05000352/LER-2001-002
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| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3522001002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 and Unit 2 were in Operational Condition (OPCON) 1 (Power Operation) at 100% power.
There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On June 21, 2001, a DC system ground occurred at Limerick that resulted in the failure of power supply fuses associated with four Unit 2 NLI inverters. As a result of the event, 4 inverters were replaced with the Topaz design. It was determined that the NLI inverters failed as a result of a ground in the negative leg of the Division 1 125/250 VDC distribution system(EllS:E1).
On July 20, 2001 during a future actions review of the earlier event, it was determined that for a fire in the Main Control Room, a fire induced ground on certain diesel generator cables had the potential to fail the remote shutdown panel inverter. Safe shutdown is directed from the remote shutdown panel using the Reactor Core Isolation Cooling (RCIC) system (EIIS:BN) for inventory control for a fire in the Main Control Room. If the inverter fails as a result of the ground on the diesel generator cables, there would be no power supply available for automatic or manual control of RCIC turbine flow. RCIC is the only analyzed system available for post-fire inventory control. The same inverter failure would result in the loss of reactor, drywell and suppression pool monitoring instruments at the remote shutdown panel.
This event involved an unanalyzed condition that potentially degrades plant safety. Therefore, an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> ENS notification was made on July 20, 2001 at 21: 57 hours6.597222e-4 days <br />0.0158 hours <br />9.424603e-5 weeks <br />2.16885e-5 months <br /> pursuant to 10CFR50.72(b)(3)(ii)(B). Also, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(ii)(B) since an unanalyzed condition existed which degraded plant safety.
Analysis of the Event
There were no actual safety consequences associated with this event since there was no fire in the control room. The potential safety consequences of this event were also minimal.
The design basis fire in the control room assumes an all encompassing fire which is unlikely and would more likely be limited to a single panel. Even if the RCIC panel was impacted by the fire, capabilities exist with other balance of plant and fire safe shutdown systems to achieve and maintain safe shutdown. However, for the identified inverter failure mode, these capabilities are outside those credited in the licensing basis.
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Cause of the Event
The cause of the condition was that the modification process failed to identify the design of the NLI inverters' surge suppressors was not compatible with the center tap design of the ungrounded system. A ground on the negative 125 VDC portion of the system would result in a potential across the inverters' surge suppressor devices in excess of the design rating (250Vdc vs. 170Vdc rating). The current would increase through the surge suppressors causing the supply fuses to blow, thus making the inverters unavailable.
Corrective Action Completed The Unit 1 inverter in the remote shutdown panel was modified on July 22, 2001, and the Unit 2 inverter was replaced on June 23, 2001. A review was conducted of the remaining fire safe shutdown NLI inverters installed in the plant and concluded that the identified failure mode would not impact the ability to safely shutdown after a fire.
This modification was processed in 1995. The modification process was enhanced in 1997 to implement a procedure for failure modes and effects analysis to preclude recurrence of such an event. Also, the Fire Protection Review Checklist was enhanced in December 2000 to provide additional guidance on interdependencies of fire safe shutdown.
Corrective Actions Planned None Previous Similar Occurrences None