05000445/LER-2024-002, (Cpnpp), Unit 1, Condition Prohibited by Technical Specifications Due to Inoperable Remote Shutdown System Functions
| ML24155A172 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 06/03/2024 |
| From: | Sewell S Vistra Operations Company |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| CP-202400120, TXX-24028 LER 24-002-00 | |
| Download: ML24155A172 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| 4452024002R00 - NRC Website | |
text
COMANCHE PEAK NUCLEAR POWER PLANT CP-202400120 TXX-24028 June 03, 2024 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Subject:
Comanche Peak Nuclear Power Plant (CPNPP)
Docket No. 50-445 Steven K. Sewell Nuclear Site Vice President Vistra Operations Company LLC P.O. Box 1002 6322 North FM 56 Glen Rose, TX 76043 Office: 254.897.6113 Ref 1 0CFR50. 73 Condition Prohibited by Technical Specifications due to Inoperable Remote Shutdown System Functions Licensee Event Report 1-24-002-00
Dear Sir or Madam:
Attached is Licensee Event Report (LER) 1-24-002-00, "Condition Prohibited by Technical Specifications due to Inoperable Remote Shutdown System Functions" for Comanche Peak Nuclear Power Plant (CPNPP) Unit 1.
This communication contains no new commitments regarding CPNPP Units 1 or 2.
Should you have any questions, please contact Kassie Mandrell at (254) 897-6987 or Kassie.Mandrell@vistracorp.com.
Sincerely, Steven Sewell (Jun 3, 202411:54 CDT)
Steven K. Sewell
Attachment:
Abstract
On October 20, 2023, following the performance of Integrated Test Sequencing (ITS), Unit 1, Train A, Neutron Flux Monitoring System (Gamma Metrics) failed to automatically restart. ITS is performed to verify proper response of Safeguards equipment following a Safety Injection (SI) in conjunction with a Loss of Offsite Power (LOOP). Operators questioned the reliability of the Gamma Metrics system during accident conditions and a follow-up evaluation of Gamma Metrics performance on April 3, 2024, found that both trains of the Unit 1 Gamma Metrics would be unable to automatically restart during a LOOP and had been inoperable in the past for longer than allowed by Technical Specifications (TS).
The direct cause of the Gamma Metrics failure was an overload trip of the high voltage power supply (HVPS). Vendor analysis of the component is ongoing and the specific failure mode has not been determined. Results of the failure analysis will be provided in a supplemental report. Corrective actions include a temporary modification to improve power restoration to the system and a procedure revision for compensatory operator actions.
I. DESCRIPTION OF REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
050 052
- 2. DOCKET NUMBER
- 3. LER NUMBER I
445 Cl NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 002 1-~
This event is reportable under 1 0CFR50. 73(a)(2)(i)(B), "Any operation or condition which was prohibited by the plant's Technical Specification." CPNPP failed to meet Technical Specifications (TS) 3.3.4 for the Remote Shutdown System. The Remote Shutdown System operability requires one Neutron Flux Monitor. It was determined that Function 1 of table 3.3.4-1 (Neutron Flux Monitor) was not met, where condition A is 30 days and condition B requires a shutdown to MODE 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, if condition A is not met.
B. PLANT CONDITION PRIOR TO EVENT Comanche Peak Nuclear Power Plant (CPNPP) Unit 1 was in MODE 1, operating at 100% power at time of discovery.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE INOPERABLE AT THE START OF THE EVENT AND CONTRIBUTED TO THE EVENT There were no structures, systems, or components that were inoperable prior to the event which contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES During the refueling outage in October 2017, the high voltage power supply (HVPS) [EIIS: (IG)(JX)] was replaced on Unit 1, Train B. It was subsequently replaced on Unit 1, Train A on July 7, 2020. The following Unit 1 refueling outage (1 RF21) in October 2020 resulted in a trip of the Neutron Flux Monitoring System (Gamma Metrics) [EIIS: (IG)(MON)]
during the performance of Integrated Test Sequencing (ITS). ITS is performed to verify proper response of Safeguards equipment following a Safety Injection (SI) in conjunction with a Loss of Offsite Power (LOOP). The HVPS was reset to restore the signal prior to plant start-up. The same failure occurred during the next two Unit 1 refueling outages in 2022 and 2023.
Following the failure on October 20, 2023, Operators questioned the reliability of the Gamma Metrics system during accident conditions and potential actuation of SI. On April 3, 2024, Engineering evaluated the condition and found that the Unit 1 Gamma Metrics is not reliable and will trip during power restoration from any loss of the 6.9 kilovolt (KV) Safety Bus.
A past operability evaluation was then completed on April 11, 2024 and determined that this condition had existed for longer than allowed by Technical Specifications (TS). On April 18, 2024, a temporary modification was installed to add a 30 second delay to the power restoration of both trains of the HVPS. As a result of the temporary modification, the system was declared operable.
II. COMPONENT OR SYSTEM FAILURES A. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE The evaluation determined that voltage spikes occur when the 6.9KV bus is lost and the Emergency Diesel Generator (EOG) breaker closes. This cycle occurs in approximately 7 seconds on Unit 2 and approximately 6 seconds on Unit 1. The Unit 1 HVPS [EIIS: (IG)(JX)] cannot sustain the shorter cycle, resulting in an overvoltage protection trip. This condition does not affect the Unit 2 Gamma Metrics.
The specific cause of this failure has not been determined and the component was sent to the vendor for further analysis.
Results of the analysis will be provided in a supplement to this LER.
B. FAILURE MODE, MECHANISM, AND EFFECTS OF EACH FAILED COMPONENT This failure was first questioned by the Operations department when performance of the ITS testing resulted in a train-related failure of the Gamma Metrics. A follow-up evaluation by Engineering found the system to be inoperable.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF COMPONENTS WITH MULTIPLE FUNCTIONS This event did not involve systems or secondary functions which were affected by the Gamma Metrics failure.
D. FAILED COMPONENT INFORMATION
The Gamma Metrics is a redundant Class 1 E system consisting of fission chamber detectors, cables, amplifiers, signal processors and indicators. The Gamma Metrics measures the neutron flux leakage outside but adjacent to the reactor vessel and is provided to detect and indicate neutron flux in two ranges: Source Range (from 0.1 to 1 0E5 counts per second (CPS)) and Wide Range (from 1 E-8 to 200% of full power). It is required to provide the operator with a visual indication of neutron flux condition. The Gamma-Metrics is available at the Remote Shutdown panel.
Ill. ANALYSIS OF THE EVENT A. SAFETY SYSTEM RESPONSES THAT OCCURRED No safety system responses occurred due to this event.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY The evaluation completed on April 3, 2024, identified that Unit 1 was operating outside of Technical Specification requirements associated with completion times for Remote Shutdown inoperability since October 6, 2017.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT The Final Safety Analysis Report (FSAR) was reviewed for the extent to which the Gamma Metrics are credited for accident mitigation in the analysis. In Chapters 6, "Engineered Safety Features" and 15, "Accident Analysis", the Gamma Metrics are not discussed as an indicator required to mitigate the consequences of an accident. There are also no accidents in these chapters which credit a control room evacuation and the use of the remote shutdown panel.
Additionally, the Gamma Metrics does not include an overpower trip function for reactor protection and are not credited in TS 3.3.3 as Post Accident Monitoring Instrumentation. In FSAR, Chapter 7, "Instrumentation and Controls", section 7.4, "Systems Required for Safe Shutdown," the design basis for Control Room evacuation does not consider a concurrent Condition II, "Faults of Moderate Frequency", Condition Ill, "Infrequent Faults" or Condition IV, "Limiting Faults" event, nor a single failure. Mitigation of core damage is achieved through control rod insertion, boron addition and chemistry sampling. In the event that the Gamma Metrics is needed at the remote shutdown panel, operations would also have the ability to manually restart the system. This event had no impact on nuclear safety, reactor safety, radiological safety, environmental safety or the safety of the public. This event has been evaluated as not meeting the definition of a safety system functional failure per 10 CFR 50. 73(a)(2)(v).
IV. CAUSE OF THE EVENT
- 2. DOCKET NUMBER
- 3. LER NUMBER I
445 Cl NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 002 1-~
The direct cause of the Gamma Metrics failure was an overload trip of the high voltage power supply. Vendor analysis of the HVPS is ongoing and the specific failure mode has not been determined. Results of the failure analysis will be provided in a supplemental report. Analysis determined that Unit 2 was not affected.
V. CORRECTIVE ACTIONS
Corrective actions include the following completed items:
- 1. Revise applicable procedures to have operators manually reset associated breakers in the event of a station blackout to restore power to the Unit 1 Gamma Metrics. Action completed on April 18, 2024.
- 2. Install a temporary modification to slow power restoration to the system and prevent the overvoltage trip of the HVPS on Unit 1. Action completed on April 18, 2024.
Final results of the analysis will be provided in a supplemental report.
VI. PREVIOUS SIMILAR EVENTS
There have been no similar events or conditions related to Gamma Metrics being inoperable for a period longer than the TS action statement allowed outage time of 30 days at Comanche Peak Nuclear Power Plant. Page 4
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