05000397/LER-2010-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 12-20-2010 |
|---|---|
| Report date: | 02-18-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 46604 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3972010002R00 - NRC Website | |
Plant Condition The plant was operating in Mode 1 at 100% power. All trains of the low pressure coolant injection (LPCI) system [BO], high pressure core spray (HPCS) system [BG], and standby service water (SSW) system [KE] were operable at the time of the event.
Maintenance on the low pressure core spray (LPCS) [BM] water leg pump (LPCS-P-2) [P] was scheduled for December 20, 2010. In preparation for the water leg pump being out of service, the LPCS system was planned to be aligned to the suppression pool with the LPCS pump (LPCS-P-1) in operation to maintain pump discharge pressure. The procedure for aligning the LPCS system to the suppression pool requires declaring LPCS inoperable. LPCS was declared inoperable on December 20, 2010 at 0435 hours0.00503 days <br />0.121 hours <br />7.19246e-4 weeks <br />1.655175e-4 months <br />.
There were no structures, systems, or components that were both a) inoperable at the time of the event and b) contributed to the event.
Event Description
Subsequent to declaring LPCS inoperable in preparation for alignment to the suppression pool, LPCS-P-1 was started at 0437 hours0.00506 days <br />0.121 hours <br />7.225529e-4 weeks <br />1.662785e-4 months <br />. The minimum flow valve (LPCS-FCV-11) [FCV] for LPCS-P-1 lost indication when the valve was expected to open. In addition, annunciation for "LPCS OUT OF SERVICE" and "MOV NETWORK PWR LOSS/OL" was received indicating that LPCS-FCV-11 had lost power. LPCS-P-1 was secured at 0438 hours0.00507 days <br />0.122 hours <br />7.242063e-4 weeks <br />1.66659e-4 months <br /> and LPCS-FCV-11 was declared inoperable. LPCS-FCV-11 functions as a primary containment isolation valve (PCIV) and the Technical Specification (TS) Required Action for Limiting Condition for Operation (LCO) 3.6.1.3 Condition C, one or more penetration flow paths with one PCIV inoperable, was complied with by isolating the affected penetration flow path by closing the LPCS minimum flow isolation valve (LPCS-V-52). In addition, PCIVs are required to have operable position indication in accordance with TS Table 3.3.3.1-1 Function 7. The TS Required Action for LCO 3.3.3.1 Condition A, one or more functions with one required channel inoperable, was complied with by closing LPCS-V-52 within the required completion time.
An investigation was initiated to determine the cause of the loss of power to the valve. The three line power fuses [FU] for the reversing starter [MSTR] for the minimum flow valve motor operator were found to be cleared. The circuit was meggered satisfactorily indicating that a short-circuit condition did not exist. The valve was manually unseated to allow it to be stroked for measurements of starting and running motor current amps. The fuses were replaced and the valve stroked open and closed normally; the fuses did not clear.
At the conclusion of the troubleshooting activities, the valve stroke time surveillance was completed satisfactory at 2300 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.7515e-4 months <br /> and the valve was declared operable. The surveillance establishing LPCS operability was completed on December 21, 2010 at 0024 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and LCPS was declared operable. The elapsed time from the occurrence of the event until LPCS was returned to service was 19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> 49 minutes.
Immediate Corrective Actions
Immediate corrective actions consisted of protecting the alternate sources of emergency core cooling system (ECCS) injection/spray including residual heat removal (RHR), which performs the LPCI function, and HPCS. The supporting systems of SSW and the diesel generators IEK] were also protected.
Assessment of Safety Consequences
The safety functions for LPCS are to provide inventory makeup and spray cooling during large breaks in the reactor coolant system that uncover the core. All remaining ECCS subsystems were operable and at no time did this event result in the loss of a safety function. The low pressure injection function was not challenged due to all three loops of RHR system LPCI mode being operable while the core spray function was satisfied by the operable HPCS system.
This event did not pose a threat to the health and safety of the public. The TS Required Action for LCO 3.5.1 Condition A, one low pressure ECCS injection/spray subsystem inoperable, was complied with by restoring the LPCS system to operable within the allowed completion time. In addition, the TS Required Actions for LCO 3.6.1.3 Condition C and LCO 3.3.3.1 Condition A were completed within the required completion time. Thus, Columbia Generating Station was never in a condition prohibited by TS.
This event is being reported under 10 CFR 50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident for a single train system. The minimum flow valve supports LPCS operability by providing a flow path to prevent pump damage during situations where the LPCS pump has been started in response to a transient but reactor vessel pressure is not low enough to allow LPCS injection.
Historically, when LPCS has been inoperable at Columbia Generating Station, it was not considered to be reportable as a single train system. This treatment is consistent with the plant safety analysis and the associated system and safety function groupings which do not identify LPCS as a single train system. There are two pertinent groupings in the safety analyses which are aligned with the credited safety functions of LPCS. The two groupings are the low pressure injection system function (combined with LPCI), and a core spray system function (combined with HPCS). Industry precedent has been consistent with the historical position. However, recent NRC interpretations have considered safety function at the lowest system level, which results in LPCS being considered a single train performing a safety function.
Causes The direct cause of LPCS-FCV-11 failing to open as expected was a loss of power that occurred when the overload elements cleared within the three line power fuses. Possible reasons to fail or trigger the overload elements in the fuses are binding within the flow control valve, binding within the motor operator, sticking or dirty contacts within the reversing starter, or a premature failure of a fuse. Once the fuses were replaced, normal operation was restored with no observed binding of the valve or motor operator or abnormalities with the reversing starter. Diagnostic testing of the motor operator was completed which showed that the amps drawn by the motor were under the fuse manufacturer's time-current curve.
The failed fuses had been installed approximately two years ago but were 12 years old. An examination of the three failed fuses revealed a poor solder joint in the trigger assembly in one of the failed fuses. The apparent cause of this event is attributed to premature failure of one of the three installed power line fuses from momentary inrush current at a current value under the fuse curve. This conclusion is based on the examination of the fuse trigger solder which displayed evidence of cold unattached solder and trapped resin flux in the trigger solder joint. Also, no heating damage of the PET (polyethylene terephthalate) insulating spool was observed. Two of the fuses obviously failed from current overload on the resulting failure of the first fuse. These two fuses were determined to have functioned normally.
Initially, this event was considered to be not reportable until discussions were held with the NRC Resident Inspector and Regional staff. Upon determination of reportability of this event, a prompt notification was made to the NRC in accordance with 10 CFR 50.72(b)(3)(v)(D) via Event Notification #46604. As such, the event investigation was conducted as an apparent cause rather than a root cause evaluation. A root cause analysis is in progress but has not yet been completed for this event.
If new information is gained, which invalidates the stated cause or corrective actions, this report will be revised to incorporate the new information.
Further Corrective Actions The fuses that failed were Cooper/Bussmann Fusetron dual-element, time-delay, current limiting, 600 Volt, 1.25 Amp fuses (model FRS-R-1-1/4). Since the fuse overload element was found to be triggered, the operating history of the LPCS-FCV-11 motor operator and starter was investigated: 1) In March 2008, a diagnostic test of the motor operator was completed satisfactorily with no abnormalities noted. 2) In March 2009, an inspection of the reversing starter was completed satisfactorily. The fuses were replaced at this time. 3) In April 2010, the motor operator was lubed and inspected with no problems found. 4) The last quarterly LPCS surveillance test prior to the event was completed satisfactorily on September 29, 2010.
Warehouse and tool crib stock of the Bussmann FRS-R-1-1/4 fuses from the same GIO date code lot as that of the three failed fuses were quarantined and segregated to prevent further installation of suspect fuses. This lot of fuses was received at Columbia Generating Station in 2000. Enhancements to the Bussmann fuse dedication process to require additional receipt inspections and testing are planned.
A search was performed of all safety-related ECCS motor operated valves which may possibly contain the same Bussmann 1.25 Amp fuses from the same lot. Inspections were performed and no additional fuses of the same lot were found to be installed.
Similar Events No similar events have been reported by Columbia Generating Station.