05000318/LER-2024-003-01, Automatic Reactor Trip Due to Main Turbine Loss of Load
| ML25083A223 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 03/21/2024 |
| From: | Constellation Energy Group |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25083A221 | List: |
| References | |
| LER 2024-003-01 | |
| Download: ML25083A223 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| 3182024003R01 - NRC Website | |
text
Abstract
On October 10, 2024, at 0557, 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.
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 analysis for this event determined that the Troubleshooting team/personnel for the July 18, 2024 trip failed to establish appropriate bridging, mitigating, and monitoring parameters in accordance with the "Conduct of Troubleshooting" procedure to adequately manage risk until the planned 2025 refueling outage when the elimination strategy was scheduled to be implemented.
PLANT AND SYSTEM IDENTIFICATION
- 2. DOCKET NUMBER YEAR 05000318 2024 -
- 3. LER NUMBER SEQUENTIAL NUMBER 003 REV NO.
01 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: October 10, 2024 Power level: 100 percent Mode: Unit 2 was in Mode 1 when the event occurred.
B. DESCRIPTION OF EVENT
At 0557 on 10/10/2024, the Control Room received multiple alarms. Following the multiple alarms Unit 2 reactor automatically tripped from 100 percent power. The Unit 2 Control Room staff implemented immediate post trip actions and successfully responded to the 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 as being the direct cause for a loss of field / loss of excitation on the Main Generator which led to a Main Generator lockout and 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
October 10, 2024 05:56:59: Generator lockout/ loss of field. Control Room received multiple alarms, notably 1 C01 alarm "A-11" Main Gen *Loss of Field *Out of Step.
05:56:59: U-2 Loss of Load Trip.
05:57:00: All Unit 2 trip circuit breakers (TCB) verified Open.
05:57:00: EOP-0 entered 06:07:00: EOP-0 exited with all safety functions checked SAT 08:45:00: The Shift Manager completed Event Notification#57372 to satisfy the reporting requirements of 10 CFR 50. 72(b)(2)(iv)(B) for the reactor protection system actuation.
D. CAUSE OF EVENT
- 2. DOCKET NUMBER YEAR 05000318 2024
- 3. LER NUMBER SEQUENTIAL NUMBER 003 REV NO.
01 The method of discovery for this event was self-revealing and is documented in the site's Corrective Action Program (CAP) under Issue Report (IR) 04808192. The cause of the Unit 2 automatic trip was the loss of an Exciter stationary field pole electrical connection resulting in a complete loss of excitation on the Main Generator.
The root cause investigation for the Unit 2 main turbine loss of load trip due to the failure of the exciter electrical connection identified one root cause and one contributing cause associated with this event:
Root Cause(s): Troubleshooting team/personnel (war room, Outage Control Center, Operations, Maintenance, Engineering) for July 18, 2024 trip failed to establish appropriate bridging, mitigating, and monitoring parameters in accordance with MA-AA-716-004 "Conduct of Troubleshooting" to adequately manage risk until the planned 2025 refueling outage when the elimination strategy was scheduled to be implemented.
Contributing Cause(s): Inadequate use of technical human performance fundamentals by station personnel to sufficiently challenge vendor inputs, ensure data-based decision making and intolerance to long-standing degraded conditions.
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 10 CFR 50. 73, paragraph (a)(2)(iv)(B).
Specifically, for this event, the RPS actuated. Therefore, this event is reportable pursuant to 10 CFR
- 50. 73(a)(2)(iv)(A).
The automatic trip was uncomplicated and there were no safety consequences as a result of the event. All safety systems functioned and operated as designed.
No SSCs that were inoperable at the start of the event and contributed to the event and the failure mode, mechanism, and effect of each failed component is known.
F. CORRECTIVE ACTIONS
During the forced outage following the October 10, 2024, Unit 2 automatic trip, the following immediate actions were taken prior to restarting the unit:
The Exciter stationary field pole that experienced the failed connection was replaced.
A "glass block" modification on the pole jumper was installed to increase strength and resistance to cycle fatigue.
A balance shot was installed to reduce overall vibration of the exciter.
A monitoring plan was initiated for exciter performance until the root cause was identified and corrected.
- 2. DOCKET NUMBER
- 3. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO.
05000318 2024 -
003 01 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 10/13/24.
The root cause investigation identified the following key corrective actions to address the identified root and contributing causes of the event:
Corrective Action to Preclude Repetition (CAPR): Improve station utilization of Troubleshooting and Operational Decision-Making Processes, consistent with MA-AA-716-004 and OP-AA-106-101-1006, such that all potential failure mechanisms are identified, evaluated, and dispositioned.
CAPR: The troubleshooting procedure will be revised to require clear guidance for monitoring and mitigation actions post trip Corrective Action (CA): Provide a read and sign document for standards and expectations regarding the utilization of Troubleshooting an Operational Decision-Making processes to identify, evaluate, and disposition all potential failure mechanisms.
CA: Implement training on a recurring basis. The training will include an overview of the lessons learned from the events described in the root cause analysis under IR 04797040 with a focus on enforcement of thi Management Model processes and the requirements for the conduct of troubleshooting. The training will bE developed for individual contributors, leaders, and include a dynamic learning activity that involves multiple levels of personnel.
G. PREVIOUS OCCURRENCES
A review of Calvert Cliffs' events was performed. A reactor trip due to a failure of a Main Generator Exciter occurred on July 18, 2024, and was reported under LER-2024-002-00 and supplemented by LER-2024-002-01.
There were no other previous occurrences identified.
H. COMPONENT FAILURE DATA
Component Exciter IEEE 803 FUNCTION ID EXC IEEE805 SYSTEM ID TL Page_4_ of _4_