05000387/LER-2001-004
| Susquehanna Steam Electric Station - Unit 1 | |
| Event date: | 11-11-2001 |
|---|---|
| Report date: | 01-09-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3872001004R00 - NRC Website | |
EVENT DESCRIPTION
On November 11, 2001, with Unit 1 in Mode 1 (Power Operation) at 100% power, the Control Room received the High Pressure Coolant Injection (HPCI; EllS Code: BJ) steam line drain pot high level alarm. Operations personnel observed that valve position indication was lost on the HPCI steam line drain to condenser inboard isolation valve (Susquehanna component number HV-155-F028) and the HPCI barometric condenser pump discharge drain valve (HV-156-F026). Operations declared HPCI inoperable due to the formation of condensate at the turbine inlet with no drain path available.
Maintenance personnel investigated the loss of indication and found a failed fuse. To determine the cause of the failed fuse, Maintenance personnel checked the resistances of solenoid coils supplied by the circuit. A failed solenoid valve coil was discovered (SV-15626). The system was restored. The restoration activities included replacing the failed solenoid valve with a better designed solenoid valve.
CAUSE OF EVENT
The cause of the failure was accelerated aging due to a normally energized coil. This failure mechanism is described in EPRI NP-7414, Solenoid Valve Maintenance and Application Guide, as a dominant electrical failure mechanism. The heat from the coil degraded the insulation on the coil windings. This reduced coil resistance and increased current draw and heat. This promoted further degradation until the coil resistance had degraded from 275 Ohms to 2 Ohms. The current draw of the degraded coil on SV-15626 after nine years of operation was high enough to exceed the capacity of the 10 amp fuse protecting the circuit.
The failed fuse caused the circuit including SV-15628 to lose power. When SV-15628 lost power, the HPCI steam line drain to condenser inboard isolation valve (HV-155-F028) closed, isolating the inlet steam line drain pot from the Main Condenser. The inlet steam line drain pot then began to fill up with condensate from the steam supply line. After a period of time, the water level in the drain pot was high enough to bring in the HPCI steam line drain pot high level alarm.
ANALYSIS I SAFETY SIGNIFICANCE
The loss of Unit 1 HPCI System had very low significance. The redundant safety systems were available for the period of time that HPCI was not available. There were no actual demands on HPCI during that period of time. Given the short duration of the event compared to an entire cycle, the total cycle Core Damage Frequency was increased less that one-half of one percent. The 9.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> that HPCI was not available was much less than the allowed outage time given by Technical Specification 3.5.1 (14 days). This out of service time was an insignificant portion of the 2.5% unavailability assumed by plant PRA analysis. Therefore, this event has very low significance.
PREVIOUS OCCURENCES
A similar failure occurred on the corresponding Unit 2 solenoid valve (SV-25626) in December 1998 while HPCI was out of service. This failure was an important consideration in the review of the most recent event. Together these failures prompted a broader review of the status of normally energized Circle Seal solenoid valves.
On December 15, 1993, PPL issued a letter to the NRC, R. G. Byram to C. L. Miller, entitled "Failures of and Continuing Service Problems with Circle Seal Controls Solenoid Valves". This letter covered problems encountered with non-functioning of the solenoid due to coil shorting and subsequent burnout.
Many of these failures occurred within a short time of installation, typically in as little as hours or days.
The root cause of these failures was faulty design and manufacturing. These were a different model of Circle Seal solenoid valves and the failures were not due to accelerated aging. One of these valves did fail while in service and LER 93-009 was issued dated 9/20/1993.
CORRECTIVE ACTIONS
Corrective actions that have been completed are:
- The failed solenoid valve and fuse were replaced and HPCI was restored. The original solenoid valve used in this application was a Circle Seal Model SV31S-9101-4. This solenoid had a coil resistance of 275 Ohms. The replacement solenoid valve is manufactured by Automatic Valve Company (AVCO) and has a coil resistance of 948 Ohms. This higher coil resistance results in lower current and allows the replacement solenoid valve to function while generating one-third the heat.
- A failure analysis was performed on the failed solenoid valve. The physical appearance and terminal resistance indicated heat damage to the insulation. Therefore, the short was coil winding to coil winding.
- A broad review was conducted of the status of Circle Seal solenoid valves currently in service on both units. Modification work was planned and in progress to replace certain Circle Seal solenoid valves (model SV31S-9101-4). Of the eight valves that remain to be replaced under this modification, only one solenoid is normally energized. This solenoid valve is the corresponding valve on Unit 2 HPCI (SV-25626). This valve was replaced with a Circle Seal solenoid valve (same model) in December of 1998.
Corrective Actions to be completed are:
- Replace the Circle Seal model SV31S-9101-4 solenoid valve on Unit 2 HPCI (SV-25626) with an AVCO solenoid valve. Expected completion is 9/30/2002. This completion date was selected since this valve will have less than four years of service when it is replaced, and the Unit 1 valve had accumulated nine years of service before it failed.
ADDITIONAL INFORMATION
Past Similar Events: See Previous Occurrences above.
Failed Component: SV15626 Manufacturer: � Circle Seal Model:
� SV31S-9101-4