05000263/LER-2010-003

LER-2010-003, Secondary Containment Briefly Dearaded

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v), Loss of Safety Function
2632010003R01 - NRC Website

Event Description

On August 5, 2010, at approximately 1145 hours0.0133 days <br />0.318 hours <br />0.00189 weeks <br />4.356725e-4 months <br />, with the plant operating in Mode 1 at 100% power, DOOR-72 and DOOR-82 for airlock [AL] 413 (985 foot Pump Room) were inadvertently opened simultaneously, breaching the Secondary Containment (SCT) [VF] boundary. Personnel immediately identified the situation and closed both doors [DR] within approximately five seconds (estimated).

Shift supervision was notified. With both doors open, the station's Technical Specification (TS) Surveillance Requirement (SR) 3.6.4.1.3 (verify one Secondary Containment access door in each access opening is closed) was not met. The TS Limiting Condition for Operation (LCO) 3.6.4.1 was declared not met and Action A, (Restore secondary containment to Operable status) entered. The doors were verified closed. With SCT restored, TS 3.6.4.1 was met at 1219.

This event was not the result of a cognitive error. The Fix-It-Now Mechanical team was performing work in the 985 foot Pump Room (outside Secondary Containment) to investigate P-32, Waste Collector Pump, suction pressure. The low dose waiting area is within Secondary Containment, so personnel exited the pump room during work breaks/stopping points. At 1145 hours0.0133 days <br />0.318 hours <br />0.00189 weeks <br />4.356725e-4 months <br />, maintenance and radiation protection personnel attempted to enter the airlock simultaneously from the pump room and from within Secondary Containment. The doors were immediately (within approximately five seconds) closed and Operations shift supervision notified. Neither door has a window, so neither employee could see the other employee operating the opposite door. Both door electromagnets are normally de-energized on this airlock; and when a door opens, the opposite door electromagnet energizes to prevent the door from opening. When both doors opened simultaneously, each door moved away from the electromagnet before it could energize.

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 because for approximately five seconds the SCT TS LCO was not met. A subsequent Secondary Containment Capability Test performed on April 27, 2011 and evaluation, EC 18336, confirmed that the Standby Gas Treatment (SBGT) system remained capable of performing its safety function in the plant configuration that existed on August 5, 2010, including consideration for penetrations that were or may have been open at the time. It was determined that SCT does not lose safety function when both DOOR-72 and DOOR-83 are simultaneously opened, provided minimal other penetrations are open. The analysis demonstrates the initial flow into SCT during pressure equalization and subsequent inleakage from the HVAC supply duct does not cause SCT differential pressure to be less negative than that required by plant Technical Specifications. Also, SBGT flow would be less than the 4,000 cfm SCT design limit. On August 5, 2010, when both doors to the 985' Pump Room airlock were briefly, simultaneously opened, SCT remained sufficiently leak tight so Standby Gas Treatment could provide a filtered, elevated release of the SCT atmosphere. SCT could have performed its safety function of controlling the release of radioactive material, thus mitigating the consequences of an accident. Therefore, this U.S. NUCLEAR REGULATORY COMMISSIONNRC FORM 366A� LICENSEE EVENT REPORT (LER)(10-2010)

CONTINUATION SHEET

event is not considered a Safety System Functional failure for the purposes of Reactor Oversight Process performance indicator reporting per the guidance in NEI 99-02.

A similar event on June 3, 2010, had inadequate interim corrective actions. The site has now implemented administrative controls to obtain permission from the Work Execution Center prior to entering the airlock doors that have this vulnerability.

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). No systems supporting critical safety functions, including support systems, are impacted due to the loss of secondary containment, and initiating event frequencies are not impacted. Large Early Release Frequency is not significantly impacted since CDF is not affected, and the duration of the secondary containment breach is very small. Based on the above, the safety significance is minor.

An engineering evaluation determined that SCT would be able to perform its safety function with these doors open.

Cause

The cause of the event was the design of the interlock between the doors in the 985 foot Pump Room allows simultaneous entry under specific timing conditions (i.e., the doors are opened at exactly the same time — otherwise, the interlock prevents one door from opening if the other is open).

Additionally, the site did not take adequate interim corrective actions for the same event that occurred on June 3, 2010.

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).

• The station initiated administrative controls on all airlocks with the same design Work Execution Center permission is required to open these airlock doors.

• The station plans to install doors with windows on the vulnerable airlocks.

Failed Component Identification None NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (10-2010) LICENSEE EVENT REPORT (LER)

Previous Similar Events

On June 3, 2010, the same issue occurred (LER 2010-02).

Actions to prevent recurrence were not taken following this event because the site assessed that the likelihood of occurrence was so low that corrective actions were not warranted.