05000249/LER-2021-001-01, Reactor Scram Due to Main Power Transformer Failure

Reactor Scram Due to Main Power Transformer Failure

ML22165A204
Person / Time
Site:	Dresden
Issue date:	06/14/2022
From:	Patrick Boyle Constellation Energy Company
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
SVPLTR 22-0021 LER 2021-001-01
Download: ML22165A204 (4)

LER-2021-001, Reactor Scram Due to Main Power Transformer Failure

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation
2492021001R01 - NRC Website

text

Constellation June 14, 2022 SVPLTR # 22-0021 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Dresden Nuclear Power Station, Unit 3 Renewed Facility Operating License No. DPR-25 NRC Docket No. 50-249 Dresden Nuclear Power Station 6500 North Dresden Road Moms. IL 60450 10 CFR 50.73

Subject:

Licensee Event Report 249/2021-001-01, Reactor Scram due to Main Power Transformer Failure Enclosed is Licensee Event Report 249/2021-001-01, Reactor Scram due to Main Power Transformer Failure.

There are no regulatory commitments contained in this submittal.

Should you have any questions concerning this letter, please contact Mr. Ken Mack, Regulatory Assurance Manager, at (815) 416-2800.

Respectfully,

/!~I~

Site Vice President Dresden Nuclear Power Station

Enclosure:

Licensee Event Report 249/2021 -001-01 CC:

Regional Administrator - NRC Region Ill NRC Senior Resident Inspector - Dresden Nuclear Power Station

Abstract

On October 16, 2021, at 0428 CDT, an automatic reactor protection system actuation occurred due to failure of the main power transformer. The safety significance of this event was minimal. All control rods inserted to their full-in position. Following the reactor scram, all systems operated as expected.

The cause of this event was due to a sudden, unanticipated, catastrophic failure of the main power transformer high voltage oil impregnated paper bushing.

Corrective actions included replacement of the failed main power transformer and restricting use and/or procurement of the specific type of bushing that failed.

This event is being reported in accordance with 10 CFR 50. 73(a)(2)(iv)(A), an event that resulted in automatic actuation of a system listed in paragraph 10 CFR 50. 73(a)(2)(iv)(B), specifically the reactor protection system including reactor scram.

PLANT AND SYSTEM IDENTIFICATION

General Electric - Boiling Water Reactor, 2957 megawatts thermal rated core power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

A. CONDITIONS PRIOR TO EVENT

Unit: 3 Reactor Mode: 1 Event Date: October 16, 2021 Mode Name: Power Operation

B. DESCRIPTION OF EVENT

Event Time: 0428 CDT Power Level: 100%

SEQUENTIAL NUMBER 001 REV NO.

01 On October 16, 2021, at 0428 CDT, an automatic reactor protection system [JC] actuation occurred due to failure of the main power transformer (MPT) [EL]. All control rods [AA] inserted to their full-in position.

Following the reactor scram, all systems operated as expected.

The MPT experienced a phase-to-ground fault. There is no evidence of a plant event or grid event as the initiator. There were no alarms indicative of a transformer failure as a precursor to the failure.

Additionally, a dissolved gas analysis of the transformer oil sample performed two hours prior to the event revealed no alert nor degrading trend.

The MPT "A" phase high voltage bushing, lightning arrester, and control cabinet suffered catastrophic damage. Ancillary components and equipment of the transformer suffered various degrees of damage.

As a result of a fire associated with the MPT failure, an Unusual Event was declared due to a fire in the protected area not extinguished in less than 60 minutes. The Unusual Event was terminated at 0709 CDT.

The transformer was replaced with a transformer of a different design and a different manufacturer.

This event is being reported in accordance with 10 CFR 50. 73(a)(2)(iv)(A), an event that resulted in automatic actuation of a system listed in paragraph 10 CFR 50.73(a)(2)(iv)(B), specifically the reactor protection system including reactor scram.

C. CAUSE OF EVENT

The cause of the event was due to a sudden, unanticipated, catastrophic failure of the MPT high voltage oil impregnated paper bushing. This conclusion is based on the failure history associated with Trench Condenser Oil Transformer ANSI High Voltage Oil-Impregnated Paper (COTA HV OIP) bushings, the physical characteristics of the catastrophic failure, and the various analyses performed.

D. SAFETY ANALYSIS

SEQUENTIAL NUMBER 001 REV NO.

01 The safety significance of this event was determined to be minimal based upon the availability of the required systems to

1) Shutdown the reactor and maintain it in a safe shutdown condition,

2) Remove residual heat,

3) Control the release of radioactive material, or

4) Mitigate the consequences of an accident.

There were no safety consequences impacting plant or public safety as a result of this event. There was no loss of safety function for this event.

E. CORRECTIVE ACTIONS

Corrective actions included replacement of the failed MPT with a transformer of a different design and from a different manufacturer. Additionally, Trench COTA HV OIP bushings, the type of bushing that failed, are identified as do not use and/or do not procure.

Other corrective actions were developed to provide a method of rapid cut-out of control cabinet power supplies for the Main Power Transformers, Unit Auxiliary Transformers, and Reserve Auxiliary Transformers. These corrective actions address fire re-flashing within the transformer control cabinets due to energized circuits faulting.

F. PREVIOUS OCCURRENCES

The most recent transformer failure at Dresden was in April 2014 (LER 05000237/2014-002-02). During that event the Unit 2 MPT failed due to an internal fault with the most probable cause determined to be a combination of insulation issues. The event resulted in the main turbine tripping and a reactor scram.

The transformer was replaced with a transformer of a different design and a different manufacturer.

G. COMPONENT FAILURE DATA

I Device I Manufacturer I Model I S/N I Type I

I Main Power Transformer I Siemens I ELIN I 1731659 I TDQ-A27D9K-99 I Page_3_ of _3_