05000318/LER-2024-002, Automatic Reactor Trip Due to Main Turbine Loss of Load

Automatic Reactor Trip Due to Main Turbine Loss of Load

ML24260A142
Person / Time
Site:	Calvert Cliffs
Issue date:	09/16/2024
From:	Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation
Shared Package
ML24260A140	List: 05000318/LER-2024-002, Automatic Reactor Trip Due to Main Turbine Loss of Load
References
LER 2024-002-00
Download: ML24260A142 (1)

LER-2024-002, Automatic Reactor Trip Due to Main Turbine Loss of Load

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3182024002R00 - NRC Website

Abstract

On July 18, 2024 at 1524, Calvert Cliffs Nuclear Power Plant Unit 2 incurred an uncomplicated automatic reactor trip from 100 percent power due to a Main Turbine Loss of Load trip initiated by the Reactor Protection System (RPS). The Loss of Load trip was the result of an internal Main Generator Exciter failure that led to a loss of field on the Main Generator that in turn led to a load reject trip from the Main Turbine. The direct cause of the Exciter failure was a failed stationary field pole electrical connection. An engineering analysis performed to determine the failure mechanism for the stationary failed field pole electrical connection identified cyclic fatigue as the cause for the failure. Impacted Exciter components were replaced and an extent of condition inspection on the other field pole electrical connections was performed prior to returning the Exciter to service. The root cause evaluation for this event is still in progress at this time.

PLANT AND SYSTEM IDENTIFICATION YEAR 2024 -

SEQUENTIAL NUMBER 002 REV NO.

00 Calvert Cliffs Nuclear Power Plant, Unit 2, is a Combustion Engineering Pressurized Water Reactor with a licensed maximum power level of 2737 megawatts thermal. The Energy Industry Identification System code used in the text is identified as [TL].

A. CONDITION PRIOR TO EVENT Unit: 2 Date: July 18, 2024 Power level: 100 Mode: Unit 2 was in Mode 1 when the event occurred.

B. DESCRIPTION OF EVENT At 1524 on 07/18/2024, the Control Room received an alarm on Unit 2 for Main Generator Loss of Field / Out of Step. In response to receipt of this unexpected alarm, the Unit 2 Control Room Operators identified erratic reactive load swings on both Unit 1 and Unit 2 as indicated on Control Room panel indications. As the Operators were retrieving the associated alarm manual response manual and commencing detailed monitoring activities, the Unit 2 reactor was automatically tripped (scrammed) by RPS due to a Loss of Load actuation.

The Unit 2 Control Room staff implemented immediate post trip actions and successfully responded to an uncomplicated reactor trip.

During troubleshooting activities to determine the cause of the Unit 2 Loss of Load trip, a local inspection of the Unit 2 Exciter identified a failed stationary field pole connection. The failed stationary field pole connection was identified at this time as being the direct cause for a loss of field / loss of excitation on the Main Generator which led to a Main Generator lockout that resulted in a load reject trip of the Main Turbine and the subsequent RPS Loss of Load automatic trip.

C. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES July 18, 2024 15:23:34: Unit 1 and Unit 2 begin to experience reactive load swings as identified on the site's three 500KV high line reactive load trends (identified during post-trip data review).

15:24:43: Generator lockout/ loss of field indication was received on the Unit 2 Plant Computer.

15:24:43: Turbine load reject indication was received on the Unit 2 Plant Computer.

15:24:44: RPS initiates an automatic trip for Unit 2.

18:33: The Shift Manager completes Event Notification#57233 to satisfy the requirements of 1 0CFR50. 72(b)(2)(iv)(B).

D. CAUSE OF EVENT

2. DOCKET NUMBER YEAR 05000318 2024 -

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

00 The method of discovery for this event was self-revealing and is documented in the site's Corrective Action Program (CAP) under IR 04788102. The direct cause cited for the Unit 2 automatic trip is the loss of an Exciter stationary field pole electrical connection resulting in a complete loss of excitation on the Main Generator. The current root cause is identified as cyclic fatigue to the point of failure on the Exciter stationary field pole electrical connection. The cyclic fatigue is attributed to a combination of long-term vibration of the Exciter and long-term out of specification Exciter air temperature.

The root cause evaluation for the Unit 2 main turbine loss of load trip due to the failure of the exciter electrical connection is still in progress at the time of this Licensee Event Report (LER) submittal; therefore, a supplement to the LER will be submitted following completion of the associated CAP product.

E. SAFETY ANALYSIS The subject event satisfies the criteria in NUREG-1022, Revision 3, for any event or condition that resulted in manual or automatic actuation of any of the systems listed in 1 0CFR50. 73, paragraph (a)(2)(iv)(B).

Specifically, for this event, the Reactor Protection System actuated. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(iv)(A). The resultant automatic trip was uncomplicated and there were no safety consequences as a result of the event. All safety systems functioned and operated as designed.

F. CORRECTIVE ACTIONS During the forced outage following the July 18, 2024 Unit 2 automatic trip, the following immediate actions were taken prior to restarting the unit:

Two Exciter stationary field poles were replaced - the stationary field pole that experienced the failed connection and the adjacent stationary field pole that had indications of a partial failure at the corresponding connection location. The two connection locations for the replaced stationary field poles are connected to one another by a jumper (metal bar). The jumper was determined to not be impacted by the failure and was reused.

An extent of condition inspection of the remaining Exciter stationary field poles was performed and no other issues or concerns were identified.

Following completion of the forced outage to effect repairs to the Exciter, Unit 2 was paralleled to the grid on 7/23/24.

As part of the continuing root cause evaluation, corrective actions will be developed as appropriate to address the identified root and contributing causes of the event.

G. PREVIOUS OCCURRENCES

2. DOCKET NUMBER YEAR 05000318 2024 -

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

00 A review of Calvert Cliffs' events was performed. No events were identified from this review that involved a reactor trip due to a failure of a Main Generator Exciter.

H. COMPONENT FAILURE DATA Component Exciter IEEE 803 FUNCTION ID EXC IEEE805 SYSTEM ID TL Page_4_ of _4_