the RBMK reactor of Chernobyl and U.S. LWRs, the specific accident mechanisms involved at Chernobyl have no exact parallel in U.S. reactors. However, this task, outlined in Chapter 5 of NUREG-1252,¹¹⁷⁴ called for the staff to assess Chernobyl phenomena for analogous implications of radionuclide releases, steam explosions, and combustible gas generation and deflagration control in U.S. reactors.

ITEM CH5.1: SOURCE TERM

This item consists of two recommendations that are evaluated separately below.

ITEM CH5.1A: MECHANICAL DISPERSAL IN FISSION PRODUCT RELEASE

DESCRIPTION

The initial release of fission products that occurred at Chernobyl was the result of mechanical dispersion. Such a mechanism is possible in LWRs within the containment during energetic events such as high pressure melt ejection, steam explosions, and hydrogen combustion. Although such events are being studied with regard to their likelihood of occurrence and their consequences, associated mechanical releases of fission products have not been quantified in current source term models and the study of such releases has only just begun to receive attention. Because some of these phenomena appear to have played a dominant role in the releases at Chernobyl, it is important to understand these phenomena more completely. This issue called for the staff to introduce the Chernoby lessons into ongoing work to improve the understanding of mechanical dispersal phenomena and to improve the modeling in NRC source term assessment codes.

Current research on mechanical dispersion is being performed in three specific areas: direct containment heating (or high pressure melt ejection), steam explosions, and hydrogen combustion. For direct containment heating, the scope of current research is to develop a capability to analyze the consequences of this phenomenon. This can be accomplished by generating an experimental data base and, by developing an analytical model based on this data base which will be subsequently incorporated in an integrated code for containment analyses. In the area of hydrogen combustion, present work includes a scoping study on mechanisms of aerosol re-suspension and volatilization during hydrogen combustions. Specifically, experiments are being conducted to investigate the re-suspension of aerosols (radioactive or otherwise) that have been previously deposited on containment surfaces, by mechanical or thermal processes during the occurrence of hydrogen combustion, and to investigate the volatilization and expulsion of airborne aerosols in the containment by similar processes. The new information will subsequently be incorporated into the lumped parameter code HECTR and the finite difference code HMS-BURN for consequence analyses.

In pursuing this issue, the staff is expected to increase its knowledge, certainty, and understanding of safety issues in order to increase its confidence in assessing levels of safety. Therefore, this issue is considered to be a licensing issue.

CONCLUSION

This item is being pursued by the staff.

ITEM CH5.1B: STRIPPING IN FISSION PRODUCT RELEASE

DESCRIPTION

The late enhanced release of fission products during the Chernobyl accident may be attributable to the chemical and/or thermal stripping of UO₂ fuel. Such mechanisms have been observed in in-pile and out-of-pile experiments when UO₂ fuel rods were exposed to steam or high temperatures and other severe degraded core conditions. During the process of thermal stripping, for example, fission products were released in proportion to the amount of UO₂ vaporized. The rate of fission product release is thus controlled by UO₂ vaporization.

Fission product release by chemical and thermal stripping mechanisms is not modeled in current severe accident source term codes. The Chernobyl accident has demonstrated that such mechanisms can be important in fission product release under some conditions. This issue called for the staff to introduce Chernobyl lessons into the continuing research on chemical and thermal stripping and to obtain sufficient data for model development and assessment.

The scope of present research on UO₂ stripping is to complete ongoing experiments investigating thermal stripping mechanisms, to collect and review experimental data on chemical stripping mechanisms from Severe Fuel Damage Program participants, and to apply both the thermal stripping and chemical stripping data to improve present fission product release codes. For chemical stripping, the present experimental program may have to be expanded to study UO₂ stripping by air oxidation. This recommendation involves coordination to assure that the ongoing work adequately reflects the Chernobyl lessons.

In pursuing this issue, the staff is expected to increase its knowledge, certainty, and understanding of safety issues in order to increase its confidence in assessing levels of safety. Therefore, this issue is considered to be a licensing issue.

CONCLUSION

This item is being pursued by the staff.

ITEM CH5.2: STEAM EXPLOSIONS

This item consists of one recommendation that is evaluated below.

ITEM CH5.2A: STEAM EXPLOSIONS

DESCRIPTION

No specific research is currently underway or planned on reactivity insertion accident (RIA) prompt-burst steam explosions with fuel-vapor-driven fragmentation and mixing of the molten fuel and water that are relevant to the Chernobyl accident. Such work is currently not believed to be necessary, subject to confirmation in the light of results of the Chernobyl follow-up reactivity transient study (Item 2.1A).

The vapor-driven fragmentation and mixing of the interspersed fuel and coolant in prompt-burst power excursions in the Chernobyl accident has been strongly contrasted in the past to the pouring mode of contact found in the slow meltdown situations relevant to current U.S. commercial reactors. Hence the Chernobyl accident has little relevance to the staff's current treatment of steam explosions and alpha-mode containment failure. This issue called for the staff to characterize RIA steam explosions.

Current steam explosion research consists primarily of developing and assessing the semi-mechanistic Integrated Fuel Coolant Interaction (IFCI) computer model, which includes hydrogen generation, for integration into an in-vessel melt progression code. IFCI provides a mechanistic treatment of both the pre-explosion mixing phase and the explosion phase (if conditions permit), but IFCI does require a parametric input trigger for the explosion. Work is also continuing on using existing experimental data for modeling the non-explosive mixing phase of the interaction.

If further work for U.S. reactors on RIA steam explosions is found to be needed, this would be performed as part of an overall investigation of RIAs and it is in this context that the specific work scope would be planned. Currently work is underway to assess the effect of in-vessel steam explosions on in-vessel core melt progression in light-water reactor accidents.

In pursuing this issue, the staff is expected to increase its knowledge, certainty, and understanding of safety issues in order to increase its confidence in assessing levels of safety. Therefore, this issue is considered to be a licensing issue.

CONCLUSION

This item is being pursued by the staff.

ITEM CH5.3: COMBUSTIBLE GAS

DESCRIPTION

The Soviet RBMK design utilizes large amounts of zirconium and graphite in the reactor core, both of which may oxidize under certain conditions resulting in the generation of large quantities of combustible gases, principally hydrogen and carbon monoxide. The generation of large quantities of combustible gases was not apparently considered as part of the Soviet containment design. The Chernobyl accident produced reactor core conditions that may have led to the generation of large quantities of combustible gases which, in turn, may have influenced the evolution and consequences of the accident.

The need to deal with the generation of combustible gas, principally hydrogen, as a consequence of reactor accidents has been recognized in the U.S. since the early days of LWRs. The burning and/or detonation of combustible gases are of concern in reactor safety for several reasons. First, a large enough energy release might threaten the integrity of the containment. Second, even if the containment survived, important safety equipment might be irreparably damaged, thus increasing the severity of the accident. Furthermore, since significant amounts of hydrogen can be generated early in the evolution of a severe reactor accident (i.e., before the reactor vessel fails), combustion can result in containment failure before expulsion of the molten core, leading to the largest radioactivity releases to the environs.

CONCLUSION

In summary, although the conditions that existed during the Chernobyl accident may have caused large amounts of combustible gases to generate, it cannot be concluded from the available data that these gases were generated by some new or different mechanisms or produced consequences not previously investigated as part of severe-accident analyses for U.S. reactors. It is difficult to apply observations from the Chernobyl accident to U.S. plants because of significant design differences between the RBMK and nuclear power reactors in the United States; furthermore, the NRC staff still lacks detailed accident data. Considering the preliminary evaluation, it does not appear that any additional work is warranted solely on the basis of the Chernobyl event. The staff concludes that its current and proposed research program on combustible gas phenomena in conjunction with the study of severe accidents would be adequate for addressing this issue in U.S. reactors.