05000325/LER-2008-002

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LER-2008-002, Loss of Two Control Room Air Conditioning (AC) Subsystems
Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No Bsep, Unit 2 05000324
Event date: 06-03-2008
Report date: 08-04-2008
Reporting criterion: 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
Initial Reporting
ENS 44262 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
3252008002R00 - NRC Website
v • d • e

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

INTRODUCTION

At 1313 hours0.0152 days <br />0.365 hours <br />0.00217 weeks <br />4.995965e-4 months <br /> EDT on June 3, 2008, two Control Room Air Conditioning subsystems [VI], required by Technical Specification (TS) 3.7.4, "Control Room Air Conditioning (AC) System," tripped during the performance of a planned preventive maintenance activity. The loss of the two AC subsystems occurred during the replacement of a solenoid valve for the Unit 1 Cable Spread Room supply/exhaust fan dainpers.

The trip occurred when the control air supply line was momentarily uncapped to enable the installation of the control air supply to the newly installed solenoid valve, causing the dampers on the two operating AC subsystems to close. With the third Control Room AC subsystem in standby with the damper in the closed position, all Control Room AC subsystems were shut down. Because BSEP has a shared Control Room, both Unit 1 and Unit 2 immediately entered TS Limiting Condition of Operation (LCO) 3.0.3 in accordance with TS 3.7.4, Required Action E.1.

Operability of the Control Room AC subsystems was restored at 1325 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.041625e-4 months <br /> when maintenance activities to install the solenoid valve were completed and the AC subsystems were returned to service. No power reduction took place on either unit as a result of the LCO 3.0.3 entry.

At 1926 hours0.0223 days <br />0.535 hours <br />0.00318 weeks <br />7.32843e-4 months <br />, the NRC was notified of this event (i.e., Event Number 44262) in accordance with 10 CFR 50.72(b)(3)(v)(D), as an event or condition that at the time of discovery 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. As a result of the planned maintenance activity, both Units 1 and 2 had entered TS 3.7.3, "Control Room Emergency Ventilation (CREV) System," Required Action B.1 (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 1259 hours0.0146 days <br />0.35 hours <br />0.00208 weeks <br />4.790495e-4 months <br />, approximately 14 minutes prior to the event.

Discussion On June 3, 2008, planned maintenance activities were in-progress to replace seven solenoid valves that control various dampers in the Control Building Heating, Ventilation, and Air Conditioning (HVAC) system [VI]. The valves were being replaced as part of a preventive maintenance corrective action, and the first two solenoid valve replacements were completed successfully.

EVENT DESCRIPTION (continued) At 1259 hours0.0146 days <br />0.35 hours <br />0.00208 weeks <br />4.790495e-4 months <br /> EDT on June 3, 2008, the Control Room authorized replacement of solenoid valve 1-VA-SV-928, affecting the Unit 1 Cable Spread Room supply/exhaust fan dampers. The work rendered two subsystems of the Control Room Emergency Ventilation (CREV) system [VI] inoperable. 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.

No isolation exists for solenoid valve 1-VA-SV-928, and the replacement required momentarily removing the control air supply line from the solenoid valve, stopping (i.e., capping) the air leakage from the system, and then reconnecting the control air supply to the newly installed valve. At 1313 hours0.0152 days <br />0.365 hours <br />0.00217 weeks <br />4.995965e-4 months <br />, during the performance of the planned solenoid valve replacement, the two operating Control Room Air Conditioning subsystems unexpectedly tripped off-line. The trip of the two operating Control Room AC units occurred when the isolated control air supply line was momentarily uncapped to enable the installation of the control air supply to the newly installed solenoid valve. The moment the control air supply line was uncapped, Control Room AC dampers 1-VA-1A-D-CB and 2-VA-2A-D-CB went closed, shutting down all Control Room AC. Because Brunswick has a shared Control Room, the following action statement was entered for both Units 1 and 2:

Further investigation showed that the Control Room AC units shut down when control air pressure to the Flow Control Valves (FCVs) 1-VA-FCV-1026 and 2-VA-FCV-1028 dropped below their switchover pressure. A small momentary drop in the overall control air header pressure, created when the control air supply line was uncapped, resulted in the control air pressure at the FCV actuators to drop below the switchover pressure. Once the FCVs dropped below the switchover pressure, the dampers in the Control Room AC subsystems closed, shutting down the two operating AC units.

Operability of the Control Room AC subsystems was restored, and LCO 3.0.3 was exited, at 1325 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.041625e-4 months <br /> when maintenance activities to install the solenoid valve were completed and the AC subsystems were returned to service. CREV system operability was then restored at 1333 hours0.0154 days <br />0.37 hours <br />0.0022 weeks <br />5.072065e-4 months <br />, and LCO 3.7.3 was exited.

No power reduction took place on either unit as a result of the LCO entries.

A number of additional factors were identified that contributed to the drop in overall control air header pressure. An air line (i.e., polyflow tubing) in the pneumatic control cabinets was found kinked due to improper mounting. This restriction could have adversely altered the response of the air header for the FCV accumulators. Additionally, testing determined that some leaking is most likely occurring through the FCV diaphragms. The combination of the leaking FCV diaphragms, the restricted polyflow tubing, and the uncapping of the control air supply line to solenoid valve 1-VA-SV-928, provided the perturbation to create EVENT DESCRIPTION (continued) a small momentary drop in the overall control air header pressure, causing the closing of the Control Room AC dampers and tripping off of the two operating Control Room AC subsystems.

EVENT CAUSE

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 and lack of recognition that additional compensatory measures were warranted. A kinked air line in the pneumatic control cabinets due to improper mounting, and the leaking diaphragms of the FCVs, contributed to lowering of the control air header pressure. When the control air supply line was uncapped during the maintenance activity to replace the solenoid valve, the control air header pressure decreased enough to provide a momentary drop below the switchover pressure of the FCVs, causing the two operating Control Room AC subsystems to trip.

SAFETY ASSESSMENT

The safety significance of this condition is considered minimal. The Control Room AC subsystems provide cooling for the Control Room. The condition existed for approximately 12 minutes. The plant staff took immediate and proper actions to return the equipment to service. For the brief time that the Control Room AC subsystems were inoperable, performance of plant personnel and equipment in the Control Room was not adversely affected. The maximum temperature in the Control Room backpanel area during this event was approximately 73°F. There was no nuclear or industrial safety consequence from this event.

CORRECTIVE ACTIONS

The following corrective action to prevent recurrence will be taken.

  • The preventive maintenance tasks for the Control Room AC system solenoid valves will be revised to include the use of a Temporary Modification whenever the solenoid valves are being replaced, to allow the Control Room AC system to run on total loss of Control Room instrument air. These actions are scheduled to be completed by October 15, 2008.

Additional corrective actions include the following.

  • Operability of the Control Room AC subsystems was restored at 1325 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.041625e-4 months <br /> on June 3, 2008, when the replacement of solenoid valve 1-VA-SV-928 was completed and the system returned to service.

CORRECTIVE ACTIONS (continued)

  • Control air tubing (Polyflow) in the pneumatic control cabinet was repaired.
  • An EC will be developed to replace all four Flow Control Valves (FCVs) and a Preventive Maintenance (PM) schedule will be developed for the FCVs. These actions are scheduled to be completed by September 15, 2008.
  • All four FCVs will be replaced. These actions are scheduled to be completed by October 15, 2008.
  • A revision to Brunswick plant procedure OAP-025, "BNP Integrated Scheduling," will be made to ensure proper risk assessment of Control Room HVAC on-line work. The procedure change is scheduled to be completed by October 30, 2008.

PREVIOUS SIMILAR EVENTS

A review of LERs and corrective action program condition reports for the past three years did not identify any previous similar occurrences.

COMMITMENTS

No regulatory commitments are contained in this report.

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