05000362/LER-2009-002
| San Onofre Nuclear Generating Station Unit 3 | |
| Event date: | 12-12-2009 |
|---|---|
| Report date: | 2-10-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3622009002R00 - NRC Website | |
Background
On December 11, 2009 at 00:25 PST, Operators declared Unit 3 Train B Emergency Diesel Generator (EDG) 3G003 [DG] inoperable due to multiple control room alarms. The alarms occurred when a technician performing maintenance grounded a wire, causing the fuse to open and the annunciator relays to actuate. Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1 B was entered for one EDG inoperable. The TS Required Actions include completing a common cause evaluation or testing the operable (opposite train) Train A EDG 3G002 within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
Event
On December 12, 2009 [Event Date], at 00:25 PST, TS Condition 3.8.1.F was entered since the Required Actions were not completed within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. At 01:05 PST Train A EDG 3G002 failed to start in compliance with the TS Required Action. As a result, Unit 3 entered a TS required shutdown. A shutdown of Unit 3 from 96% power was initiated on December 12, 2009 at 01:26 PST. Reactor power was reduced to 40% before Train B EDG 3G003 was declared operable at 05:11 PST. Unit 3 was returned to 95% power by 02:10 PST on December 13, 2009. Unit 2 was in the Cycle 16 refueling outage and was not affected by the event.
The initiation of a TS required shutdown of Unit 3 under TS 3.8.1 required declaration of an Unusual Event (UE) under the SONGS Emergency Plan. The report of the UE and a 10 CFR 50.72 (b)(3)(v) eight-hour report were made under NRC Operations Log #45564.
SCE is reporting this occurrence as a loss of safety function (on-site emergency power) under 10 CFR 50.73(a)(2)(v)(D).
Cause of the Event
Train A EDG 3G002 The cause of the Train A EDG 3G002 failure to start on December 12, was found to be a failed open capacitor [CAP] in one of the two power supplies for the local panel annunciator (Panalarm model 70B-PCD-100 DC-DC converter). The failed capacitor caused excessive ripple voltage and spiking on the speed switch power supply circuit, causing the speed switch to malfunction. The voltage spikes caused the speed switch to change state prematurely, preventing the engine start. There was no direct indication to the control room that the speed switch was blocking a start signal.
The annunciator power supplies [RJX] of Train A EDG 3G002 had previously shown signs of age related degradation in June 2009, producing noise in the speed switch and also preventing a start of Train A EDG 3G002. These power supplies are continually energized and the capacitors normally degrade slowly over time. The power supply noise was caused by capacitors which had failed open-circuit. Two replacement power supplies were obtained and installed in the Train A EDG FORM 366A� U.S. NUCLEAR REGULATORY COMMISSION (7-2001) San Onofre Nuclear Generating Station (SONGS) Unit 3 05000-362 3G002 control panel. As part of the pre-installation testing of the power supplies, the capacitor in the power supply that subsequently failed on December 12, 2009 had been removed, tested, and reinstalled. Following the June 2009 power supply replacement, corrective action was initiated to replace all of the annunciator power supplies with newer units, containing capacitors which were no more than two years old at the time of receipt.
After the December 12, 2009 event, the power supply from Train A EDG 3G002 was removed for failure analysis testing. The failed capacitor was found to be in contact with an adjacent power resistor. Testing determined the amount of heat generated by the contact with the power resistor was not sufficient to account for the premature failure of the capacitor, and suggests it may have been caused by a latent manufacturing defect. There was no prior evidence of the capacitor failure until Train A EDG 3G002 failed to start. The failed capacitor was manufactured by Illinois Capacitor, part number TTAM 220uf/160 VDC.
Train B EDG 3G003 The direct cause of the fuse opening in the Train B EDG 3G003 local panel, was poor work practices by a maintenance technician, coincident with a pre-existing ground condition.
On December 11, a technician was working to correct a failure of a lube oil low temperature sensing switch on Train B EDG 3G003. While performing the work, the technician dropped an energized wire, grounding the circuit, opening the fuse, and activating the alarm panel relays.
On May 3, 2009 the same fuse had opened in the Train B EDG 3G003 panel and the cause was not found at that time. After the December 11 event, it was concluded a pre-existing ground condition caused the alarm when the technician dropped the energized wire. Troubleshooting located a ground in one of the power supplies where the diode mounting screw contacted a wire.
Corrective Action Trouble shooting located the pre-existing ground in the Train B EDG 3G003 local panel power supply.
The preventative maintenance program will be revised to include a check of the ground detection circuit in the local EDG panels.
All of the EDG annunciator power supplies on all the EDGs in both Units 2 and 3, have been replaced with more recently manufactured units as a result of the December 12, 2009 failure.
Voltage readings were taken on these power supplies at the time of installation to confirm acceptable performance.
Safety Significance
The event was of low safety significance. At the time the Train A EDG 3G002 failed to start, the Train B EDG 3G003 remained fully functional during the event and was not prevented from receiving an automatic start signal, or being manually started to provide on-site AC power.
FORM 366A� U.S. NUCLEAR REGULATORY COMMISSION (7-2001) San Onofre Nuclear Generating Station (SONGS) Unit 3 05000-362 Operating Experience SONGS LER 3-2007-001 reported an EDG failure caused by a defective soldered connection on the engine speed probe. The failure was not similar to the December 12, 2009 event, other than it was associated with a speed sensing circuit.
OE29575 - Diesel Trip due to Excessive Voltage Noise from Refurbished Power Supply to Speed Switch (San Onofre). This June 2009 event was discussed in the Cause section.
The annunciator power supplies for the EDGs have caused events at other plants, refer to the OE below:
occurred on October 22, 2009 and was reported on December 21, 2009, after the SONGS event had occurred.
OE27329, “Power Supply Failure From Blown Fuse Affected Main Control Board EDG Annunciators“ (Summer) OE18444, “Emergency Diesel Generator Locked-Out due to Speed Switch Actuation caused by Electrical Noise from Failed Power Supply Capacitor” (Turkey Point) OE16045, “Diesel Speed Switch Erratic Operation” (Summer) OE17548, “Update to OE 16119 - DC Voltage Spikes” (Summer)