05000263/LER-2023-001, Main Steam Isolation Valve Leakage Exceeds Technical Specification Requirements Due to Stem Scoring

Main Steam Isolation Valve Leakage Exceeds Technical Specification Requirements Due to Stem Scoring

ML23137A275
Person / Time
Site:	Monticello
Issue date:	05/17/2023
From:	Domingos C Northern States Power Company, Minnesota, Xcel Energy
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
L-MT-23-023 LER 2023-001-00
Download: ML23137A275 (1)

LER-2023-001, Main Steam Isolation Valve Leakage Exceeds Technical Specification Requirements Due to Stem Scoring

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2632023001R00 - NRC Website

text

2807 West County Road 75 Monticello, MN 55362

May 17, 2023 L-MT-23-023 10 CFR 50.73

ATTN: Document C ontrol D esk U.S. Nuclear Re gulatory Co mmission Washington, D C 2 0555- 0001

Monticello Nuclear G enerating Plant Docket N o. 50-2 63 Renewed F acility Operating License No. D PR-22

LER 20 23- 001-00 Main Steam I solation Valve Leakage Exceeds T echnical Specification Requirements Due to Stem Scoring

Pursuant t o 10 CFR 50.73(a)(2)(i)(B), N orthern States Power C ompany, a Minnesota corporation, d oing busi ness as X cel Energy (hereafter " NSPM"), hereby s ubmits M onticello Nuclear Generating Plant ( MNGP) Licensee Event Report (LER) 2023-0 01-0 0.

If you have any questions about this submittal, please contact R ick Loeffler, Senior Nuclear Regulatory Engineer, at r ick.a.loeffler@xcelenergy.com or at 612-3 42-8 981.

Summary of C ommitments

This l etter makes n o new c ommitments a nd no revisions t o existing co mmitments.

Christopher P. D omingos Site Vice President, M onticello and P rairie Island Nuclear G enerating Plants Northern States Power C ompany Minnesota

Enclosure (1)

cc: Administrator, Region I II, USNRC Project Manager, Monticello, USNRC Resident Inspector, Monticello, USNRC State of Minnesota ENCLOSURE 1

MONTICELLO NUCLEAR GENERATING PLANT LICENSEE EVENT REPORT 50 -263/2023- 001-00

3 pages follow

Abstract

A packing leak on valve AO-2-86C, 13 Outboard Main Steam Isolation Valve (MSIV), exceeded the Technical Specification (TS) individual MSIV leakage limit of Surveillance Requirement (SR) 3.6.1.3.12 on March 4, 2023, based on subsequent evaluation. The condition was discovered on March 21, 2023, when it was concluded that a reasonable expectation of operability was not met, and the MSIV was declared inoperable in accordance with TS 3.6.1.3 Condition E. During a maintenance outage on March 26, 2023, the Local Leak Rate Test as-found leakage was 987.1 standard cubic feet per hour (scfh) which exceeded the TS SR limit for individual MSIV leakage. This condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the TSs. The cause of the packing leakage is attributed to scoring on the valve stem. During the planned maintenance outage, the MSIV was repacked to restore compliance with TS, and the valve packing gland was bored out and the stem was replaced during the 2023 Refuel Outage.

EVENT DESCRIPTION

While in Mode 1 at approximately 99 percent power, a packing leak being monitored on valve AO-2-86C, 13 Outboard Main Steam Isolation Valve (MSIV), (i.e., the outboard MSIV on the C main steam line, hereafter referred to as the C outboard MSIV) exceeded the Technical Specification (TS) individual MSIV leakage limit of Surveillance Requirement (SR) 3.6.1.3.12 on March 4, 2023, based on subsequent evaluation. On March 21, 2023, engineering concluded that a reasonable expectation of operability for the C outboard MSIV [SB][ISV] was not met and operators declared the valve inoperable in accordance with TS 3.6.1.3 Condition E. During a planned maintenance outage on March 26, 2023, the 10 CFR 50, Appendix J Local Leak Rate Test (LLRT) as-found leakage for the C outboard MSIV was 987.1 standard cubic feet per hour (scfh) which exceeded the 100 scfh limit of SR 3.6.1.3.12 for individual MSIV leakage (when tested at greater than or equal to 44.1 psig). This condition is being reported within 60 days from the date of discovery, March 21, 2023, in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the TSs as discussed in the event analysis.

Chronology

In May 2021, the as-left LLRT leakage from the 2021 Refuel Outage (RFO) for the C outboard MSIV was 33.9 scfh. In June 2021 a packing leak on this valve was identified and an Adverse Condition Monitoring Plan established to monitor the leak. On March 4, 2023, packing material was observed on camera being ejected from the packing gland on this valve.

On March 10, 2023, a four-degree Fahrenheit step change increase in main steam chase temperature occurred. On March 14, 2023, at 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-4 months <br />, due to an increase in leakage, operators entered the procedure for steam leaks outside primary containment, and on March 16, 2023, a prompt operability determination was requested. On March 18, 2023, the C inboard MSIV was closed to isolate the leak. On March 21, 2023, engineering concluded that a reasonable expectation of operability for the C outboard MSIV was not met due to the estimated leakage, and operations declared the valve inoperable in accordance with the TS 3.6.1.3 Condition E. It was concluded that March 4, 2023, was likely the date the TS individual MSIV leakage limit had been exceeded. On March 26, 2023, during the maintenance outage the as-found LLRT for the C outboard MSIV was 987.1 scfh which exceeded the 100 scfh limit of SR 3.6.1.3.12 for individual MSIV leakage (when tested at greater than or equal to 44.1 psig). Following repacking of the C outboard MSIV during this outage an as-left LLRT leakage rate of 67.7 scfh was measured. The C main steam line was returned to service with startup from the maintenance outage until the 2023 RFO began. During the 2023 RFO the C outboard MSIV packing gland was bored out to provide additional clearance and the valve stem was replaced, and the valve tested satisfactorily.

EVENT ANALYSIS

The event was determined to be reportable in accordance with 10 CFR 50.73(a)(2)(i)(B), Any operation or condition which was prohibited by the plant's Technical Specifications. Specifically, this occurrence is reportable as a condition prohibited by Specification 3.6.1.3, "Primary Containment Isolation Valves," because the MSIV had exceeded the individual leakage limit and had been inoperable for greater than the 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> Completion Time of Condition E and for greater than the Completion Times specified in Limiting Condition for Operation (LCO) 3.0.3, i.e., to be in Mode 2 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />, Mode 3 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />, and Mode 4 in 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />, respectively, when Condition E was not met.

This event is not classified as a safety system functional failure since the C inboard MSIV was fully operational.

SAFETY SIGNIFICANCE

Each of the four main steam lines have an inboard and an outboard MSIV. The MSIVs safety function is to close in the event of a main steam line break and provide containment isolation. Two valves are provided in series to ensure the safety function is met even if one valve were to fail to close.

There were minimal safety consequences associated with this condition. During this event there was no equipment out of service or otherwise inoperable that contributed to the leakage. The MSIV leakage rate is assumed to be the actual pathway leakage rate for the flow path. The C inboard MSIV as-left leakage rate was 12.70 scfh during the 2021 RFO (the last outage prior to the planned maintenance outag e) and the as-found leakage rate at the end of the two year operating cycle was 25.0 scfh for the 2023 RFO. The C inboard MSIV would have closed on an isolation signal ensuring the C main steam line flow path was isolated and within analyzed leakage limits.

The main steam pathway total minimum pathway leakage following the 2021 RFO was 67.811 scfh which is less than the SR 3.6.4.3.13 limit of less than or equal to 200 scfh (when tested at greater than or equal to 44.1 psig). Therefore, the primary containment isolation capability of the main steam lines remained operable which ensured the required isolation safety function was maintained.

CAUSE

The cause of the packing leakage for the C outboard MSIV is attributed to scoring on the valve stem caused by contact with the packing gland follower.

CORRECTIVE ACTION

During the planned maintenance outage, the C outboard MSIV (Anchor Darling 18-inch venturi, Model No. W9324183) was repacked and tested restoring compliance with TS leakage limits. During the 2023 RFO the valve stem was replaced, the packing gland was bored out to provide additional clearance, and the valve tested satisfactorily to meet TS leakage limits.

PREVIOUS SIMILAR EVENTS

Monticello Nuclear Generating Plant LER 2017-003-00, Main Steam Isolation Valve Leakage Exceeds Technical Specification Requirements, dated June 14, 2017, discusses where this same valve exceeded the TS individual MSIV leakage rate limit.

ADDITIONAL INFORMATION

The Institute of Electrical and Electronics Engineer codes for equipment are denoted by [XX].