Structural Integrity of Spent Fuel Pool Structures Subject to Heavy Loads Drops

A heavy load drop into the spent fuel pool, or onto the spent fuel pool wall, can affect the structural integrity of the spent fuel pool. A loss-of-inventory from the spent fuel pool could occur as a result of a heavy load drop. The staff evaluated heavy loads as Generic Technical Activity A-36, which resulted in the publication of NUREG-0612 [Ref. 1]. Cask handling is expected to be the dominant heavy load operation at a decommissioning plant. We revisited NUREG-0612 to review the evaluation and the supporting data then available. We identified two additional sources of information and used them to reassess the heavy load drop risk: The 1990s Navy data encompassed primarily bridge cranes with lift capacities of 20,000 lb to 350,000 Ib, both at shipyards and at non-shipyard sites. The data are summarized in Table 20-1 by incident type and incident cause. Improper operation caused 38% of the events, improper rigging 30%, procedures 20%, equipment failures 5%, and other causes 8%. Improper rigging was further subdivided: (a) 70% were identified as rigging errors and (b) 30% were rigging-related failures resulting from the crane operation. Reported load drops occurred in about 9% of the accidents, 3% related to the crane and its operation and 6% to improper rigging. The fault trees used to assess a heavy load drop leading to a loss-of-inventory are shown in Figure 2C-1 (taken from NUREG-0612). Table 20-1 includes the grouping of the incidents type for use in the fault tree quantification. Based on the July 1999 SFP workshop, we assume that there will be a maximum of 100 cask lifts per year. Using the 1990s Navy database, for 100 lifts, about 3 lifts may lead to a load drop for the evaluation of the "failure of crane" event (CF). Using the new Navy database, for 100 lifts, about 6 lifts may lead to a load drop for the evaluation of the "failure of rigging" event (CR). In NUREG-0612, which was based on 200 lifts per year, the range of lifts leading to a load drop was estimated by the staff to be between 10 and 4 (5% to 2%). The handling system failure rate was estimated in NUREG-0612 to be in the range of 1.5x10-4 to 1.0xxl 0-incidents per year based on the 1970s Navy crane incident data and a staff estimate of the total number of lifts per year. The staff's evaluation included …