05000458/LER-2015-006
| River Bend Station - Unit 1 | |
| Event date: | 7-17-2015 |
|---|---|
| Report date: | 09-14-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4582015006R00 - NRC Website | |
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REPORTED CONDITION
On July 17, 2015, with the plant operating at 92 percent power, it was determined that an operability evaluation previously performed for a safety-related instrument in the primary containment isolation circuitry was in error, which resulted in the failure to take actions required by the Technical Specifications. That initial evaluation performed on July 8 had determined that the affected circuitry was operable, when in fact it was not actually capable of performing its design function. This condition existed for longer than the allowable outage time, and is, thus, being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as operations prohibited by Technical Specifications.
INVESTIGATION AND IMMEDIATE CORRECTIVE ACTIONS
On July 8, 2015, a scheduled surveillance test was performed on one channel of the primary containment isolation logic. The channel is part of the logic that automatically isolates the reactor water cleanup (RWCU) system if the area temperature in the RWCU heat exchanger room exceeds operating limits. If this channel is found to be inoperable, the channel must be placed in the tripped condition within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, or else the affected primary containment path must be isolated within one hour.
During the test, an error message was displayed on the associated trip unit. The operators and technicians researched the vendor manual, consulted the cognizant engineers, and determined that the error message was not indicative of any inability of the system to perform its design safety function. A condition report was written to document the situation and provide for further review. The surveillance test was deemed a success, and the trip channel was restored to an operable status.
Subsequent review found that the first operability determination on the condition report was in error, and that the trip channel was not actually capable of performing as designed. The trip unit was declared inoperable, and taken out of service to be replaced. The channel was again declared operable on July 18 at 2:44 a.m. The elapsed time between the receipt of the error message on July 8 and the restoration to an operable status exceeded the allowable outage time of Technical Specifications.
CAUSAL ANALYSIS
The cause of the error in the first operability determination was the use of an outdated vendor manual for the initial troubleshooting.
The trip unit is of a slightly different design than the original, having been installed in the past as part of a system modification. The vendor manual for the old trip unit is still available in the document control system as a reference. This investigation determined that the operators' understanding of the process for retrieving controlled documents was deficient, in that they did not know the correct method of differentiating the status of an historical document versus a current controlled document. The similarity between the old and new trip unit led others involved in the initial troubleshooting to think they were using the correct manual. It was during the engineers' follow-up evaluation of the first operability determination that the error was discovered.
Contributing to this event was the inappropriately lengthy amount of time allowed the engineers for performance of their follow-up evaluation. That is, a more timely due date for that review could have resulted in the error being discovered prior to the expiration of the Technical Specification required actions. Sufficient guidance existed in the governing procedure that should have resulted in a due date commensurate with the significance of the situation.
CORRECTIVE ACTIONS to PREVENT RECURRENCE These actions were taken (or are planned) to address the direct causes of this event, and are documented in the corrective action program:
- Senior operators were briefed on the proper use of the document control operating system.
- A user's guide was developed for the document control operating system.
- Senior operators will be briefed regarding expectations for timeliness of due dates on operability evaluations.
PRIOR OCCURRENCE EVALUATION
No previous similar events have been reported at River Bend Station in the last three years.
SAFETY SIGNIFICANCE
During the time that the trip unit was inoperable, redundant channels in the isolation logic remained capable of performing the safety function. This event was thus of minimal safety significance to the health and safety of the public.