05000306/LER-2011-003
| Prairie Island Nuclear Generating Plant Unit 2 | |
| Event date: | 06-27-2011 |
|---|---|
| Report date: | 08-23-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3062011003R00 - NRC Website | |
For PINGP Unit 2, there are four possible paths between the offsite transmission system and the Safeguard 4160V busesl. Each path is capable of providing the required power to shutdown the reactor and maintain it in a shutdown condition.
At 13:10 CDT on June 27, 2011, Transformer 2RY was locked out. The lockout of the 2RY Transformer2 occurred when the 51G/2RY Transformer ground detection relay3 actuated. The actuation of the, relay was caused by a 2RY bus phase to ground fault. PINGP Unit 2 entered Technical Specification (TS) 3.8.1, Condition A and a single path to the transmission system4.
At 13:44 CDT, the site was notified by transmission systems operator (TSO) that the 345 KV grid voltage could not be maintained at the minimum voltage required per the Electrical Power System Security Analysis procedure (C20.3). The path to the transmission system was declared inoperable and Unit 2 entered Technical Specification 3.8.1, Condition C. Although inoperable, transmission system sources remained connected to Unit 2; emergency diesel generators5 were available but not required to run..
By securing a cooling tower pump and fans6, the required minimum transmission system voltage was met and determined to be sustainable. TS 3.8.1, Condition C, was exited on June 28, 2011 at 00:38 CDT.
On July 2, 2011 a visual inspection discovered that the lower exterior vertical section of the secondary 2RY bus duce had a hole. The bus duct was repaired and returned to service on July 23, 2011.
EVENT ANALYSIS
Initial troubleshooting focused on the cause of the 51G/2RY relay actuation. The 2RY transformer and bus duct were separated and tested. On July 2, 2011 a visual inspection discovered that the lower exterior vertical section of the bus duct had a hole with evidence of an electrical fault around it.
It was determined that the secondary bus ducts are susceptible to moisture intrusion due to their location outdoors and exposure to moisture and rain. The bus duct design includes removable panels and expansion joints that are sealed by gasketing material.
EIIS System Code EB 2 EIIS Component Identifier: XFMR 3 EIIS Component Identifier: RLY 4 EIIS System Code:
FK 5 EIIS Component Identifier: EK 6 EllS System Code:
BS 7 EllS Component Identifier: BU 3/29/10 QF-1118 form request # 181 was rejected due to the fact the work could be performed online.
3/31/10 A PM Change Request (PMCR) was generated to defer PM Task (PMRQ). Deferral requested until 12/31/10.
4/01/10 A General Note was added by the PM Coordinator to the PMCR stating verbal approval had been received from the engineering manager to defer maintenance until 12/31/10. A Corrective Action Request (CAP) was written to document the deferral of a PM without using the risk identification form (QF0922).
12/01/10 General Note added by system engineer to the PMCR stating "Deferral of this PMRQ can go to the end of 2011.
12/03/10 General Note added by PM Coordinator to the PMCR stating per the system engineer deferral had been extended to the end of 2011.
5/02/11 A PMCR was assigned to the system engineer requesting a PMRQ deferral.
5/19/11 PMCR deferral form (QF-0922) completed by system engineer.
It was determined that the 2RY secondary bus duct ground faulted due to a failed gasket on the expansion joint of the 2RY bus duct. The failed gasket allowed moisture intrusion into the bus duct which created a path for the electrical fault to occur. After additional analysis, it was determined that this condition was a safety system function failure for Unit 2 and reportable under 10 CFR 50.73(a)(2)(v)(D).
SAFETY SIGNIFICANCE
This event did reduce the nuclear safety margin, however, the transmission system sources remained connected to Unit 2 (even though they were declared inoperable). In addition, the emergency diesel generators were available but not required to run. By taking appropriate measures, PINGP was able to exit TS 3.8.1, Condition C (Two paths inoperable) in a timely manner. Therefore there were no RC FORM 366A ( N 10-2010) LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
U.S. NUCLEAR REGULATORY COMMISSION 05000 306 YEAR 2011 - 003
NO
- 00 radiological, environmental, or industrial impacts associated with this event and it did not affect the health and safety of the public.
CAUSE
The causal evaluation determined that a less than adequate review of the PM Deferral Process delayed the bus duct inspections that would have likely identified and corrected the deficient gasket material.
CORRECTIVE ACTION
Management will reinforce the Preventative Maintenance (PM) process of procedure adherence to the Preventative Maintenance Program procedure (FP-PE-PM-01). PM deferral decisions will be made with the proper risk information which is solicited by performing the risk assessment form QF- 0922.
Bus duct inspections will be performed under currently planned work orders and scheduling future inspections per the maintenance frequency will alleviate maintenance issues.
The outdoor bus duct maintenance procedures will be modified to require the replacement of all inspection cover and expansion joint gasketing.
PREVIOUS SIMILAR EVENTS
A LER search was conducted and no similar events involving age-related failures of bus duct gaskets were identified in the last three years at PINGP. However, in November 2001, the 2RX transformer bus duct failed due to the presence of moisture inside the bus duct.