05000316/LER-2002-005
| Donald C. Cook Nuclear Plant Unit 2 | |
| Event date: | 05-12-2002 |
|---|---|
| Report date: | 1-12-2004 |
| 3162002005R01 - NRC Website | |
17. TEXT (11 more space requrred use adchticrrai copies of NRC Form (3668) Conditions Prior to Event Unit 2 — Made 1, 100 percent power Unit 1 — Mode 6, 0 percent power
Description of Event
At 2301 hrs on May 12, 2002, Unit 2 tripped due to an instrument rack power supply failure. Specifically, the second of two redundant 24-volt direct current (VDC) power supplies to reactor control instrumentation control group cabinet 2-PS-CGC-16 failed. The failure of both power supplies caused steam generator (SG) feedwater regulating valve 2-FRV-210 to close. Unit 2 subsequently tripped on low water level in SG-21 coincident with low feedwater flow.
The post trip Investigation indicated that all control rods fully inserted. The three auxiliary feedwater (AFW) pumps automatically started as expected. The turbine-driven AFW pump was manually stopped to minimize reactor coolant system (RCS) cooldown. The SG stop valves were manually dosed to stabilize RCS temperature in accordance with plant procedures. Automatic pressurizer pressure control was disabled due to the failure of the 24- VDC power supplies. As such, RCS pressure was manually controlled using the pressurizer heaters and spray.
Due to the failure of the 2-PS-CGC-16, 24-VDC power supplies, the automatic function of the power operated relief valves (PORV) was disabled as well, but manual control remained available.
The volume control tank (VCT) level instrument 2-QLC-451 bi-stables deenergized due to the failure of the 24-VDC power supplies. This caused 2-ORV-303, "RCS letdown to VCT diversion valve: to open to full divert to the hold up tanks, allowing the running centrifugal charging pump (CCP) to significantly reduce VCT level. The Joss of 24-VDC disabled automatic makeup to the VCT. Automatic transfer of the CCP suction from the VCT to the refueling water storage tank (RWST) had also been disabled by the loss of the 24-VDC power supply. VCT level dropped to approximately two to three percent, and suction to the charging pumps was manually switched from the VCT to the RWST. Additionally, operators aligned QRV-303 to the VCT position. These manual actions avoided a draindown of the VCT and potential damage to the inservice CCP.
The boration flow path from the RWST was declared inoperable due to the failure of the RWST sequence signal ability to actuate. Both boration flowpaths from the boric acid storage tanks were declared inoperable due to VCT pressure and then declared inoperable again, due to loss of the plant heating steam when the auxiliary building area temperatures fell below the minimum area temperature requirements.
In accordance with 10 CFR 50.72 (b)(2)(iv)(B), a four-hour ENS notification (Event #38915) was made to the NRC on May 13, 2002, at 0255 hours0.00295 days <br />0.0708 hours <br />4.21627e-4 weeks <br />9.70275e-5 months <br /> for a condition that resulted in an actuation of the reactor protection system (RPS) when the reactor is critical. As such, this LER is being submitted in accordance with the requirements of 10 CFR 50.73 (a)(2)(iv)(A) for a condition that resulted in an automatic actuation of the RPS system.
Cause of Event
The preliminary cause of this event was age-related failure of components within the control group 24-VDC power supplies. A failure evaluation was performed on the two failed power supplies. The evaluation identified that the failed power supplies were originally installed in 1994 and contained components (capacitors, resistors, etc.) manufactured as early as 1989. As such, the power supplies were four to five years old when first installed and energized in 1994.
17. TEXT (if more spare Is required, use additional copies of NRC Form (356A) A contributing factor was that no provisions existed for periodic monitoring of the control group 24-VDC power supplies. Additionally, the wiring configuration in the control group 24-VDC power supply racks did not allow on-line replacement of individual failed power supplies.
Analysis of Event
Several control functions were lost due to the failure of the control group 24-VDC power supplies: 1) SG feedwater regulation with valve 2-FRV-210, 2) VCT automatic make-up, 3) automatic switchover of the CCP suction to the RWST on low-low VCT level, and 4) automatic pressurizer pressure control using the pressurizer spray valves and heaters and automatic operation of the PORVs on high pressure.
1. Closing of the feedwater regulating valve 2-FRV-210 is not a significant safety concern for the following reasons:
■ On a reactor trip or loss of normal feedwater, the AFW system starts and supplies the required feedwater flow to shutdown the unit. This function was not affected due to the control group 24-VDC power supply failures because it does not rely on valve 2-FRV-210.
- 2-FRV-210 failed closed on a loss of the control group 24-VDC power supplies, but this is the desired valve position if a SG tube rupture were to occur or when isolating a faulted SG.
2. VCT level instrument 2-QLC-451 bi-stables deenergized due to the loss of the control group 24-VDC, resulting in the diversion of the RCS letdown from the VCT to the hold up tanks. Deenergizing the 2-QLC-451 hi-stables also disabled the automatic make-up to the VCT and automatic switchover of the CCP suction to the RWST.
Moreover, loss of 24-VDC caused the -VCT level hi divert to holdup tanks' alarm and "volume control tank level low" alarm to annunciate and also disabled the `VCT level lo lo refueling water sequence' alarm. Nonetheless, VCT level instrument 2-01X-451 continued to provide accurate indication of VCT level. in addition, VCT level instrument 2-QLC-452 remained unaffected. Thus, while automatic make-up and switch over to the RWST functions were disabled, control room personnel were able to monitor VCT level and manually switch the CCP suction from the VCT to the RWST.
3. Automatic pressurizer pressure control was disabled, however, manual control of the pressurizer heaters and spray was still available. The PORVs were not available for automatic action, but were available in manual.
4. The boration flowpaths Technical Specification action statements were met.
Based on the discussion above, the safety significance of this event was minimal since plant procedures and operator training provided sufficient direction for control room personnel to shutdown the plant and maintain it in a safe shutdown condition. There was no impact on the health and safety of the public as a result of this event.
Corrective Actions
The failed control group 24-VDC power supplies in control group CGC-16 were replaced. The remaining 24-VDC control group power supplies in Unit 2 were inspected and one power supply in control group 2-CG2-19 was replaced.
Prior to the startup of Unit 1 on June 8, 2002, all 24-VDC control group power supplies in Unit 1 were replaced.
Routine tasks have been established to verify the availability of the control group 24-VDC power supplies for both Unit 1 and Unit 2.
2002 � 005 � 01 17. TEXT (if more space Is required, use additional copies of NRC Form (.366A) A modification was performed on the Unit 1 control group 24-VDC power supply racks to allow on-line replacement of failed power supplies. To improve system reliability, a similar modification has been scheduled for Unit 2 during the next refueling outage.
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