05000296/LER-2009-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No N/A N/A | |
| Event date: | 11-12-2009 |
|---|---|
| Report date: | 07-28-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2962009002R01 - NRC Website | |
I. PLANT CONDITION(S)
At the time of discovery, Browns Ferry Nuclear Plant (BFN) Units 1, 2, and 3 were at 100 percent power.
IL DESCRIPTION OF EVENT
A. Event:
On November 12, 2009, 1235 hours0.0143 days <br />0.343 hours <br />0.00204 weeks <br />4.699175e-4 months <br /> Central Standard Time (CST), Unit 3 Operations personnel completed the performance of 3-SR-3.5.1.7, High Pressure Coolant Injection (HPCI) [BG] Main and Booster Pump Set Developed Head and Flow Rate Test at Rated Reactor Pressure, and commenced returning of the HPCI turbine to standby readiness. While securing the HPCI turbine, BFN Operations personnel received an alarm indicating high water level in the HPCI turbine exhaust steam drain pot. In accordance with the applicable Alarm Response Procedure, Operations personnel opened the HPCI system condensate level control valve and subsequently dispatched personnel to the HPCI pump to drain the condensate from the drain pot through the drain pot level switch instrument test drain. Operations personnel removed in excess of 80 gallons of condensate from the HPCI turbine exhaust drain pot to clear the alarm. BFN Chemistry personnel analyzed the condensate and determined it was from the suppression pool.
On November 12, 2009, at 1711 hours0.0198 days <br />0.475 hours <br />0.00283 weeks <br />6.510355e-4 months <br /> CST BFN Unit 3 Operations Personnel declared the Unit 3 HPCI system inoperable due to excessive water in the turbine exhaust line drain pot. BFN Operations personnel entered Technical Specification (TS) 3.5.1, Action C, which requires the Reactor Core Isolation Cooling (RCIC) [BN] system to be immediately verified by administrative means to be operable and the HPCI system restored to operable status in 14 days.
On November 14, 2009, at 0630 hours0.00729 days <br />0.175 hours <br />0.00104 weeks <br />2.39715e-4 months <br /> CST, during troubleshooting and return to service activities for the HPCI system, BFN Operations personnel received indication that the RCIC Electronic Flow Controller failed. BFN Operations personnel immediately declared RCIC inoperable. With RCIC inoperable, TS 3.5.1 Required Action C.1, which requires verification by administrative means that the RCIC system is operable, was no longer met. As a result, BFN Operations personnel were required to enter TS 3.5.1 Action G, which requires the unit to be placed in Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and reactor steam dome pressure to be reduced to less than or equal to 150 psig within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. With the RCIC system inoperable, BFN Operations personnel immediately entered TS 3.5.3 Action A, which requires the HPCI system to be immediately verified by administrative means to be operable and the RCIC system to be restored to operable status in 14 days. Since HPCI was inoperable, TS 3.5.3 Required Action A.1 was not met.
Operations personnel were then required to enter TS 3.5.3 Action B, which requires the unit to be placed in Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and reactor steam dome pressure to be reduced to less than 150 psig within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
BFN Maintenance personnel determined the RCIC flow controller failure was the result of a loose ribbon cable connector to the power supply. As soon as the connector was reconnected the RCIC controller began to indicate normally. At 0830 hours0.00961 days <br />0.231 hours <br />0.00137 weeks <br />3.15815e-4 months <br /> CST, on November 14, 2009, BFN Operations personnel declared RCIC operable and exited TS 3.5.3 Action B. Following the restoration of RCIC to an operable status, TS 3.5.1 Action C remained in effect until HPCI could be returned to operable status.
On November 16, 2009, at 0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br /> CST, as part of the troubleshooting activities, the HPCI system was placed in service. While the HPCI system was in operation the "HPCI drain pot level high" alarm came in; however, once the HPCI System was secured, the alarm cleared.
Unit 3 Operations and Engineering personnel have determined that following the November 12, 2009, performance of 3-SR-3.5.1.7, the suppression pool water siphoned back into the turbine exhaust drain pot via the HPCI Turbine Drain Line causing the High water level alarm. BFN Operations personnel, in order to prevent recurrence, isolated HPCI turbine drain pot drain line from the suppression pool on November 18, 2009, at approximately 0100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> CST. At 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> CST, on November 18, 2009, as a post maintenance test for isolating the suppression pool from the HPCI drain line, BFN Operations Personnel commenced performance of 3-SR-3.5.1.7. At 0830 hours0.00961 days <br />0.231 hours <br />0.00137 weeks <br />3.15815e-4 months <br /> CST, following successful completion of 3-SR-3.5.1.7, and final review of the HPCI Functional Evaluation, Operations exited TS 3.5.1 Condition C and declared the HPCI System Operable.
The Tennessee Valley Authority (TVA) is submitting this report in accordance with 10 CFR 50.73(a)(2)(v)(D), as any 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.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None C. Dates and Approximate Times of Major Occurrences:
November 12, 2009 at 1130 hours0.0131 days <br />0.314 hours <br />0.00187 weeks <br />4.29965e-4 months <br /> CSTU Unit 3 Operations personnel completed performance of 3-SR-3.5.1.7 and commenced returning of the HPCI turbine to standby readiness.
November 12, 2009 at 1235 hours0.0143 days <br />0.343 hours <br />0.00204 weeks <br />4.699175e-4 months <br /> CSTU Unit 3 Operations personnel receive alarm indicating the turbine exhaust drain tank level is high.
November 12, 2009, at 1711 hours0.0198 days <br />0.475 hours <br />0.00283 weeks <br />6.510355e-4 months <br /> CSTU Unit 3 Operations personnel declared Unit 3 HPCI inoperable.
November 12, 2009, at 2136 hours0.0247 days <br />0.593 hours <br />0.00353 weeks <br />8.12748e-4 months <br /> CSTU Operations personnel made a Non-Emergency Notification System report in accordance with 10 CFR 50.72(b)(3)(v).
November 14, 2009, at 0630 hours0.00729 days <br />0.175 hours <br />0.00104 weeks <br />2.39715e-4 months <br /> CSTU Unit 3 Operations personnel receive alarm indicating the RCIC flow controller had failed and immediately declared RCIC inoperable.
November 14, 2009, at 0830 hours0.00961 days <br />0.231 hours <br />0.00137 weeks <br />3.15815e-4 months <br /> CSTU Upon return of the RCIC flow controller to service, Unit 3 Operations personnel declared RCIC operable.
November 18, 2009, at 0830 hours0.00961 days <br />0.231 hours <br />0.00137 weeks <br />3.15815e-4 months <br /> CSTU Following successful completion of 3-SR-3.5.1.7, and final review of the HPCI Functional Evaluation, Operations declared the HPCI System Operable.
D. Other Systems or Secondary Functions Affected
None
E. Method of Discovery
Operations personnel received a main control room alarm indicating the HPCI turbine steam exhaust drain tank level was high. Operations personnel received a main control room indication that the RCIC Flow Controller had failed.
F. Operator Actions
The operator opened the HPCI turbine exhaust condensate pot level control valve from the main control room. An Auxiliary Unit Operator was dispatched to the HPCI pump to manually drain the condensate from the HPCI turbine exhaust drain pot.
G. Safety System Responses
None
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of the HPCI system inoperability was high water level in the HPCI turbine exhaust drain pot. The cause of the RCIC system inoperability was a loose ribbon cable connector in the RCIC flow controller.
B. Root Cause
The root cause for HPCI inoperability was siphoning of water from the suppression pool. The siphoning resulted in water flowing back from the suppression pool to the HPCI drain pot via the HPCI drain pot drain line. No root cause for the RCIC loose ribbon cable connector has been determined.
C. Contributing Factors
The original design of the HPCI exhaust drain piping contributed to the event. The design relied on gravity for the flow of condensate from the HPCI turbine exhaust drain pot to the suppression pool. However, the BFN piping configuration rises in elevation from the turbine exhaust drain pot to the suppression pool. The drain line penetrates the top of the torus and terminates below the suppression pool water line; thus, requiring a check valve to prevent the siphoning of water out of the suppression pool. The drain path afforded by the drain line is functional only during turbine operation. Some of the later vintage plants do not utilize the drain line.
IV. 'ANALYSIS OF THE EVENT The HPCI exhaust steam line contains a steam pot and steam trap drain line arrangement that is connected to the low point of the HPCI turbine exhaust line. The drain pot collects condensate present in the steam and discharges it through the steam trap to the suppression pool through a two inch line (HPCI system turbine exhaust drain line) or bypasses flow to the gland steam condenser through a one-inch line. A check valve and isolation valve installed in the HPCI turbine exhaust drain line should prevent suppression pool water from flowing back into the HPCI turbine.
A drain pot level control valve regulates the condensate level in the drain pot sending the majority of the condensate to the gland seal condenser. The HPCI turbine exhaust drain pot level control valve automatically opens on receipt of a high drain pot level signal via two HPCI turbine exhaust drain line pot level switches. In this event, suppression pool water flowed back to the HPCI turbine exhaust drain pot following HPCI system operation.
The RCIC system failed when the flow controller failed because of a loose connector to the power supply in a panel mounted enclosure. The design is such that the internally mounted rack mount modules are typically not accessible when installed (slid into panel mounted enclosure). The complete power supply rack mount modules which are connected with the ribbon connectors are not typically disconnected (at the ribbon connector) from the panel mounted enclosures. The ribbon cable found partially connected/seated is typical of the ribbon cable assemblies used extensively throughout the plant to supply power rack components. The connector involved is designed to be installed by simply pushing the connector assembly down onto a imbedded receptacle at the rear of a "rack" assembly.
As soon as the connector was reseated, the RCIC controller began to indicate normally.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. BFN TS 3.5.1, Required Action C.2, allows continued power operation for up to 14 days with the HPCI system inoperable as long as the RCIC system is operable. In this condition, the other required Emergency Core Cooling Systems were operable and remained capable of mitigating design basis accidents and transients assumed in the UFSAR. In addition, with the exception of approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> that RCIC was inoperable, the RCIC system would have automatically provided makeup water to the reactor if required, at most reactor operating pressures. Therefore, TVA concludes that there was no significant reduction in the protection of the public by this event.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions Immediate corrective actions included isolating the turbine exhaust drain line between the turbine exhaust drain pot and the suppression pool. BFN Operations and Maintenance personnel commenced troubleshooting and return to service activities. The connector to the RCIC Flow Controller was reseated on to the power supply and proper controller output voltage was verified.
B. Corrective Actions to Prevent Recurrence - The corrective actions to prevent recurrence are being managed by TVA's corrective action program.
TVA is planning to remove the HPCI turbine exhaust drain line from service for all three units.
There is no evidence that the failure of the ribbon cable connector to remain seated RCIC controller is a generic or recurring problem. Only one similar previous occurrence was identified in 2004 on a non-critical piece of equipment. As an enhancement to current rack component work related practices, the rear ribbon cable connector will be visually inspected for proper seating prior to and after maintenance which involves removing rack mounted devices from/into panel installation slots.
VII. ADDITIONAL INFORMATION
A. Failed Components
None B. Previous LERs On Similar Events No previous similar LERS were identified. In 1995 an event that occurred on Unit 2 resulted in between 15-20 gallons of water in the Unit 2 HPCI turbine exhaust line. The evaluation did not identify the capability of the drain line to siphon water from the suppression pool. WA determined HPCI system maintained functionality with this amount of water in the exhaust line.
Another similar event occurred on Unit 1 on September 20 and again on September 21, 2007.
Operations Personnel had main control room indication of high level in the exhaust drain pot. A review of the main control room annunciators had found that the annunciator had been receive each time HPCI was run since Unit 1 restart in May of 2007. The annunciator clears after the turbine is shut down. Additionally, on Units 2 and 3, the annunciator was received and clears several times during the performance of the surveillance. TVA's evaluation of these events concluded that the HPCI system remained fully functional. The high velocity of the exhaust steam exiting the HPCI turbine entrains the condensate from the exhaust line and delivers it to the suppression pool.
C. Additional Information
Corrective action documents for this report are Problem Evaluation Reports 207915, 208077 and 234082.
D. Safety System Functional Failure Consideration:
This event is classified as a safety system functional failure according to NEI 99-02.
E. Scram With Complications Consideration:
The event described was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None