05000339/LER-2004-001
| Document Numbersequential Revisionmonth Day Year Year Month Day Year 05000-Number Number | |
| Event date: | 05-09-2004 |
|---|---|
| Report date: | 06-29-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3392004001R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER 6) PAGE (3) 1.0 DESCRIPTION OF THE EVENT On May 9, 2004, at 2105 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.009525e-4 months <br />, with North Anna Unit 2 in Mode 6 (Refueling), both emergency diesel generators (EDG) (EIIS System EK, Component DG) were declared inoperable.
On May 9, 2004, at 1428 hours0.0165 days <br />0.397 hours <br />0.00236 weeks <br />5.43354e-4 months <br />, the Unit 2 "H" EDG, 2-EE-EG-2H, was removed from service for scheduled maintenance. During maintenance activities, it was identified that all four shims on the engine exhaust stack seismic support (EIIS Component SPT) on the south wall had fallen out of place. In addition, both lower anchor bolt nuts had worked loose and fallen to the floor and one upper anchor bolt nut had worked loose.
An inspection of the Unit 2 "J" EDG, 2-EE-EG-2J, commenced and it was identified that three of four shims on the engine exhaust stack seismic support on the south wall had fallen out of place. The remaining shim had become loose and rotated approximately 90 degrees. In addition, one of the upper anchor bolt nuts had worked loose and fallen to the floor.
On May 9, 2004, at 2105 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.009525e-4 months <br />, the Unit 2 "J" EDG was conservatively declared inoperable because the as found condition of the engine exhaust stack seismic support could not be immediately analyzed for operability. The Unit 2 "J" EDG was subsequently tagged out to perform the support repairs. When the Unit 2 "J" EDG was tagged out to perform the support repairs, both Unit 2 EDGs were rendered inoperable. The applicable Technical Specification Action requirements were entered due to both EDGs being declared inoperable.
With both Unit 2 EDGs being inoperable the safety functions needed to maintain the reactor in a safe shutdown condition, remove residual heat, and mitigate the consequences of an accident would have been impaired in the event of a loss of offsite power. An 8-hour notification was made to the NRC Operations Center on May 10, 2004, at 0011 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br />, in accordance with 10 CFR 50.72(b)(3)(v)(D). This event is reportable pursuant to 10 CFR 50.73 (a)(2)(v)(A), (B) and (D), for any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shut down the reactor and maintain it in a safe shutdown condition, remove residual heat, and mitigate the consequences of an accident.
Actions were implemented by 2313 hours0.0268 days <br />0.643 hours <br />0.00382 weeks <br />8.800965e-4 months <br /> on May 9, 2004 to restore the Unit 2 "J" EDG to operable.
FACILITY NAME (1) DOCKET LER NUMBER 6) PAGE (3) 2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event posed no significant safety implications because an engineering calculation subsequently determined the as found condition of the EDG exhaust stack seismic supports did not affect prior operability of the Unit 2 "H" and "J" EDGs. The Unit 2 "J" EDG was only inoperable for approximately two hours. However, normal power was available to the Service Water System (EIIS BI), Main Control Room/Emergency Switchgear Room Emergency Ventilation System (EIIS VI), Emergency Core Cooling System Pump Room Exhaust Air Cleanup System (EIIS VF), and Component Cooling System (EIIS CC). In addition, the Station Blackout (SBO) EDG was operable and capable of carrying loads on an emergency bus. Also, there was no movement of recently irradiated fuel during the time frame both Unit 2 EDGs were inoperable. Therefore, the health and safety of the public were not affected by this event.
3.0 CAUSE A root cause evaluation of the event was performed. The root cause evaluation identified modifications to the EDG exhaust stacks that were implemented over the years and the cumulative effects of the modifications were not fully considered as described below.
The original design of the EDG engine exhaust stacks included silencers. Deterioration of the silencers resulted in a modification, which removed them and capped the pipe inside the exhaust bunker wall and sealed the opening through that wall with a missile resistant cover plate. The support structure in question still serves as both a horizontal restraint for the old, capped exhaust pipe and a vertical support for the entire exhaust stack.
The cause of the missing shims was determined to be unanticipated interaction between the redesigned vertical exhaust piping and the old exhaust capped piping with the silencer removed. The vibration characteristics of the old capped exhaust pipe were not adequately analyzed to ensure the exhaust pipe support structure bolting would remain tight.
Contributing causes are: 1) No steps taken to prevent the vibration from the operating EDG from causing the bolts to loosen. In a.vibratory environment the nuts should have been tightened to a specified torque value to prevent their loosening. The design should have had additional measures implemented to maintain bolt/shim tightness other than just the specified torque. 2) The shims were not tack welded or otherwise prevented from rotating about the anchor bolts if the clamping force was relaxed. The design of the shim was intended for easy installation however, a shim designed with a hole in it would ensure capture by the anchor bolt.
FACILITY NAME (1) DOCKET LER NUMBER 6) PAGE (3) 4.0 IMMEDIATE CORRECTIVE ACTION(S) The applicable Technical Specification Action requirements were entered due to both Unit 2 EDGs being declared inoperable.
5.0 ADDITIONAL CORRECTIVE ACTIONS Engineering calculations determined that with the as found condition of the EDG exhaust stack supports remained capable of performing its intended design function of supporting the 24 inch diesel exhaust piping for all postulated loading conditions.
A root cause evaluation was performed. The following corrective actions were implemented.
- All Emergency Diesel Generators have been inspected for the loose exhaust support condition. The as-found condition of the supports has been analyzed by Engineering and all were determined to have been operable in that condition.
- The missing shims on the Unit 2 "J". EDG exhaust stack seismic support were re installed and the anchor bolt nuts were re-torqued to a minimum of 125 ft-lbs. The EDG was declared operable on May 9, 2004 at 2313 hours0.0268 days <br />0.643 hours <br />0.00382 weeks <br />8.800965e-4 months <br />. Applicable Technical Specifications Actions were then cleared.
- Three of the four missing shims for the Unit 2 "H" EDG exhaust stack seismic support were located, re-installed and the anchor bolt nuts were re-torqued to a minimum of 125 ft-lbs. The EDG was determined to be operable with three of four shims installed.
An additional shim was fabricated and subsequently installed. The Unit 2 "H" EDG was declared operable following completion of originally scheduled maintenance activities.
On May 22, 2004, a follow-up inspection of the EDG exhaust stack seismic support was performed after the 24-hour maintenance run of the Unit 2 "J" EDG. The shims were in place as installed.
6.0 ACTIONS TO PREVENT RECURRENCE Corrective actions to address the cause of the event are being reviewed by management.
Upon management approval, corrective actions, applicable to North Anna Units 1 and 2, will be tracked to completion in the Corrective Action System.
FACILITY NAME (1) DOCKET � LER NUMBER 16) � PAGE (3) 7.0 SIMILAR EVENTS EDG was out of service during preplanned maintenance and the Unit 2 "H" EDG became inoperable during testing.
EDG was inoperable to perform surveillance testing and the Unit 1 "H" EDG was inoperable due to an engine cooling fan failure.
"J" EDG was removed from service for maintenance during a refueling outage and the Unit 1 "H" EDG was subsequently determined to be inoperable due to hydraulic lock.
8.0 ADDITIONAL INFORMATION The issue with the EDG exhaust piping is limited to the support that is anchored to the south wall of the EDG exhaust bunker. The purpose of this support is to provide for seismic restraint of the old EDG exhaust pipe. The configuration of the support structure is the same for each EDG. Inspection and repair of the subject EDG exhaust supports to the original design configuration addressed the seismic implications of the issue. The problem is not believed to extend to any other supports in the EDG exhaust bunker since the other supports are not subject to the same loading conditions as the subject supports.
On April 27, 2004, inspection of the Unit 1 "J" EDG identified one shim on the engine exhaust stack seismic support on the south wall had fallen out of place. This condition was subsequently analyzed and the support was found to retain its operability. The shim was replaced and the anchor bolts were re-torqued to a minimum of 125 ft-lbs.
On May 10, 2004, inspection of the Unit 1 "H" EDG identified two shims on the engine exhaust stack seismic support on the west wall had fallen out of place and the associated anchor bolt nuts were missing. This condition was subsequently analyzed and the support was found to retain its operability. The shims were replaced and the anchor bolts were re-torqued to a minimum of 125 ft-lbs.