05000361/LER-2005-005

LER-2005-005, Inoperable Class 1E Supply Breaker Causes Pressurizer Heater to Be Inoperable for Longer Than Allowed by Technical Specifications

Docket Number
Event date:	12-07-2005
Report date:	01-23-2006
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3612005005R00 - NRC Website

1. FACLITY NAME 2. DOCKET NUMBER 6. LER NUMBER PAGE (3) Discovery Date: December 7, 2005 Reactor Vendor: Combustion Engineering Mode: Mode 1 — Power Operation Power: 99 percent

Description of Event

On December 3, 2005 at 1235 PST, plant operators were preparing for a routine surveillance test and attempted to close pressurizer heater lE supply breaker 2B0602 from the control room. The breaker failed to close so the pressurizer heater bank 2E129 was declared inoperable and a 72-hour action statement was entered. Troubleshooting was performed, the breaker was replaced, and the new breaker tested satisfactorily. The pressurizer heater bank 2E129 was returned to operable status at about 1950 PST on December 3, 2005.

On December 7, 2005 at 1345 PST (discovery date), breaker 2B0602 again failed to close upon actuation from the control room and operators declared pressurizer heater bank 2E129 inoperable.

This was the first time an attempt was made to operate breaker 2B0602 since it was replaced and satisfactorily tested on December 3, 2005. Upon investigation, Southern California Edison (SCE) concluded that interferences and misalignments between the breaker and breaker cubicle were causing the intermittent discontinuity in the secondary contacts. (See cause discussion below.) The breaker removed on December 3, 2005 was modified to eliminate the interferences and the cubicle breaker cradle alignment was adjusted prior to installing the modified breaker in the cubicle. A successful test was performed and the pressurizer heater bank 2E129 was returned to operable status on December 9, 2005 at 1235 PST.

Based on the cause evaluation (below), SCE concluded that the breaker, and its supported pressurizer heater bank, were inoperable from 1235 PST on December 3, 2005 until 1235 PST on December 9, 2005. If one group of pressurizer Class lE powered heaters is inoperable for greater than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, Technical Specification (TS) 3.4.9 requires plant operators to place the plant in Mode 3 in the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 4 in the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Because SCE did not discover this period of inoperability until after the breaker was returned to service on December 9, 2005, plant operators were not able to complete the actions required by TS 3.4.9. This submittal is being made in accordance with 10CFR50.73(a)(2)(i)(B) as a condition prohibited by the TS.

Cause of Event

The original breaker installed in cubicle 2B0602 operated, without a similar failure, from plant startup until October 16, 2002 (almost 20 years) when it was removed for overhaul under the preventive maintenance program. The breaker installed on October 16, 2002 had operated successfully during surveillance tests performed quarterly until it failed on December 3, 2005. Both the breaker installed on October 16, 2002 and its replacement installed on December 3, 2005 (that failed on December 7, 2005) functioned within specifications when tested outside the cubicle.

After the replacement breaker failed to close upon actuation from the control room on December 7, 2005, further investigation revealed that the intermittent discontinuity was due to reduced contact pressure between the secondary contacts of the breaker and the cubicle. SCE has identified two causes of the reduced contact pressure:

1. Interference between the breaker and the cubicle While reinstalling the breaker in the cubicle, the breaker contact support plates were observed to be interfering with bolts at the rear of the cubicle that hold the secondary contact carrier to the cubicle. The interference contributed to reducing contact pressure between the secondary contacts.

2. Marginal alignment Upon detailed post-event examination, it was found that the breaker cradle installed in the cubicle was slightly misaligned. This particular switchgear cubicle was designed to accommodate either a 600 or a 1600 Amp breaker. For a 1600 Amp breaker, the cubicle rails are nearly flush with the walls of the cubicle, which prevents significant misalignment. For a 600 Amp breaker, which is the installed size in 2B0602, the rails must be inset from the walls of the cubicle. This allows greater room for misalignment of the cradle rails in the cubicle. The cradle rails were not adjusted until after the event on December 7, 2005; therefore, SCE concluded the breaker installed on December 3, 2005 had sufficiently different physical dimensions, within tolerances, to cause a marginally proper alignment (and corresponding reduction in contact pressure) between the secondary contacts of the breaker and the cubicle.

The marginal alignment combined with the cubicle-to-breaker interference reduced the contact pressure enough to cause an intermittent discontinuity in this particular breaker/cubicle combination.

The cause evaluation of this event remains open until further inspections are completed.

Corrective Actions

After the December 7, 2005 occurrence, SCE performed the following corrective actions:

1. The breaker was removed and the alignment of the breaker cradle in the cubicle was adjusted.

2. The breaker that had been installed from October 16, 2002 until December 3, 2005 was modified by trimming the corners of the contact support plates to eliminate the interference with the bolts at the rear of the cubicle. This minor modification was performed in accordance with plant procedures and with verbal concurrence from the manufacturer.

3. The cubicle contacts were cleaned and the modified breaker was installed in the cubicle. No interferences were observed and the breaker tested satisfactorily.

4. Until the start of the current Unit 2 Cycle 14 refueling outage, a recorder was installed to monitor the continuity of the close coil circuit by recording any occurrence of the voltage falling below the specified setpoint.

The following additional actions are planned:

1. Breaker 2B0602 and other cubicles in 2B06 will be inspected for alignment issues with vendor assistance during the current Unit 2 Cycle 14 refueling outage and will be adjusted/repaired as required.

2. The preventive maintenance procedures for the breakers and switchgear (including breaker cubicles) will be revised, as necessary, based on results of the inspections performed during the outage.

1. FACLITY NAME� 2. DOCKET NUMBER 6. LER NUMBER� PAGE (3) Additional corrective actions will be implemented if they are identified.

Safety Significance

Technical Specification 3.4.9 requires two groups of pressurizer heaters, with a minimum capacity of 150 kW each, to be operable in Modes 1, 2, and 3 to ensure reactor coolant system (RCS) pressure can be maintained at normal pressure. Safety analyses presented in the Updated Final Safety Analysis Report do not take credit for pressurizer heater operation to mitigate the initial phase of the accident. The need to maintain subcooling in the long term during loss of offsite power is the reason for their inclusion in the Technical Specifications (TSs).

There are a total of eight banks of pressurizer heaters installed at SONGS. Two of these banks are powered from safety related buses (Class lE buses). Either of these two Class lE powered heater banks are adequate to maintain RCS pressure control during a natural circulation cooldown on a complete loss of offsite power. Although one bank of Class lE powered heaters was inoperable, the redundant Class lE powered heater bank remained available to perform the required safety function throughout the duration of this event.

In addition, although breaker 2B0602 failed to operate upon actuation from the control room, it was able to be closed upon manual manipulation at the breaker. Also, positive indication of the heater operating status is available in the control room via indicating lights and a current meter.

Because the redundant bank of Class lE powered pressurizer heaters was available to perform its required safety function throughout this event and because the inoperable pressurizer heater Class lE supply breaker could have been operated manually the safety significance of this event was low.

Additional Information

In the past three years, SCE has not reported any events for Class lE powered pressurizer heaters being inoperable for longer than allowed by TSs.