05000397/LER-2005-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 41779 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3972005003R00 - NRC Website | |
Plant Condition The plant was operating in Mode 1 at 100 percent power at the time of this event.
Event Description
On June 15, 2005 at 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br />, the reactor [RCT] tripped from 100% power. The trip resulted from a Reactor Protection System (RPS) [JC] actuation when the four turbine throttle valves (TVs) [FCV] simultaneously stroked from full open to full close. The RPS actuates when two of four TVs are 95% open with power greater than 30% power. All rods fully inserted as expected in response to the RPS actuation.
Nineteen minutes later, all four TVs reopened with no operator action. During the time from the reactor trip to the TVs reopening, the main turbine [TA] failed to trip as designed. At thirty minutes following the reactor trip, plant operators manually tripped the main turbine from the front standard resulting in the re-closure of TVs at 1430 hours0.0166 days <br />0.397 hours <br />0.00236 weeks <br />5.44115e-4 months <br />.
At 1538 hours0.0178 days <br />0.427 hours <br />0.00254 weeks <br />5.85209e-4 months <br />, the NRC was notified of the RPS actuation per 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.72(b)(3)(iv)(A) (reference event notification 41779). This LER is submitted pursuant to 50.73(a)(2)(iv)(A) as an event or condition that resulted in manual or automatic actuation of the reactor protection system.
Immediate Corrective Action Following the event, plant personnel performed numerous troubleshooting activities.
Although no specific DEH system [JJ] failure could be identified, the three circuit cards [ECBD] providing the control signals to all four turbine throttle valves were replaced. These cards were identified as the most likely source of the DEH system failure.
Cause
The root cause of this event is the DEH Control System design has single point vulnerabilities and the cards in this system do not exhibit a predictable failure mechanism which would allow replacement prior to failure.
Failure analysis on the three cards replaced was unable to identify any component failure for this event.
A significant contributing cause is the design of this system occurred at a time when the impacts of Balance of Plant system failures were not emphasized. This resulted in a system with single failure vulnerabilities.
S.
Assessment of Safety Consequences
This event posed no threat to the health and safety of the public or plant personnel. AU safety equipment was available during this transient and performed as expected. Local manual action was required to trip the main turbine, but there were no safety consequences associated the failure of the turbine to trip automatically. Thus this event was not safety significant.
Similar Events The relevant recent LERs, PERs, and CR records for DEH Control System circuit card failures include: LER 2004-004-00, PERs 204-0969 and 205-0424, and CRs 2-04-04824, 2 04-05205, 2-05-05314 and 2-05-05564.
Columbia's DEH circuit cards have failed at a rate of about 11/200 over a 7 year period. Of these failures, two have resulted in plant scrams, both occurring within 11 months of each other. This historical failure rate is an indicator; however, the data is insufficient to provide an accurate predictor of future performance.
The internal experience shows the failures are random and the elimination of these failures would require DEH system replacement. A project to evaluate the replacement of this system has been initiated.
EIIS information denoted as IXX1 26158 R2