05000325/LER-2008-004
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No Bsep, Unit 2 05000324Property "Site" (as page type) with input value "Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No Bsep, Unit 2 05000324" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process. | |
| Event date: | 06-19-2008 |
|---|---|
| Report date: | 0-8-2008"0-8-2008" contains a sequence that could not be interpreted against an available match matrix for date components. |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 44309 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3252008004R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
INTRODUCTION
On June 19, 2008, at 1641 hours0.019 days <br />0.456 hours <br />0.00271 weeks <br />6.244005e-4 months <br /> Eastern Daylight Time (EDT), the Control Room authorized post- maintenance testing following replacement of solenoid valves affecting the Control Room Emergency Ventilation (CREV) [VI] subsystem. This test inputs a simulated high radiation signal into the logic for the Control Building Ventilation system to ensure that the CREV subsystem automatically aligns to the radiation/smoke protection mode. At 1735 hours0.0201 days <br />0.482 hours <br />0.00287 weeks <br />6.601675e-4 months <br />, during performance of this test, the 2D Control Building exhaust fan damper failed to close and the associated Control Building exhaust fan failed to trip as expected. These functions are provided to maintain a positive pressure in the Control Building in the event of high radiation or smoke intrusion into the Control Building. Because BSEP has a shared Control Room, both Unit 1 and Unit 2 immediately entered TS Limiting Condition of Operation (LCO) 3.7.3, "Control Room Emergency Ventilation (CREV) System," Required Action B.1 for two CREV subsystems inoperable (i.e., be in Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />). At 1910 hours0.0221 days <br />0.531 hours <br />0.00316 weeks <br />7.26755e-4 months <br />, the 2A CREV subsystem was manually placed in the radiation/smoke protection mode restoring the subsystem to operable status, and LCO 3.7.3, Required Action B.1 was exited. No reactor power reduction took place on either unit as a result of the LCO entry.
On June 20, 2008, at 0015 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br />, the NRC was notified of this event (i.e., Event Number 44309) in accordance with 10 CFR 50.72(b)(3)(v)(D), as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
EVENT DESCRIPTION
Initial Conditions Prior to the event, both Units 1 and 2 were in Mode 1 operating at approximately 100 percent rated thermal power.
Discussion On June 3, 2008, a series of solenoid valves associated with the CREV dampers were being replaced as part of Preventive Maintenance (PM) activities. It was intended that a total of seven CREV solenoid valves be replaced, but due to an unanticipated isolation of two Control Room Air Conditioning subsystems (i.e., LER 1-2008-002), this activity was stopped after replacement of only three. One of the three solenoid valves replaced was 2-VA-SV-917, Control Building Exhaust Fan 2D Damper Solenoid, also referred to as the EVENT DESCRIPTION (continued) Control Room washroom exhaust damper. Following replacement on June 3, 2008, the solenoid valve was tested satisfactorily by using its manual control switch to open and then close the damper. Following replacement of additional solenoid valves and a damper shaft repair, a post-maintenance test was performed to ensure proper response of the CREV components and verify that the radiation monitor was restored.
On June 19, 2008, at 1641 hours0.019 days <br />0.456 hours <br />0.00271 weeks <br />6.244005e-4 months <br /> EDT, the Control Room authorized post-maintenance testing following replacement of solenoid valves affecting the 2B CREV subsystem. The post-maintenance test, OPT-23.1, "Control Room Emergency Filtration System Operability Test," was required to restore the subsystem to operable status following the planned maintenance. The test inputs a simulated high radiation signal into the logic for the Control Building ventilation system to ensure that the CREV subsystem automatically aligns to the radiation/smoke protection mode. At 1735 hours0.0201 days <br />0.482 hours <br />0.00287 weeks <br />6.601675e-4 months <br />, during performance of the post-maintenance test, the 2D Control Building exhaust fan damper failed to close and the associated Control Building exhaust fan failed to trip as expected. These functions are provided to maintain a positive pressure in the Control Building in the event of high radiation or smoke intrusion into the Control Building. Because Brunswick has a shared Control Room, the following action statement was entered for both Units 1 and 2:
- TS 3.7.3, Required Action B.1, be in Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, for two CREV subsystems inoperable.
At 1910 hours0.0221 days <br />0.531 hours <br />0.00316 weeks <br />7.26755e-4 months <br />, on June 19, 2008, the 2A CREV subsystem was manually placed in the radiation/smoke protection mode restoring the subsystem to operable status, and LCO 3.7.3, Required Action B.1 was exited. No reactor power reduction took place on either unit as a result of the LCO entry.
control board's (i.e., the RTGB) damper and flow indicating lights remaining red. The expected green indication would have signaled that the exhaust fan damper is closed and the exhaust fan is off. All other equipment monitored in the OPT-23.1 test responded satisfactorily. Once the post-maintenance test was completed, the fan and damper were isolated using the control switch on the Unit 2 RTGB. Each Unit has a separate control switch which can be used to operate this fan and damper combination.
Further evaluation determined five scenarios that could have contributed to intermittent sticking or binding and possibly account for the observed indications. These scenarios included sticking of the solenoid valve, sticking of the limit switch, sticking of the damper actuator, sticking of the damper, and a combination of two failures within parallel control circuits associated with the simulated high radiation signal.
Subsequent troubleshooting was unable to recreate the failure and no obvious faults were observed. The troubleshooting activities involved cycling the actuator, control switch, and limit switch's actuation arm. All appeared to move freely and appropriately, and actuated the control board lights per design. The damper and its actuator were observed to operate without any indication of binding or resistance immediately after EVENT DESCRIPTION (continued) the event. The solenoid valve and limit switch were removed and examined without any observable causes for sticking. The last scenario involves a duel failure of components which have been functionally checked each quarter and are channel checked every cycle. Since the failure cleared prior to the beginning of troubleshooting and has not been able to be recreated, the actual failure mechanism has not been identified.
EVENT CAUSE
An exact root cause of this event has not been determined. The select cause of this event was determined to be failure to identify appropriate Preventive Maintenance (PM) routes for the CREV system components.
The most probable cause of this event was age related degradation causing intermittent operation of one or more of the components in the Control Building exhaust damper's control scheme. PM's will be generated to ensure limit switches, dampers, damper actuators, and relays relied upon to satisfy the Control Room envelope Technical Specification requirements, or provide verifying indications, have suitable PM routes.
SAFETY ASSESSMENT
The safety significance of this condition is considered minimal. The affected functions of the CREV system are to provide isolation, positive pressurization, and emergency filtration of the Control Room in the event of high radiation or smoke intrusion into the Control Building. For the brief time that the CREV subsystems were inoperable, performance of plant personnel and equipment in the Control Room was not adversely affected. The plant staff took immediate and proper actions to return the equipment to service.
There were no nuclear or industrial safety consequences from this event.
CORRECTIVE ACTIONS
The following corrective action to prevent recurrence will be taken.
- Develop Preventive Maintenance tasks for the Control Building ventilation system dampers, actuators, limit switches, and relays.
Additional corrective actions include the following.
- The 2D Control Building exhaust damper solenoid valve 2-VA-SV-917 and limit switch 2-VA-ZS- 917 were replaced.
PREVIOUS SIMILAR EVENTS
A review of LERs and corrective action program condition reports for the past three years identified the following previous similar occurrences.
- LER 1-2006-001, dated March 9, 2006, "Control Room Emergency Ventilation (CREV) and Air Conditioning (AC) Inoperable Due to Loss of Control Air," documents an event where the CREV and AC systems were declared inoperable for Unit 1 and 2 due to the Control Building Instrument Air compressor failing to maintain pressure. The root cause of the event was determined to be ineffective condition monitoring of compressor oil pressure to detect hydraulic unloader degradation. The corrective actions associated with LER 1-2006-001 addressed the oil pressure trending and monitoring, and could not have reasonably been expected to prevent the condition reported in this LER.
- LER 1-2008-002, dated August 4, 2008, "Loss of Two Control Room Air Conditioning (AC) Subsystems," documents an event where the two operating Control Room AC subsystems tripped while replacing solenoid valves for the Control Building supply/exhaust fan dampers. The root cause of this event was determined to be a reliance on an Engineering Change calculation which did not adequately consider the impact of system material condition and resulted in an inadequate assessment of risk. The corrective actions revised the Preventive Maintenance tasks to use a Temporary Modification whenever the Control Room AC system solenoid valves are being replaced, which will allow the Control Room AC system to run on total loss Control Room instrument air. The corrective actions associated with LER 1-2008-002 could not have reasonably been expected to prevent the condition reported in this LER.
COMMITMENTS
No regulatory commitments are contained in this report.