05000306/LER-2011-003, Regarding Offsite Power Sources Declared Inoperable as a Consequence of the Loss of the 2RY Transformer Bus Duct

Regarding Offsite Power Sources Declared Inoperable as a Consequence of the Loss of the 2RY Transformer Bus Duct

ML112360593
Person / Time
Site:	Prairie Island
Issue date:	08/23/2011
From:	Schimmel M Northern States Power Co, Xcel Energy
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
L-PI-11-066 LER 11-003-00
Download: ML112360593 (6)

LER-2011-003, Regarding Offsite Power Sources Declared Inoperable as a Consequence of the Loss of the 2RY Transformer Bus Duct

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3062011003R00 - NRC Website

text

@ Xcel Energym August 23,201 1 U S Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 L-PI-11-066 10 CFR 50.73 Prairie lsland Nuclear Generating Plant Unit 2 Docket: 50-306 Renewed License No. DPR-60 LER 50-306l2011-003-00, Unit 2 Offsite Power Sources Declared Inoperable As A Consequence of the Loss of the 2RY Transformer Bus Duct Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, herewith encloses Licensee Event Report (LER) 50-3061201 1-003-00.

On June 27, 201 1, the off-site AC power sources to Unit 2 Prairie lsland Nuclear Generating Plant (PINGP) were declared inoperable as a result of Transformer 2RY lockout and less than the required minimum voltage on the transmission system. The paths to the transmission system was declared inoperable. Although inoperable, transmission system sources remained connected to Unit 2; emergency diesel generators were available but not required to run.

The overcurrent ground detection relay actuated due to a bus phase to ground fault resulting from failed gasket material resulting in the Transformer lockout. It was determined that a less than adequate review of the Preventive Maintenance (PM)

Deferral Process delayed the bus duct inspections that would have likely identified and corrected the deficient gasket material.

Summaw of Commitments This letter contains no new commitments and no changes to existing commitments.

Mark A. Schimmel Site Vice President, Prairie lsland Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:

Administrator, Region Ill, USNRC Project Manager, Prairie lsland Nuclear Generating Plant (PINGP), USNRC Resident Inspector, PINGP, USNRC Department of Commerce, State of Minnesota 171 7 Wakonade Drive East Welch, Minnesota 55089-9642 Telephone: 651.388.1 121

ENCLOSURE LICENSEE EVENT REPORT 50-3061201 1-003-00 4 Pages Follow

, the NRC ma) not wnduct or sponsor, and a person is not required to respond to, the information wilectlon.

I.

FACILITY NAME 1 2. DOCKET NUMBER 1 3. PAGE Prairie Island Nuclear Generating Plant Unit 2 05000 306 1OF 4

4. TITLE Unit 2 Offsite Power Sources Declared Inoperable As A Consequence of the Loss of the 2RY Transformer Bus Duct

5. EVENT DATE I

6. LER NUMBER I

7. REPORT DATE I

8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER SEQUENTIAL REV MONTH DAY YEAR YEAR NUMBER NO MONTH DAY YEAR FACILITY NAME DOCKET NUMBER 06 27 2011 2011 - 003 - 00 08 23 2011 I

I I

I I

I

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 5: (Check all that apply)

Mode 1

10. POWER LEVEL 100%

50.73(a)(2)(vii) 50,73(a)(2)(viii)(A) 50,73(a)(2)(viii)(B) 50,73(a)(2)(ix)(A) 50.73(a)(2)(x) 73.71 (a)(4) 73.71 (a)(5)

OTHER I

20.2203(a)(Z)(vi) 50873(a)(2)(i)(B) a 50.73(a)(2)(v)(D)

Specify in Abstract below or in 12. LICENSEE CONTACT FOR THlS LER JAME Sam J. DiPasquale, P.E.

TELEPHONE NUMBER (Include Area Code) 651.388.1 121 x7350 I MANU-REPORTABLE MANU-REPORTABLE

CAUSE

SYSTEM COMPONENT FACTURER TO EPlX

CAUSE

SYSTEM COMPONENT FA CTURER TO EPlX A

FK BU 0000 Y

14. SUPPLEMENTAL REPORT EXPECTED

15. EXPECTED MONTH DAY YEAR SUBMISSION 3 YES (If yes, complete 15. EXPECTED SUBMISSION DATE).

O NO DATE IBSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) 3n June 27, 201 I, the off-site AC power sources to Unit 2 Prairie Island Nuclear Generating Plant

PINGP) were declared inoperable as a result of Transformer 2RY lockout and less than the required ninimum voltage on the transmission system. The paths to the transmission system was declared noperable. Although inoperable, transmission system sources remained connected to Unit 2; 3mergency diesel generators were available but not required to run.

The overcurrent ground detection relay actuated due to a bus phase to ground fault resulting from

'ailed gasket material resulting in the Transformer lockout. It was determined that a less than adequate review of the Preventive Maintenance (PM) Deferral Process delayed the bus duct nspections that would have likely identified and corrected the deficient gasket material.

'JRC FORM 366 (10-2010)

EVENT DESCRIPTION LICENSEE EVENT REPORT (LER)

U.S. NUCLEAR REGULATORY COMMISSION (10-2010)

CONTINUATION SHEET For PlNGP Unit 2, there are four possible paths between the offsite transmission system and the Safeguard 41 60V buses'. Each path is capable of providing the required power to shutdown the reactor and maintain it in a shutdown condition.

1. FACILITY NAME Prairie Island Nuclear Generating Plant Unit 2 At 13:lO CDT on June 27,201 1, Transformer 2RY was locked out. The lockout of the 2RY

~ransformer* occurred when the 51 GRRY 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..

2. DOCKET NUMBER 05000 306 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, 201 1 at 00:38 CDT.

On July 2, 201 1 a visual inspection discovered that the lower exterior vertical section of the secondary 2RY bus duct7 had a hole. The bus duct was repaired and returned to service on July 23, 201 1.

6. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO 2011 -

003

- 00

EVENT ANALYSIS

3. PAGE 2OF4 Initial troubleshooting focused on the cause of the 51GMRY relay actuation. The 2RY transformer and bus duct were separated and tested. On July 2, 201 1 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.

Ells System Code 2

E B 3 Ells Component Identifier: XFMR Ells Component Identifier: RLY Ells System Code:

5 FK 6 Ells Component Identifier: EK Ells System Code:

B S Ells Component Identifier: BU

The 2RY bus duct maintenance timeline was reviewed and the following was determined:

2/02 Bus duct 2R-YS/3000BD was inspected in February 2002 and no issues were found.

The next performance of the bus duct inspection was due on 11/17/08 with a late date of 511 911 0. LICENSEE EVENT REPORT (LER)

U.S. NUCLEAR REGULATORY COMMISSION (10-201 0)

CONTINUATION SHEET 311 611 0 Outage Scope change Request Scope Control (QF-1 I 1 8) request # 181 filled out to add Preventive Maintenance (PM) task to 2R26 (Unit 2 refueling outage, Cycle 26).

1. FACILITY NAME Prairie Island Nuclear Generating Plant Unit 2 312911 0 QF-I 1 18 form request # 1 81 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.

2. DOCKET NUMBER 05000 306 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 201 1."

6. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO 2011 -

003

- 00 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 201 1.

3. PAGE 30F4 5/02/11 A PMCR was assigned to the system engineer requesting a PMRQ deferral.

511 911 1 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, PlNGP was able to exit TS 3.8.1, Condition C (Two paths inoperable) in a timely manner. Therefore there were no

radiological, environmental, or industrial impacts associated with this event and it did not affect the health and safety of the public. LICENSEE EVENT REPORT (LER)

U.S. NUCLEAR REGULATORY COMMISSION (10-201 0)

CONTINUATION SHEET

CAUSE

1. FACILITY NAME Prairie Island Nuclear Generating Plant Unit 2 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

2. DOCKET NUMBER 05000 306 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.

6. LER NUMBER SEQUENTIAL REV YEAR NUMBER NO 2011 -

003

- 00 The outdoor bus duct maintenance procedures will be modified to require the replacement of all inspection cover and expansion joint gasketing.

3. PAGE 4 0 F 4

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.