Resolution of Generic Safety Issues: Item A-48: Hydrogen Control Measures and Effects of Hydrogen Burns on Safety Equipment (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
Following a LOCA in an LWR, combustible gases, principally hydrogen, may accumulate inside the primary reactor containment as a result of: (1) metal-water reaction involving the fuel element cladding; (2) the radiolytic decomposition of the water in the reactor core and the containment sump; (3) the corrosion of certain construction materials by the spray solution; and (4) any synergistic chemical, thermal and radiolytic effects of post-accident environmental conditions on containment protective coatings and electric cable insulation. Although hydrogen control measures in connection with a design basis LOCA had been required by 10 CFR 50.44 well before the TMI-2 accident, metal-water reactions generated hydrogen during the accident that were in excess of the amounts specified in 10 CFR 50.44. As a result, it became apparent that additional hydrogen control and mitigation systems would have to be considered in power reactors with small containment structures. This concern was first raised in NUREG-057857 and later in TMI Action Plan48 Item II.B.7. The issue was declared a USI in February 1981 and published in NUREG-0705.44
A detailed action plan for resolving the issue was published in NUREG-0649, Rev. 1,1061 and was limited to near-term rulemaking efforts which included: (1) the BWR Mark I and Mark II containments hydrogen inerting rule; (2) the ice condenser/Mark III containment hydrogen control rule; and (3) the near-term construction permit/manufacturing license (CP/ML) rule. The CP/ML rule specified licensing requirements for pending CP and ML applications. The rule requiring inerting of BWR Mark I and Mark II containments as a method for hydrogen control was published in December 1981.1225 The BWR Mark I and Mark II containments have operated for a number of years with an inerted atmosphere (by addition of an inert gas, such as nitrogen) which effectively precludes combustion of any hydrogen generated. USI A-48 has been fully implemented at BWR plants with Mark I and Mark II containments.
The rule for BWRs with Mark III containments and PWRs with ice condenser containments was published in January 1985.1226 This rule required that the affected plants be provided with a means for controlling the quantity of hydrogen produced by a 75% fuel-cladding metal-water reaction, but did not specify the control method. In addition to the promulgation of rules on hydrogen control, the action plan for USI A-48 provided for plant-specific reviews of lead plants for reactors with Mark III and ice condenser containments.
Concurrent with the development of regulations, both the NRC and the industry have conducted extensive research programs since early 1980 on hydrogen igniter systems and effects of hydrogen combustion on safety-related equipment. A number of research programs were started to investigate the control of large quantities of hydrogen in reactors with small volume containments. The staff has also sponsored a peer review of the hydrogen research programs by the National Research Council under the auspices of the National Academy of Sciences (NAS). The NAS report, "Technical Aspects of Hydrogen Control and Combustion in Severe Light-Water Reactor Accidents," published in 1987 presents findings on the hydrogen research by both industry and the NRC.1227 The committee concluded that, for most accident scenarios, current regulatory requirements make it highly unlikely that hydrogen detonation would be the cause of containment failure. It was also concluded that inerting is adequate for reactors with Mark I and Mark II containments and that igniters are a reasonable way to reduce the probability of hydrogen detonation in medium volume containments (BWR Mark III and PWR ice condenser).
Large dry PWR containments were excluded from USI A-48 because they have a greater ability to accommodate the large quantities of hydrogen associated with a recoverable degraded core accident than the smaller MARK I, II, III and ice condenser containments. Most dry containments have about two million or more cubic feet of net free volume and have a design pressure which ranges from about 45 to 60 psi. Analyses which were performed to determine the pressure in a dry containment resulting from the combustion of hydrogen corresponding to a 75% metal-water reaction, following onset of a degraded core accident and while the containment was still near its peak pressure, indicated that the peak total containment pressure was below the failure pressure. Furthermore, analyses indicated that essential equipment would function during and after a large deflagration in a dry containment. This conclusion was supported by the TMI-2 experience.
In December 1984, the staff concluded that rulemaking with regard to hydrogen control for LWRs with large, dry containments could be safely deferred due to the inherent capability of these containments to accommodate large quantities of hydrogen. This concern is covered under Issue 121. In the staff's plans for resolving Issue 121, any recommendations for further modifications to 10 CFR 50.44 related to LWRs with large, dry containments will be provided at the conclusion of ongoing research. In April 1989, SECY-89-1221227 was forwarded to the Commission documenting the results of the staff's efforts in resolving USI A-48. Thus, this issue was RESOLVED and new requirements were established.