05000390/LER-2017-010

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LER-2017-000, Actuation of Turbine Driven Auxiliary Feedwater Pump Due to Loss of 6.9kV Shutdown Board
Watts Bar Nuclear Plant, Unit 1
Event date: 08-17-2017
Report date: 10-16-2017
Reporting criterion: 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material

10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident

10 CFR 50.73(a)(2)(iv)(A), System Actuation
3902017000R00 - NRC Website
LER 17-010-00 for Watts Bar Nuclear Plant, Unit 1 Regarding Actuation of Turbine Driven Auxiliary Feedwater Pump Due to Loss of 6.9kV Shutdown Board
ML17289B089
Person / Time
Site: Watts Bar Tennessee Valley Authority icon.png
Issue date: 10/10/2017
From: Simmons P
Tennessee Valley Authority
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML17289B089 (8)


comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to 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.

I. PLANT OPERATING CONDITIONS BEFORE THE EVENT

Watts Bar Nuclear Plant (WBN) Unit 1 was at 100 percent rated thermal power (RTP) .

II. DESCRIPTION OF EVENT

A. Event Summary On August 17, 2017, at 1205 Eastern Daylight Time (EDT), the Watts Bar Nuclear Plant (WBN) lost power to the 1B-B 6.9kV Shutdown Board (SDBD){EIIS:SWGR}, which is part of the safety- related medium-voltage power system {EIIS:EB}. The loss of power to this safety related bus resulted in an automatic start of the Unit 1 Turbine Driven Auxiliary Feedwater Pump (TDAFWP){EIIS:P}. The motor driven AFW pumps (MDAFWPs) did not start and were not expected to start for this event. No other system actuations occurred as a result of loss of power to the 1B-B 6.9kV Shutdown Board. Power to the 1B-B SDBD was restored at 1505 EDT on August 17, 2017.

During the loss of power to the 1B-B SDBD, a reduction in containment and control rod drive mechanism cooling occurred. At 1233 EDT, lower containment average temperature exceeded Technical Specification (TS) limits, and TS 3.6.5 Condition A was entered for containment average air temperature not within limits. Lower containment average temperature was restored to within limits at 1525 EDT on August 17, 2017. This is reportable as a potential loss of safety function.

Other than several common Unit TSs having not been met, Unit 2 was not operationally impacted by the loss of power to the 1B-B Shutdown Board and remains in Mode 1 at 100% power.

This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(iv)(A) as a valid safety system actuation and under 10 CFR 50.73(a)(2)(v)(C) and (D) as an event or condition that could have prevented fulfillment of a safety function.

B. Inoperable Structures, Components, or Systems that Contributed to the Event No inoperable equipment contributed to this event.

C. Dates and Approximate Times of Occurrences Events described below occurred on August 17, 2017.

Time Event 1205 Power lost to 1B-B 6.9kV Shutdown Board. Entered multiple TS including 3.3.5 Loss of power to Diesel Generator (DG) start instrumentation, 3.3.6 Containment ventilation isolation instrumentation, 3.5.2 Emergency Core Cooling system (ECCS) Operating for one train inoperable, 3.8.4 DC Sources Operating for one vital DC electrical power subsystem inoperable, 3.8.7 Inverters Operating for one inverter channel inoperable, and 3.8.9 for Distribution Systems Operating for one or more AC electrical power distribution subsystems inoperable. The Unit 1 TDAFWP automatically starts on loss of power to the 1B-B Shutdown Board.

1206 Entered 0-A01-43.02, Loss of U1 Train B Shutdown Boards.

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Time Event 1233 Entered TS 3.6.5 Containment Air Temperature is not met.

1258 Entered TS 3.1.8, Rod Position Indication not met due to more than one Analog Rod Position Indication (ARPI) outside of +/-12 steps from group demand position due to increased containment temperatures. Entered LCO 3.0.3 due to no condition existing for more than one ARPI outside of limits.

1320 Authorized Work Order (WO) 118948922 to Troubleshoot 1B DG Potential Transformer (PT) and Degraded Voltage PT Drawers and Fuses.

1331 Crew briefed for 1-AOI-39, Rapid Load Reduction 1405 Entered TS 3.8.4 DC Sources Operating, Condition B due to expiration of TS 3.8.4 Condition A.

1432 Entered TS 3.7.5 AFW System is not met for Unit 1, one train AFW not operable due to taking the 1B-B MDAFWP to pull-to-lock (PTL) during 1B-B Shutdown board restoration.

1433 Entered TS 3.7.8, Essential Raw Cooling Water (ERCW) Systems is not met. One ERCW train inoperable due to taking E-B and G-B ERCW to PTL for 1B-B Shutdown board restoration.

1505 Re-energized the 1B-B Shutdown Board from offsite power in accordance with 0-A01- 43.02 and 0-S01-211.02. TS 3.3.5, TS 3.3.6, TS 3.8.7, and TS 3.8.9 now met.

1525 TS 3.6.5 Containment Air Temperature is now met.

1542 TS 3.1.8, Rod Position Indication is now met. Exited TS Limiting Condition for Operation (LCO) 3.0.3 due to restoring ARPI to within limits.

1556 Exited TS 3.7.5, AFW system due to restoring 1B-B MDAFWP to operable.

1557 TS 3.5.2, ECCS Operating is now met due to restoring power to B train ECCS components.

1559 TS 3.7.8 ERCW Systems is now met due to restoring E-B and G-B ERCW pumps to Operable status.

1601 Exited 1-AOI-39 1758 Exited 1-A01-43.02 after completion D. Manufacturer and Model Number of Components that Failed During the Event There were no failed components that contributed to this event.

E. Other Systems or Secondary Functions Affected

The loss of the 1B-B Shutdown board resulted in a reduction in containment cooling capability, which led to lower containment average temperature exceeding TS limits.

F. Method of discovery of each Component or System Failure or Procedural Error The control room received multiple alarms due to the loss of the 1B-B 6.9kV SDBD.

G. Failure Mode and Effect of Each Failed Component No actual equipment failures occurred during this event.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to 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.

H. Operator Actions

Operator actions to recover from the loss of power to the 1B-B SDBD are described in Section II.0 of this report.

I. Automatically and Manually Initiated Safety System Responses The loss of power to the 1B-B SDBD led to the start of the TDAFWP.

III. CAUSE OF THE EVENT

A. The cause of each component or system failure or personnel error, if known.

Due to an alignment condition of the EDG 1B PT drawer, an inordinate amount of force was required to be used during drawer closure. This action impacted connections in the protective relay PT drawer immediately above and adjacent to the EDG PT drawer such that it caused the protective relays to sense and respond to the loss of voltage to the 6.9kV SDBD 1B. Since the EDG 1B was out of service due to a component outage, the EDG 1B was not able to start and maintain voltage on the 6.9kV SDBD.

B. The cause(s) and circumstances for each human performance related root cause.

No human performance causes are attributed to this event.

IV. ANALYSIS OF THE EVENT

On August 17, 2017 at 1205 EDT, Unit 1 received multiple alarms due to the loss of the 6.9kV SDBD 1B.

The EDG 1B did not re-energize the 6.9kV SDBD 1B due to being tagged out for maintenance.

Restoration from the EDG 1B component outage was underway and the clearance involving the EDG 1B PT fuses was being worked.

of force was required to close the EDG PT drawer. Panel 7 also contains a PT drawer directly above the EDG 1B PT drawer which is associated with the degraded and loss of voltage relays. The PT drawers contain fuses and stabbed electrical connectors such that proper seating of the drawer is required to complete the electrical circuit. When this extra force was applied to the EDG 1B PT drawer, the protective relay PT drawer directly above received enough vibration to sense an undervoltage/loss of voltage condition. The loss of voltage relays were found tripped and required resetting prior to restoration of the 6.9kV SDBD 1B. The type of jarring force applied to the EDG 1B PT draw was of a different frequency band than that associated with normal seismic events. Normal seismic events consist of a higher more rhythmic frequency which the Shutdown Boards are designed to withstand.

Upon the event of an undervoltage condition, the 6.9kV SDBDs are designed to trip all the board feeder breakers, start the EDG and then tie the board on to the EDG. Each 6.9kV SDBD is supplied by a physically and electrically separate EDG. During this event, the EDG 1B was under a component outage.

The EDG 1B was unable to start due to a clearance in place, and the actuation of the loss of voltage protective relays prevented a transfer to the alternate feeder breaker for the 6.9kV SDBD 1B. The 6.9kV comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to 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.

SDBD 1B normal feeder breaker, alternate feeder breaker, and associated protective relay scheme functioned as designed.

V. ASSESSMENT OF SAFETY CONSEQUENCES

From an accident standpoint, the most significant item was the increase in lower containment average temperature above the accident analysis assumptions (peak average indicated value of 121.3 degrees Fahrenheit). A review of the containment analysis indicates, for the normal 120 degree limit in lower containment, that the limiting pressure for a large break loss of coolant accident (LOCA) is 9.36 psig and limiting temperature is 235 degrees Fahrenheit. The small temperature increase above the lower containment temperature limit is expected to have a small impact that is within design limits. If the conclusion of the final evaluation is different, a supplement to this LER will be submitted.

During the event, TS 3.0.3 was entered for multiple ARPI being outside limits, which is a condition not addressed by TS 3.1.8. TS 3.0.3 was exited in less than three hours, which is well within the limit of seven hours before Mode 3 must be entered. All ARPI were restored to within limits, and no rod movement occurred during this event.

A risk review performed for this event indicated that the increase in core damage probability was less than 1E-6.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The 1A-A Shutdown board remained operable during this event.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident Not applicable.

C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service The 1B-B Shutdown board, and associated components, were without power for about

3 hours
3.472222e-5 days
8.333333e-4 hours
4.960317e-6 weeks
1.1415e-6 months

.

VI. CORRECTIVE ACTIONS

This event was entered into the Tennessee Valley Authority (WA) Corrective Action Program and is being tracked under Condition Report (CR) 1329397.

A. Immediate Corrective Actions

Upon receiving alarms for a loss of shutdown board 1B-B, efforts commenced to manage the event and restore power to the 1B-B shutdown board. Power was restored approximately

3 hours
3.472222e-5 days
8.333333e-4 hours
4.960317e-6 weeks
1.1415e-6 months

after it was lost.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to 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.

3. LER NUMBER

2017 - 00 010 B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future During SDBD maintenance, the loss of voltage relays will be isolated as part of the clearance process to prevent a similar occurrence. Plant procedures associated with maintenance of the PT drawers will be revised to verify smooth opening and closing operation of the drawers.

VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE

On July 15, 2016, WBN Unit 1 reported a loss of voltage to the 1B-B 6.9kV Shutdown board in LER 390/2016-008. On May 17, 2016 while restoring from a plant modification, the feeder breakers for the 6.9kV Shutdown Board 1B-B tripped resulting in a loss of bus voltage. The feeder breakers tripped due to actuation of the loss of voltage relays in the shutdown board protective relay trip logic circuit resulting in separation of offsite power from the 6.9kV Shutdown Board 1 B-B. The cause was attributed to an inadequate process due to the order of AC and DC fuse restoration. While similar, the 2016 event was process related, and the 2017 event is attributed to mechanical vibration causing relay actuation.

VIII. ADDITIONAL INFORMATION

None.

IX. COMMITMENTS

None.