05000336/LER-2024-002, Unit 2, Two Main Steam Safety Valves Failed to Lift within the Acceptance Criteria Resulting in a Condition Prohibited by Technical Specifications
| ML24331A015 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 11/26/2024 |
| From: | Petty J Dominion Energy Nuclear Connecticut |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 24-304 LER 2024-002-00 | |
| Download: ML24331A015 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3362024002R00 - NRC Website | |
text
Dominion Energy Nuclear Connecticut, Inc.
Millstone Pow.er Station 314 Rope Ferry Road, Waterford, CT 06385 Dominion Energy.com U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 NOV 2 6 2024 DOMINION ENERGY NUCLEAR CONNECTICUT. INC.
MILLSTONE POWER STATION UNIT 2 LICENSEE EVENT REPORT 2024-002-00 Serial No.:
24-304 MPS Lie/JP RO Docket No.:
50-336 License No.: DPR-65 TWO MAIN STEAM SAFETY VALVES FAILED TO LIFT WITHIN THE ACCEPTANCE CRITERIA RESULTING IN A CONDITION PROHIBITED BY TECHNICAL SPECIFICATIONS This letter forwards Licensee Event Report (LER) 2024-002-00, documenting a condition that was discovered at Millstone Power Station Unit 2 (MPS2) on October 1, 2024. This LER is being submitted pursuant to 10 CFR 50.73 (a)(2)(i)(B) as a condition prohibited by technical specifications.
A supplemental report will be provided upon completion of the cause determination.
There are no regulatory commitments contained in this letter or its enclosure.
Should you have any questions, please contact Ms. Lori Kelley at (860) 447-1791 x 6520.
Sincerely, f J F-at. "JAfo\~'"S t l!T"f'(
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1'c.T,,1t. s,.,P Jam s T.
etty Site Vice President - Millstone Enclosure: LER 336/2024-002-00
cc:
U.S. Nuclear Regulatory Commission Region I 475 Allendale Road, Suite 102, King of Prussia, PA 19406-1415.
R. V. Guzman NRG 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 NRG Senior Resident Inspector Millstone Power Station Serial No.24-304 Docket No. 50-336 Licensee Event Report 2024-002-00 Page 2 of 2
ATTACHMENT Serial No.24-304 Docket No. 50-336 Licensee Event Report 2024-002-00 LICENSEE EVENT REPORT 2024-002-00 TWO MAIN STEAM SAFETY VALVES FAILED TO LIFT WITHIN THE TECHNICAL SPECIFICATION ACCEPTANCE CRITERIA RESULTING IN A CONDITION PROHIBITED BY TECHNICAL SPECIFICATIONS MILLSTONE POWER STATION UNIT 2 DOMINION ENERGY NUCLEAR CONNECTICUT, INC.
Abstract
On October 1, 2024, Millstone Power Station Unit 2 (MPS2) was in Mode 1 at 100 percent reactor power, while performing Unit 2 Main Steam Safety Valve (MSSV) set pressure testing when two safety valves 2-MS-249 and 2-MS-251 failed to lift within the (+/- 3%) ({As Found" acceptance criteria of Technical Specification (TS) 3.7.1.1 and were declared inoperable. Operations entered the four-hour limited condition of operation (LCO) Technical Specification action statement. 2-MS-249 was adjusted, and both safety valves were returned to operable status by two consecutive successful as-left set pressure tests. The cause for the failure of the MSSVs to lift within the acceptance criteria is still under evaluation. Engineering performed an evaluation that concluded that there is reasonable assurance that valves 2-MS-249 and 2-MS-251 will lift at their design set pressures throughout the coming surveillance interval. This event is reported pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the plant's Technical Specifications. A supplemental to this LER will be provided when the cause is determined.
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~-I 002 1-G On October 1, 2024, Millstone Power Station Unit 2 (MPS2) was in Mode 1 at 100 percent reactor power, when set pressure testing of Main Steam Safety Valves (MSSVs) was conducted per plant procedure SP 2730B, "Main Steam Safety Valve Testing". During the surveillance testing, valves 2-MS-249 and 2-MS-251 did not meet the required "as-found" set pressure test acceptance criteria of+/- 3% per MPS2 Technical Specification (TS) requirements and were declared inoperable. Operations entered Technical Specification action statement (TSAS) 3.7.1.1.a.1 for 2-MS-249 at 1251 hours0.0145 days <br />0.348 hours <br />0.00207 weeks <br />4.760055e-4 months <br /> and for 2-MS-251 at 1510 hours0.0175 days <br />0.419 hours <br />0.0025 weeks <br />5.74555e-4 months <br />.
The failed MSSVs, 2-MS-249 (S/N BN4966) and 2-MS-251 (S/N BN4974) are manufactured by Dresser Industries. 2-MS-249 was adjusted, and both valves were retested in accordance with SP 2730B. Set pressures were returned within the "As-Left" acceptance criteria of+/- 1 % of the valve nameplate set pressure, and the valves were declared operable.
Operations exited TS 3.7.1.1.a.1 on October 1, 2024, at 1445 hours0.0167 days <br />0.401 hours <br />0.00239 weeks <br />5.498225e-4 months <br /> and 1524 hours0.0176 days <br />0.423 hours <br />0.00252 weeks <br />5.79882e-4 months <br />, respectively.
The as-found test result for 2-MS-249 was found 4% lower than the nameplate set pressure of 1000.3 psig, or 10.2 psi outside of the acceptance range. Following the test failure, adjustments were made until two successive tests were performed with results meeting the required "As-Left" test acceptance criteria of+/- 1 %. The as-found test result for 2-MS-251 was 3.3% lower than the nameplate set pressure of 1020.3 psig, or 3.0 psi outside of the acceptance range.
Following the test failure, the valve was retested without adjustment until 2 successive tests were performed with results meeting the required "As-Left" test acceptance criteria of+/- 1 %. Each valve was declared operable within the four hours allowed by TSAS 3.7.1.1.a.1.
Test scope expansion was required, and all other Main Steam Safety Valves were tested on "A" Main Steam header. All the MSSVs as-found lift settings were found to be within +/- 3% of their set pressures. Additionally, two valves from the "B" Main Steam header were selected and tested. After testing was completed, all tested MSSVs as-left set pressures were within +/- 1 % of TS acceptance criteria.
Plant Technical Specification (TS) 3.7.1.1 requires that all MSSVs be OPERABLE with lift settings as specified on Table 4.7-1. Table 4.7-1 allows a+/- 3% setpoint tolerance (allowable value) on the as-found lift setting for OPERABILITY.
During this testing, the MSSVs are OPERABLE provided that the actual as-found lift settings are within +/- 3% of the required lift setting. Table 4.7-1 requires that the as-left lift setting shall be within+/- 1 % of the required lift setting. Since the as-found lift pressures for the two valves were not within the+/- 3% TS allowable values, these valves were not OPERABLE.
NUREG 1022 provides guidance regarding multiple test failures. Multiple MSSVs exceeding the allowable TS acceptance criteria limits is an indication that the discrepancies may have occurred during the operating cycle and were therefore likely inoperable for greater than the TS allowed outage time of 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. This condition is reportable under 10 CFR 50.73(a)(2)(i)(B), "any operation or condition prohibited by the plant's technical specifications."
CAUSE
The cause determination has not been completed at this time. A supplemental to this LER will be submitted.
ASSESSMENT OF SAFETY CONSEQUENCES
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~-I 002 1-G The FSAR Chapter 14 limit on main steam system pressurization is 110% of the design pressure or 1100 psia.
FSAR Chapter 14 was reviewed for events that credited Main Steam Safety Valves (MSSV) opening. Events that credit the MSSV's for secondary side overpressure protection would be bounded by a lower MSSV opening setpoint and therefore would not have an adverse effect on event outcomes. MSSV's lifting earlier also have the potential to increase offsite dose and decrease steam generator inventory. The following FSAR Chapter 14 events were examined for consequences of a lower MSSV lift setpoint.
A Steam Generator Tube Rupture with a Loss of Offsite Power assumes the ruptured steam generator MSSV setpoints are -3.0% of their respective lift setpoint to establish a worst case bounding offsite dose. 2-MS-249 and 2-MS-251 would still be bounded by this analysis as the lower MSSV's on the same steam generator, 2-MS-247 and 2-MS-254, open at 3%
below the setpoints of 1000 PSIA (970 PSIA) and 1005 PSIA (974.85 PSIA), respectively. The lower MSSVs would be the valves controlling pressure on that steam generator after the initial turbine trip until Atmospheric Dump Valves (ADV's) begin controlling pressure 1800 seconds into the event.
A Control Rod Ejection Accident (CREA) at power with a subsequent Loss of Offsite Power event credits all MSSV's opening for overpressure protection. 2-MS-249 and 2-MS-251 opening at a lower setpoint is still bounded under this condition. Additionally, the radiological consequences are analyzed to ensure adherence to 10 CFR 50.67 and Regulatory Guide 1.183 limits. After the initial high-pressure condition, both MSSV's and ADV's are credited for decay heat removal.
2-MS-249 and 2-MS-251 having lower setpoints would not lead to additional fission product inventory being released as the decay heat removal and primary to secondary leakage assumptions are independent and don't change. The analysis of this event would continue to be bounded and within 10 CFR 50.67 and Regulatory Guide 1.183 limits.
A Loss of Normal Feed (LONF) with a subsequent Loss of Offsite Power event utilizes the MSSV's for decay heat removal. This analysis assumes the MSSV's adhering to a +3.0% above lift setpoint. While 2-MS-249 and 2-MS-251 would open at a lower steam generator pressure than is assumed in FSAR Chapter 14, the rate of decay heat generation and proportionally steam generator inventory removal would remain the same. The analysis would still be bounded in that the secondary side overpressure protection is met and the steam generators would not reach dry-out conditions during a LONF event.
Based on this assessment of the analyzed events, the as found condition of 2-MS-249 and 2-MS-251 was not safety significant.
CORRECTIVE ACTION
2-MS-249 and 2-MS-251 were declared operable after two successive tests with results within +/-1 % of the set pressure.
An engineering evaluation determined that there is reasonable assurance that 2-MS-249 and 2-MS-251 will meet their set pressures throughout the coming surveillance interval. Scope expansion was completed with all valves meeting as found allowable values. Additional corrective actions will be taken in accordance with the station's corrective action program.
PREVIOUS OCCURRENCES I
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- - I I -0 002 A similar condition occurred at MPS2 where two main steam valves, 2-MS-242 and 2-MS-245, failed the as-found test criteria. The cause of the as-found test failure was due to disc to seat micro-bonding (seat sticking), resulting in the valves lifting at higher pressure. This was documented under LER 2020-001-00.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES SB Main Steam RV Relief Valve Page 4
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