05000353/LER-2014-004
| Limerick Generating Station, Unit 2 | |
| Event date: | 04-04-2014 |
|---|---|
| Report date: | 06-03-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3532014004R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
2. DOCKET
Unit Conditions Prior to the Event Unit 2 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 Friday, April 4, 2014, Limerick Unit 2 was operating at 100% power. At approximately 0100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 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) setpoint is 0.20 inches of vacuum water gauge (WG) and the reactor enclosure low pressure alarm did not actuate during the event. The airlock was open for less than 10 seconds; therefore, the airlock open main control room alarm did not actuate. The 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 material through the airlock that connects Unit 2 reactor enclosure and Unit 2 turbine enclosure on the 269 foot elevation.
One worker opened the outboard door before the other worker had fully closed and latched the inboard 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.
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 inches of vacuum (WG) 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 is capable of performing its safety function during events when the airlock is 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 is designed to limit the inleakage to 200% of each zone's 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 inches 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
The workers failed to verify the inboard airlock door was latched fully closed prior to a co- worker opening the outboard airlock door. The workers failed to use the self-check fundamental. 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 airlock doors were closed to restore reactor enclosure secondary containment integrity.
The workers were coached regarding use of the self-check fundamental.
The lessons learned were communicated to the site.
Previous Similar Occurrences Unit 2 LER 2014-003, 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.