05000458/LER-2010-003
| Docket Number Sequential Rev •Month Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 06-25-2010 |
|---|---|
| Report date: | 08-19-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4582010003R00 - NRC Website | |
On June 25, 2010, at 8:24 p.m. CDT, with the plant operating at 100 percent power, the unit cooler (**CLR**) for the high pressure core spray (HPCS) pump room was shut down by the operators following the report of noise and light smoke coming from the fan motor. This action required that the HPCS (BG) system be declared inoperable, as the unit cooler (VF) is necessary to support continued operation of the HPCS pump. This condition is being reported in accordance with 10CFR50.73(a)(2)(v)(D) as the loss of a system needed to mitigate the consequences of an accident.
The HPCS pump room unit cooler is designed to maintain area temperature within the design criteria to ensure equipment operability and safety of plant personnel. The unit consists of a fan and cooling coils. The 50 horsepower fan motor is manufactured by Reliance Electric Co. (Model No. 326TCZ).
The HPCS system was restored to its standby condition at 10:30 a.m. CDT on June 29.
CAUSAL ANALYSIS
On August 6, 2009, the unit cooler began to emit an abnormal high-pitched noise. Motor (**MO**) vibration levels were measured, and no notable increase above baseline levels were detected. It was concluded that the unit cooler remained capable of performing its design function.
On June 19, 2010, the vibration test engineer was requested to assist Operations in evaluating a report of increased noise from the unit cooler. Multiple sets of vibration data were collected. Based on the analysis of the data, it was determined that the increase in noise coming from the fan was not resulting in a subsequent related increase in fan vibration levels.
It was determined during the investigation of the motor failure that the method of collecting vibration data for this component may have not been adequate to accurately assess potential bearing degradation. To obtain the data, a hand-held magnet mount vibration sensor was used on the outside of the fan shroud enclosure, which may have been adversely influenced by signals from other contributors to the overall vibration signature:
Disassembly of the fan motor revealed that the inboard bearing had failed, while the outboard bearing remained intact. The motor bearings were sent to an independent laboratory for failure analysis. The findings concluded that the bearings exhibited signs of lubrication failure.. There were also wear patterns in the outboard bearing indicating that it had not been installed correctly.
The unit cooler fan motor had been in service since 1995. The motor's installation at that time constituted a design change, since the motor being replaced was a 40 HP model made by a different manufacturer. When this change was made, the preventive maintenance program was not updated with recommendations from Reliance. The task for periodic lubrication of the motor bearings should have been changed from 3 years to 9 months, and the task for bearing replacement should have been changed from 26 years to 10 years.
In summary, the cause of this event was the failure of lubrication in the fan motor bearings resulting from inadequate implementation of vendor recommendations for preventive maintenance.
Contributing factors were the inaccurate data collected during vibration monitoring, and improper installation of the original bearings in the motor.
IMMEDIATE CORRECTIVE ACTIONS
The unit cooler motor was fitted with new bearings and returned to service.
CORRECTIVE ACTIONS TO PREVENT RECURRENCE
Revisions to the preventive maintenance program are being processed to incorporate the recommendations from Reliance for the lubrication and periodic replacement of the motor bearings.
The method of collecting vibration data from this fan is being evaluated for potential improvements.
These actions are being tracked in the station's corrective action process.
PREVIOUS OCCURRENCE EVALUATION
There have been no similar events reported by RBSsince January 1, 2005.
SAFETY SIGNIFICANCE
Two of three divisions of emergency core cooling systems (ECCS) are required for the RBS loss of coolant accident analyses. While HPCS was out of service, Division 1 and Division 2 ECCS systems and the automatic depressurization system were available, and would have met the ECCS performance criteria of 10CFR50.46.
Evaluation of the postulated rise in pump room temperature with the unit cooler out of service indicates that the HPCS pump could likely have performed its safety function for at least an hour from the start of the design basis accident had it been necessary.
The HPCS system was returned to service within the time limit of the Required Action in the plant's Technical Specifications. This event was of minimal safety significance with respect to the health and safety of the public.
(NOTE: Energy Industry Component Identification codes are annotated as (**XX**).)