05000324/LER-2023-001, Two Main Steam Line Safety Relief Valves Inoperable

Two Main Steam Line Safety Relief Valves Inoperable

ML23199A224
Person / Time
Site:	Brunswick
Issue date:	07/17/2023
From:	Krakuszeski J Duke Energy Progress
To:	Document Control Desk
References
RA-23-0138 LER 2023-001-00
Download: ML23199A224 (1)

LER-2023-001, Two Main Steam Line Safety Relief Valves Inoperable

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3242023001R00 - NRC Website

text

U.S. Nuclear Regulatory Commission Page 2 of 2

cc (with enclosure):

Ms. Laura Dudes, NRC Regional Administrator, Region II Mr. Luke Haeg, NRC Project Manager Mr. Gale Smith, NRC Senior Resident Inspector

Abstract

On May 23, 2023, BSEP received test results for the eleven main steam line safety relief valves (SRVs) removed from Unit 2 during the spring 2023 refuel outage (RFO). Two of the eleven valves were found to have as found pilot valve lift setpoints outside the +/-3 percent tolerance required by Technical Specification (TS) 3.4.3, which requires ten of the eleven installed SRVs to be operable. All eleven of the SRV pilot valves were replaced with certified spares before Unit 2 startup from the spring 2023 RFO. At the time the test results were received, Unit 2 was operating in Mode 1 at approximately 100% power.

Corrosion bonding of the SRV pilot disc to the pilot seat likely caused one of the out of tolerance as-found lift setpoints. The other out of tolerance lift setpoint was likely a result of a combination of degraded pilot valve assembly sub-components.

Since less than ten SRVs were operable and the failure modes likely arose over a period of time during Unit 2 operation, this event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the plants TS.

There was no impact on the health and safety of the public or plant personnel.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

Background

Initial Conditions

At the time the test results were received, Unit 2 was in Mode 1 (i.e., Power Operation), at approximately 100 percent rated thermal power.

Reportability Criteria

Unit 2 Technical Specification (TS) 3.4.3 requires at least ten of eleven main steam [SB] line safety relief valves (SRVs) to be operable. Per Surveillance Requirement (SR) 3.4.3.1, each valve is required to open within +/- 3 percent of its opening setpoint. As-found testing of the valves indicated that two of the valves had lift setpoints outside this tolerance. Since the failure modes likely arose over a period of time during Unit 2 operation, the plant likely operated in a condition which is prohibited by the TS, that is, with fewer than the required number of SRVs having lift setpoints within the +/-3 percent tolerance. Therefore, the condition is being reported per 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the plants TS.

Event Description

During the spring 2023 Unit 2 refueling outage, all eleven pilot valve assemblies in the SRVs were replaced with certified spares. The removed SRV pilot valves were sent to NTS Labs, LLC, to determine the as-found set pressure.

On May 23, 2023, test results were reported to BSEP. The test results showed that two of the eleven valves actuated at pressures outside of the +/-3 percent tolerance allowed by TS SR 3.4.3.1. Specifically, pilot valve with Serial Number (SN) 1278 lifted 3.8 percent above the nameplate set pressure, and pilot valve with SN 1223 lifted 5 percent below the nameplate set pressure.

Event Cause

For the SN 1278 pilot valve, corrosion bonding of the SRV pilot disc to the pilot seat likely caused the +3.8 percent out of tolerance as-found lift setpoint. Corrosion bonding between the pilot disc and seat is an inherent problem with the Target Rock two-stage SRV design used at BSEP that has been effectively mitigated by coating each pilot disc surface with Platinum. However, due to the complexities associated with the Platinum coating process and the harsh environment in which the SRVs operate, corrosion bonding may occur periodically.

For the SN 1223 pilot valve, the -5 percent out of tolerance lift setpoint was likely a result of a combination of degraded pilot valve assembly sub-components which were noted during disassembly inspections. Minor steam cutting was noted on the pilot disc which likely resulted from slight seat leakage. Minor scoring was noted between the guide assembly and the seal bellows rod assembly indicating side-loading of the spring. In addition, the set pressure spring was not a matched set with the spring follower which like ly resulted in the spring repositioning after being lifted and contributed to the side-loading of the spring. Minor steam cutting and minor scoring are considered normal wear. It should be noted that the mismatched set pressure spring assembly produced satisfactory results during the

certification process and no leakage was observed during said certification. Therefore, it is unlikely that any one of these conditions alone would result in the out of tolerance lift setpoint; however, the combined effect of the degraded components likely resulted in the -5 percent out of tolerance lift setpoint.

Safety Assessment

The purpose of the SRVs is to provide overpressure protection for the reactor coolant system. The as-found condition of the Unit 2 SRVs was compared to cases analyzed in the Brunswick Unit 2 Cycle 25 Reload Safety Analysis Report (RSAR). Each as-found SRV was bounded by the opening pressures assumed in the analysis; therefore, the overall RSAR analysis bounds the overall as-found condition of the SRVs.

The results from the RSAR analysis demonstrate that the pressure limits are not exceeded for the overpressurization events. Therefore, it is concluded that the Unit 2 Cycle 25 SRVs always remained capable of performing their safety function of preventing overpressurization of the reactor coolant system.

Based on the foregoing analysis, it is concluded that this event had no adverse impact on nuclear safety.

Corrective Actions

All eleven of the SRV pilot valves were replaced with certified spares before Unit 2 startup from the spring 2023 refueling outage. Also, it was confirmed that all other valves removed from Unit 2 during the spring 2023 refueling outage have matching spring sets and rebuild paperwork for all valves in operation in Unit 1 and Unit 2 document matching spring sets.

In addition to the aforementioned completed corrective actions, the SRV pilot valve assemblies with lift setpoints outside +/- 3 percent will be rebuilt and certified for use. The rebuild process restores the pilot disc surface finish to a satisfactory condition and recoats the pilot disc with Platinum to prevent corrosion bonding. The rebuild process will also ensure that degraded pilot valve assembly sub-components are restored to acceptable conditions, and the set pressure spring, retainer, and follower are a matched set. The rebuild campaign is scheduled to complete prior to the end of 2023.

Any changes to corrective actions or completion schedules w ill be made in accordance with the sites corrective action program.

Previous Similar Events

A review of LERs for the past three years identified the following previous similar event associated with SRVs with as-found lift setpoints outside of TS allowable limits.

LER 1-2020-004, dated September 29, 2020, reported Unit 1 operation prohibited by TSs due to three of the eleven valves found with lift setpoints of their pilot valves outside the +/-3 percent tolerance required by TS SR 3.4.3.1.

The direct cause of this event was corrosion bonding. The root cause was attributed to the procedures in place at the time these valves were refurbished lacking sufficient detail to ensure consistent surface preparation and proper quality checks of surface condition prior to Platinum coating. The procedures were revised and utilized during the rebuild process for the subject pilot valves.

Commitments

No regulatory commitments are contained in this report.