05000321/LER-2007-001
| Docket Number(S) | |
| Event date: | 02-07-2007 |
|---|---|
| Report date: | 4-9-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3212007001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric — Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EIIS Code XX).
DESCRIPTION OF EVENT
On 2/7/07 at 1640 EST, Unit 1 was in the Run mode at a power level of approximately 2804 CMWT (100 percent rated thermal power). At that time, a control room operator was performing a panel walk-down when he noticed the position indicating light for the High Pressure Coolant Injection (HPCI, EIIS Code BJ) system's minimum flow valve was extinguished. The valve was in its normal standby line-up, closed, at the time of discovery. The light bulb was immediately checked and, as is common practice, a new bulb was placed into the green indicating lamp socket. The bulb replacement did not correct the problem, and so the valve breaker was turned off and back on. Following this action, the reset pushbutton on the breaker door was depressed. However, the position indicating light did not illuminate.
Subsequent investigations by Maintenance personnel found the control power fuse (1E41A-F202) for the breaker blown, resulting in a loss of power to the valve's motor control center (MCC). This rendered the minimum flow valve incapable of operating. Consequently, HPCI was declared inoperable and Technical Specifications (TS) Action statement 3.5.1.0 entered.
The control power fuse was replaced and HPCI declared Operable on 2-7-07 at 2052 EST.
On 2/8/07 at 0431 EST, Unit 1 was in the Run mode at a power level of approximately 2804 CMWT (100 percent rated thermal power). Control room personnel were performing the HPCI pump Operability surveillance, a TS required surveillance which is performed once every 92 days. As operators attempted to initiate the system, the HPCI turbine steam supply valve, 1E41-F001, failed to open. A double indication was received as the operators positioned the 1E41-F001 switch to 'open', but the turbine did not come up to speed. Shortly thereafter, the `11PCI Valve Overload' annunciator was received. At this time, the operators manually tripped the HPCI turbine. When the control switch for the steam supply valve was taken to the `close' position, a ground was received on the IV station service 125/250 battery. Personnel in the HPCI turbine area reported that the motor was hot to the touch. They also noted a burnt odor in the area. Further investigations by Maintenance personnel confirmed that the valve motor had failed.
As is routinely done, the HPCI system had been administratively declared inoperable prior to beginning the HPCI pump operability surveillance. This is due to the HPCI to Condensate Storage Tank Test Valve, a normally closed valve, being placed in the open position before the test. Due to the problem with the steam supply valve, however, the HPCI system remained in TS Action Statement 3.5.1.0 following the test.
Following replacement of the valve motor, HPCI was declared Operable on 2-13-07 at 0113 EST.
The failed motor and the valve were both inspected. Inspection of the motor revealed a portion of the armature winding had failed from apparent heat and smoke damage. Inspection of the valve discovered the packing bushing to be degraded and deformed. However, based on diagnostic testing performed on the steam supply valve with the degraded bushing installed, this does not appear to have caused the stem to bind and the subsequent motor failure. Further inspections of the valve revealed no significant problems. However, upon reassembly, while the packing was being consolidated, a light marring (rub mark) was found on the valve stem. Maintenance personnel discovered that the pressure seal bonnet was off-center. This likely resulted in the valve stem loading the packing bushing, resulting in its deformation. Further investigation revealed that the pressure seal gasket retainer ring was bent. According to the vendor, the most likely explanation for the bent ring was that the valve had been filled with a fluid and overpressurized.
Historical records also show that the motor had been exposed to various packing leaks over a number of years, which could have potentially accelerated motor thermal aging and degradation. The failed motor had been in service since 1987.
CAUSE OF EVENT
The likely cause of the failure of the minimum flow valve to open was a degraded resistor in the position indicating light circuit.
Investigations by the Maintenance staff discovered a blown control power fuse in the minimum flow valve's MCC. Additionally, the resistor on the back of the green open position indicating light was found blackened as if it had been burned. It is likely that this resistor had degraded with age. This resulted in an increase in the current flowing through the resistor and through the circuitry associated with the indicating lamp. The excess current caused the fuse to blow, resulting in a loss of power to the MCC.
The cause of failure of the HPCI turbine steam supply valve to open is a failure of the valve motor. The likely cause of failure of the motor is past bonnet pressurization events and thermal aging.
If water leaks into the turbine steam supply valve when the HPCI steam line is filled with water, the potential for bonnet pressurization exists. After water enters the bonnet, pressurization could occur on a subsequent heating of the piping, which would cause an expansion of the entrapped liquid. The steam supply valve is a parallel disk gate valve. Pressurization of the valve bonnet could cause the wedge disks to press tightly against the seats, resulting in binding of the disks. No water was found in the valve during this event.
However, there have been past events where water may have entered the valve, for example a Unit 2 event in 2000 where water reached the steam lines. Also, in 2004 and 2006, in-service inspection pressure testing resulted in water entering the HPCI steam lines. As a result of these events, as well as thermal aging, the motor could have been degraded to a point such that it failed during this Operability test.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
These events are reportable under the provisions of 10 CFR 50.73(a)(2)(v)(B) in that, in each case, an event occurred which could have prevented fulfillment of a safety function. Specifically, a failed minimum flow control valve and a failed turbine steam supply valve could have, separately, prevented operation of the HPCI system.
The HPCI system is designed to provide adequate core cooling to limit fuel clad temperature in the event of a small break in the nuclear supply system that does not result in rapid depressurization of the reactor vessel.
The Automatic Depressurization System (ADS, EIIS Code JE) is the back-up for the HPCI system and is initiated on a low reactor water level condition coincident with a Primary Containment high pressure condition. Upon initiation of ADS, the reactor is depressurized to a point where either the Low Pressure Coolant Injection (LPCI, EIIS Code BO) system or the Core Spray (CS, EIIS Code BM) system can operate to maintain adequate core cooling.
In one event described in this LER, the HPCI minimum flow valve was incapable of functioning properly.
The minimum flow valve is designed to provide a flow path to a centrifugal pump during a low flow condition, such as the normal discharge valve being closed, or the pump otherwise operating at its shutoff head. In those situations, the minimum flow path provides cooling to the pump, preventing damage to the pump internals that could result from overheating.
The other event involves a failure of the turbine steam supply valve to open, which would prevent the HPCI turbine from coming up to speed, rendering the HPCI system unavailable.
However, the ADS, CS, and LPCI systems were all Operable during these events. Consequently, had a small break Loss of Coolant Accident occurred during the time the HPCI system was unavailable, these systems were available to provide adequate core cooling.
Furthermore, the Reactor Core Isolation Cooling system (RCIC, MS Code BN) is a steam turbine driven system similar to the HPCI system. It (RCIC) is designed to provide core cooling to the reactor pressure vessel upon a loss of the feedwater/condensate supply. The RCIC system is not a credited accident mitigation system and is a much lower volume system (approximately 400 gpm vs. 4000 gpm for HPCI). However in the event of a loss of feedwater event both the HPCI and RCIC systems are designed to automatically initiate.
Had a loss of feedwater occurred with the HPCI system inoperable, the RCIC system was Operable at the time, capable of alone providing adequate core cooling.
CORRECTIVE ACTIONS
The degraded resistor and the blown fuse were replaced.
The resistance of the resistors associated with indicating lamps for DC motor operated valves of the HPCI and RCIC systems with wiring similar to that of the HPCI minimum flow valve will be checked. If any values are found less than the rating of the resistors, they will be replaced. This action will be completed by 7-31-07.
The failed motor for the 1E41-F001 valve was replaced.
The Unit 1 and Unit 2 procedures 34S0-E41-001, "High Pressure Coolant Injection (HPCI) System," have been revised to require stroking the E41-F001 valve after water has been drained from the steam line. This will allow any water that may have gotten into the valve to drain out.
ADDITIONAL INFORMATION
Other Systems Affected:
No systems other than those previously described in this report were affected by this event.
Failed Components Information:
Master Parts List Number: 1E41-F001 EIIS System Code: BJ Manufacturer: Anchor Darling Reportable to EPIX: Yes Model Number: E-6550-1-1 Root Cause Code: X Type: 20 EIIS Component Code: 20 Manufacturer Code: A391 Master Parts List Number: 1E41-F202 EIIS System Code: BJ Manufacturer: General Electric Reportable to EPIX: Yes Model Number: 165A7844P3 Root Cause Code: X Type: Modifier, Current EIIS Component Code: IM Manufacturer Code: 6080 Commitment Information: There are no commitments provided in this Licensee Event Report.
Previous Similar Events:
There were no previous events in the past two years, where a HPCI system resistor failed resulting in a blown control power fuse or where bonnet pressurization resulted in a pressure lock situation resulting in an inability to cycle the steam supply valve.