ULNRC-06130, LER 14-003-00 for Callaway, Unit 1 Inverter NN11 Inadvertantly Transferred to Its Alternate AC Source

LER 14-003-00 for Callaway, Unit 1 Inverter NN11 Inadvertantly Transferred to Its Alternate AC Source

ML14219A726
Person / Time
Site:	Callaway
Issue date:	08/07/2014
From:	Bradley G Ameren Missouri
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ULNRC-06130 LER 14-003-00
Download: ML14219A726 (11)

Text

'W Ameren MISSOURI Callaway Plant ***********************************

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August 7, 2014 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 ULNRC-06130 10 CFR 50.73(a)(2)(i)(B)

Ladies and Gentlemen:

DOCKET NUMBER 50-483 CALLA WAY PLANT UNIT 1 UNION ELECTRIC CO. FACILITY OPERATING LICENSE NPF-30 LICENSEE EVENT REPORT 2014-003-00 INVERTER NNll IN ADVERT ANTLY TRANSFERRED TOITSALTERNATEACSOURCE The enclosed licensee event report (LER) is submitted in accordance with 10 CFR 50.73(a)(2)(i)(B).

LER 2014-003-00 is submitted to report a condition prohibited by Technical Specifications due to the inoperability of 120-volt vital AC instrument system power inverter NN11 for a period oftime longer than that allowed by Technical Specification 3.8.7. Other Technical Specifications were affected and are identified in the enclosure.

As indicated in the LER, a supplement is expected to be submitted by March 31, 2015. This letter does not contain new commitments.

Enclosed: LER 2014-003-00

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Sincerely, Gregory Bradley Director Engineering Systems PO Box 620 Fulton , MO 65251 AmerenM i ssour i.com STARS* Alliance **************

ULNRC-06130 August 7, 2014 Page 2 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 ofNuclear Reactor Regulation U.S. Nuclear Regulatory Commission Mail Stop 0-8B 1 Washington, DC 20555-2738 ULNRC-06130 August 7, 2014 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 S. A. Maglio T. B. Elwood J. W. Hiller J. B. Little M.D. Hudson D. S. Turley T. G. Fugate M. A. Killebrew N.J. Fisher G. J. Reinhard G. L. Bradley 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 NRC FORM366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/2017 (01-2014)

Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry.

Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001 , or by LICENSEE EVENT REPORT (LER) 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 (See reverse for required number of of Management and Budget , Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB digits/characters for each block) control number , the NRC may not conduct or sponsor, and a person is not required to respond to , the information collection. 1. FACILITY NAME 2. DOCKET NUMBER 13. PAGE Callaway Plant Unit 1 05000483 1 OF 8 4. TITLE INVERTER NN11 INADVERTANTL Y TRANSFERRED TO ITS ALTERNATE AC SOURCE 5. EVENT DATE 6. LER NUMBER 7. REPORT DATE 8. OTHER FACILITIES INVOLVED YEAR I SEQUENTIAL I REV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR NUMBER NO. MONTH DAY YEAR FACILITY NAME DOCKET NUMBER 06 09 2014 2014 -003 -00 08 07 2014 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)

10. POWER LEVEL D 20.2203{a){2){ii)

D 50.36{c){1 ){ii){A) D 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) 0 OTHER D 20.2203{a){2){vi) 181 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 (Include 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 MANU-REPORTABLE CAUSE SYSTEM COMPONENT MANU-REPORTABLE FACTURER TOEPIX FACTURER TO EPIX G EF INVT Solid State y Controls 14. SUPPLEMENTAL REPORT EXPECTED 15.EXPECTED MONTH DAY YEAR 181 YES (If yes , complete 15. EXPECTED SUBMISSION DATE) 0NO SUBMISSION DATE 03 31 2015 ABSTRACT (Limit to 1400 spaces , i.e., approximately 15 single-spaced typewritten lines) On 6/9/2014, during a maintenance activity , inverter NN11 unexpectedly transferred from its normal direct current (DC) source to its bypass alternating current (AC) source. This inverter provides power to the NN01 bus which is one of four vital 120-VAC instrument buses at Callaway Plant. In addition to other vital instrumentation , the bus provides power to cabinet SB038 which supports instrumentation and controls for systems such as the Reactor Trip System (RTS) and the Engineered Safety Feature Actuation System (ESF AS). Investigation identified a loose mounting screw that secures disconnect switch NNO 1-11 to bus NNO 1. Maintenance work in the area of the loose termination led to a momentary interruption of power to cabinet SB03 8 , which appeared as a fault condition to the inverter, thus causing the inverter to transfer to its alternate power source. The cabinet , bus , and inverter are seismically qualified and are required to be capable of performing their design basis accident mitigation functions following a Safe Shutdown Earthquake (SSE). Per Callaway's licensing basis , this capability is an Operability requirement.

With the degraded electrical termination , which is assumed to have existed for an extended period oftime before discovery and repair, the inverter and SB038 loads would not have been capable of performing their design basis function following an SSE , thus rendering the components inoperable. Examination and further evaluation of the disconnect switch, SB038 mounting screw , and interface with the NN01 bus is required to identify a definitive root cause. This activity is planned for the refueling outage that begins this fall. 6 (10-2010)

NRC FORM 366A (10-2010)

LICENSEE EVENT REPORT (LER)u.s.

NUCLEAR REGULATORY coMMISSION CONTINUATION SHEET 1. FACILITY NAME 2. DOCKET YEAR Callaway Plant Unit 1 05000483 2014 NARRATIVE

6. LER NUMBER I SEQUENTIAL I NUMBER 003 REV NO. 00 1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S)

AND COMPONENT(S):

3. PAGE 2 OF The onsite Class 1E AC, DC, and AC vital bus electrical power distribution systems at Callaway are divided by train into two redundant and independent AC, DC, and AC vital bus electrical power distribution subsystems.

The AC electrical power subsystem for each train consists of an Engineered Safety Feature (ESF) 4.16-kV bus [EllS system EB], 480-V buses [EllS system ED], and load centers. Each 4.16-kV ESF bus may be supplied power from either of two offsite power sources or from a dedicated onsite emergency diesel generator (DG) source. Each 4.16-kV ESF bus is normally connected to a preferred offsite source. After a loss ofthe preferred offsite power source to a 4.16-kV ESF bus, the onsite emergency DG automatically supplies power to the bus. A transfer to the alternate offsite source is accomplished by manually repositioning breakers, if required.

Control power for the 4 .16-k V breakers is supplied from the Class 1E DC system. The 120-V AC vital buses [EllS system EF] are arranged in two load groups per train and are normally powered through the inverters from the 125-VDC electrical power subsystem

[EllS system EJ]. The 125-VDC electrical power distribution system is arranged into two buses per train. Each 125-VDC bus is supported by a 125-VDC Class 1E battery bank [EllS system EJ] and associated 125-VDC Class 1E battery charger which is supported by a 480-VAC source. The table below provides the overall scheme for the above-described distribution buses. AC and DC Electrical Power Distribution Systems TYPE VOLTAGE TRAIN A TRAINB AC safety buses 4160V ESF BusNB01 ESFBusNB02 480V Load Centers Load Centers NG01, NG03 NG02, NG04 DC buses 125 v BusNK01 BusNK02 BusNK03 BusNK04 AC vital buses 120V BusNN01 BusNN02 BusNN03 BusNN04 The four independent Class 1 E 120-V AC vital buses independently provide power to the four NRC FORM 366A (10-2010) 8 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 2014 NARRATIVE

6. LER NUMBER I SEQUENTIAL I NUMBER 003 REV NO. 00 3. PAGE 3 OF channels of the protection systems and reactor control systems. The power supply for each I20-V AC vital bus consists of one inverter/uninterruptible power supply (UPS) and one external standby regulating transformer which can be connected to the bus through the manual transfer switch. Each inverter!UPS

[EllS component INVT] (i.e., NNII, NNI2, NN13 and NNI4) consists of a 7.5-kVA solid-state inverter with an integral480-VAC to I20-VAC single-phase regulating transformer

[EllS component XFMR] for use as a backup/alternate source, an automatic static transfer switch that will switch to the backup source in the event of inverter failure, and a manual maintenance bypass switch that can be used to switch to the external backup supply (external standby regulating transformer) during maintenance activities or in the event of an inverter failure. The normal supply for each UPS/inverter is from the associated Class IE DC bus. The UPS/inverter backup source (i.e., the integral regulating transformer) for each unit is supplied from Class IE 480 VAC. 2. INITIAL PLANT CONDITIONS:

On 06/09/20I4, Callaway Plant was in Mode 1 (Power Operations) at IOO-percent rated thermal power and normal operating temperature and pressure when the event (described below) occurred. No other significant equipment was concurrently inoperable.

3. EVENT DESCRIPTION:

On 06/09/20I4, thermography inspections and clamp-on ammeter readings were being conducted on disconnect switch NNOI-II. During this activity, an unexpected transfer of inverter NNll from its normal DC source to its bypass AC source occurred. The NNll inverter provides power to the NNOI bus which is one of four vitali20-volt AC instrument buses at Callaway Plant. In response to the inverter transfer, which is considered to render the inverter and its uninterruptible power supply (UPS) function inoperable, the applicable Condition and Required Action under Technical Specification (TS) 3.8.7, "Inverters-Operating," were entered. In addition, the applicable Conditions and Required Actions under the following Technical Specifications were entered for normally deenergized bistables that are powered by the NNOI bus: TS 3.3.2, "Engineered Safety Feature Actuation System (ESF AS) Instrumentation," for the affected Refueling Water Storage Tank Low Low Level and Containment Pressure-High 3 trip channels, and TS 3.3.5, "Loss of Power (LOP) Diesel Generator (DG) Start Instrumentation," for the affected 4.I6-kV bus undervoltage relaying. Following the event, troubleshooting was performed which identified that a screw that secures the NNO I bus to the line side of disconnect switch NNO I-II was loose. Electrical Maintenance subsequently tightened the mounting screw I 0 revolutions. This disconnect switch is part of the feed from the NNOI bus to the SB038 cabinet, which supports instrumentation and controls for the Reactor Trip System (RTS), Engineered Safety Feature Actuation System (ESF AS), Post Accident NRC FORM 366A (10-2010) 8 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 REV OF I SEQUENTIAL I NUMBER NO. YEAR Callaway Plant Unit 1 05000483 4 2014 003 00 NARRATIVE Monitoring System (P AMS), and the Remote Shutdown System. It was thus determined that the loose termination, in conjunction with the maintenance work, contributed to a disturbance of power to cabinet SB038. The momentary interruption in power to cabinet SB038 resulted in a power transient on bus NN01 that appeared as a fault condition to the NN11 inverter.

The detected fault condition resulted in the inverter transferring to its alternate AC power source. Cabinet SB038, vital bus NN01, and inverter NN11 are required to be seismically qualified and are thus designed to remain capable of performing their design basis accident mitigation functions following a Safe Shutdown Earthquake (SSE). In light of this design requirement, it was determined that, in the event of a seismic event with the degraded electrical termination condition present, the inverter would be subject to a spurious transfer and loss of its UPS function, and the SB038 cabinet would be subject to a power supply disruption.

Consequently, it could not be assumed that the NN11 inverter and the loads associated with the SB038 cabinet would be capable of performing their design basis function following an SSE, with the degraded electrical termination condition present. The identification of the loose screw provides a plausible explanation for the NN11 inverter transfer.

However, it is unknown why the disconnect screw was not fully engaged prior to the occurrence of this event. Potential causes may be attributed to worker practices during replacement of the disconnect switch in October 2008 or to degradation of the mounting interface between the disconnect switch and the NNO 1 bus. Additional inspections/evaluations of the internals of the disconnect switch and its interface with the NNO 1 bus will be performed in the upcoming refueling outage currently scheduled for October 2014. Additional information gained from those further inspections/evaluations will be provided, as applicable, in a supplement to this Licensee Event Report. The supplement is expected to be submitted by March 31, 2015. At present, a high level of confidence exists in the integrity of the electrical termination for the noted disconnect switch since the mounting screw was tightened on the evening of06/09/2014.

No abnormalities have been noted since that time. Affected Components and Associated Technical Specifications As detailed above, the SB038 cabinet and its loads, along with the NN11 inverter, are designed to be seismically qualified.

With the degraded electrical termination condition present, the inverter was subject to spurious transferring and the SB038 cabinet was subject to power supply disruption, in the event of a seismic event. Consequently, it cannot be assumed that with the degraded electrical termination condition present, the NN11 inverter and the loads associated with the SB038 cabinet would be capable of performing their design basis function following an SSE. Section 3.1.2 of the Callaway Final Safety Analysis Report (FSAR) provides a summary of the key assumptions, including single-failure assumptions, that were factored into the design and accident NRC FORM 366A (10-2010) 8 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 REV OF I SEQUENTIAL I NUMBER NO. YEAR 05000483 Callaway Plant Unit 1 5 2014 003 00 NARRATIVE analysis for Callaway Plant. As noted therein, "In designing for and analyzing for DBAs [design basis accidents], ... for a LBLOCA [large break loss-of-coolant accident]

... no credit is taken for the functioning of non-seismic Category 1 components." This guidance is part of Callaway's licensing basis wherein seismic qualification is considered to be a requirement for Operability of systems, structures and components needed to mitigate a DBA LOCA. Per Callaway's accident analysis as described in the FSAR, a DBA is postulated to occur with a loss of offsite power, but not with a seismic event. Nevertheless, based on the above, it is a requirement that in order for SSCs to be considered Operable with respect to their required DBA mitigation functions, they must be seismically qualified.

With the degraded electrical termination condition present at the noted disconnect switch, the NN11 inverter and loads associated with the SB038 cabinet were, in effect, not seismically qualified due to their capability to be disabled by a seismic event. With this vulnerability present, the inverter and SB038 loads must be considered inoperable.

As noted previously, it is not known how long the degraded electrical termination condition existed. There is no firm evidence that points to a known point in time when the condition began. However, based on the looseness and sensitivity of the connection, and the last time when work was performed on the disconnect switch, it may be assumed that the condition existed for a significant period of time relative to the allowed out-of-service times permitted by the Technical Specifications for all of the affected components.

The affected Technical Specifications are identified as follows. The NN11 inverter is subject to the requirements of Callaway Technical Specification (TS) 3.8.7, "Inverters-Operating." Per the Limiting Condition for Operation (LCO) of this Technical Specification, all four required inverters must be Operable during plant operation, i.e., during Modes 1, 2, 3, and 4. With one required inverter inoperable, Condition A applies and associated Required Action A.1 must be entered, which requires restoring the inoperable inverter to Operable status within the specified Completion Time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Otherwise, Condition B is entered, and in this Condition a controlled plant shutdown is required such that per Required Actions B.l and B.2, the plant must be in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />, respectively.

With an inverter inoperable, other Technical Specifications (beside TS 3.8.7) are affected as well. They were identified in Section 3, wherein it was noted that when the NN11 inverter was initially declared inoperable on 6/9/2014, the applicable Conditions and Required Actions under TS 3.3.2, "Engineered Safety Feature Actuation System (ESFAS) Instrumentation," for the affected Refueling Water Storage Tank Low Low Level and Containment Pressure-High 3 trip channels, and TS 3.3.5, "Loss of Power (LOP) Diesel Generator (DG) Start Instrumentation," for the affected 4.16-kV bus undervoltage relaying, were entered due to the impact on normally deenergized bistables powered by the NN01 bus. For the affected loads associated with the SB038 cabinet, the loads are numerous. (A detailed listing is included with the corrective action document initiated in response to the event.) They primarily consist of one division/channel of various trip units (bistables) associated with the RPS, ESF AS, PAMS, and Remote Shutdown System, which are subject to the requirements ofTS 3.3.1, "Reactor Trip System (RTS) Instrumentation," TS 3.3.2, "Engineering Safety Feature Actuation System NRC FORM 366A (10-2010) 8 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 REV OF I SEQUENTIAL I NUMBER NO. YEAR Callaway Plant Unit 1 05000483 6 2014 003 00 NARRATIVE (ESFAS) Instrumentation," TS 3.3.3, "Post Accident Monitoring (PAM) Instrumentation," and TS 3.3.4, "Remote Shutdown System," respectively.

From review of the various Conditions and Required Actions under these Technical Specifications, which applied with the noted instrumentation inoperable, the most severe Completion Times (i.e., allowed out-of-service times) are on the order of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. As previously noted, the degraded terminal connection condition is conservatively assumed to have existed for an unknown but extended period of time. Based on that assumption, the resultant "past" inoperability of the various affected instruments/components identified above must be assumed to have existed for that length of time. 4. ASSESSMENT OF SAFETY CONSEQUENCES:

Although the identified condition affected numerous loads, including the NNll inverter and the various instrument channels associated with the RTS, ESFAS, PAMS, and Remote Shutdown System instrumentation, the condition did not prevent LOCA mitigation equipment from performing required mitigation functions assuming no concurrent seismic event. In addition, in the event of a seismic event, the condition would not have prevented the achievement of safe shutdown for such a hazard, since sufficient shutdown capability remained due to the availability of redundant equipment not affected by the condition.

It should also be noted that, for much of the RTS and ESFAS instrumentation potentially affected by the condition, in the event of a seismic event and the resultant loss of power to the affected instrumentation due to the identified condition, the affected bistables would trip to their tripped state due to the instrumentation's deenergize-to-trip design. Based on the above considerations, the event/condition is not considered to be safety significant.

Risk Assessment The plant risk associated with the unqualified SB038 electrical connection is related to seismic events where mitigation is complicated by the induced failure of SB038 and its affected loads. A qualitative risk assessment was conducted based on the range of seismic impacts and the effect of the unqualified SB038 electrical connection.

The range of seismic magnitudes was divided into three intervals associated with step changes in damage states. The seismic impacts were evaluated by considering the consequences, required plant response, effects of the unqualified SB038 connection and the expected frequency of the associated seismic events for these intervals.

The evaluation of overall risk associated with the effect of the unqualified SB03 8 electrical connection indicates this condition had very low risk significance.

NRC FORM 366A (10-2010) 8 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 2014 NARRATIVE

5. REPORTING REQUIREMENTS:

6. LER NUMBER I SEQUENTIAL I NUMBER 003 REV NO. 00 3. PAGE 7 OF This LER is submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) to report a condition prohibited by the plant's Technical Specifications. The Technical Specifications affected by the past inoperability of the various components due to the degraded electrical termination at the noted disconnect switch are identified in Section 3 of this LER. The allowed out-of-service times (i.e., Completion Times) specified in the Required Actions of the affected Technical Specifications are also identified or discussed. For the inoperable NN11 inverter, an allowed out-of service time of24 hours is specified in the TS 3.8.7. For some of the instrument channels made inoperable by the identified condition (as previously explained), allowed service times on the order of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> apply, under TS 3.3.1 and TS 3.3.2. Since the degraded terminal connection has been conservatively assumed to exist for an unknown but extended period of time, the resultant

" past" inoperability of the various affected instruments/components (including the NN11 inverter itself) must be assumed to have existed for a period oftime that exceeds the allowed out-of-service times permitted by the Technical Specifications. The condition must thus be reported as an operation or condition prohibited by the plant's Technical Specifications. 6. CAUSE OF THE EVENT: The most probable cause of the event/condition is attributed to a loose bus mounting screw that secures the disconnect switch NN01-11 to the NN01 bus. This loose termination contributed to an interruption in power to the circuit load (SB038) when personnel accessed this disconnect switch compartment for thermography inspections and clamp-on ammeter readings on 06/09/14. The mounting screw was subsequently tightened 10 turns following initial troubleshooting assessments by Maintenance personnel.

Further evaluation of the disconnect switch, SB038 mounting screw, and interface with NNO 1 bus is required to identify a definitive root cause. Since these activities cannot be performed without risk to plant operation, they are planned to be completed during the upcoming refueling outage scheduled for October 2014. 7. CORRECTIVE ACTIONS: Since a definitive root cause analysis is delayed until the upcoming refueling outage, corrective actions have not been developed.

Upon further investigation, these actions will be developed and submitted in a supplemental report. 8. PREVIOUS SIMILAR EVENTS: Although there have been previous instances of inverters spuriously transferring to their alternate NRC FORM 366A (10-2010) 8 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 2014 NARRATIVE

6. LER NUMBER I SEQUENTIAL I NUMBER 003 REV NO. 00 3. PAGE 8 OF source at Callaway, the root cause for this current issue is not definitively known. Therefore, it cannot be determined with certainty whether previous similar events have occurred at Callaway.

There have not been previous cases of inverter failure at Callaway that were found to have been caused by a degraded bus mounting screw. NRC FORM 366A (10-2010) 8