05000483/LER-2014-003-02, Regarding Inverter NN11 Inadvertently Transferred to Its Alternate AC Source

Regarding Inverter NN11 Inadvertently Transferred to Its Alternate AC Source

ML15218A567
Person / Time
Site:	Callaway
Issue date:	08/06/2015
From:	Cox B Ameren Missouri
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ULNRC-06237 LER 14-003-02
Download: ML15218A567 (11)

LER-2014-003, Regarding Inverter NN11 Inadvertently Transferred to Its Alternate AC Source

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function
4832014003R02 - NRC Website

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'WAmeren MISSOURI August 06, 2015 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 Callaway Plant ULNRC-06237 10 CFR 50.73(a)(2)(i)(B)

Ladies and Gentlemen:

DOCKET NUMBER 50-483 CALLAWAY PLANT UNIT 1 UNION ELECTRIC CO.

RENEWED FACILITY OPERATING LICENSE NPF-30 LICENSEE EVENT REPORT 2014-003-02 INVERTER NN11 IN ADVERT ANTL Y TRANSFERRED TOITSALTERNATEACSOURCE On August 7, 2014, Ameren Missouri submitted Licensee Event Report (LER) 2014-003-00 for Callaway Plant to report a condition prohibited by Technical Specifications in accordance with 10 CFR 50.73(a)(2)(i)(B). Specifically, 120-volt vital AC instrument system power inverter NN11 was inoperable for a period of time longer than that allowed by Technical Specification 3.8.7.

An expected supplemental LER to update the causes and corrective actions for the identified condition was submitted to the NRC on March 05, 2015.

The enclosed supplemental LER, 2014-003-02, is hereby submitted to include additional information in the event description regarding the momentary de-energization of an instrumentation and control power cabinet.

This letter does not contain new commitments.

Enclosure Sincerely,

~~

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

• Alliance

ULNRC-06237 August 06, 2015 Page2 cc:

Mr. Marc L. Dapas Regional Administrator U.S. Nuclear Regulatory Co~ission 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. L. John Klos Project Manager, Callaway Plant Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Mail Stop 0-8B 1 Washington, DC 20555-2738

ULNRC-06237 August 06, 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 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01 /31 /2017 (01-2014)

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- 5 F53),

U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by LICENSEE EVENT REPORT (LER) internet e-mail to infocollects.Resource@nrc~ov, and to the Desk Officer, Office of Information and Regulatory Affairs, N OB-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.

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 - 02 08 06 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) 0 20.2203(a)(3)(ii)

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

D 50.73(a)(2)(viii)(A) 0 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) 0 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)(S) 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 channels of the protection systems and reactor control systems. The power supply for each 120-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., NN11, NN12, NN13 and NNI4) consists of a 7.5-kVA solid-state inverter with an integral480-VAC to 120-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 1E DC bus. The UPS/inverter backup source (i.e., the integral regulating transformer) for each unit is supplied from Class 1E 480 VAC.

2.

INITIAL PLANT CONDITIONS

On 06/09/2014, Callaway Plant was in Mode 1 (Power Operations) at 100-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 06/09/2014, thermography inspections and clamp-on ammeter readings were being conducted on disconnect switch NN01-11. During this activity, an unexpected transfer of inverter NN11 from its normal DC source to its bypass AC source occurred. The NN11 inverter provides power to the NN01 bus which is one of four vital120-volt AC instrument buses at Callaway Plant. The transfer caused a momentary de-energization of cabinet SB038 (explained later), which resulted in the following plant impacts:

Control Rod insertion of 6 Y2 steps, with an associated pressurizer level and pressure perturbation and subsequent xenon transient, Opening of the 'A' Centrifugal Charging Pump suction from the Reactor Water Storage Tank isolation valve due to momentary loss of the associated Volume Control Tank level channel, and Steam Generator Environmental Allowance Modifier (EAM) circuit actuation resulting in the resetting of the low level setpoint from 17% to 21% narrow range level.

None of these effects posed a significant challenge to plant operators.

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

6. LER NUMBER I

SEQUENTIAL I NUMBER 003 REV NO.

02

3. PAGE 4

OF Conditions and Required Actions under the following Technical Specifications were entered for normally deenergized bistables that are powered by the NN01 bus: 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.

Fallowing the event, troubleshooting was performed which identified that a screw that secures the NN01 bus to the line side of disconnect switch NN01-11 was loose. Electrical Maintenance subsequently tightened the mounting screw 1 0 revolutions. This disconnect switch is part of the feed from the NNO 1 bus to the SB03 8 cabinet, which supports instrumentation and controls for the Reactor Trip System (RTS), Engineered Safety Feature Actuation System (ESFAS), Post Accident 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 SB03 8 cabinet would be capable of performing their design basis function following an SSE, with the degraded electrical termination condition present.

Removal of the disconnect switch and inspections of the bus bar mounting interface were performed during the subsequent refueling outage in October 2014. During the refuel, evaluations of the switch compartment were performed. The evaluations confirmed that 1 0 revolutions of the bus bar termination screw results in removal from the bus bar connection. With this bus termination screw removed, slight agitation of the back of the switch resulted in complete interruption of circuit continuity. Inspection of the bus bar with the switch subsequently removed confirmed that the threads in the bus bar were not stripped.

It was thus concluded that insufficient thread engagement at this mating surface resulted in a configuration that was not seismically qualified. This configuration was determined to have most likely been established when the disconnect switch was last replaced in 2008.

1. FACILITY NAME

2. DOCKET Callaway Plant Unit 1 05000483 YEAR 2014

6. LER NUMBER I

SEQUENTIAL 1 NUMBER 003 REV NO.

02 Affected Components and Associated Technical Specifications

3. PAGE 5

OF 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 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 SB03 8 loads must be considered inoperable.

As noted previously, the degraded electrical termination condition was determined to have most likely been established when the disconnect switch was last replaced in 2008. Based on the last time when work was performed on the disconnect switch, it may be presumed 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, ESFAS, 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 (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 />.

The degraded terminal connection condition was determined to have most likely been established when the disconnect switch was last replaced in 2008. The resultant "past" inoperability of the various affected instruments/components identified above must be presumed to have existed for that length of time.

4.

ASSESSMENT OF SAFETY CONSEQUENCES

Although the identified condition affected numerous loads, including the NN11 inverter and the various instrument channels associated with the R TS, ESF AS, P AMS, 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

6. LER NUMBER I

SEQUENTIAL I NUMBER 003 REV NO.

02

3. PAGE 7

OF 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 SB03 8 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 SB03 8 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.

5.

REPORTING REQUIREMENTS

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 out-of-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 presumed to exist since 2008, the resultant "past" inoperability of the various affected instruments/components (including the NN11 inverter itself) must be presumed to have existed for a period of time 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 direct cause of the event/condition is attributed to inadequate thread engagement of the screw that secures disconnect switch NN01-11 to the NN01 bus. This loose termination contributed to an interruption in power to the circuit load (SB03 8) when personnel accessed this disconnect switch compartment for thermography inspections and clamp-on ammeter readings on 06/09/14.

7.

CORRECTIVE ACTIONS

Corrective actions include enhancements to job planning aids. Specifically, location planning notes will be added to specify that verification of proper alignment and thread engagement of the screw securing the switch to the bus bar is performed.

In addition, the bus maintenance procedure will be revised to include back panel verification for proper switch alignment I thread engagement for bus bar termination screws that have been disturbed during bus cleaning I inspection activities.

8.

PREVIOUS SIMILAR EVENTS

Although there have been previous instances of inverters spuriously transferring to their alternate source at Callaway, there have not been previous cases of inverter failure at Callaway that were found to have been caused by a degraded (inadequate thread engagement) bus mounting screw. 8