05000277/LER-2024-001, Automatic Reactor Scram Due to an Invalid Generator Lockout
| ML24081A121 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 03/21/2024 |
| From: | Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24081A118 | List: |
| References | |
| CCN: 24-14, ENS 56936 LER 2024-001-00 | |
| Download: ML24081A121 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 2772024001R00 - NRC Website | |
text
Abstract
On 01/29/2024 at 1202 hours0.0139 days <br />0.334 hours <br />0.00199 weeks <br />4.57361e-4 months <br />, Unit 2 experienced a Generator lockout, Turbine trip and automati c Reactor Scram. All control rods inserted, and all safety systems responded as designed. Reactor level was initially maintained by Feedwater and pressure controlled with bypass valves. During the scram response, condenser vacuum degraded due to the loss of Turbine steam seals, resulting in level and pressure control being transferred to Reactor Core Isolation Cooling (RCIC) and High Pressure Coolant Injection (HPCI), respectively.
The cause of the Generator lockout is under investigation, with the suspected cause being a spurious trip of the Audio Tone system. The system has been removed from ser vice with a redundant system providing the protective function. The Audio Tone system is scheduled for replacement in the 2024 refueling outage.
This event is reportable under 10CFR50.73(a)(2)(iv)(A) due to manual or automatic actuation of systems listed in 10CFR50.73(a)(2)(iv)(B) including the Reactor Protection System, containment isolation signals, actuation of an emergency core cooling system and Reactor Core Isolation Cooling. An ENS notification was made within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the event and was updated within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, reference ENS 56936.
Plant Operating Conditions Before the Event
Prior to the event, Peach Bottom Atomic Power Station, Unit 2 w as operating at approximately 100% power in MODE 1. There were no structures, systems or components (SSC) that were inoperable at the start of the event that contributed to the event.
Event Description
On 01/29/2024 at 1140 hours0.0132 days <br />0.317 hours <br />0.00188 weeks <br />4.3377e-4 months <br />, the Main Control Room (MCR) received a Generator Tone Trouble Alarm, which immediately reset once acknowledged. A second Generator Tone Trouble Alarm was received, which remained locked in for a short period of time (approximately 15 seconds) before resetting.
At 1202 hours0.0139 days <br />0.334 hours <br />0.00199 weeks <br />4.57361e-4 months <br />, Unit 2 experienced a Generator lockout, Turbine trip and automatic Reactor Scram. Turbine control valves (EIIS: TA) closed as expected. Safety relief va lves (EIIS: SB) were used to control the expected pressure increase and were not reopened after the initial trans ient. Primary Containment Group II and III Isolations (EIIS: JM) occurred as expected. Reactor vessel level was initially controlled by Feedwater (EIIS:
SJ). Condenser vacuum slowly degraded due to an issue with the turbine steam seal system (EIIS: TC) and, therefore, after approximately 20 minutes level control was transferred to the Reactor Core Isolation Cooling (RCIC) system (EIIS: BN). Pressure control was maintained by t urbine bypass valves (EIIS: SO) and the High Pressure Coolant Injection (HPCI) system (EIIS: BJ), which was only used for pressure control and not injection. Steam seals were restored and condenser vacuum was re-established. Vessel level control was transferred to Condensate (EIIS: SD) and the unit was safely transitioned to a cold shutdown condition.
A review of the plant event logs confirmed that a generator lockout and power-load unbalance occurred. The 2-50F-386A, Main Transformer Over Head Line Protection Lock Out Relay was the first actuating device. The TTR-1 Audio Tone drawers indicated that a trip signal was tra nsmitted from the Audio Tone drawers in the Cable Spreading Room to the drawers in the substation. The sub station drawers opened the CB-225 and CB-215 breakers (EIIS: EL) upon receipt of the trip signal.
At this time, the cause of the Generator lockout is still under investigation by a Root Cause analysis. The most probable cause is a failure of the Audio Tone trip system cable s which are planned for abandonment in the upcoming 2024 refueling outage as part of a planned system upgr ade. Currently, there is no plan to supplement this LER; however, this LER will be supplemented if the conclusions of the Root Cause analysis substantially change the information presented in this LER.
An investigation was performed and the cause of the loss of con denser vacuum is attributed to the setting of the sealing steam header pressure control valve. This resulted in the valve not opening to admit steam to the sealing steam header following the trip of the main turbine, wh ich provided a path for air in-leakage and degraded condenser vacuum.
Unit 3 was unaffected by this event. Unit 2 start up was comme nced at 2028 hours0.0235 days <br />0.563 hours <br />0.00335 weeks <br />7.71654e-4 months <br /> on 01/30/2024 and the unit was synchronized to the grid at 1427 hours0.0165 days <br />0.396 hours <br />0.00236 weeks <br />5.429735e-4 months <br /> on 01/31/2024.
This event is reportable under 10CFR50.73(a)(2)(iv)(A) due to manual or automatic actuation of systems listed in 10CFR50.73(a)(2)(iv)(B) including RPS, containment isolation signals, actuation of an emergency core
cooling system and RCIC. An ENS notification was made within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the event and was updated within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, reference ENS 56936.
Safety Consequences
The reactor Scram occurred as designed and resulted in no safet y consequences. All safety systems responded as expected. As described above, the temporary degradation in condenser vacuum resulted in use of the RCIC system to control level for a portion of the respon se, but this did not challenge the ability to safely shutdown and cool down the unit.
Corrective Actions
Immediate corrective actions were taken to remove the affected Audio Tone system (TTR-1) from service and utilize the fully redundant TTR-2 system. The TTR-1 Audio Tone drawer has been replaced and a new set of cables has been identified and tested. After the completion of additional testing, this system will be maintained in a standby condition as a spare. The Audio Tone system is scheduled for a complete replacement in the upcoming 2024 refueling outage.
The corrective action to address the condenser vacuum degradati on is to revise the procedure which sets the sealing steam pressure control valve to ensure it will receive the appropriate demand signal when needed.
Previous Similar Events
Peach Bottom has not experienced a Scram due to a Generator loc kout in at least 10 years. The station experienced a trip of the Audio Tone system for an offsite sour ce due to cable degradation in 2001 (Ref. EN 38241).