05000336/LER-2023-002, Failed Check Valve Resulted in an Unanalyzed Condition

Failed Check Valve Resulted in an Unanalyzed Condition

ML23243B033
Person / Time
Site:	Millstone
Issue date:	08/31/2023
From:	O'Connor M Dominion Energy Nuclear Connecticut
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
23-209 LER 2023-002-00
Download: ML23243B033 (1)

LER-2023-002, Failed Check Valve Resulted in an Unanalyzed Condition

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
3362023002R00 - NRC Website

text

Serial No. 2 3-209 Docket No. 50 -336 Licensee Event Report 2023-002 -00 Page 2 of 2

cc: U.S. Nuclear Regulat ory Commission Region I 475 Allendale Road, Suite 102, King of Prussia, PA 19406-1415.

R. V. Guzman NRC Project Manager Millstone Units 2 and 3 U.S. Nuclear Regulatory Commission One White Flint North, Mail Stop 08 C2 11555 Rockville Pike Rockville, MD 20852-2738

NRC Senior Resident Inspector Millstone Power Station Serial No.23-209 Docket No. 50- 336 Licensee Event Repor t 2023-002-00

ATTACHMENT

LICENSEE EVENT REPORT 2023- 002- 00 FAILED CHECK VALVE RESULTED IN AN UNANALYZED CONDITION.

MILLSTONE POWER STATION UNIT 2 DOMINION ENERGY NUCLEAR CONNECTICU T, INC.

Abstract

On July 6, 2023, at 0938, Millstone Power Station Unit 2 was in Mode 1 at 100 perc ent reactor power, when it was identified that the turbine driven auxiliary feedwater (TDAFW) pump supply check valve from #2 steam generator, 2-MS-4B, may not be able to prevent reverse flow in the event of a main steam line break in the #2 steam generator main steam line upstream of the main steam isolation valves. To verify its operability, the 2-MS-4B closure test was performed on July 6, 2023, at 1202, with unsatisfactory results. 2-MS-4B was disassembled and the bolt that holds the disc to swing arm was found to have failed.

The direct cause of the mechanical damage to the disc bolt was check valve chatter. 2-MS-4B internals were replaced and reactor power was reduced to 86% to reduce the valve chatter as an interim action. 2-MS-4B was replaced with a normally open nozzle check valve and reactor power was returned to 100%. The inability of 2-MS-4B to prevent reverse flow, constituted an unanalyzed condition that could result in exceeding the containment design pressure in response to a main steam line break. This report is being submitted as a condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety pursuant to 10 CFR 50.73(a)(2)(ii)(B).

On July 6, 2023, at 0938, Millstone Power Station Unit 2 was in Mode 1 at 100 percent reactor power, when it was identified that the turbine driven auxiliary feedwater (TDAFW) pump supply check valve from #2 steam generator, 2-MS-4B, may not be able to perform its safety function to prevent reverse flow in the event of a m ain steam line break (MSLB) in the #2 steam generator main steam line upstream of the main steam isolation valves. To verify its operability, the 2-MS-4B closure test was performed on July 6, 2023, at 1202, with unsatisfactory results. The TDAFW pump steam supply line containing 2-MS-4B was isolated by closing steam supply stop valve, 2-MS-202, from #2 steam generator on July 6, 2023, at 1206. 2-MS-4B was disassembled and the disc bolt holding the dis c to the swing arm was found to have failed with the disc sitting ins ide the valve body.

On July 3, 2023, at 2005, in Mode 1 during power ascension from a refueling outage, chattering was noted on 2-MS-4B.

Station personnel performed troubleshooting in an effort to eliminate or reduce the valve chatter by reconfiguring plant alignment and isolating steam traps on the Unit 2 main steam piping. Operations performed 2-MS-4B closure test on July 4, 2023, at 0359, which was satisfactory, demonstrating that the valve was capable to perform its safety function.

Continued valve chattering was observed on July 6, 2023, at 0250, providing a reasonable assurance that the valve disc was still attached to the swing arm and valve 2-MS-4B would be able to per form its isolation function at that time.

On July 6, 2023, upon no longer hearing valve chattering, the 2-MS-4B closure test was performed which required isolating one path of steam to the TDAFW Pump and entry into Technical Specification Action Statement (TSAS) 3.7.1.2.a at 1202. Results of the surveillance were unsatisfactory and the main steam line was maintained isolated. To support internal inspection of 2-MS-4B, both steam supply lines to the TDAFW Pum p were required to be isolated and TSAS 3.7.1.2.c was entered on July 6 at 1727 hours0.02 days <br />0.48 hours <br />0.00286 weeks <br />6.571235e-4 months <br />. The 2-MS-4B internals were replaced and post maintenance tests were completed including a satisfactory valve closure test on July 8, 2023. At 1729 on July 8, 2023, TSAS 3.7.1.2.c was exited. Troubleshooting was perfor med and an alternate plant configuration that involved lowering reactor power to 86%

that reduced valve chatter as an interim measure on July 11, 2023. 2-MS-4B closure test was performed with satisfactory results

The plant was maintained at 86% power awaiting an Engineering Change to replace the 2-MS-4B swing check valve with a normally open nozzle check valve. The 2-MS-4B nozzle check valve installation and post maintenance tes ting at 100% reactor power was completed on July 27, 2023, at 1307.

Evaluation determined that this condition, inability of 2-MS-4B to prevent reverse flow, constituted an unanalyzed condition that could result in exceeding the containment design pressure in response to a main steam line break. This failure would have resulted in the blowdown of both steam generators during a main steam line break in #2 steam generator main steam line upstream of the main steam isolation valves until the operators could manually isolate the faulted steam generator.

NRC resident inspectors were notified and an 8-hour non-emergency notification that the plant had been in an unanalyzed condition for the failure of 2-MS-4B was made on July 6, 2023, at 1648.

This 60-day report is being submitted to report that on July 6, 2023, from the time of discovery at 0938 until 1206, when the TDAFW pump steam supply line containing 2-MS-4B was isolated by closing steam supply stop valve, 2-MS-202, a condition existed that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety pursuant to 10 CFR 50.73(a)(2)(ii) (B).

CAUSE

The direct cause of the valve failure was mechanical damage to the disc bolt caus ed by check valve chatter.

ASSESSMENT OF SAFETY CONSEQUENCE S

The as found condition of 2-MS-4B resulted in the inability of the check valve to prevent reverse flow from the #1 steam generator main steam line in the event of a MSLB in the #2 steam generator main steam line upstream of the main steam isolation valves. Additionally, the potential for the 2-MS-4B separated disc to block steam flow from the #2 steam generator main steam line through 2-MS-4B to the TDAFW pump has not been conclusiv ely ruled out.

In the event of a MSLB in the #2 steam generator main steam line upstream of the main steam isolation valves with 2-MS-4B failing to prevent reverse flow, then the FSAR Section 14.8.2 MSLB analyses for the containment response would no longer be bounding. In this scenario, additional mass and energy releases fr om the intact #1 steam generator main steam line would continue until the operators isolate this steam flow path in accordance with the existing Emergency Operating Procedure guidance. This would result in an increase in mass and energy releases to containment with the predicted containment pressure exceeding the c ontainment design pressure of 54 psig. While the containment design pressure would have been exceeded, the predicted peak containment pressure would be less than the containment lower bound failure pressure of 102 psig established in the Millstone 2 Individual Plant Examination.

In the event the 2-MS-4B separated disc blocked all steam flow fr om the #2 steam generator main steam line through 2-MS-4B to the TDAFW pump, the other flow path from the #1 steam generator main steam line through 2-MS-4A to the TDAFW pump was available. The steam flow path through 2-MS-4A to the TDAFW pump is adequate for the pump to perform its design function of delivering auxiliary feedwater to the steam generators in the event of a loss of all AC power (station black out) and for all FSAR Chapter 14 Safety Analysis scenarios, except for MSLB in either main steam line upstream of the main steam isolation valves. In this MSLB scenario, the two remaining motor driven auxiliary feedwater pumps would be adequate to satisfy the reactor coolant system (RCS) decay heat removal safety function.

CORRECTIVE ACTION

The TDAFW pump steam supply swing disk check valve, 2-MS-4B, was replaced with a normally open nozzle check valve. Additional corrective actions will be taken in accordanc e with the stations corrective action program.

PREVIOUS OCCURRENCES

LER 2021-002-00

On Oct 23, 2021, during valve overhaul of 2-MS-4B, the valve disc was found separated from the disc arm. The direct cause of the event was mechanical damage due to valve disk chattering. Two interim corrective actions were implemented. An alternate system alignment was established which significantly reduced 2-MS-4B chatter. Also, 2-MS-4B was redesigned to have a more robust valve disc retention mechanism to reduce likelihood of its failure.

Following troubleshooting and discussions with third party vendors, an Engineering Change was implemented in May 2023 during the Unit 2 refueling outage to impr ove the piping configuration design to prevent the most probable cause, turbulent flow, from initiating check valve chatter. The piping configuration change was not successful in stopping the chatter. An Engineering Change to change the check valve was not pursued due to previous successes in preventing check valve chattering and indus try OE.

ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES

SB Main Steam V Valve