05000443/LER-2005-004
| Docket Number | |
| Event date: | 03-29-2005 |
|---|---|
| Report date: | 05-19-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4432005004R00 - NRC Website | |
I. Description of Event
During operation at 95% power on March 29, 2005, plant personnel discovered an earlier failure to comply with Techncial Specification (TS) 3.8.1.1, AC Sources. On February 14, 2005 at 0525, the circuit breaker [EB, 52] for the reserve auxiliary transformer (RAT) [EA, XFMR] supply to the train A 4160 volt emergency bus (EB, BU] was removed from service while the bus remained energized from the unit auxiliary transformer (UAT). This activity rendered inoperable one of the two, independent offsite AC sources [EA] required by TS 3.8.1.1. The station had historically implemented the TS such that only one offsite AC source was required for each train of emergency buses, provided the sources to the two trains of emergency buses were physically independent.
Consequently, the unavailability of the RAT supply to the emergency bus was not considered a loss of an offsite AC source. As a result, the periodic surveillance requirement, verification of the operability of the remaining offsite AC source within one hour and every eight hours thereafter, specified in the actions of TS 3.8.1.1 was not performed. The failure to comply with the actions while in noncompliance with the TS limiting condition for operation (LCO) resulted in a condition prohibited by the TS. This condition exceeded the 72-hour allowed outage time permitted by action a of TS 3.8.1.1 by continuing for approximately 9.5 days until the RAT supply breaker to the emergency bus was returned to operable status at 1538 on February 23, 2005. In addition, in August 2003, the plant operated with the train B emergency bus energized from the RAT to support maintenance on the UAT from 0321 on August 18 until 1703 on August 21. In this condition, the offsite AC source through the UAT was inoperable for approximately 85 hours9.837963e-4 days <br />0.0236 hours <br />1.405423e-4 weeks <br />3.23425e-5 months <br />, which exceeded the 72-hour TS allowed outage time.
II. Cause of Event
The cause of this event was a misapplication of the electrical configuration needed to meet the TS requirement for two independent offsite AC sources. Since initial licensing, the station implemented the TS such that only one offsite AC source is required for each train of emergency buses, provided the sources to the two trains of emergency buses are independent. However, after a review of the plant licensing basis and meeting with the NRC staff, it was determined that the historical implementation of the TS was incorrect and an operable offsite AC source must be aligned to both trains of emergency buses. Subsequent evaluation identified the two instances of noncompliance with both the LCO and actions of TS 3.8.1.1.
III. Analysis of Event
The Seabrook Station design includes three offsite lines and two independent offsite AC sources: (1) one offsite circuit through each UAT to both trains of emergency buses, and (2) a second independent offsite circuit through each RAT to both trains of emergency buses. A separate UAT and RAT supply each bus. A total of four independent transformers, with individual breakers, supply power to the two emergency buses. Historically, the station considered that the minimum requirements of TS 3.8.1.1 were met with one UAT and one RAT inoperable if the operable UAT and RAT were connected to the onsite lE distribution system. This connection was considered acceptable since there were still two physically independent circuits (one UAT and one RAT) from the transmission network to the onsite distribution system. On this basis, the actions of TS 3.8.1.1 were not applied to the loss of the RAT supply to the train A emergency bus. However, the licensing basis review and NRC meeting determined that the two independent power supplies should be provided to each train of the lE distribution system.
For this reason, the unavailability of the RAT supply to the emergency bus rendered one offsite AC source inoperable.
This event is of regulatory significance because it met the reporting criterion of 10 CFR 50.73 (a)(2)(I)(B) for a condition prohibited by the TS. In addition, the event was reported to the NRC on March 29, 2005 at 1524 (event # 41543) in accordance with the Facility Operating License Condition 2.G for a violation of the TS.
This event had no adverse impact on the plant or on the health and safety of the public. No plant transients, systems actuations, or consequences resulted from this event. This event did not involve a Safety System Functional Failure since two independent offsite AC sources remained available to the train B emergency bus. No inoperable structures, systems, or components contributed to the event.
IV. Corrective Actions
The corrective actions for this event included:
1. Revising the TS bases and the UFSAR as necessary to reflect that operability of an offsite AC source requires the source be available to both trains of emergency buses, and 2. Communicating to station managers, supervisors, and licensed operators the revised criteria for operability of offsite AC sources.
Similar Events Seabrook Station has had one previous occurrence of an event in which incorrect procedure implementation resulted in a condition prohibited by the TS. On August 15, 2003, LER 2003-001 reported that station personnel incorrectly concluded that surveillance testing of an emergency diesel generator was not required to meet the actions of a TS. A review of the condition determined that such testing was required, and the failure to meet the actions while in noncompliance with the LCO resulted in a condition prohibited by the TS.