05000382/LER-2024-002, Automatic Reactor Trip Due to Transformer Failure

Automatic Reactor Trip Due to Transformer Failure

ML24137A335
Person / Time
Site:	Waterford
Issue date:	05/16/2024
From:	Twarog J Entergy Operations
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
W3F1 -2024-0023 LER 2024-002-00
Download: ML24137A335 (1)

LER-2024-002, Automatic Reactor Trip Due to Transformer Failure

Event date:
Report date:
3822024002R00 - NRC Website

text

S) entergy John Twarog Manager Regulatory Assurance 504-739-67 4 7

W3F1 -2024-0023 10 CFR 50.73

May 16, 2024

ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject: Licensee Event Report 50-382/2024-002-00, Automatic Reactor Trip Due to Transformer Failure Waterford Steam Electric Station, Unit 3 NRC Docket No. 50-382 Renewed Facility Operating License No. NPF-38

Entergy Operations, Inc. (Entergy) submits the enclosed Licensee Event Report (LER) 50-382/2024-002-00 for Waterford Steam Electric Station, Unit 3 (Waterford 3). The events reported herein are reportable in accordance with 1 O CFR 50.73(a)(2)(iv)(A), Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B); Reactor protection system (RPS) including: reactor scram or reactor trip, general containment isolation signals, emergency core cooling systems (ECCS),

emergency feedwater system and emergency ac electrical power systems.

The LER describes the Main Transformer 'B' fire, reactor trip, loss of off-site power to the 'B' train components, and the automatic actuations of Safety Injection Actuation Signal (SIAS),

Containment Isolation Actuation Signal (CIAS) and Emergency Feedwater Actuation Signal (EFAS).

This letter contains no new commitments.

Should you have any questions concerning this issue, please contact me at 504-739-6747.

Respectfully,

John Twarog

JRT/ahv

Entergy Operations, Inc., 17265 River Road, Killona, LA 70057 W3F1 -2024-0023 Page 2 of 2

Enclosure: Licensee Event Report 50-382/2024-002-00

cc: NRG Region IV Regional Administrator NRG Senior Resident Inspector - Waterford Steam Electric Station, Unit 3 NRG Project Manager-Waterford Steam Electric Station, Unit 3 Louisiana Department of Environmental Quality Enclosure

W3F1-2024-0023

Licensee Event Report 50-382/2024-002-00

Abstract

At 2328 CDT on March 21, 2024, with Waterford Steam Electric Station, Unit 3 (WF3), operating in Mode 1 at 98% power, Main Transformer B (MT-B} experienced a failure that resulted in a fire and automatic reactor trip. The MT-B failure resulted in a loss of offsite power to the 'B' train components. After the reactor trip, a safety injection actuation signal was received due to Reactor Coolant System cooldown (RCS). RCS pressure did not degrade to the point of injection.

The direct cause of MT-B failure has been determined to be a failure of a high voltage bushing. Confirmation of the failure mode is still in progress with an external vendor. The causal investigation identified gaps in implementing industry accepted preventative maintenance and testing strategies to detect early failure of high voltage bushings.

Corrective actions include replacement of the MT-B, repairs to the 'B ' Startup Transformer, and to the MT-B deluge system.

This event is being reported in accordance with 10 CFR 50. 73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B).

Event Description

At 2328 CDT on March 21, 2024, with Waterford Steam Electric Station, Unit 3 (WF3), operating in Mode 1 at 98% power, a Main Transformer B (MT-B) [XFMR:EL] failure resulted in a fire and automatic reactor trip. The MT-B fire caused extensive damage to the Startup Transformer 'B' (SUT-B} [XFMR:EA] preventing a transfer of the 'B' train components from the 'B' Unit Auxiliary Transformer [XFMR:EA]. The loss of offsite power to the 'B' train components resulted in the 'B' safety-related bus being powered by 'B' Emergency Diesel Generator (EDG) [DG:EK].

Upon notification of MT-B fire, Operations dispatched the fire brigade and a notification was made to the local fire station for additional assistance. The Emergency Director's request for additional assistance was based on information available at the time. The fire brigade was able to put out the transformer fire in 41 minutes without the assistance of the local fire station. As a result of the request for additional assistance, an Unusual Event (HU4.4) was declared at 2337 CDT (Event Number57042). The Unusual Event was later retracted by the site on March 26, 2024.

Following the reactor trip, the steam generator (SG) feedwater control system (FWCS) [JB] experienced a level deviation resulting in all feedwater regulating valves going to manual control which blocks the valves automatic response to a reactor trip. The control room operators had to manually perform the reactor trip override function that closes the feedwater regulating valves to a lower flow position. Prior to taking the manual action to lower feedwater flow, a Reactor Coolant System (RCS) cooldown occurred, due to the high feedwater flow, and RCS pressure lowered to less than 1684 psia resulting in a safety injection actuation signal (SIAS) and a containment isolation actuation signal (CIAS). RCS pressure recovered prior to the point of injection from the High-Pressure Safety Injection (HPSI) pumps. Emergency feedwater actuation signal 2 (EFAS) was also received on the reactor trip due to the level shrink in the steam generators.

This event was reported on March 22, 2024, at 0402 EDT in accordance with 10 CFR 50.72(b)(2)(iv)(B) for Reactor Protection System actuation and 10 CFR 50.72(b)(3)(iv)(A) as a valid actuation of the Emergency Feedwater System, Emergency Core Cooling System, Containment Isolation and EDGs (EN 57042).

Event Cause

The direct cause of the ASEA Brown Boveri (ABB) MT-B failure was a result of the H1 high voltage bushing short to ground. The cause is currently being confirmed by an external vendor. If vendor failure analysis determines a different direct cause a supplement will be provided. The causal investigation identified gaps in routine maintenance practices for large transformers, incorporation of Operating Experience, and upholding standards for routine maintenance.

Waterford did not adequately implement the preventative maintenance program for large transformers as recommended by the Institute of Electrical and Electronics Engineers and Electric Power Research Institute. ME-004-051, "Main Transformer B," did not provide adequate guidance on which testing needed to be performed. The procedure also lacked information on trending data on the power factor and capacitance acceptance criteria. The investigation also determined that transformer yard corona scans were performed at an inadequate frequency.

The site failed to incorporate actions from SOER 10-1, "Large Power Transformer Reliability and IER 21-04, "Improving Plant Reliability." These actions include failing to incorporate step or rate of change limits, testing on winding resistance, testing on core ground and lightning arrestors, and high standards in vendor and supplemental oversight. Additionally, internal OE was not properly considered and incorporated into preventative maintenance strategies.

The risk associated with critical components was improperly managed by the site. MT-B was categorized as a Single Point Vulnerability (SPV), as a SPV component a mitigation strategy was developed that included maintenance and testing each refueling outage. The mitigation strategy did not include a sufficient preventative maintenance scope and acceptance criteria. The strategy did not include replacement of key components such as bushings, lighting arrestors, CTs, and other components that would prolong the life of the transformer.

Safety Assessment

The actual consequence of the high voltage bushing failure on the MT-B resulted in a complete loss of MT-B, failure of SUT-B resulting in a partial loss of offsite power, and 'B ' safety related bus being powered by the 'B' EDG. An automatic SIAS was received due to the cooldown of the RCS. The RCS cooldown was arrested with RCS pressure above the shutoff head of the HPSI pumps therefore no safety injection flow to the reactor occurred. There were no actual consequences to general safety of the public, nuclear safety, industrial safety, and radiological safety for this event.

Corrective Actions

(1) The site will replace the failed MT-B with a temporary transformer. The transformer is scheduled to be operational by the end of May.

(2) Repairs were made to SUT-B lightning arrestors, HV bushings, open phase detection current transformers, Calvert bus raceway cover, wiring, and other miscellaneous components.

(3) Procedures related to large transformers will be revised to ensure the transformers are adequately maintained.

(4) Create Preventive Maintenance tasks to replace large power transformers and sub-components at frequency determined by industry best practices.

(5) Repairs to the MT-B deluge system solenoid, pressure switches, corroded conduit and supply header.

(6) Corona Testing frequency will be revised to be performed every 6 months and after startup from a outage (refuel or forced).

Previous Similar Occurrences

No events were identified that were similar to the event described in this LER