Resolution of Generic Safety Issues: Issue 121: Hydrogen Control for Large, Dry PWR Containments (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
In December 1984, the staff recommended in SECY-83-357B861 that rulemaking with regard to H2 control for LWRs with large, dry containments could be safely deferred due to the greater inherent capability of these containments to accommodate large quantities of H2. This issue was identified to document the staff's research on H2 control in large, dry PWR containments.
NRC experimental and analytical programs were initiated to provide data to supplement the experiments that were carried out at the Nevada Test Station (NTS), the experience on H2 burn during the TMI-2 accident, and earlier H2 burn experiments in order to support a final recommendation on whether safe shutdown equipment is likely to survive an H2 burn. Experiments were planned to determine the significance of factors not included in the NTS experiments (e.g., preconditioning of equipment to simulate aging, enclosing equipment in conduits or protective heat shields, and energized equipment). Experimental and analytical studies based in part on NTS-generated data were to be used to determine the local environmental stresses on equipment produced by convective and radiant heat flux to be expected in an H2 burn.
In addition to the above, the staff intended to explore the possibility of forming local detonable concentrations in large, dry PWRs and the probable consequences regarding containment and equipment survivability. These items received attention from the standpoint of fundamental detonation phenomena; however, the research effort was extended to develop the capability for predicting conditions in realistic configurations.
After consideration of the safety concerns associated with this issue and the NRC resources that had been expended prior to 1984 in pursuit of a resolution, this issue was given a high priority ranking.
Staff studies showed that H2 combustion was not a significant threat to the dry containments evaluated and the potential for global detonation was considered to be very unlikely for these containments; these findings were documented in NUREG-1150.1081 Furthermore, the H2 igniter systems for Zion and Surry were evaluated in NUREG/CR-56621464 and found to be not cost-effective. Therefore, there was no basis for new generic H2 control measures. However, it was believed that plant-specific vulnerabilities may exist mainly due to the effects of local H2 detonation. Activities for estimating the likelihood of local H2 detonation and assessing the consequences would require plant-specific information and are included as part of the IPE program. Moreover, the staff is investigating H2 control strategies via the accident management research program. In view of the above activities, the staff concluded that:
(1) The NUREG-11501081 findings of no problems on H2 control for the two PWR plants investigated, the IPE plant-specific examinations for vulnerabilities to H2 risk, and the long-term implementation of an accident management program formed the basis for resolution of this issue for existing plants.
(2) No new regulatory guidance on H2 control was required for existing PWR dry containments.
(3) For future plants, the need for H2 control requirements will be examined as part of ongoing case-by-case review (certification reviews) and the generic efforts associated with 10 CFR 50.59 rule changes.
Thus, this issue was RESOLVED and no new requirements were established.1451 In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not affect the resolution.