ULNRC-06178, LER 14-006-00 for Callaway Plant, Unit 1, Regarding Main Generator Excitation Transformer Faulted to Ground, Causing Turbine and Reactor Trip

LER 14-006-00 for Callaway Plant, Unit 1, Regarding Main Generator Excitation Transformer Faulted to Ground, Causing Turbine and Reactor Trip

ML15033A215
Person / Time
Site:	Callaway
Issue date:	02/02/2015
From:	Cox B L Ameren Missouri, Union Electric Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ULNRC-06178 LER 14-006-00
Download: ML15033A215 (7)

Text

W Ameren MISSOURI Callaway Plant ....................................................................................................................................................................................................................................................

February 2, 2015 ULNRC-06178 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 Ladies and Gentlemen:

10 CFR 50.73 DOCKET NUMBER 50-483 CALLAWAY PLANT UNIT 1 UNION ELECTRIC CO. FACILITY OPERATING LICENSE NPF-30 LICENSEE EVENT REPORT 2014-006-00 MAIN GENERATOR EXCITATION TRANSFORMER FAULTED TO GROUND, CAUSING REACTOR TRIP The enclosed licensee event report is submitted in accordance with 10CFR50.73(a)(2)(iv)(A) to report a reactor protection system actuation while critical and an auxiliary feed water system actuation due to a plant trip caused by a ground fault in the main generator excitation transformer.

This letter does not contain new commitments.

DRB/nls Enclosure

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Sincerely, Barry Cox Senior Director, Nuclear Operations PO Box 620 Fulton, MD 65251 AmerenMissouri.com

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ULNRC-06178 February 2, 2015 Page2 cc: Mr. Marc L. Dapas Regional Administrator U.S. Nuclear Regulatory Commission Region IV 1600 East Lamar Boulevard Arlington, TX 76011-4511 Senior Resident Inspector Callaway Resident Office U.S. Nuclear Regulatory Commission 8201 NRC Road Steedman, MO 65077 Mr. Fred Lyon Project Manager, Callaway Plant Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Mail Stop 0-8B 1 Washington, DC 20555-2738 ULNRC-06178 February 2, 2015 Page 3 Index and send hardcopy to QA File A160.0761 Hardcopy:

Certrec Corporation 4150 International Plaza Suite 820 Fort Worth, TX 76109 (Certrec receives ALL attachments as long as they are non-safeguards and may be publicly disclosed.)

Electronic distribution for the following can be made via LER ULNRC Distribution:

F. M. Diya D. W. Neterer L. H. Graessle B.L.Cox T. E. Herrmann S.M. Maglio T. B. Elwood Corporate Oversight Corporate Communications NSRB Secretary Performance Improvement Coordinator Resident Inspectors (NRC) STARS Regulatory Affairs Mr. John O'Neill (Pillsbury Winthrop Shaw Pittman LLP) Missouri Public Service Commission NRCFORM366 (01-2014)

U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/2017 LICENSEE EVENT REPORT (LER) (See reverse for required number of digits/characters for each block) Estimated burden per response to comply with this mandatory collection request: ao hours. Reported lessons learned are incorporated into the licensing process and fed back to industry.

Send comments burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to infocollects.Resource@nrc.gov , and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-1 0202 , (3150-01 04), Office of Management and Budget, Washington , DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number , the NRC may not conduct or sponsor , and a person is not required to respond to , the information collection. 1. FACILITY NAME Callaway Plant Unit 1 2. DOCKET NUMBER 05000483 13. PAGE 1 OF 4 4. TITLE Main Generator Excitation Transformer Faulted to Ground, Causing Turbine and Reactor Trip 5. EVENT DATE 6. LER NUMBER 7. REPORT DATE 8. OTHER FACILITIES INVOLVED YEAR 'SEQUENTIAL I REV MONTH FACILITY NAME DOCKET NUMBER MONTH DAY YEAR NUMBER NO. DAY YEAR FACILITY NAME DOCKET NUMBER 12 03 2014 2014 -006 -00 02 01 2015 9. OPERATING MODE 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR§: (Check all that apply) D 20.2201 (b) D 20.2203(a)(3)(i)

D 50.73(a)(2)(i)(C)

D 50.73(a)(2)(vii) 1 D 20.2201 (d) D 20.2203(a)(3)(ii)

D 50. 73(a)(2)(ii)(A)

D 50. 73(a)(2)(viii)(A)

D 20.2203(a)(1)

D 20.2203(a)(4)

D 50.73(a)(2)(ii)(B)

D 50.73(a)(2)(viii)(B)

D 20.2203(a)(2)(i)

D 50.36(c)(1)(i)(A)

D 50.73(a)(2)(iii)

D 50.73(a)(2)(ix)(A) 20.2203(a)(2)(ii)

D 50.36(c)(1)(ii)(A) 181 50.73(a)(2)(iv)(A)

D 50.73(a)(2)(x)

D 20.2203(a)(2)(iii)

D 50.36(c)(2)

D 50.73(a)(2)(v)(A)

D 73.71 (a)(4) D 20.2203(a)(2)(iv)

D 50.46(a)(3)(ii)

D 50. 73(a)(2)(v)(B)

D 73.71 (a)(5) 100 D 20.2203(a)(2)(v)

D 50.73(a)(2)(i)(A)

D 50.73(a)(2)(v)(C)

D OTHER D 20.2203(a)(2)(vi)

D 50.73(a)(2)(i)(B)

D 50.73(a)(2)(v)(D)

Specify in Abstract below or in NRC Form 366A 12. LICENSEE CONTACT FOR THIS LER FACILITY NAME 'TELEPHONE NUMBER (Inclu de Area Code) T.B. Elwood, Supervising Engineer, Regulatory Affairs and Licensing 314-225-1905

13. COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT-CAUSE SYSTEM COMPONENT FACTURER REPORTABLE TO EPIX CAUSE SYSTEM COMPONENT FACTURER REPORTABLE TO EPIX B TL EXC Magnetic Technologies y 14. SUPPLEMENTAL REPORT EXPECTED 0 YES (If yes, complete 15. EXPECTED SUBMISSION DATE) 181 NO ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) 15.EXPECTED SUBMISSION DATE MONTH DAY YEAR At 00:22 hours on December 3, 2014, during normal power operations, A turbine and reactor trip occurred, when the main generator excitation transformer faulted to ground. The reactor trip was classified as "uncomplicated." Safety systems performed as designed.

During recovery the valve providing flow from Motor Driven Auxiliary Feedwater Pump "B" to Steam Generator "D" failed to throttle closed afterwards.

Repair of the excitation transformer was completed and the plant returned to power operations on December 6, 2014. The construction of the excitation transformer includes high voltage jumper cables between termination points inside its protective enclosure and the winding taps of the transformer coils. These jumper cables are routed above the iron core of the transformer and are supported by insulating boards as well as restrained by nylon cable ties. The fault to ground was caused when a jumper cable dropped onto the iron transformer core after failure of the nylon cable ties. These cable ties were an original part of the transformer installed in 2007. The root cause of the transformer failure was inadequate design (routing cables above the transformer core) and material selection (use of nylon cable ties) during the manufacture of the transformer.

NRC FORM 366 (1 0-201 0)

NRC FORM 366A (10-2010)

LICENSEE EVENT REPORT (LER)u.s.

NUCLEAR REGULATORY COMMISSION CONTINUATION SHEET 1. FACILITY NAME 2. DOCKET 6. LER NUMBER 3. PAGE YEAR Callaway Plant Unit 1 05000483 2014 NARRATIVE I SEQUENTIAL I NUMBER 006 REV NO. 00 1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S)

AND COMPONENT(S):

2 OF The Main Generator Excitation System Power Transformer, XMB01, [EllS system TL] takes power at the 25-kV level and steps it down to the 880-Vac level for use in the static excitation system which provides a source of excitation to the rotating field in the Main Generator.

Power is conducted to XMBO 1 by the Main Isophase Bus, which also carries the Main Generator output to the generator step-up transformers.

XMBO 1 is a dry transformer that was manufactured in 2006 by Magnetic Technologies Corporation and installed during the Callaway Plant Refuel 15 outage in the spring of 2007. The construction of the transformer includes high voltage jumper cables between the termination points inside its protective enclosure (to which power is conducted from the 25-kV generator output isophase bus) and the winding taps of the transformer coils. These jumper cables are routed above the iron core of the transformer and situated on top of insulating boards which are slightly wider than the cable diameter.

These in turn are supported by insulators.

The cables were restrained by low-grade nylon cable ties that were subject to environmental degradation.

2. INITIAL PLANT CONDITIONS:

Prior to the event, the plant was in Mode 1, at 100% power. There were no activities going on at that time that could have caused this event. There was no equipment out of service that would have had an impact on this event. 3. EVENT DESCRIPTION:

At 00:22 on December 3, 2014, a turbine trip and reactor trip occurred.

The "first out" alarm was 113F, "Unit Trip Turbine Trip". No other "first out" alarms were lit or should have been lit for conditions observed.

There were no activities going on at that time to cause the trip. The operating crew implemented the trip response procedures (E-0, "Reactor Trip or Safety Injection," and ES-0.1, "Reactor Trip Response")

to verify the plant's response to the trip signal from 100% power. The plant was stabilized in Mode 3 at 00:35. Plant equipment was secured per the Emergency Operating Procedures.

The Auxiliary Feedwater Actuation Signal was reset, and both Motor-Driven Auxiliary Feedwater Pumps were secured at 01:30. The Feedwater Isolation Signal was reset, and the Feedwater Isolation Valves were re-opened.

Feedwater supply was transferred from Auxiliary Feedwater to the Start-Up Feedwater Pump at 04:25. It should be noted, however, that "D" Steam Generator level started rising following the trip, and valve ALHV0005 (Motor-Driven Auxiliary Feedwater Pump "B" to Steam Generator "D" Hand Valve) was identified as not throttling closed as expected.

An operator was dispatched and closed the valve manually.

Technical Specification (TS) 3.7.5 "Auxiliary Feedwater (AFW) System," Condition "C" was entered. The valve was repaired prior to the end of the 72-hour Completion Time. NRC FORM 366A (10-2010) 4 NRC FORM 366A (10-2010)

LICENSEE EVENT REPORT (LER)u.s.

NUCLEAR REGULATORY COMMISSION CONTINUATION SHEET 1. FACILITY NAME 2. DOCKET 6. LER NUMBER 3. PAGE Callaway Plant Unit 1 05000483 YEAR 2014 NARRATIVE

4. ASSESSMENT OF SAFETY CONSEQUENCES:

I SEQUENTIAL I NUMBER 006 REV NO. 00 3 OF 4 The trip occurred without complications, and safety systems responded as required.

Appropriate action was taken to restore the required flow to the "D" Steam Generator after the associated failure of ALHV0005 to throttle closed. This event was evaluated with the Callaway PRA model. The evaluation determined the increase in core damage frequency of this event was less than 1E-6; therefore, this event was of very low risk significance.

Use of the PRA model to evaluate the event provides for a comprehensive, quantitative assessment of the potential safety consequences and implications of the event, including consideration of alternative conditions beyond those analyzed in the FSAR. 5. REPORTING REQUIREMENTS:

This LER is submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A) to report a reactor protection system actuation while critical and an auxiliary feedwater system actuation.

Specifically, 10 CFR 50. 73(a)(2)(iv) states in part, "The licensee shall report: (A) Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section ... (B) The systems to which the requirements of paragraph (a)(2)(iv)(A) ofthis section apply are: (1) Reactor protection system (RPS) including:

reactor scram or reactor trip .... ( 6) PWR auxiliary or emergency feed water system." The RPS was actuated at 00:22 on December 3, 2014, during normal power operations (from 100% power). This fulfills the reporting requirement of 10 CFR 50.73(a)(2)(iv)(A) by actuation of the system specified in 10 CFR 50. 73(a)(2)(iv)(B)(l

). A valid auxiliary feedwater system actuation was received as a direct consequence of the turbine and reactor trip. This also fulfills the reporting requirement of 10 CFR 50.73(a)(2)(iv)(A) by actuation of the system specified in 10 CFR 50. 73(a)(2)(iv)(B)(6).

6. CAUSE OF THE EVENT: The root cause of the transformer failure was inadequate design and material selection during the manufacture of the transformer.

The design was inadequate due to critical cables being routed precariously above the transformer core, and the material selected was inadequate because it relied upon low-grade nylon cable ties for restraint.

7. CORRECTIVE ACTIONS: The corrective action to prevent recurrence is to add lacing to supplement the cable ties used to restrain the jumper cables inside the XMBO 1 transformer enclosure.

The root cause team determined that the use of lacing would prevent the jumper cable from dropping on the transformer core and causing an electrical short. The lacing would have a high temperature rating and would not require periodic replacement.

Lacing is scheduled to be installed during the next refueling outage, planned for the spring of2016. It should be noted that as a remedial action, all nylon cable ties have been replaced with Tefzel cable ties which are designed for higher operating temperatures and a longer life expectancy.

NRC FORM 366A (10-2010)

NRC FORM 366A (10-2010)

LICENSEE EVENT REPORT (LER)u.s.

NUCLEAR REGULATORY COMMISSION CONTINUATION SHEET 1. FACILITY NAME 2. DOCKET Callaway Plant Unit 1 05000483 YEAR 20?? NARRATIVE

8. PREVIOUS SIMILAR EVENTS: 6. LER NUMBER I SEQUENTIAL I NUMBER -00? -REV NO. 00 3. PAGE 4 OF No electrical plant events have occurred due to failed cable ties at Callaway.

However, in 2003, severely embrittled and broken cable tie wraps were discovered in the diesel generator room. These tie wraps were on a power feed, but the cable was not displaced.

Corrective actions included the use of heavy duty aqua-blue (Tefzel) safety-related cable ties. Recent significant industry events include an age-related failure of plastic cable ties at Palo Verde Unit 1, which allowed a shield conductor to contact a 13.8kv bus. This resulted in catastrophic failure of a 480 volt AC load center as well as a reactor power cutback from 100% to 60% and declaration of an Unusual Event. NRC FORM 366A (10-2010) 4