05000348/LER-2008-002
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 04-15-2008 |
|---|---|
| Report date: | 06-11-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3482008002R00 - NRC Website | |
Westinghouse -- Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX]
Description of Event
On April 15, 2008 control room operators identified several Emergency Power Board (EPB) alarms for the Service Water structure Load Control Center (LCC) 1-2L [EC]. Along with the alarms the operator noted that the lights on the EPB for the 1-2L LCC were dimming. Troubleshooting efforts were initiated and voltage on the 1-2L LCC was found to be fluctuating below the Technical Specifications (TS) minimum. LCC 1-2L was declared inoperable at 0849 hours0.00983 days <br />0.236 hours <br />0.0014 weeks <br />3.230445e-4 months <br />. This resulted in and Unit 2 was inoperable. TS LCO 3.0.6 was entered due to the loss of B-Train SW which supports the removal of process and operating heat from safety related components during a Design Basis Accident (DBA). TS 5.5.15, "Safety Function Determination Program (SFDP)" was performed for both Units. The SFDP indicated that A-Train of Residual Heat Removal (RHR) for Unit 1 was inoperable due to a planned equipment outage and B-Train RHR was inoperable due to RHR. TS 3.0.3 was entered due to both trains of RHR being inoperable on Unit 1. At 1029 hours0.0119 days <br />0.286 hours <br />0.0017 weeks <br />3.915345e-4 months <br /> Unit 1 exited TS 3.0.3 when A-Train RHR was returned to service. No A-Train safety related equipment was out of service on Unit 2.
Troubleshooting efforts on April 15, 2008 identified a loose connection for the phase 2 bus bar on the Unit 1 side of this split LCC. Troubleshooting identified the loose connection, when the LCC was supplied from either the Unit 1 or Unit 2 4160 volt supply bus with the tie breaker on 1-2L LCC between Unit 1 and Unit 2 closed. This alignment required current to flow through the loose bus bar connection, and resulted in the indications described above. With the tie breaker on 1-2L LCC open, the safety related portion of the bus (the Unit 2 side) returned to normal voltage range.
Voltages of the safety related portion of 1-2L LCC were monitored and determined to be stable.
LCC 1-2L was returned to service on April 15, 2008 at 16:20 with 1-2L LCC tie breaker open.
Cause of Event
The 125 V-DC system for the Service Water area consists of two independent and redundant subsystems (A-Train and B-Train) which support both Unit 1 and Unit 2. Each subsystem consists of two battery/charger sets and two DC distribution panels. Either battery/charger set is capable of providing 100-percent power to both DC distribution panels while recharging its batteries. One battery/charger set provides power to the DC distribution panels while the other is on standby. The two DC distribution panels of each subsystem feed Unit land Unit 2 loads.
AC power is supplied to the A-Train subsystems from 600 volt LCC 1-2K while the B-Train subsystems are supplied from 600 volt LCC 1-2L. For the A-Train, LCC 1-2K is a split bus design that allows power to be supplied from either Unit 1 or Unit 2 via a tie breaker within 1-2K LCC. The feed from 1-2K LCC to the A-Train battery chargers comes off the Unit 1 side of 1-2K LCC.
� Similarly, the B-Train LCC 1-2L is a split bus design that allows power to be supplied from either Unit 1 or Unit 2 via a tie breaker within 1-2L LCC. The feed from 1-2L LCC to the B-Train battery chargers comes off the Unit 2 side of 1-2L LCC while the only load off the Unit 1 side of 1-2L LCC is a relatively small non-safety related load (screen wash pump).
This split bus design allows power for a given subsystem to come from Unit 1 or Unit 2, and thus provides a means for maintenance to be performed on the off service power supply. The normal supply breaker alignment for 1-2L LCC is from Unit 2 which is also the side of the LCC that supplies power to the B-Train battery chargers.
On April 11, 2008, the 1-2L LCC was swapped from its normal source (Unit 2 power supply) to the alternate source (Unit 1 power supply) to allow for maintenance on the Unit 2 4160 volt supply breaker to 1-2L LCC. This alignment placed the battery charger load across the 1-2L LCC tie breaker resulting in a higher demand on the Unit 1 side of the LCC. This alternate alignment was maintained until April 15, 2008 when the unexpected bus alarms and indications were observed.
Troubleshooting efforts identified a loose connection for the phase 2 bus bar on the Unit 1 side of this split LCC. Evidence of severe arcing was present on the bus bar connection bolts and a gap was present between the washer and horizontal bus bar. The washers that are used on the bus bar are similar to Belleville washers in that they are convex so that they will maintain a torque even when temperature gradients, vibration or other conditions exist that could cause a bolt to loosen.
Review of work order history indicates that no work has been performed on this portion of the LCC since initial installation.
Therefore, it was determined that the bus bar bolted connection was not torqued correctly at the time of initial installation. The heat and arcing associated with the loose bolting caused degradation over the life of the plant which lead to the deterioration and subsequent failure of the bus bar.
A broadness review determined that similar bolted connections are found on the bus bars of all of the 4160 volt switchgear, 600 volt LCCs, and the 600 volt Motor Control Centers (MCC). It is rare that maintenance is performed on the bolted connections of these bus bars. The bus bars on the 600 volt LCCs and the bolted connections on the 4160 volt switchgear are not readily accessible.
Review of maintenance history has shown that the problem of loose connections in the bus bars has not been an issue at FNP. While current SNC inspection and testing methods meet industry standards, SNC is developing test methods to ensure the integrity of the bolted connections on the bus bars at FNP.
� Safety Assessment This event had no adverse effect on the safety and health of the public.
The RHR system at FNP consists of two independent, redundant, 100% capacity, high volume low pressure pumps that supply water from the Refueling Water Storage Tank (RWST) or the containment sump, to the Reactor Coolant System. The Service Water system consists of two independent, multi-pump, 100% capacity supporting systems that provides a cooling medium to the Engineered Safety Feature which includes the associated RHR Pump Room Coolers and heat exchangers.
During the 100 minutes that the B-Train Service Water system was declared inoperable and the A- Train Unit 1 RHR pump was inoperable due to planned maintenance, no demands were placed on the Unit 1 RHR or Service Water systems and the B-Train battery was at full capacity. The basic function of the battery system at Service Water is to provide control power to breakers. Even if an event had required the repositioning of Service Water breakers, adequate power was available in the B-Train on service battery with an additional 100% capacity available in the standby B-Train battery with operator action. These two fully charged batteries provided 100% redundancy. In addition, Operations and Maintenance personnel were present at the Service Water structure troubleshooting the problem with the 1-2L LCC. If required power could have been restored to the battery chargers in short order by opening the 1-2L LCC tie breaker and supplying the battery chargers from the Unit 2 side of the LCC. Thus this event did not adversely affect the safety and health of the public.
Corrective Action The 1-2L LCC has been fully inspected and the damaged bus bar replaced.
Based on the extent of condition review, a corrective action plan has been developed to perform periodic testing or inspection of the 4160 volt switchgear, 600 volt LCCs, and 600 volt MCCs to ensure the integrity of the bolted connections on the bus bars.
The A-Train LCC 1-2K will be inspected by June 30, 2008.
Additional Information
Mechanism-Operated Cell Switch U