05000219/LER-2006-004
| Docket Number121- 08�- - 2006,, -‘ 2006 -�004 = -�00 - ; 02 02 2007' | |
| Event date: | |
|---|---|
| Report date: | |
| 2192006004R00 - NRC Website | |
Unit Conditions Prior to the Event.
The unit was in the Power Operation Operational Condition at 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
At 13:51 on 12/8/06 the 'D' EMRV opened for about 57 seconds and re-closed without operator action.
EMRV actuation was confirmed by operator monitoring of the valve's acoustic monitors, EMRV solenoid actuated control room indication, relief valve tailpiece temperature, decrease in reactor power, decrease in generator output, decrease in reactor pressure and various alarms. Prior to the event the reactor was at 100% power, 1020 psig and normal water level. When the EMRV re-closed, the resultant pressure increase caused reactor power to increase to 102.46%, as indicated on the Average Power Range Monitors (APRMs), for approximately 9 seconds. Reactor power was reduced below 100% when the control room operator decreased the reactor recirculation flow rate.
EMRVs actuate on one of three signals: manually from the Control Room; reactor high pressure, as sensed on its associated IA83 pressure switch; and Automatic Depressurization System (ADS) initiation, which requires'ir three simultaneous signals of Reactor Triple Lo Level, High Drywell Pressure, and Core Spray Booster Pump pressure differential. None of these conditions for ADS initiation existed at the time of actuation.
Prior to the event Instrument Technicians were in the process of performing the Reactor Triple Lo Water Level Calibration, 619.3.006, and had just completed the calibration of level sensor RE18B, which was valved out of service when the EMRV actuated. The Instrument Technicians' actions were confirmed to be consistent with the surveillance procedure and could not explain EMRV's actuation.
RE18B was returned to service after the EMRV closed. The IA83D pressure switch, which actuates the 'D' EMRV on high pressure, was inspected for signs of unintentional impact from concurrent work in the area. No sign of impact was found, and the personnel who had been working in the area were interviewed but no inadvertent contact with the pressure switch was reported.
Procedure ABN-40, Stuck Open EMRV, was entered, but the EMRV closed without operator action prior to reaching the steps that would have required repositioning the EMRV's control switch to the OFF position.
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were also minimal. Operators took prompt action to decrease reactor power, which terminated the overpower condition in approximately 40 seconds.
The peak reactor power of 102.46% was of very short duration and would have a negligible impact on the decay heat for a Loss of Coolant Accident (LOCA). The LOCA analysis assumes that the initial conditions of the event are at 102% power at steady state, due to uncertainties in measurement and detection of core power. These analysis assumptions bound this event.
Opening of an EMRV at power is an evaluated event that is significantly bounded by the limiting events evaluated each reload and the associated operating limits documented in the Core Operating Limits Report. As such, no reactor vessel (pressure boundary) or fuel safety limits or other Specified Acceptable Fuel Design Limits (SAFDLs) were challenged A Prompt Investigation has been performed in accordance with current plant procedure "Event Response Guidelines". The investigation team determined that there was no evidence that this event was influenced by human performance. The investigation determined that the most likely cause of this event was a malfunction of the IA83D Pressure Sensor. A review of the as left data for EMRV Pressure Sensor test and calibration surveillance indicates that during the last surveillance test prior to this event, the set point was within specification. However, set point drift of the pressure switch since last surveillance occurred, lowering the set points by approximately 20-25 psig. The as found set point for IA83D after the event was still above the operating pressure at the time of the event. Component history review indicates that in July and August 2005, the IA83D pressure switch had excessive drift identified during the EMRV Pressure Sensor Test and Calibration. This component had a history of drifting in the past year. Furthermore, Operating Experience from Oyster Creek in 1993 indicates an EMRV pressure switch failed due to dirty contacts from natural aging. Based on the above, the degraded pressure switch was considered to be the most probable cause of the spurious opening of the D EMRV.
Cause of the Event
The most probable cause of the event was malfunction of the pressure switch connected to the D EMRV.
Corrective Action Completed.
The IA83D pressure switch and its associated relay were replaced.
Corrective Action Planned.
The new pressure switch performance will be monitored for a year to determine if periodic replacement of the pressure switches is warranted.
Previous Similar Occurrences There were no previous occurrences of a spurious EMRV opening that caused reactor power to exceed the licensed maximum power level.
Component Data.
Component: IA83D, Reactor High Pressure Cause: Pressure switch degradation System: SB (Main/Reheat Steam System) Component: PS (Pressure Switch) Manufacturer: Barksdale Model number: B2S-M12SS