05000263/LER-2010-004

LER-2010-004, Secondary Containment Briefly Inoperable Due to Simultaneous Opening of Airlock Doors

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v), Loss of Safety Function
2632010004R00 - NRC Website

On November 4, 2010, at 11:25 hours, with the plant operating in Mode 1 at 93% power, DOOR-62 (DR) and DOOR-63 for Airlock (AL) 124 from Main Access were inadvertently opened simultaneously, breaching the Secondary Containment (SCT) boundary. This occurred when personnel attempted to enter the airlock from Secondary Containment before the outboard door fully closed. The local alarm sounded for approximately one to two seconds, and the plant employees noticed DOOR-62 was not fully closed. Plant personnel immediately closed the doors and notified the Control Room that SCT was momentarily breached (for approximately 5 seconds). While both doors were open simultaneously, Surveillance Requirement (SR) 3.6.4.1.3, verify one Secondary Containment access door in each access opening is closed every 31 days, was failed. Technical Specification (TS) Limiting Condition for Operation (LCO) 3.6.4.1 was declared not met and Condition A was entered at 1125. The required action, A.1, is to restore secondary containment to Operable status, which was completed at 1125, as the report from the worker was that the issue had occurred and restoration had already been acted on.

This was not a cognitive error and appears to be caused by an intermittent failure of the interlock's magnetic bond sensor (IEL).

EVENT ANALYSIS

The event is reportable to the NRC under 10 CFR 50.73(a)(2)(v)(C and D) — Event or Condition that could have Prevented Fulfillment of a Safety Function. The station reported the event to the NRC under 10 CFR 50.72 (b)(3)(v)(C and D) on November 4, 2010.

This event is considered a Safety System Functional Failure because for approximately five seconds there was an unplanned SCT TS LCO not met condition.

SAFETY SIGNIFICANCE

There were no nuclear, radiological or industrial safety significant consequences related to this event.

The Monticello risk assessment group reviewed the event for risk impact. Defeating the airlock feature for Secondary Containment has no direct or indirect impact on the frequency of core damage (CDF).

For the period the airlock was inoperable, the conservative assumption was made that all core damage events will cause a large early release event. Additionally, a conservative time of 60 seconds, many times actual, was used for duration. The incremental large early release probability was calculated to be 1.34E-11, well below the risk significance threshold of 1E-7. Therefore, the conclusion is that the safety significance in terms of reactor safety and radiological release to the environment from this event is considered to be very low.

The apparent cause for Airlock 124 breach is intermittent failure of the magnetic bond sensor due to a lack of periodic maintenance. Discussions with the vendor verified this could be the case.

CORRECTIVE ACTION

The following actions were taken or are planned and will be tracked in the Monticello Corrective action program

• The doors were closed (within approximately 5 seconds) and appropriate TS actions taken.

• An Apparent Cause Evaluation was conducted to determine the cause.

• Troubleshooting was performed and the equipment worked as required.

• A work order has been generated to replace the magnets and switches in the interlock to positively assure equipment reliability.

• Periodic maintenance items, such as replacing magnets in this type of interlock, will be developed as appropriate and placed into the Preventive Maintenance Program.

PREVIOUS SIMILAR EVENTS

Similar events occurred on June 3 and August 5, 2010 (LER 2010-002-00 and LER 2010-003-00).

These two events occurred with a different type of airlock interlock.