05000263/LER-2014-009

LER-2014-009, Both Emergency Filtration Trains Inoperable

Monticello Nuclear Generating Plant
Event date:	08-05-2014
Report date:	09-30-2014
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2632014009R00 - NRC Website

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EVENT DESCRIPTION

At the time of event, the plant was operating in Mode 1 at approximately 88% rated thermal power.

On August 5, 2014, the 'A' train of emergency filtration (EFT) was out of service for charcoal filter [FLT] replacement work. The '13' EFT was placed in service at 14:34 hours to supply fresh filtered air to the Control Room.

At 14:46 hours the Control Room received a low flow alarm for '13' EFT which initiated an automatic 'B' EFT fan [FAN] trip and damper [DMP] to close which isolated the filter train. This isolation resulted in both trains of EFT being inoperable. As a result, Technical Specification 3.7.4 was not met and Limiting Condition for Operation (LCO) 3.0.3 was entered at 16:01 hours.

LCO 3.0.3 was exited following successful completion of post maintenance testing on the 'A' train of EFT at 17:07 hours. There was no addition of negative reactivity to the reactor during the time the plant was in LCO 3.0.3.

EVENT ANALYSIS

This condition is being submitted in accordance with 10 CFR 50.73(a)(2)(v)(D) "Event or Condition that Could Have Prevented Fulfillment of a Safety Function." With both emergency filter trains inoperable, Control Room habitability would not have been assured should a radiological release have occurred. The emergency filtration safety trains mitigate the consequences of an accident by assuring Control Room habitability.

As a result of both trains of EFT being inoperable concurrently, this event is considered a safety system functional failure.

SAFETY SIGNIFICANCE

The EFT system has two functions. The first function is to allow manual isolation of outside air from the Control Room and first and second floors of the EFT Building which places the system in full recirculation.

The second function is to automatically isolate outside air and provide filtered/pressurizing air to the Control Room and first and second floors of the EFT Building. With both filter trains inoperable, Control Room habitability may not have been maintained during an unfiltered radioactive release. During this events' time frame, Control Room breathing air supply system was available to provide three hours of air for up to eight personnel, but was not required to be used.

CAUSE

The 'B' EFT fan tripped because the damper actuator failed due to poor quality of vendor refurbishment.

The damper actuator was in service (operating train) for approximately 250 hours0.00289 days <br />0.0694 hours <br />4.133598e-4 weeks <br />9.5125e-5 months <br /> prior to failure. Normal life expectancy for rebuilt actuators is greater than five years.

The 'B' EFT trip caused the plant to enter LCO 3.0.3 due to the legacy operating procedure that permitted Operators to start the 'B' EFT while 'A' EFT was inoperable for maintenance.

CORRECTIVE ACTION

The failed actuator was subsequently replaced and post maintenance testing was satisfactorily completed.

The damper actuator has been returned to the vendor for failure analysis. Additional corrective actions may be required based on the results of those analyses.

The EFT procedures will also be revised to restrict operation of the standby train while in protected status.

PREVIOUS SIMILAR EVENTS

There were no previously similar Licensee Event Reports in the past three years.