05000293/LER-2010-002
| Event date: | 03-25-2010 |
|---|---|
| Report date: | 5-20-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 2932010002R00 - NRC Website | |
BACKGROUND
The Pilgrim Station secondary containment system is designed, in conjunction with other engineered safeguards and nuclear safety systems, to limit the release of radioactive material during normal plant operations within the limits of 10 CFR 20 and to limit the release of radioactive material so that offsite doses from a postulated design basis accident will be below the guideline values in 10 CFR 100.
The secondary containment system consists of four subsystems: (1) the Reactor Building, (2) the Reactor Building Isolation Control System (RBICS), (3) the Standby Gas Treatment System (SGTS), and (4) the Main Stack. The Reactor Building completely encloses the primary containment system which includes the Drywell (containing the Reactor) and Torus (containing the Suppression Pool). The function of the RBICS is to trip the Reactor Building ventilation supply and exhaust fans, isolate the normal Reactor Building ventilation system, and provide initiation signals to start the SGTS in the event of a postulated Loss of Coolant Accident (LOCA) inside the Drywell or a postulated fuel handling accident in the Reactor Building. The SGTS dischArges processed gases to the Main Stack via underground piping. The Main Stack provides an elevated release point for the processed gases. The mission time for the secondary containment system is 30 days.
The SGTS consists of ductwork and instrumentation and includes two full capacity, parallel air filtration assemblies that are located outside the Reactor Building. Each filter assembly includes an in-series air-operated supply damper, demister, an electrical heating coil, high efficiency particulate absorber (HEPA), two charcoal filter beds, a final HEPA filter, exhaust fan, motor-operated adjustable damper, fixed back draft damper, and an air- operated discharge damper. Cross connections between the trains and a restricting orifice are provided to maintain the required decay heat removal cooling air flow through the charcoal filter assembly in the inactive train.
Each train of SBGT contains demister doors surrounding the charcoal beds. The demister door allows access to the demister section of the SBGT Train. The demister is a section of the SBGT that removes the water droplets and drains the water to the Turbine Building sump. The demister acts as a dehumidifier to maintain the charcoal filter efficiency.
At normal reactor power operation, the SGTS Train 'A' is maintained in a standby status with the Train 'A' control switch in the AUTO position. Train 'B' is maintained in a standby status with the Train 'B' control switch in the STANDBY position. In this configuration and if an RBICS initiation signal occurs, the. Train 'A' heater and fan starts, the Train 'A' supply and discharge air-operated dampers open, Train 'B' starts (fan and heater) and the Train 'B' supply and discharge air-operated dampers open. Train 'B' automatically shuts down if sufficient flow exists through Train 'A' after a preset time delay. If a failure of the Train 'A' heater or fan occurs during Train 'A' operation, Train automatically starts. If the SGTS Train 'A' is removed from standby service with the control switch in the OFF position and Train 'B' control switch in the MAINTENANCE position and an RBICS initiation signal occurs, Train 'B' starts (heater and fan) and the Train 'B' air-operated supply and discharge dampers open.
Pilgrim Station Technical Specification 3/4.7.B.1 governs the operability requirements of the SGTS. The specification requires that with certain exceptions, the SGTS shall_be operable during_periods that include reactor power operation (RUN, STARTUP, and HOT SHUTDOWN modes), during movement of irradiated fuel assemblies in secondary containment, during movement of new fuel over the spent fuel pool, during CORE ALTERATIONS, and during operations with a potential for draining the reactor vessel. For reactor power operation, the specification allows a 7-day limiting condition for operation (LCO) if one train is made or found to be inoperable for any reason provided the other train and associated diesel generator is operable and that if the system is not fully made operable within 7 days, the specification requires the initiation of a reactor shutdown and cold shutdown within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. If both trains are inoperable, the specification requires the initiation of a reactor shutdown with cold shutdown within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
Licensed operators monitor drywell-torus differential pressure, and the SGTS is manually operated as necessary to maintain the differential pressure required by Technical Specifications.
EVENT DESCRIPTION
On March 25, 2010, at 12:12 hours Operations entered into a Limiting Condition of Operation (LCO) for SBGT instrument calibration and functional test for Train 'B' in accordance with Pilgrim Nuclear Power Station (PNPS) Procedure 8.M.3-18, Attachment 2. The allowed out-of-service time (AOT) in accordance with PNPS TS 3.7.B.1.c is 7 days, with one train in verified operable condition. Accordingly, the procedural steps of PNPS 8.C.34 were performed to verify the required systems were operable for the Train 'B' AOT and Train 'A' was aligned to provide the safety-related function during the surveillance. 8.M.3-18, Attachment 2 Steps 12-15 were completed and demister door on Train 'B' was opened to perform the required surveillance. At 16:07 hours Instrumentation and Control Technicians reported to the Control Room the completion of the surveillance in accordance with PNPS 8.M.3-18, Attachment 2. At 17:10 hours SGTS Train 'A' was placed in service for torus purging in accordance with PNPS Procedure 2.2.70. At 17:45 hours SGTS 'A' was secured and placed in its normal standby status.
At 22:55 hours, a demister door on Train 'B' was discovered open. The door was immediately closed upon discovery. The door was opened during performance of the scheduled surveillance and subsequently not closed at the completion of surveillance. With this door open, the 13' Train of SGTS was inoperable. Because of the physical configuration of the SBGT system, it could not be immediately verified that the 'A' SBGT Train would be capable of performing its safety-related function. There exists a probability that 'A' SGTS Train could draw suction through the open demister door of the 'B' SGTS Train and a normally open crosstie between the Trains 'A' and 13' without providing the required full suction flow from the secondary containment —a safety function called for under the TS 3.7.B.1. Condition Report PNP-2010-1079 was written. The LCO (LCO ACT 1-10-0063) was revised to reflect a new initiation time (12:12 hours) due to the fact that the 'B' Train SGTS demister door was open. The door was closed and the LCO was exited at 22:55 hours, and both 'A' and 'B' Trains of SBGT were declared operable.
CAUSE
The cause of this event was identified as a human performance error, failure to verify the closure of the demister door prior to completion of the surveillance in accordance with the Procedure 8.M.3-18, Attachment 2.
CORRECTIVE ACTION
Immediate corrective actions taken included: closing the demister door, coaching the affected personnel who performed the surveillance, and enhancing procedural steps to provide clarification in the surveillance procedure.
The longer term corrective action taken to preclude recurrence includes: a case study to review expectations for verification practices and implementation of standardized approach for verification practices during the performance of surveillances.
These corrective actions are tracked by the site Corrective Action Program (CAP) under CR-PNP-2010-1079
EXTENT OF PROBLEM
The SGTS surveillance that was performed is a frequently performed surveillance and the existing procedure provides steps to conduct the required surveillance. The open demister door is a unique condition due to a human error; it does not extend to equipment or common cause failure. Thus the extent of the problem is related to this single event. An Extent of Condition and Extent of Cause was performed as part of the Root Cause Evaluation issued for this event.
SAFETY CONSEQUENCES
The condition posed no threat to public health and safety.
The event occurred during normal power operation while at 100% power with the mode switch in the RUN position. The reactor vessel pressure was approximately 1030 psig with reactor water temperature at saturation temperature for that pressure.
During the time the demister door on Train 'B' was open, both trains were aligned to provide a suction path to maintain secondary containment integrity in the event a design basis event were to occur. However with the demister door open on Train 'B', there was a potential that the system could not have drawn the required vacuum to achieve -0.25 inches of water pressure in the secondary containment. The Train 'B' open demister door condition lasted for approximately six hours, which impacted the operability of the system to fulfill its normal safety function to draw the required vacuum on the secondary containment. During this event there were no other adverse safety conditions impacting secondary containment. Thus, the risk presented by this condition was negligible because of the very short duration of the demister door open event and that there were no other adverse conditions present at the time.
REPORTABILITY
This report was submitted in accordance with 10 CFR 50.73(a)(2)(v)(C) and (D) for an event or condition that could have prevented fulfillment of a safety function.
SIMILARITY TO PREVIOUS EVENTS
A review was conducted of Pilgrim Station LERs issued since 2001. The review focused on LERs that involved a similar event or condition involving the SGTS or similar cause. The review identified no similar event or condition.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) CODES The El IS codes for this report are as follows:
COMPONENTS � CODES Demister Door � DR
SYSTEMS
Standby Gas Treatment System (SGTS) � BH