05000298/LER-2024-006, Human Performance Errors Cause a Condition Prohibited by Technical Specifications and Common Cause Inoperability of Independent Reactor Protection System Channels
| ML24354A318 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 12/19/2024 |
| From: | Dia K Nebraska Public Power District (NPPD) |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NLS2024074 LER 2024-006-00 | |
| Download: ML24354A318 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2982024006R00 - NRC Website | |
text
N Nebraska Public Power District "Always there when you need us" 10 CFR 50.73 NLS2024074 December 19, 2024 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Subject: Licensee Event Report No. 2024-006-00 Cooper Nuclear Station, Docket No. 50-298, Renewed License No. DPR-46 The purpose of this correspondence is to forward Licensee Event Report 2024-006-00.
This letter does not contain regulatory commitments.
Sincerely, *
/bk
Attachment:
Licensee Event Report 2024-006-00 cc:
Regional Administrator w/attachment USNRC - Region IV Cooper Project Manager w/attachment USNRC - NRR Plant Licensing Branch IV Senior Resident Inspector w/attachment USNRC-CNS SRAB Administrator w/attachment NPG Distribution w/attachment INPO Records Center w/attachment via IRIS entry SORC Chairman w/attachment CNS Records w/attachment COOPER NUCLEAR STATION 72676 648A Ave/ P.O. Box 98 I Brownville, NE 68321 http://www.nppd.com
Abstract
On October 24, 2024, during document review of functional testing for Reactor Protection System (RPS) electrical protection monitoring assemblies (EPA) performed on October 22, 2024, the underfrequency trip as-left values for RPS-EPA-1 B1 and RPS-EPA-1 B2 were noted outside the associated Technical Specification (TS) limit.
RPS-EPA-1 B1 and RPS-EPA-1 B2 were declared inoperable. It was identified that single control rod movements occurred during the time frame that RPS-EPA-1 B1 and RPS-EPA-1 B2 were in service and inoperable, violating TS 3.3.8.2. This is a condition prohibited by TS and a common cause inoperability of independent RPS channels.
The values were calibrated back into tolerance and EPAs declared operable. The direct cause was as-found data that was within tolerance was adjusted without procedural guidance. Causal factors were rigor and formality in the use of peer checks were not in accordance with station standards and review of the surveillance results by supervision was inadequate. To address the direct cause and causal factors, Cooper Nuclear Station will develop and implement procedure use and adherence training for electrical maintenance personnel and will additionally review electrical maintenance procedures for allowances for adjusting in-tolerance parameters and revise procedures accordingly.
PLANT STATUS SEQUENTIAL NUMBER 006 REV NO.
00 At the time the event was discovered (i.e., October 24, 2024), Cooper Nuclear Station (CNS) was in Mode 5, Refueling.
There were no systems, structures, or components inoperable at the time of the event that contributed to the event.
BACKGROUND The Reactor Protection System (RPS) [EIIS:JC] limits the uncontrolled release of radioactive material from the fuel and reactor coolant pressure boundary by terminating excessive temperature and pressure increases through the initiation of an automatic scram.
The RPS Electric Power Monitoring System isolates the RPS bus [EIIS:BU] from the motor generator (MG) [EIIS:MG] set or an alternate power supply [EIIS:JX] in the event of overvoltage, undervoltage, or underfrequency condition The RPS electric power monitoring assembly (EPA) [EIIS:MON] monitors and detects abnormal high or low voltage or low frequency condition in the outputs of the two MG sets or the alternate power supply and will de-energize its respective RPS bus, thereby causing safety functions normally powered by this bus to de-energize. Some of the essential equipment powered from the RPS busses includes the RPS logic relays [EIIS:RLY] and scram solenoids [EIIS:SOL].
In the event of a failure of an RPS Electric Power Monitoring System (e.g., both in-series EPAs), the RPS loads may experience significant effects from the unregulated power supply. Deviation from the nominal conditions can potentially cause damage to the scram solenoids and other Class 1 E devices.
Technical Specifications (TS) limiting condition for operation (LCO) 3.3.8.2, RPS Electric Power Monitoring, requires two RPS EPAs shall be operable for each in-service RPS MG set or alternate power supply. LCO 3.3.8.2 is applicable in Mode 5, Refueling, with any control rod withdrawn from a core cell containing one or more fuel assemblies.
EVENT DESCRIPTION
During dayshift on October 22, 2024, electrical maintenance performed Surveillance Procedure 6.2RPS.313, RPS Electrical Protection Assemblies Calibration and Functional Test (Division 2). During performance of the surveillance procedure, as-found values were within calibration tolerance and TS limits for both RPS-EPA-1 B1 and RPS-EPA-1 B2 but the undervoltage trip values were near the lower limit of being out of calibration tolerance. The technician contacted maintenance supervision, and the decision was made to document it as a discrepancy and allow the technician to adjust the as-found values closer to the midpoint of the calibration tolerance range. Adjustments were made to undervoltage trip, underfrequency trip/delay for RPS-EPA-1 B1 and undervoltage trip, overvoltage trip, and underfrequency trip for RPS-EPA-182.
In the sign-off and review section of the surveillance procedure, the electrical maintenance technician documented the discrepancy and made the adjustments. The completed surveillance procedure was reviewed and signed by maintenance supervision and delivered to the control room for final shift manager review which was performed during nightshift on October 22, 2024. Page 2 of 4 (04-02-2024)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 04/30/2027 LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/read i nq-rm/doc-collections/nuregs/staff /sr1022/r3/)
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LER NUMBER Cooper Nuclear Station 05000-298 YEAR SEQUENTIAL NUMBER REV NO.
2024 006 00 Subsequently, on October 24, 2024, the surveillance coordinator reviewed the completed surveillance procedure and identified the documented as-left underfrequency trip values for both RPS-EPA-1 B1 and RPS-EPA-1 B2 were left low outside of calibration tolerance and TS limits. This information was immediately provided to the control room and at 1230 on October 24, 2024, operations declared RPS-EPA-1 B1 and RPS-EPA-1 B2 inoperable due TS surveillance requirement 3.3.8.2.1 not being met. LCO 3.3.8.2, RPS Electric Power Monitoring, is applicable in Mode 5 with any control rod withdrawn from a core cell containing one or more fuel assemblies.
At the time the condition was identified by the surveillance coordinator on October 24, 2024, the reactor remained in Mode 5 and no control rod movement was occurring. However, between the performance of the surveillance procedure until the point the surveillance coordinator identified the condition and notified the control room, numerous control rods had been withdrawn and inserted from a core cell containing fuel assemblies (one control rod at a time) for the performance of control rod withdrawal and operability testing. These single control rod movements did occur during the time frame that RPS-EPA-1 B1 and RPS-EPA-1 B2 were in service and inoperable, violating TS 3.3.8.2 for approximately 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.
Upon being notified of the condition, a caution order was implemented to prevent control rod movement. By 1650 on October 24, 2024, operations had transferred RPS B to the alternate power supply thus removing RPS-EPA-1 B1 and RPS-EPA-1 B2 from service and meeting LCO 3.3.8.2 and allowing control rod movement to resume.
Surveillance Procedure 6.2RPS.313 was re-performed and RPS-EPA-1 B1 and RPS-EPA-1 B2 were recalibrated and restored to operable status on October 29, 2024, at 1127.
A human performance evaluation was performed and identified the following:
Rigor and formality in the use of human performance tools, specifically a self-check and peer checks, were not performed in accordance with station standards.
Maintenance first line supervisor review of the completed surveillance was inadequate. The supervisor had a preconceived mindset during their review that as-found data was within range and the items that had a discrepancy had only been adjusted to the midpoint of the calibration tolerance.
Control room supervisor had a preconceived mindset that the adjustments were only to place values closer to the midpoint of the calibration tolerance. This mindset impacted their review of documented surveillance as-left data.
A maintenance procedure for relay testing and adjustment has a note allowing for as-found values within range to be adjusted to the midpoint of range. This guidance instilled a work practice standard that is not acceptable in all surveillance activities.
BASIS FOR REPORT The event is reportable under 1 O CFR 50. 73(a)(2)(i)(B) as any operation or condition prohibited by Technical Specifications.
With both RPS-EPA-1 B1 and RPS-EPA-1 B2 inoperable, this is also reportable under 10 CFR 50.73(a)(2)(vii) as a common cause inoperability of independent channels. Page 3 of 4 (04-02-2024)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 04/30/2027 LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/readinq-rm/doc-collections/nureqs/staff/sr1022/r3/}
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LER NUMBER Cooper Nuclear Station 05000-298 YEAR 2024
SAFETY SIGNIFICANCE
SEQUENTIAL NUMBER 006 REV NO.
00 There were no actual consequences to general safety of the public, nuclear safety, industrial safety, or radiological safety.
Although there were numerous control rod movements from a core cell containing fuel assemblies when RPS-EPA-1 B1 and RPS-EPA-1 B2 were left improperly calibrated, the one-rod-out interlock was enforced with the reactor mode switch in the Refuel (Mode 5) position. This allowed only one control rod in core cells containing one or more fuel assemblies to be withdrawn at a time thus ensuring shutdown margin was maintained.
CAUSE
The direct cause was as-found data that was within tolerance was adjusted without procedural guidance. Causal factors were rigor and formality in the use of peer checks were not in accordance with station standards and review of the surveillance results by maintenance and operations supervision was inadequate.
CORRECTIVE ACTIONS
The surveillance procedure was reperformed and RPS-EPA-1 B1 and RPS-EPA-1 B2 were recalibrated and restored to operable status.
A stand-down was conducted with electrical maintenance personnel to reinforce compliance of procedure use and adherence and use of peer checks. Additionally, procedural guidance for surveillance test reviews was reinforced with senior reactor operators.
The following corrective actions are planned:
Develop and implement procedure use and adherence training for electrical maintenance personnel.
Review electrical maintenance procedures for allowances in steps to adjust in-tolerance parameters and revise procedures accordingly.
Review requirements of the work package process with maintenance supervisors.
Review surveillance program requirements for surveillance test reviews with senior reactor operators.
PREVIOUS EVENTS A review of CNS licensee event reports for the last three years was performed. There were no similar occurrences identified. Page 4 of 4