05000423/LER-2015-001
| Millstone Power Station Unit 3 | |
| Event date: | 02-19-2015 |
|---|---|
| Report date: | 04-20-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 50836 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4232015001R00 - NRC Website | |
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1. EVENT DESCRIPTION:
On February 19, 2015, with Millstone Power Station Unit 3 (MPS3) at 100% power and in operating mode 1, an individual on a fire watch rove processed through a dual train high energy line break (HELB) door normally and upon checking the door after passage the individual noted the door did not latch. The Control Room was promptly notified. An operator was dispatched to investigate. The operator exercised the door lock-set mechanism freeing the latch allowing the door to properly latch.
The door was inoperable for approximately 7 minutes. Technical Specification 3.0.3 was entered and exited appropriately.
This event was reported to the NRC pursuant to 10 CFR 50.72(b)(3)(v)(D), (NRC event # 50836) as a condition that could have prevented the fulfillment of a safety function for systems needed to mitigate the consequences of an accident. This event is also being reported pursuant to 10 CFR 50.73(a)(2)(v)(D), as a condition that could have prevented the fulfillment of a safety function for systems needed to mitigate the consequences of an accident.
BACKGROUND:
This door fulfills the requirements of a Security Door, Technical Requirement Manual Fire Door, CO2 Door, Dual Train Protection Door, and a HELB Door. It is a key card actuated door with a crash bar on one side and a thumb latch on the other side. The door is part of the HELB barrier for the A and B 480 volt switchgear.
2. CAUSE:
Although no definite failure mechanism was identified, the door was experiencing high usage due to compensatory fire watch roves entering/exiting the door. Further the door design has the door swing such that the HELB event would act to open the door when the latch fails.
3. ASSESSMENT OF SAFETY CONSEQUENCES:
Given the low likelihood of an Auxiliary Building HELB occurring during the time the door was not properly latched (7 minutes), the consequences of this event was of very low safety significance.
4. CORRECTIVE ACTION:
Since this event occurred on the back shift, a maintenance technician was called in to inspect the door lock-set mechanism and affect any necessary repairs. The technician reported his inspection was satisfactory. He exercised the door lock-set mechanism from both the crash bar and the thumb release mechanisms approximately 30 times without any repeat indications of the latch sticking or not functioning. He also noted he tightened one screw on the mechanism that he found loose during this inspection. Continued exercises of the door mechanism after tightening the screw showed no difference in the smooth and proper operation of the door lockset mechanism.
It was identified that the door was experiencing high usage due to compensatory fire watch roves entering/exiting the door. Equipment repairs have been completed eliminating the need for this high frequency fire rove activity. Additionally, the preventive maintenance for the door lock-set mechanism has been changed.
A design change to reverse the door swing such that the HELB event would cause the door to close and thus not rely on the lock-set mechanism is being considered.
Additional corrective actions are being taken in accordance with the station's corrective action program.
5. PREVIOUS OCCURRENCES:
- MPS3 LER 2014-004-00, Unlatched Dual Train HELB Door Results in Potential Loss of Safety Function.
6. Energy Industry Identification System (EIIS) codes:
- Door — DR
- Switchgear — SWGR