NOC-AE-240040, Response to Apparent Violation in NRC Inspection Report 5000498/2024050 and 05000499/2024050; EA-24-117
| ML24355A074 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 12/19/2024 |
| From: | Tomlinson J South Texas |
| To: | NRC Region 4, Document Control Desk |
| References | |
| EA-24-117, NOC-AE-24004082, STI: 35684445, IR 2024050 | |
| Download: ML24355A074 (1) | |
Text
December 19, 2024 NOC-AE-24004082 File No.: G25 STI: 35684445 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Units 1 and 2 Docket Nos. STN 50-498, STN 50-499 Response to Apparent Violation in NRC Inspection Report 05000498/2024050 and 05000499/2024050; EA-24-117
References:
Letter, G. Miller (NRC) to K. Harshaw (STP), South Texas Project Electric Generating Station, Units 1 and 2 - NRC Special Inspection Report 05000498/2024050 and 05000499/2024050, November 21, 2024, (AE-NOC-24003423) (ML24320A137).
The purpose of this letter is to confirm the actions STP Nuclear Operating Company (STPNOC) is taking in response to Nuclear Regulatory Commission (NRC) Special Inspection Report 05000498/2024050 and 05000499/2024050, dated November 21, 2024 (ML24320A137).
STPNOC verbally notified Ms. Patricia Vossmar, Reactor Projects Branch A Chief, on December 1, 2024, that STPNOC is not requesting a Regulatory Conference on this matter and will be submitting this written response. Please see the attachment for STPNOCs reply to the referenced NRC Inspection Report.
Corrective steps identified in the attachment will be tracked and implemented in accordance with the STP Corrective Action Program.
There are no commitments in this letter.
If there are any questions about this report, please contact Chris Warren at (361) 972-7293 or me at (361) 972-8945.
Jason R. Tomlinson Site Vice President
Attachment:
Response to Apparent Violation in NRC Inspection Report 05000498/2024050 and 050004999/2024050; EA-24-117 Jason Tomlinson Digitally signed by Jason Tomlinson Date: 2024.12.20 08:09:18
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NOC-AE-24004082 Page 2 of 2 cc:
Director, Division of Operating Reactor Safety U.S. Nuclear Regulatory Commission, Region IV 1600 E. Lamar Boulevard Arlington, TX 76011-4511 Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511 Thomas Byrd Project Manager U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Division of Operating Reactor Licensing Licensing Project Branch 4 John Vera Senior Resident Inspector, South Texas Project U.S. Nuclear Regulatory Commission Leanne Flores Resident Inspector, South Texas Project U.S. Nuclear Regulatory Commission R4Enforcement@nrc.gov
NOC-AE-24004082 Attachment Page 1 of 3
Attachment:
Response to Apparent Violation in NRC Inspection Report 05000498/2024050 and 050004999/2024050; EA-24-117 In the U.S. Nuclear Regulatory Commission (NRC) letter to South Texas Project Nuclear Operating Company (STPNOC), South Texas Project Electric Generating Station, Units 1 and 2 - NRC Special Inspection Report 05000498/2024050 and 05000499/2024050 and Preliminary White Finding, (ADAMS Accession No. ML24320A137), dated November 21, 2024, the NRC issued preliminary White apparent violation EA-24-117 to STPNOC as summarized below:
Performance Deficiency: The inspectors determined the failure to establish adequate preventive maintenance instructions pertaining to the calibration of protective relays and the maintenance of hand switch auxiliary contacts, was a performance deficiency that was within the licensees ability to foresee and correct. Specifically, the licensee failed to incorporate relevant vendor recommendations in the preventive maintenance program as required by Procedure 0PGP03-ZM-0002, Preventive Maintenance Program, revision 43.
This led to an inadvertent actuation of a protective relay, subsequent reactor trip with partial loss-of-offsite power, and failure of a safety-related load center breaker to close when called upon by the load sequencer.
Screening: The inspectors determined the performance deficiency was more than minor because it was associated with the Procedure Quality attribute of the Initiating Events cornerstone and adversely affected the cornerstone objective to limit the likelihood of events that upset plant stability and challenge critical safety functions during shutdown as well as power operations. Specifically, the failure to incorporate vendor calibration guidance into the relays calibration procedure led to an inadvertent actuation of the relay and subsequent reactor trip.
Significance: The inspectors assessed the significance of the finding using Manual Chapter 0609, appendix A, The Significance Determination Process (SDP) for Findings At-Power. Using Exhibit 1 - Initiating Events Screening Questions, the inspectors determined that the finding caused a reactor trip and the loss of mitigating equipment (offsite power to two safety buses and the Load Center E2A1 supply breaker). As a result, a Detailed Risk Evaluation was required.
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Conclusion:==
The incremental conditional core damage probability (CCDP) resulting from the failure to establish adequate instructions pertaining to the calibration of protective relays and the preventive maintenance of hand switch auxiliary contacts which led to a reactor trip with a loss of the main condenser and a coincident breaker failure, was estimated to be 1.9E-6, or of low to moderate safety significance (White).
NOC-AE-24004082 Attachment Page 2 of 3 Enforcement:
Violation: Title 10 CFR 50.65(a)(1), requires, in part, that the holders of an operating license shall monitor the performance or condition of Structures, Systems, and Components (SSCs) within the scope of the rule, against licensee-established goals, in a manner sufficient to provide reasonable assurance that such SSCs, as defined by 10 CFR 50.65(b), are capable of fulfilling their intended functions.
Title 10 CFR 50.65(a)(2) states, in part, that monitoring as specified in 10 CFR 50.65(a)(1) is not required where it has been demonstrated that the performance or condition of an SSC is being effectively controlled through the performance of appropriate preventive maintenance, such that the SSC remains capable of performing its intended function.
Contrary to the above, as of May 12, 2024, the licensee failed to demonstrate that the performance of an SSC, load center E2A1 breaker, had been effectively controlled through the performance of appropriate preventive maintenance such that the SSC remained capable of performing its intended function. Specifically, the licensee failed to implement adequate preventive maintenance procedures such that the load center E2A1 hand switch HS-0001 remained capable of performing its intended function by not preventing a close signal from reaching the breaker.
Enforcement Action: This violation is being treated as an apparent violation pending a final significance (enforcement) determination.
This attachment provides STPNOCs reply to preliminary White apparent violation EA-24-117.
Reason for the Violation:
On May 12, 2024, Unit 2 was at 15% power following completion of a planned refueling outage.
Unit 2 automatically tripped at 1641 due to a partial Loss of Offsite Power (LOOP) from the unit auxiliary transformer due to inadvertent actuation of backup distance relay 21/G1. While Standby Diesel Generators (SBDG) 21 and 23 actuated and energized all three Engineered Safety Feature (ESF) busses as designed, the reactor trip was complicated by Load Center (LC) E2A1 breaker failure to close automatically and manually, which resulted in the unavailability of several safety-related loads.
A Root Cause Evaluation (RCE) was performed for the inadvertent actuation of backup distance relay 21/G1 and determined that the relay was susceptible to mechanical agitation to spuriously trip and seal in the fault condition when no restraining voltages were present, and the protection scheme was not updated to bypass the trip function of the 21/G1 relay when the Main Generator Circuit Breaker (MGCB) was open.
Additional investigations are planned to determine the causes of LC E2A1 breaker failure to close and identify causes of inadequacies and corrective actions to eliminate the inadequacies in the STPNOC Preventative Maintenance (PM) program. A supplement to this letter will be provided once all investigations have been completed and corrective actions identified.
NOC-AE-24004082 Attachment Page 3 of 3 Corrective Steps Taken and Results Achieved:
STPNOC has completed the following corrective steps as a result of the Unit 2 automatic trip:
- 1. Engineering Change (EC) 24-4884-24 was developed and implemented in Unit 1 in October 2024. The EC includes a design change to isolate the trip signals from relays 21/G1 and 62/G1 when the main generator is disconnected from the off-site power system to prevent spurious trips.
- 2. Incorporated Vendor instructions for calibration of KD-10 and KD-11 relays into applicable site maintenance procedures.
- 3. The handswitch for Load Center E2A1 breaker has been replaced.
Additional corrective steps that have been completed will be added to the supplement to this letter.
Corrective Steps That Will Be Taken:
STPNOC will take the following corrective steps to further address the spurious trip of the 21/G1 relay, the failure of Load Center E2A1 breaker failure to close, and inadequacies in the PM program. Additional corrective steps will be added to a supplement to this letter once they are identified during planned investigations.
- 1. Unit 1 - Develop, issue, and implement EC 24-4884-61 to install a permanent solution to block the 21/G1 relay trip signal when the Main Generator Circuit Breaker is open.
- 2. Unit 2 - Develop, issue, and implement EC 24-4884-60 to install a permanent solution to block the 21/G1 relay trip signal when the Main Generator Circuit Breaker is open.
- 3. Establish a PM strategy for all risk-ranked High and Medium handswitches for periodic replacement and clean/inspect activities based on vendor recommendations and industry benchmarking.
Date When Full Compliance Will Be Achieved:
STPNOC will achieve full compliance by May 31, 2026, when all design changes for the 21/G1 relay are installed in each Unit. STPNOC will establish the handswitch PM strategy by February 28, 2025.