05000336/LER-2023-002, 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) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3362023002R00 - NRC Website | |
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Dominion Energy Nuclear Connecticut, Inc.
Millstone Power Station 314 Rope Ferry Road, Waterford, CT 06385 DominionEnergy.com U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 AUG 3 *1 2023 DOMINION ENERGY NUCLEAR CONNECTICUT, INC.
MILLSTONE POWER STATION Unit 2 LICENSEE EVENT REPORT 2023-002-00 A Dominion ii# Energy Serial No.:
23-209 MPS Lie/ JP RO Docket No.: 50-336 License No.: DPR-65 FAILED CHECK VALVE RESULTED IN AN UNANALYZED CONDITION.
This letter forwards Licensee Event Report (LER) 2023-002-00, documenting a condition that was discovered at Millstone Power Station Unit 2 (MPS2) on July 6, 2023. This LER 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).
There are no regulatory commitments contained in this letter or its enclosure.
Should you have any questions, please contact Mr. Dean E. Rowe at (860) 444-5292.
Sincerely,
~~d'~
Michael J. O'Connor Site Vice President - Millstone Enclosure: LER 336/2023-002-00
Serial No.23-209 Docket No. 50-336 Licensee Event Report 2023-002-00 Page 2 of 2 cc:
U.S. Nuclear Regulatory 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 Report 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 CONNECTICUT, INC.
Abstract
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 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).
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- 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
052 050
ASSESSMENT OF SAFETY CONSEQUENCES
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 conclusively 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 from 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 containment 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 from 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 accordance 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 improve 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 industry OE.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES SB Main Steam V Valve Millstone Power Station - Unit 2 336 2023 002 00
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