05000352/LER-2014-001
| Limerick Generating Station, Unit 1 | |
| Event date: | 10-30-2013 |
|---|---|
| Report date: | 01-08-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3522014001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at 100% power. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Sunday, November 10, 2013, Limerick Unit 1 was operating at 100% power. At 1351 hours, the main control room supervisor was notified that both doors (EIIS:DR) on one reactor enclosure airlock had been briefly opened. The reactor enclosure low pressure alarm (EIIS:ALM) did not actuate during the event and the reactor enclosure pressure did not drop below the Technical Specification limit of 0.25 inch of vacuum water gauge. However, reactor enclosure secondary containment (EIIS:NH) integrity was declared inoperable for the period when both doors were open.
An investigation confirmed that two workers had simultaneously opened the inboard and outboard doors on the reactor enclosure 217 foot elevation airlock for a period of less than 10 seconds. The workers immediately re-closed both doors and the breach of secondary containment was terminated. The workers notified Operations shift management of the unexpected containment breach.
Technical Specification (TS) 3.6.5.1.1 Reactor Enclosure Secondary Containment Integrity surveillance requirement 4.6.5.1.1.a requires verification that reactor enclosure pressure is greater than or equal to 0.25 inch of vacuum water gauge which is performed on a 24 hour frequency. The TS surveillance requirement 4.6.5.1.1.b.2 requires at least one door in each access to the reactor enclosure be verified closed which is performed on a 31 day frequency. TS 3.6.5.1.1 is applicable in operational conditions (OPCON) 1, 2 and 3.
An 8-hour NRC ENS notification was required by 10CFR50.72(b)(3)(v)(C) for a condition that at the time of discovery could have prevented fulfillment of a safety function of structures or systems needed to control the release of radioactive material. The ENS notification (#49526) was completed on Sunday, November 10, 2013, at 1615 hours ET.
This LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(v)(C) for a condition that could have prevented the fulfillment of the safety function of structures or systems needed to control the release of radioactive material.
VRC FORM 366A (10-2010) == Analysis of the Event ==
There was no actual safety consequence associated with this event.
The potential safety consequences of this event were minimal. Both doors on the airlock were open simultaneously for less than 10 seconds.
To prevent an unplanned breach of secondary containment each reactor enclosure airlock is equipped with door open indicating lights which are used to locally verify the door status. If both doors are opened simultaneously a local alarm is actuated. If both doors remain open for greater than 10 seconds an alarm is actuated in the main control room and operators are dispatched to verify the airlock doors are closed.
UFSAR 6.2.3.2.1 describes the secondary containment design. The reactor enclosure secondary containment (Zones I and II) are designed to limit the inleakage to 200% of their zone free volume per day, and the refueling area secondary containment (Zone III) is designed to limit the inleakage to 50% of its zone free volume per day. These inleakage rates are based on a negative interior pressure of 0.25 in wg, while operating the standby gas treatment system (SGTS). Following a LOCA the affected zone is maintained at this negative pressure by operation of the SGTS.
Cause of the Event
This event was caused by a weakness in the design of the reactor enclosure airlocks since there is no mechanical interlock and the door open indicating light does not prevent simultaneous opening of both airlock doors.
Corrective Action Planned A modification is being evaluated for the airlock doors.
Previous Similar Occurrences Unit 2 LER 2013-003 was submitted due to a reactor enclosure airlock breach caused by a weakness in the airlock design. Unit 2 LER 2013- 002 was submitted due to a reactor enclosure airlock breach caused by an non-functional airlock door open indicating light not providing the correct door status.
VRC FORM 366A (10-2010)