05000339/LER-2008-001
| North Anna Power Station | |
| Event date: | 02-08-2008 |
|---|---|
| Report date: | 04-01-2008 |
| Initial Reporting | |
| ENS 43972 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3392008001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On February 8, 2008, Unit 2 was in Mode 3, zero percent power, preparing for restart following a planned maintenance outage. At 1759 hours0.0204 days <br />0.489 hours <br />0.00291 weeks <br />6.692995e-4 months <br />, a rod control urgent failure alarm was received while withdrawing the "A" Shutdown Bank control rods (EIIS System AA, Component ROD). Rod withdrawal was stopped and it was noted that the "A" Shutdown Bank Group Step Counters (EIIS System AA, Component CTR) were three steps apart (61 steps on Group 1 and 58 steps on Group 2). As a result, at 1812 hours0.021 days <br />0.503 hours <br />0.003 weeks <br />6.89466e-4 months <br /> a manual reactor trip was initiated by opening the reactor trip breakers (EIIS System AA, Component BKR) in accordance with Technical Requirement 3.1.3.
An Urgent Failure in Power Cabinet 2AC (EIIS System ED, Component CAB) immediately prevented Shutdown Bank "A" Group 2 rod motion. However, it did not prohibit continued rod withdrawal of Shutdown Bank "A" Group 1 since no Urgent Failure existed in Power Cabinet 1AC. Since the Urgent Failure occurred in Power Cabinet 2AC, four suspect movable cards (Regulation, Phase Control, Firing and Failure Detection) were replaced in Power Cabinet 2AC. Following card replacement the reactor trip breakers were closed.
Shutdown Banks "A" and "B" were fully withdrawn with no concerns. On February 9, 2008, at 0136 hours0.00157 days <br />0.0378 hours <br />2.248677e-4 weeks <br />5.1748e-5 months <br /> reactor startup commenced and 100 percent power was achieved at 2222 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.45471e-4 months <br />.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event posed no significant safety implications because the reactor was subcritical when the reactor trip breakers were opened. Compliance with all Technical Requirements was maintained. Therefore, the health and safety of the public were not affected by this event.
On February 8, 2008, at 1959 hours0.0227 days <br />0.544 hours <br />0.00324 weeks <br />7.453995e-4 months <br /> a non-emergency 8-hour report was made in accordance with 10 CFR 50.72(b)(3)(iv)(A) due to actuation of the Reactor Protection System. This event is reportable pursuant to 10 CFR 50.73 (a)(2)(iv)(A) for a condition that resulted in manual actuation of the Reactor Protection System..
3.0 CAUSE No definitive root cause has been identified for the Urgent Failure that caused the Rod Control Group Step Counter deviation. The four cards that were in service during the event were sent to Westinghouse for testing and failure analysis. Westinghouse performed thorough, card testing that did not reveal any failed cards or components.
Further investigation has determined that it is possible an intermittent failure in the signal path between the Power Cabinet Movable Regulation Card and the Failure Detection Card could have generated the Movable Regulation Failure alarm. Additionally, two other logic � cabinet cards that were not replaced following this event could have caused the Urgent Failure alarm. An intermittent short in the output of I/O AC amplifier could result in a movable signal being sent to the Movable Firing Card for a period longer than required.
This possibility was validated by Westinghouse in their inspection and test for the returned cards. It is possible an intermittent failure in either the Logic Cabinet I/O AC Amplifier Card A805 or Slave Cycler Movable Decoder Card A503 could have generated the Movable Regulation Failure alarm. Further evaluation is necessary to determine if the failure is an intermittent failure on the I/O AC Amplifier Card A805 or the Slave Cycler Movable Decoder Card A503 in the Unit 2 Logic Cabinet, or the signal path between the Power Cabinet Movable Regulation Card and the Failure Detection Card.
4.0 IMMEDIATE CORRECTIVE ACTION(S) Control Room personnel responded to the event in accordance with emergency procedure E-0, Reactor Trip or Safety Injection. The unit remained stable at Mode 3 conditions.
5.0 ADDITIONAL CORRECTIVE ACTIONS Four suspect moveable cards (Regulation, Phase Control, Firing and Failure Detection) were replaced in Power Cabinet 2AC. Following card replacement the reactor trip breakers were closed. Shutdown Banks "A" and "B" were fully withdrawn with no concerns.
6.0 ACTIONS TO PREVENT RECURRENCE Further evaluation/testing is necessary both while on-line and while shutdown to determine if the failure is an intermittent failure on the I/O AC Amplifier Card A805 or the Slave Cycler Movable Decoder Card A503 in the Unit 2 Logic Cabinet, or the signal path between the Power Cabinet Movable Regulation Card and the Failure Detection Card. The I/O AC Amplifier Card A805 and the Slave Cycler Movable Decoder Card A503 will be replaced, if necessary, during the next unit shutdown of sufficient duration.
If the exact cause is determined following additional evaluation/testing, a supplement to this LER will be submitted.
7.0 SIMILAR EVENTS The following events were reported via LERs. The cause for items 1 and 2 was equipment failure and Item 3 was caused by an improper position of the demand step counter cover.
The cause for item 4 was internal connections noted to be inadequate resulting in intermittent operation.
1. LER 50-339/1998-001-00 dated 04/30/98, Manual Reactor Trip Due To Control Bank "B" Group 2 Step Counter Inoperable.
2. LER 50-339/2001-002-00 dated 04/24/01, Manual Reactor Trip Due To Control Bank "B" Group 2 Step Counter Inoperable.
3. LER 50-339/2004-002-00 dated 07/19/04, Manual Reactor Trip Due To Control Bank "D" Group 2 Step Counter Inoperable.
4. LER 50-339/2006-001-00 dated 05/24/06, Manual Reactor Trip Due To Shutdown Bank "A" Group 2 Step Counter Inoperable.
8.0 ADDITIONAL INFORMATION At the time of this event, North Anna Unit 1 was operating at 100 percent power and was not affected by this event.
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