05000250/LER-2002-001
| Turkey Point Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2502002001R00 - NRC Website | |
EVENT DESCRIPTION
On June 5, 2002, with the unit operating at approximately 100% reactor power, a quarterly surveillance performed under Technical Specification (TS) 4.8.2.1.b recorded a 3A vital battery [Ej, BTRY] individual cell voltage that was below the Technical Specification allowable value. This condition was recognized during a review of the quarterly surveillance test results by an Electrical Maintenance Supervisor at 1340 on June 17, 2002. This was reported immediately to the Nuclear Plant Supervisor (NPS), and the 3A battery was declared inoperable. With one of the required battery banks inoperable, TS 3.8.2.1 action b. allows two hours to restore the required battery banks to operable status or be in at least hot standby within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in cold shutdown within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. Since the condition existed for a longer time than permitted by Technical Specification action statement 3.8.2.1.b, the plant entered TS 3.0.3 on June 17, 2002 at 1340. The spare battery was placed in service on June 17, 2002 at 1423 completing the required recovery actions within the time allowed by Technical Specification 3.0.3. Technical Specification 3.0.3 was exited. This event is reported per 10CFR50.73(a)(2)(i)(B) due to operation of the unit in a condition prohibited by the plant's TS.
BACKGROUND
Emergency power for vital instrumentation and controls is supplied by a station DC power system containing five safety related 125VDC batteries and four DC distribution panels. Two battery banks are associated with each unit. A spare battery bank can be substituted for any of the other four battery banks to allow for testing or maintenance. Each battery has been sized to support operation of its required loads for two hours without terminal voltage falling below its minimum required value.
Technical Specification Table 4.8-2, Category B provides the battery cell voltage limit and allowable values. A battery whose parameters satisfy the allowable value, but does not meet the limits may be considered operable, provided the parameters are restored to the Category B limits within 7 days. Values that are below the allowable value render the battery inoperable. The limit and allowable value for battery float voltage are 2.13 VDC and 2.07 VDC, respectively.
The 3A vital battery consists of 60 individual cells connected in series. Cell #28 voltage of the 3A vital battery was found at 2.06 VDC, which is below the allowable limit.
SEQUENCE OF EVENTS
On June 5, 2002, a quarterly surveillance performed under Technical Specification (TS) 4.8.2.1.b recorded a 3A vital battery individual cell #28 voltage of 2.06 VDC, which is below the TS limit and allowable values of 2.13 VDC and 2.07 VDC, respectively. As required by procedure, the journeyman immediately notified his General Maintenance Leader (GML) of the unacceptable as-found condition. The GML instructed the journeyman to complete the surveillance for the rest of the battery. The journeyman noted in the remarks section of the procedure that the voltage on cell #28 did not meet acceptance criteria, and proceeded to complete the surveillance for the rest of the battery at the end of shift.
The GML has the responsibility to review the as-found cell voltage data to determine if any corrective actions are required, such as electrolyte level correction or the performance of an equalizing charge. The GML did not recognize the TS impact of not meeting the acceptance criteria on cell #28. Since the surveillance test was completed at the end of the shift, the GML turned over the work package for review to an Electrical Supervisor.
He did not alert the Electrical Supervisor of the as-found condition on cell #28. Unaware of the failed surveillance, the Electrical supervisor did not perform the surveillance data review that day.
On June 17, 2002, at 1340, an Electrical Supervisor performed the review of the as-found data and recognized that 3A battery low voltage on cell #28 rendered the vital battery inoperable. The NPS was notified at 1350; the 3A battery was removed from service and the spare vital battery was placed in service at 1423 on June 17, 2002.
CAUSES OF THE EVENT
The cause of this event is postulated to be a minor plate defect which resulted in an internal short circuit, causing the low voltage condition in cell #28 of the 3A vital battery.
A contributing cause of this event is an inadequate battery surveillance procedure, which did not alert the journeyman of the TS implications of a battery cell voltage below the TS minimum allowable voltage. The TS action statement has a two-hour allowable outage time to return the vital battery to operable status or to place the spare battery in service. In addition, the surveillance procedure did not require NPS notification when an as-found condition on a vital battery cell fails to meet the acceptance criteria.
Personnel error is another contributing cause of this event. The GML did not recognize the TS implications of an as-found battery cell voltage below the TS minimum allowable voltage, therefore, the NPS was not notified of the situation at the time of discovery, and the surveillance data review was not performed in a timely manner.
ANALYSIS OF SAFETY SIGNIFICANCE
The safety related function of the Vital AC and DC Systems is to provide power to the safety related loads required to achieve and maintain safe shutdown of both Turkey Point Units 3 and 4 during a loss of off-site power. The Vital DC System is designed to: 1) provide power, as required, to support one Auxiliary Feedwater System pump train during a loss of all off-site and on-site AC power; 2) provide power to its safety related loads during a design basis accident, with or without concurrent loss of off site power; and 3) support systems, as required, that mitigate the consequences of an accident on the affected unit, and systems that achieve and maintain safe shutdown of the unaffected unit.
The as-found condition of the 3A battery cell #28 was below the criteria for operability of TS Table 4.8-2. The as- found cell voltage was 2.06 VDC compared to the minimum allowable value of 2.07 VDC. The root cause of the low voltage is a microscopic internal short. Cells with minor internal shorts are not failed and are capable of providing high discharge currents close to their rated values, and are able to pass overall battery current.
An evaluation was performed to determine the impact of the as-found condition on the 3A battery's capability to perform its design safety related functions. The as-found specific gravity for cell #28 was 1.218, which is in the normal band and within the acceptance criteria (greater than 1.195). This parameter indicates that the cell had the chemical ability to pass its design output current on demand. Maintaining the battery on float charge prevented the suspected internal short from discharging the cell completely.
With a conservative assumption that the internal short had reduced the capacity of cell #28 to zero, the evaluation determined that the battery string was capable of meeting its design load profile. The as-found voltage and electrolyte values for the remaining 59 cells in the 3A battery support the following conclusions: 1) the 3A battery was capable of providing the capacity found during the previous performance test (104%); 2) the electrolyte solution of the battery was not degraded and was capable of passing the current from the remaining 59 cells; 3) the remaining cells had sufficient capacity to satisfy the required load profile; and 4) the minimum battery terminal voltage during a design basis event would remain above the minimum design value of 105 VDC.
Therefore, the noted degraded condition of cell #28 would not have prevented the 3A battery string from meeting its design function. Thus, the health and safety of the public were not affected by this event.
CORRECTIVE ACTIONS
The following corrective actions have been completed:
1. Replaced cell #28 of the 3A vital battery 2. Provided training to Electrical Maintenance personnel addressing the event and emphasizing the reporting requirements and management expectations.
3. Battery Surveillance Procedures have been enhanced to include caution statements requiring immediate NPS notification, if acceptance criteria is not met, and a briefing requirement prior to performing the surveillance to ensure all personnel are aware of the applicable Technical Specification and NPS notification requirements.
ADDITIONAL INFORMATION
function identifier (if appropriate)]. There have been no previous similar events at Turkey Point Units 3 and 4.