05000423/LER-2024-002, Unit 3, Door Latch Failure Resulted in Loss of Safety Function for Secondary Containment Boundary
| ML24331A033 | |
| 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 | |
| 24-348 LER 2024-002-00 | |
| Download: ML24331A033 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4232024002R00 - NRC Website | |
text
Dominion Energy Nuclear Connecticut, Inc.
Millstone Pow.er Station 314 Rope Ferry Road, Waterford, CT 06385 DominionEnergy.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 3 LICENSEE EVENT REPORT 2024-002-00 Serial No.:
24-348 MPS Lie/JP RO Docket No.: 50-423 License No.: NPF-49 DOOR LATCH FAILURE RESULTED IN LOSS OF SAFETY FUNCTION FOR SECONDARY CONTAINMENT BOUNDARY This letter forwards Licensee Event Report (LER) 2024-002-00, documenting a condition that was discovered at Millstone Power Station Unit 3 (MPS3) on October 10, 2024. This LER is being submitted pursuant to the requirements of 10 CFR 50. 73(a)(2)(v)(C) and 10 CFR 50. 73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function for systems or structures that are needed to control the release of radioactive material, and to mitigate the consequences of an accident.
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 x6520.
Sincerely,
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T. Petty Site Vice President - Millstone Enclosure: LER 423/2024-002-00
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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.24-348 Docket No. 50-423 Licensee Event Report 2024-002-00 Page 2 of 2
ATTACHMENT Serial No.24-348 Docket No. 50-423 Licensee Event Report 2024-002-00 LICENSEE EVENT REPORT 2024-002-00 DOOR LATCH FAILURE RESULTED IN LOSS OF SAFETY FUNCTION FOR SECONDARY CONTAINMENT BOUNDARY MILLSTONE POWER STATION UNIT 3 DOMINION ENERGY NUCLEAR CONNECTICUT, INC.
Abstract
On October 10, 2024, at 0902 hours0.0104 days <br />0.251 hours <br />0.00149 weeks <br />3.43211e-4 months <br />, with Millstone Power Station Unit 3 (MPS3) in Mode 1, 100 percent reactor power, a door latch mechanism failed. This rendered the Secondary Containment boundary inoperable. Maintenance repaired the door latch which returned the Secondary Containment boundary to operable on October 10,2024 at 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />. The cause of the failure was due to loosening of the set screw engagement hatch handle from frequent access and egress through the door. This condition is being reported in accordance with the requirements of 10 CFR 50. 73(a)(2)(v)(C) and 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of a radioactive material and mitigate the consequences of an accident.
EVENT DESCRIPTION
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00423 D
NUMBER NO.
I YEAR SEQUENTIAL REV
~-I 002 1-G On October 10, 2024, at 0902 hours0.0104 days <br />0.251 hours <br />0.00149 weeks <br />3.43211e-4 months <br />, with Millstone Power Station Unit 3 (MPS3) in Mode 1, 100 percent reactor power, during a plant walkdown, it was discovered that a Supplemental Leak Collection and Release System (SLCRS) door, SF-21-5, latch had fallen off. This door is latch dependent, and therefore would not close in the SLCRS air flow direction. It is located in the Unit 3 Engineered Safety Features Building (ESF) and is part of the Secondary Containment boundary.
Maintenance performed necessary repairs on the latch mechanism which returned the Secondary Containment boundary to OPERABLE on October 10, 2024, at 1200. The latching mechanism for the door was fabricated on site, and therefore does not have a unique EIIS manufacturer code.The Secondary Containment must be OPERABLE to ensure proper operation of the Supplemental Leak Collection and Release System (SLCRS) and auxiliary building filter systems to limit radioactive leakage from containment to those paths and leakage rates assumed in the accident analyses. Additionally, Plant Technical Specification (TS) 3.6.6.2 states that with Secondary Containment inoperable, restore Secondary Containment to OPERABLE status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. The Secondary Containment was inoperable for less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
The condition of this single train system is reportable pursuant to the requirements of 10 CFR 50.73(a)(2)(v)(C) and 10 CFR 50.73(a)(2)(v)(D) as a "condition that could have prevented the fulfillment of the safety function of structures or system that are needed to control the release of radioactive material and to mitigate the consequences of an accident."
CAUSE
Frequent access and egress through door SF-21-5 lead to loosening of the set screw engagement for the hatch handle.
This condition prevented the door from being secured in a closed position, creating a breach in the Secondary Containment boundary.
ASSESSMENT OF SAFETY CONSEQUENCES
The Supplementary Leak Collection and Release System (SLCRS) is designed to mitigate the radiological consequences of postulated accidents by filtering the exhaust air from the secondary containment. SLCRS is comprised of two redundant trains of exhaust fans and filter units. The safety function of SLCRS is to maintain a negative pressure in the secondary containment to ensure there is no unfiltered leakage to the outside environment from these areas in the event of a loss of coolant accident, other than those releases from SLCRS bypass and damper leakage accounted for in the dose analyses.
SLCRS is not normally in operation but is required to be OPERABLE in Modes 1-4. The SLCRS system and the auxiliary building filtration portion of the auxiliary building ventilation system start on receipt of a Safety Injection Signal. SLCRS is required to draw down the secondary containment to a negative pressure within 120 seconds after the accident. The negative pressure provided by SLCRS ensures the containment releases occur through the release pathways modeled in the design basis dose analysis. With secondary containment breached, the ability for SLCRS to achieve negative pressure was impacted. The acceptability of the condition with respect to the station dose analyses is established on the basis of the conservatisms in the existing dose analysis of record for the LOCA event and the fact that the auxiliary ventilation system was operable at the time of this event. One of the largest conservatisms in the analysis is the assumption of containment leak rate during a LOCA event. This assumption is set at the Technical Specification leak rate limit La [0.3 %
by weight of the containment air (wt.%)/day]. Replacing the assumed value with the current Integrated Leak Rate Test (ILRT) test result of 0.0506 wt. %/day and assuming 100% SLCRS bypass [i.e., no filtration and ground level release], the calculated potential doses to the public from a postulated LOCA event remain below those documented in FSAR Table 15.0-8 and the 1 0CFR50.67 criteria. Therefore, this event is determined to be of low safety significance.
CORRECTIVE ACTION
I
- 2. DOCKET NUMBER
- 3. LER NUMBER I YEAR SEQUENTIAL REV NUMBER NO.
00423
. I 20241 -I 1-0 002 The latching mechanism for the door was repaired and the Secondary Containment boundary was restored to OPERABLE on October 10, 2024, at 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />. Additional corrective actions will be taken in accordance with the station's corrective action program.
PREVIOUS OCCURRENCES There have been similar occurrences at MPS3 in the past that resulted in rendering secondary containment inoperable.
Two of these occurrences, LER 2024-001-00 and LER 2020-005-00, were due to different causes. The third occurrence, LER 2017-001-00, had a similar cause, however, the failure mechanism was different from the current occurrence, therefore, this is not considered a repeat condition.
LER 2024-001 On August 21, 2024, at 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />, 'A' waste disposal building exhaust fan, 3HVR-FN8A, which is part of the secondary containment boundary, was removed during maintenance activities. The removal of 3HVR-FN8A created a breach in the secondary containment boundary. The root cause was the lack of identifying information in the work management database for 3HVR-FN8A to identify a potential to impact SLCRS.
LER 2020-005 On October 1, 2020, at MPS3, there was a SLCRS Door in the secondary containment blocked open rendering secondary containment inoperable. The identified apparent cause was a weakness within the process of notifying the control room prior to block open doors as it is not fully bounded by the procedures and lacks clear ownership.
The contributing factor was inadequate communication.
LER 2017-001 On January 20, 2017, at MPS3, there was a failure of the hydraulic door closer for SLCRS door A-24-4. The failure prevented the door from closing and prevented movement in either direction from the failed point, rendering the Secondary Containment boundary inoperable.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES SLCRS - VF, VG, VH Door-DR Auxiliary Building - NF Page 3
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