05000353/LER-2014-003
| Limerick Generating Station, Unit 2 | |
| Event date: | 03-20-2014 |
|---|---|
| Report date: | 05-19-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3532014003R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
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2. DOCKET
Unit Conditions Prior to the Event was in OPCON 5 performing a refueling outage; therefore, Unit 1 reactor enclosure secondary containment was inoperable and not affected by this event. There were no other structures, systems, or components out of service that contributed to this event.
Description of the Event
On Thursday, March 20, 2014, Limerick Unit 2 was operating at 100% power. At 2000 hours0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br />, the main control room supervisor was notified that both doors (El IS:DR) on one reactor enclosure airlock had been briefly opened. The reactor enclosure low pressure alarm (El IS:ALM), which is set at 0.20 inches of vacuum water gauge did not actuate during the event. Reactor enclosure pressure briefly increased to 0.23 inches of vacuum water gauge which exceeded the Technical Specification (TS) limit of 0.25 inches of vacuum water gauge.
Reactor enclosure secondary containment integrity (El IS:NH) was declared inoperable for the period when both doors were open.
An investigation confirmed that workers were moving equipment through the airlock that connects the Unit 1 reactor enclosure and Unit 2 reactor enclosure on the 253 foot elevation.
One worker opened the Unit 2 door before the other worker had closed the Unit 1 door. The airlock doors were open for a period of less than 10 seconds. The workers immediately reclosed both doors and the breach of secondary containment was terminated. The workers notified Operations shift management of the containment breach.
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 inches of vacuum water gauge 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 on a 31-day frequency. TS 3.6.5.1.1 is applicable in operational conditions (OPCON) 1,2 and 3.
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.
NEI 99-02 Revision 7, Regulatory Assessment Performance Indicator Guideline, section 2.2 Mitigating Systems Cornerstone, Safety System Functional Failures, Clarifying Notes, states the following:
Engineering analyses: events in which the licensee declared a system inoperable but an engineering analysis later determined that the system was capable of performing its safety function are not counted, even if the system was removed from service to perform the analysis.
This event will not be reported in the NRC performance indicator (PI) for safety system functional failures (SSFF) since an engineering analysis (technical evaluation) was performed which determined that the system was capable of performing its safety function during events when the airlock was open for less than 10 seconds. The post-LOCA dose calculation does not credit reactor enclosure secondary containment integrity for mitigation of on-site and off- site doses for the first 15.5 minutes of the event. Therefore, this event is bounded by the existing dose calculation.
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 a 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 that 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
A worker deviated from the briefing instructions regarding one worker being responsible to open and close the airlock doors while the other workers were responsible for pushing the cart through the open door. When the worker deviated from the briefing instructions the worker did not adhere to the airlock door policy. This event was not prevented by 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 Completed The affected airlock doors were closed to restore reactor enclosure secondary containment integrity.
The worker was coached regarding compliance with the responsibilities assigned at the pre- job brief and airlock door use.
The lessons learned were communicated to the site.
Previous Similar Occurrences Unit 1 LER 2014-003, Unit 2 LER 2014-002, Unit 2 LER 2014-001, Unit 1 LER 2014-002, Unit 1 LER 2014-001, and Unit 2 LER 2013-003 were submitted due to reactor enclosure airlock breaches allowed by the airlock design. Unit 2 LER 2013-002 was submitted due to a reactor enclosure airlock breach caused by a non-functional airlock door open indicating light not providing the correct door status.