05000318/LER-2023-004, Automatic Reactor Trip from Reactor Protection System Actuation Due to Loss of Unit Service Transformer
| ML24016A205 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 01/16/2024 |
| From: | Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24016A203 | List: |
| References | |
| LER 2023-004-00 | |
| Download: ML24016A205 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3182023004R00 - NRC Website | |
text
Abstract
On November 16, 2023, the Unit 2 reactor automatically tripped due to a Reactor Protection System actuation based on an undervoltage condition. Specifically, the U-4000-22 Unit Service Transformer was lost due to a ground fault signal which resulted in the de-energization of the 22, 23, and 24 4KV buses. This resulted in a loss of both Control Element Drive Mechanism Motor Generator Sets, causing the reactor trip bus undervoltage condition.
The Main Feedwater pumps tripped; therefore, Auxiliary Feedwater was manually initiated to supply both Steam Generators. The 28 Emergency Diesel Generator automatically started and restored its bus. Heat removal remained via the normal turbine bypass valves to the main condenser. The cause of the U-4000-22 Unit Service Transformer ground fault signal occurrence is still under investigation at this time; however, all components associated with the suspected ground fault circuitry were replaced prior to placing the transformer back into service.
Independent digital relays monitoring the affected transformer and associated Voltage Regulator (2H2103REG) at the time of the event and subsequent equipment testing showed no indication of an actual ground fault condition on the 13KV system at the time.
PLANT AND SYSTEM IDENTIFICATION
- 2. DOCKET NUMBER YEAR 05000318 2023 -
- 3. LER NUMBER SEQUENTIAL NUMBER 004 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 [EA].
A. CONDITION PRIOR TO EVENT
Unit: 2 Date: November 16, 2023 Power level: 100 Mode: Unit 2 was in Mode 1 when the condition was discovered.
B. DESCRIPTION OF EVENT
At 0227 on 11/16/2023, the Unit 2 reactor automatically tripped from a Reactor Protection System (RPS) actuation based on a reactor trip bus undervoltage (UV) condition. At that time, a loss of the U-4000-22 Unit Service Transformer (UST) caused a loss of 22, 23, and 24 4KV buses. This resulted in a loss of both Control Element Drive Mechanism (CEDM) Motor Generator (MG) Sets, causing the reactor trip bus undervoltage condition. The loss of 22 and 23 4KV non-safety related (NSR) buses resulted in a trip of Main Feedwater.
Auxiliary Feedwater was manually initiated to feed both Steam Generators. The 28 Emergency Diesel Generator (EOG) automatically started and restored the 24 4KV safety related (SR) bus. Heat removal remained via the normal turbine bypass valves to the main condenser.
C. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES
November 16, 2023 0227: Unit 2 reactor automatically tripped (scrammed) on reactor trip bus undervoltage due to a loss of U-4000-22 UST. The loss of U-4000-22 UST was caused by its feeder breaker tripping open. The loss of U-4000-22 UST resulted in the following effects:
Loss of 4KV NSR buses 22 and 23, and 4KV SR bus 24 o
Loss of 22 and 23 4KV buses de-energized the CEDM MG Sets Loss of CEDM MG sets de-energized all control element assemblies (CEAs) which resulted in the CEAs inserting into the Unit 2 reactor core (SCRAM) o Loss of 22 and 23 4KV buses resulted in a trip of Condensate pumps, Condensate Booster pumps and Main Feedwater supplying the Steam Generators. Operations manually initiated Auxiliary Feedwater (AFW) to maintain Steam Generator levels. Heat removal remained through the Turbine Bypass valves to the Main Condenser.
o Loss of 24 4KV bus automatically started the 28 EOG which loaded onto the 24 4KV SR bus and repowered the bus as designed based on a valid UV signal from ESFAS.
0330: Power was restored to the 22 and 23 4KV buses via their alternate feeder breakers.
0341 : 24 4KV bus was powered from its alternate feeder breaker and the 28 EOG was separated from the bus and secured in accordance with normal operating procedures.
D. CAUSE OF EVENT
- 2. DOCKET NUMBER YEAR 05000318 2023 -
- 3. LER NUMBER SEQUENTIAL NUMBER 004 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 04718086. The Unit 2 reactor automatically tripped (scrammed) on Reactor Trip Bus Undervoltage due to a loss of the U-4000-22 UST. The root cause evaluation for the loss of the U-4000-22 UST due to a ground fault signal is still in progress at the time of the initial LER submittal and a supplement to the LER will be submitted following the completion of the associated CAP product. The ground fault signal was detected by the U-4000-22 UST's protective relaying, tripping the high-side feeder breaker to the Voltage Regulator and the U-4000-22 UST, along with its associated low-side 4KV breakers. Independent digital relays monitoring the affected transformer and associated Voltage Regulator (2H2103REG) at the time of the event and subsequent equipment testing showed no indication of an actual ground fault condition on the 13KV system at the time.
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 10CFR50.73, paragraph (a)(2)(iv)(B).
Specifically, for this event, the Reactor Protection System, the 28 Emergency Diesel Generator, and the Auxiliary Feedwater System. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(iv)(A). There were no safety consequences as a result of the event. All safety systems functioned and operated as designed.
F. CORRECTIVE ACTIONS
The ground fault protection circuit, currently suspected of being the source of the failure that led to the loss of U-4000-22, was installed with the U-4000-22 Voltage Regulator (2H2103REG) in the mid-1990's. As part of the troubleshooting activities in support of the automatic trip due to the loss of U-4000-22, the site replaced all components in the suspected ground fault protection circuit, including the ground sensor, ground sensor relay, ground relay timer, lockout relay, and accessible wiring from the ground sensor to the ground sensor relay. The site has also temporarily installed high speed recorders to monitor ground fault protection relays, associated contacts, and ground fault current transformer (CT) outputs; the recorders are providing a direct feed of monitored parameters to the Control Room. As part of the continuing root cause evaluation, an inspection plan is being developed to execute the timely inspection of the ground fault protection circuitry associated with the site's other applicable USTs.
G. PREVIOUS OCCURRENCES
A review of Calvert Cliffs' events was performed. A similar event occurred on Unit 2 on November 7, 2023 as reported in LER 318-2023-002. Although the elapsed time between the November 7, 2023 and November 16, 2023 loss of U-4000-22 events was only nine days, the respective investigations of the two events to this point have identified separate initiating events that led to each of the transformer losses. The on-going root cause evaluation for the November 16, 2023 event will be utilized to identify any commonality or tie between the events.
H. COMPONENT FAILURE DATA
Component IEEE 803
- 2. DOCKET NUMBER
- 3. LER NUMBER SEQUENTIAL REV YEAR NUMBER NO.
05000318 2023 -
004 00 IEEE805 FUNCTION ID SYSTEM ID Voltage Regulator Ground Protection Circuit XCT EA Page _4_ of _4_