05000341/LER-2011-001
| Fenni2 | |
| Event date: | 09-06-2011 |
|---|---|
| Report date: | 11-01-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 47242 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3412011001R00 - NRC Website | |
Initial Plant Conditions:
Mode 1 Reactor Power 100 percent
Description of the Event
On September 6, 2011, at approximately 0104 EST surveillance testing of the High Pressure Coolant Injection (HPCI) [BJ] system per procedure 24.202.01 "HPCI Pump Time Response and Operability Test at 1025 PSI," was in progress. The HPCI turbine was in the process of coasting down at the end of the surveillance test, and at the point where the minimim flow valve should have closed, indication was lost on the open and closed pushbuttons. Alarm 2D73 (HPCI MOVS MTR OVERLOAD / LOSS OF POWER SUPPLY) was received.
Initial troubleshooting determined the minimum flow valve main power fuses [FU] were blown. The fuses were replaced, and during a subsequent stroking of the valve, control room position indication was immediately lost when the open pushbutton was depressed. Both primary power fuses were again found blown which were subsequently replaced and the breaker re-energized. Once the breaker was re-energized the closed indication was restored in the control room.
The valve was locally verified closed to comply with the Technical Specification Limiting Condition for Operation (LCO) 3.6.1.3 to isolate the containment penetration. A 14 day LCO was entered for HPCI system inoperable per Technical Specification Limiting Condition for Operation (LCO) 3.5.1. The Reactor Core Isolation Cooling (RCIC) system remained operable during this time.
Troubleshooting activities were expanded into the valve and control circuits. The HPCI minimum flow valve motor, valve torque switch, and Motor Control Center (MCC) main and auxiliary contactors were replaced.
Circuit leads were inspected and tightened, and power fuse tightness was verified during fuse replacement.
Thermal overloads were also replaced. Post maintenance testing was successfully completed and the HPCI system was returned to service on September 10, 2011 at 2300 EST.
Significant Safety Consequences and Implications The HPCI system, a single train safety system, was rendered inoperable when the operation of the HPCI minimum flow valve was determined to be unreliable and the valve was declared inoperable. The purpose of the HPCI system is to provide emergency core cooling in the event of an accident involving loss of coolant from a small break. Reactor steam is used to drive the HPCI turbine, which in turn drives the main and booster pumps to provide a source of high pressure water to the reactor. The Reactor Core Isolation Cooling [BN] and Standby Feedwater [SJ] systems remained available for high pressure injection in the event of an emergency.
Additionally, the Automatic Depressurization System [JE] was available to reduce reactor pressure to within the capabilities of the low pressure Emergency Core Cooling Systems.
This event resulted in approximately 5 days where HPCI was inoperable. Technical Specification 3.5.1 allows HPCI to be taken out of service for planned outages for up to 14 days. This risk increase associated with HPCI being out of service for approximately 5 days has been evaluated by the Probabilistic Safety Analysis (PSA) group and determined to be low.
This report is made in accordance with 10 CFR 50.73(a)(2)(v)(D), for any event or condition that could have prevented the fulfillment of a safety function of a system needed to mitigate the consequences of an accident. An eight-hour non-emergency notification was also made pursuant to 10 CFR 50.72(b)(3)(v)(D) as a condition that at the time of discovery could have prevented the fulfillment of a safety function (EN 47242).
Cause of the Event
The HPCI minimum flow valve motor (MO) was sent for failure analysis which determined the valve motor series winding had developed turn-to-turn shorts. The shorted winding caused the main power fuses to blow. A root cause evaluation team continues to investigate the problem.
Corrective Actions
The HPCI minimum flow valve motor and main power fuses were replaced. Additionally, the HPCI minimum flow valve motor operated valve torque switch, thermal overloads and MCC main and auxiliary contactors were replaced. Circuit leads were inspected and tightened, and power fuse tightness verified during fuse replacement.
Thermal overloads were also replaced. This event has been documented in the Fermi 2 Corrective Action Program and additional actions may be taken as determined by the program.
Additional Information
A. Failed Component:
Component: Motor Function: Valve Actuator Manufacturer: Peerless Electric Model Number: Frame DK56H Failure Cause: Shorted Winding B. Previous LERs on Similar Problems:
A similar problem was identified in Licensee Event Report 2010-04, "High Pressure Coolant Injection System Inoperable Due To Inoperable Minimum Flow Valve." However, that failure was determined to be caused by a failed contactor. The HPCI minimum flow valve motor was also replaced as a precaution in December, 2010, when the contactor failed.
Since the cause of the current failure (a failed valve actuator motor) is different from the LER 2010-04 failure (failed contactor), the corrective actions for the previous LER would not be expected to be effective in addressing the problem identified in this LER.