Beyond design basis accidents in spent-fuel pools — Generic Issue 82

Abstract

Nuclear reactor plants include storage facilities for the wet storage of spent-fuel assemblies. The safety function of the spent-fuel pool (SFP) and storage racks is to cool the spent-fuel assemblies and maintain them in a subcritical array during all credible storage conditions and to provide safe means of loading the assemblies into shipping casks. Generic Issue 82 (GI-82) relates to the concern that for a postulated accident sequence that results in the loss of water from a light-water reactor (LWR) spent-fuel storage pool, a Zircaloy cladding fire could occur and propagate to older stored fuel. This issue was identified during hearings concerning SFP reracking amendments in the late 1970s when licensees were starting to use high-density storage racks. High-density racks are used to accommodate the storage of spent fuel in SFPs at reactor sites until such time as the Department of Energy (DOE) repository is available and spent fuel can be removed from the reactor sites. Maintaining a low-density storage configuration for recently discharged spent fuel would reduce the Zircaloy cladding fire probability by an order of magnitude, but at a greater cost for additional onsite storage space. The accident sequences that could result in water loss from the SFP, including beyond design basis earthquakes, various types of seal failures and dropped shipping casks, and the Zircaloy cladding fire issues have been studied by the NRC staff. The results of these studies are provided in NUREG-1353, “Regulatory Analysis for the Resolution of Generic Issue 82, Beyond Design Basis Accidents in Spent-Fuel Pools”. Although these studies conclude that most of the spent-fuel pool risk is derived from beyond design basis earthquakes, this risk is not greater than the risk from core damage accidents due to these beyond design basis earthquakes. Therefore, reducing the risk from spent-fuel pools due to events beyond the safe shutdown earthquake would still leave a comparable risk due to core damage accidents. The risk due to beyond design basis accidents in spent-fuel pools, while not negligible, is sufficiently low that the added cost involved with further risk reduction is not warranted.