05000391/LER-2021-001, Automatic Reactor Trip on Main Turbine Trip Caused by Main Feed Pump Trip Due to Low Condenser Vacuum
| ML21130A027 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 05/10/2021 |
| From: | Anthony Williams Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| WBL-21-017 LER 2021-001-00 | |
| Download: ML21130A027 (8) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3912021001R00 - NRC Website | |
text
1\\'A TENNESSEE VALLEY AUTHORITY Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 WBL-21-017 May 10, 2021 ATTN : Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 2 Facility Operating License No. NPF-96 NRC Docket No. 50-391 10 CFR 50.73
Subject:
Licensee Event Report 391/2021-001-00, Automatic Reactor Trip on Main Turbine Trip caused by Main Feed Pump Trip due to Low Condenser Vacuum This submittal provides Licensee Event Report (LER) 391/2021-001-00. This LER provides details concerning a recent event where the operating main feedwater pumps tripped on low condenser vacuum, resulting in a turbine and subsequent reactor trip. This condition is being reported as a safety system actuation of the reactor protection system and the auxiliary feedwater system in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(iv)(A).
There are no new regulatory commitments contained in this letter. Please direct any questions concerning this matter to Tony Brown, WBN Licensing Manager, at (423) 365-7720.
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nthony L. Williams IV Site Vice President Watts Bar Nuclear Plant
U.S. Nuclear Regulatory Commission WBL-21-017 Page 2 May 10, 2021
Enclosure:
LER 391/2021-001-00, Automatic Reactor Trip on Main Turbine Trip caused by Main Feed Pump Trip due to Low Condenser Vacuum cc (w/Enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Region II
ENCLOSURE Tennessee Valley Authority Watts Bar Nuclear Plant Unit 2 LER 391/2021-001-00, Automatic Reactor Trip on Main Turbine Trip caused by Main Feed Pump Trip due to Low Condenser Vacuum
Abstract
On March 17, 2021, at 1004 Eastern Daylight Time (EDT), the Watts Bar Nuclear Plant (WBN) Unit 2 experienced an automatic reactor trip on a main turbine trip due to a main feed pump trip on low condenser vacuum. Concurrent with the reactor trip, the Auxiliary Feedwater system actuated as designed. All control and shutdown rods inserted properly and all safety systems responded as designed.
The direct cause of the event was failure of a close latch to maintain an Alternate Supply Breaker open after installation in the Unit 2C Board. The Alternate Supply Breaker closed causing the Normal Supply Breaker to open, de-energizing the Unit 2C Board. The 2C Condenser Circulating Water (CCW) Pump lost power causing a reduction of CCW flow and subsequent main feed pump trip on low condenser vacuum. Corrective actions include incorporating vendor enhancements to the breaker maintenance procedures.
This condition is being reported as a safety system actuation in accordance with 10 CFR 50.73(a)(2)(iv)(A).
I.
Plant Operating Conditions Before the Event
Watts Bar Nuclear Plant (WBN) Unit 2 was at 90 percent Rated Thermal Power (RTP). Unit 1 was unaffected by this event.
II.
Description of Event
A. Event Summary
On March 17, 2021, during the performance of a breaker swap Work Order, the Alternate Supply Breaker [EIIS:BKR] on the Unit 2C Board [EIIS:EA] inadvertently closed and tripped open upon installation. The Normal Supply Breaker opened as designed when the Alternate Supply Breaker closed. This caused a loss of power to the non-safety related Unit 2C Board and subsequent de-energization of the 2C Condenser Circulating Water (CCW) pump [EIIS:P] and reduction of CCW flow to the plant.
The lower CCW flow caused condenser vacuum [EIIS:SH] to lower and resulted in a reactor trip on a main turbine trip due to Main Feed Pump (MFP) trip on low condenser vacuum.
Concurrent with the reactor trip, all control and shutdown rods fully inserted, the Auxiliary Feedwater (AWF) System [EIIS:BA] actuated as designed, and all safety systems responded as designed. There were no complications associated with the reactor trip.
This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(iv)(A) as a safety system actuation of the Reactor Protection System (RPS) and the AFW system.
B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event There were no safety related inoperable structures, components, or systems that contributed to this event.
C. Dates and approximate times (Eastern Daylight Time [EDT]) of occurrences Date Time (EDT)
Event 3/17/2021 0957 The 2C Board de-energized and 2C CCW pump lost power lowering CCW flow to the main condenser.
3/17/2021 0959 Operators started lowering reactor power in accordance with 2-AOI-39, Rapid Load Reduction in response to lowering vacuum in the main condenser.
3/17/2021 1002 Remaining condensate booster pumps tripped.
3/17/2021 1004 Automatic Main Turbine and Reactor trip occurred, Operators entered 2-E-0, Reactor Trip or Safety Injection.
3/17/2021 1048 Transitioned to 2-GO-5, Unit Shutdown from 30 percent Reactor Power to Hot Standby.
Page 3 of 5 (08-2020)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
- 3. LER NUMBER Watts Bar Nuclear Plant, Unit 2 05000391 YEAR SEQUENTIAL NUMBER REV NO.
2021
- - 001
- - 00
D. Manufacturer and model number of each component that failed during the event
General Electric - AM 7.2-500-6HB, - 6.9KV - 2000 Amp Magne-Blast Circuit Breaker, Manufacture Date 5/27/1996.
E. Other systems or secondary functions affected
No other systems or secondary functions were affected.
F. Method of discovery of each component or system failure or procedural error
The component failure became evident when the Alternate Supply Breaker inadvertently closed after installation into the Unit 2C Board.
G. Failure mode, mechanism, and effect of each failed component Upon racking in, the 2C Alternate Supply Breaker charging motor energizes to compress the breaker springs. The close latch maintains the springs in a charged state with the breaker open. In this case, the close latch did not properly engage with the charging spring cam roller to maintain the springs charged. When the engagement of the closed latch was not obtained, the springs discharged, resulting in a mechanical closure of the breaker (no electrical signal).
H. Operator actions
Operators reduced reactor power in response to lowering vacuum in the main condenser.
I.
Automatically and manually initiated safety system responses
The Main Feed Pumps tripped on low condenser vacuum, which resulted in an automatic Turbine and Reactor trip, and subsequent actuation of the AFW system.
III.
Cause of the Event
A. Cause of each component or system failure or personnel error
The most probable cause of the unintentional closure of the Unit 2C Board Alternate Supply Breaker was the inconsistent return of the close latch to the reset position as confirmed by the closing latch adjusting screw not in contact with the frame when the closing latch rotated to reset position (observation of breaker in shop during post event inspection). This was most likely caused by internal friction on the close latch shaft and bearings and could be further compounded by degraded spring tension on the latch.
This failure had no effect on any safety systems or functions.
Page 4 of 5 (08-2020)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
- 3. LER NUMBER Watts Bar Nuclear Plant, Unit 2 05000391 YEAR SEQUENTIAL NUMBER REV NO.
2021
- - 001
- - 00
No human performance root causes were identified for this event.
IV.
Analysis of the Event
At the time of the event, the Unit 2 CCW system was operating at reduced margin with the 2D CCW pump out of service for repairs. The loss of the additional CCW pump caused a pressure transient in the plant condenser systems. Operators in the control room responded to the event and began lowering reactor power to stabilize the plant. The MFP condenser vacuum lowered resulting in a MFP trip which led to an automatic Turbine and Reactor trip from approximately 90 percent RTP.
V.
Assessment of Safety Consequences
This event closely matches and is bounded by the Loss of Normal Feedwater event described in the Updated Final Safety Analysis Report (UFSAR). A probablistic risk review of this event shows the risk from this trip is very small.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event Not applicable.
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 Not applicable.
VI.
Corrective Actions
This event was entered into the Tennessee Valley Authoritys (TVA) Corrective Action Program and is being tracked under Condition Report 1679456.
A. Immediate Corrective Actions
The faulty breaker was removed from the Unit 2C Board and quarantined. As an intermediate compensatory measure, breaker swaps on Normal and Alternate Supply Breakers were put on hold pending an online risk evaluation.
Page 5 of 5 (08-2020)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
- 3. LER NUMBER Watts Bar Nuclear Plant, Unit 2 05000391 YEAR SEQUENTIAL NUMBER REV NO.
2021
- - 001
- - 00 B. Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future The overhaul procedure will be enhanced to include replacing specific internal components based on internal vendor operating experience. In addition, a step will be added to the Close Latch Reset check which will simulate breaker installation to verify proper close latch alignment.
VII.
Previous Similar Events at the Same Site
LER 391/2016-005-00 describes a Unit 2 reactor trip that occurred on June 20, 2016, as a result of a MFP trip on low condenser vacuum. This specific event was caused by a human performance error.
LER 391/2020-005-00 describes an event where a normal feeder breaker failed to close during a Shutdown Board transfer. This failure was due to a faulty switch contact internal to the breaker.
No similar inadvertent breaker closure events are known.
VIII.
Additional Information
There is no additional information.
IX.
Commitments
There are no new commitments.