05000325/LER-2008-003
| Brunswick Steam Electric Plant (Bsep), Unit 1 | |
| Event date: | 06-11-2008 |
|---|---|
| Report date: | 08-11-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3252008003R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction On June 11, 2008, as a result of a review of the Reactor Building crane structure by the original equipment manufacturer, it was determined that the structural design of the Unit 1 and Unit 2 Reactor Building cranes (1/2-BC-RB-BRIDGE-CRANE) [LR] did not ensure the crane structural integrity during a design basis seismic event. Specifically, the allowable design stresses for the design basis seismic event are exceeded in the end connector plates and bolted connections connecting the crane girders. This condition has existed since operation of the plants began.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B), as an event or condition that resulted in the plant being in an unanalyzed condition that significantly degraded plant safety.
Event Description
Initial Conditions At the time of the event, Units 1 and 2 were in Mode 1, operating at approximately 100 percent of rated thermal power.
Discussion As part of BSEP's ongoing Dry Fuel Storage project, Whiting Corporation, the original equipment manufacturer, was requested to reconstitute the design calculations for the Reactor Building cranes in order to evaluate the increased loading conditions resulting from handling the dry shielded canisters used for fuel storage.
These cranes are Whiting 125/5 Ton Bridge Cranes, Serial Numbers 10388 and 10389. The design of the cranes was based on BSEP specification 257-002. This specification was approved initially on July 6, 1971. Specification 257-002 states that all parts of the cranes are to be designed to resist seismic loads. Specifically, the crane was to be designed to withstand both an Operating Basis Earthquake (OBE) and a Design Basis Earthquake (DBE) with the maximum lift load on the hook. Maximum material stresses were to be maintained within normal American Institute of Steel Construction (AISC) Code allowable limits for OBE, and within material yield points and in accordance with AISC specification requirements for DBE loading.
As a result of this effort, on June 11, 2008, it was determined that the allowable design stresses for the design basis seismic event are exceeded in the end connector plates and bolted connections connecting the crane girders.
Event Cause The direct cause of this event is that the crane girder end connection design was not adequately evaluated during the initial design of the crane by Whiting Corporation. The crane design was performed by Whiting Corporation in the 1970's. Due to the historical nature of this condition, determining a plausible cause is not practical or feasible.
Safety Assessment The safety significance of this condition is minimal. This concern is only applicable to conditions present during an extremely unlikely DBE. However, the ability of the crane to maintain a load or structural stability during the DBE is in question. If a DBE had occurred prior to the implementation of the compensatory measures, there was a potential that the lack of structural integrity could have resulted in crane structural damage which, in turn, could have adversely impacted structures, systems, or components in the vicinity of the crane at the time. Due to the difficulty in predicting the various effects on plant equipment, this condition was conservatively considered an unanalyzed condition that significantly degraded plant safety.
Corrective Actions
Modifications have been implemented for the Unit 1 and Unit 2 Reactor Building cranes to allow continued restricted use of the cranes for loads of up to 40 tons.
Engineering Changes will be developed and implemented to restore the cranes to their original seismic design requirements. These modifications are currently scheduled to be completed by February 27, 2009, for Unit 2 and July 31, 2009, for Unit 1.
Previous Similar Events
A review of LERs and corrective action program condition reports for the past three years identified the following similar event.
- Nuclear Condition Report (NCR) 251648, originated on October 27, 2007, identified a similar concern related to the tornado wind loading design of the cranes. At the time this condition was identified, there was no indication that the original seismic design was in question. Additionally, in 1997, calculation OSEIS-0036 was performed. This calculation included a re-analysis of the Reactor Building crane structure. BSEP Engineering relied on calculation OSEIS-0036 to provide assurance that there were no seismic concerns with the cranes. However, as with the original evaluation, this analysis did not consider the effect on the girder end connectors. As a result the corrective actions for NCR 251648 focused on the wind loading issue and would not have been reasonably expected to prevent the condition discussed in this LER.
- Commitments No regulatory commitments are contained in this report.