05000271/LER-2001-005
| Vermont Yankee Nuclear Power Station (Vy) | |
| Event date: | 10-25-2001 |
|---|---|
| Report date: | 12-20-2001 |
| 2712001005R00 - NRC Website | |
DESCRIPTION:
On October 25, 2001 at 0900 with the reactor power at 100%, the "B" Containment Hydrogen/Oxygen (H2/02) Analyzer (SAH-VG-5B) was discovered exhibiting excessive noise while performing a weekly I&C minor maintenance. Closer inspection revealed a black residue inside the cabinet. This residue was due to deterioration of the pump to motor coupling spider. Flows were found low and were adjusted to within specification. A Work Request and Event Report were initiated to investigate the cause of the pump noise. At 1225, the Control Room received a trouble alarm on the "B" H2/O2 analyzer. An I&C Technician and Supervisor, in route to inspect for the Work Order, were contacted by the Control Room to investigate the alarm. The I&C Technician discovered that the tubing had broken off below the fitting on the discharge side regulator (R-2), causing a breach in primary containment. The Shift Supervisor was notified and the "B" H2/O2 Analyzer was isolated at 1244, nineteen minutes after receiving the alarm. The pumps rotating spindle had become severely damaged due to a failure of the pump to pump motor coupling. Three of the four setscrews, which hold the coupling together, had backed off and fallen out during operation. The failing coupling caused excessive vibration on the discharge tubing and resulted in a radial fracture of the tubing, at the fixed point below the R-2 regulator. The System Engineer determined this to be a breach of Primary Containment, in excess of the maximum allowable accident leak rate (La) and reported this to the Shift Supervisor. As part of a root cause analysis, a metallurgist analyzed the failed section of tubing and concluded; "the tubing failed due to the failure of the pump, which most likely subjected the line to a high stress low cycle failure".
ASSESSMENT OF SAFETY CONSEQUENCES:
"A" 112/02 Analyzer was fully operable, thirty day LCO entered for repair of "B" 112/02 Analyzer.
It has been determined that this condition would not adversely impact the plant's containment Level-2 Risk Assessment. Specifically, this failure of Primary Containment integrity would not contribute to the determination of the Large Early Release Frequency (LERF) due to the small size of the air space release flow path ( diameter piping) and the fact that this condition was corrected nineteen minutes after receiving the trouble alarm.
Therefore, there was no increased risk to public health and safety.
BACKGROUND:
The failed tubing had been in service for ten years without consequence.
CAUSE OF THE EVENT:
The Root Cause Analysis determined the cause for the breach in Primary Containment, was due to the cumulative effects of strains and stresses exerted on the tubing at the R-2 connection, amplified by vibraton from a failing pump coupling. The coupling setscrews were found to have backed out from either inadequate setscrew installation with normal vibration or from pump to motor misalignment during installation, causing increased vibration. Although assembly was completed in accordance with the Vendor's manual, it was subsequently determined that the Vendor's manual did not provide any required torque values, and did not provide any specific requirements for pump alignment. Additionally, the tubing was not configured to an ideal orientation, which caused the vibrational forces to be transmitted to one single point in a twisting motion. Three of the four setscrews, which hold the coupling together, had backed off and fallen out during operation. Specific torque values are not provided in the Vendors manual for the coupling setscrews.
The failing coupling caused excessive vibration on the discharge tubing from the pump to the R-2 regulator. The tubing failed at the first point from the pump discharge where it is restrained to a fixed point, below the R-2 regulator. The other end of the tubing is attached to the pump and is free to move with the pump vibrations. This configuration causes most of the pump vibrations to be absorbed at the fixed point. The pump suction has the same restraint as the discharge tubing when attached to the cabinet. However, that section of tubing is configured in one plane to enhance the vibrational absorption capabilities over the system, rather than transmitting them to one single fixed point. The discharge tubing is configured in such an orientation that a twisting motion was created on the tubing at the fixed point, while absorbing the vibrations of the pump. This configuration led to a radial fracture when combined with the excessive vibrations from the failing pump coupling.
ANALYSIS OF THE EVENT:
SAH-VG-5B is a closed loop system outside of the Primary Containment structure. The failure of the H2/02 analyzer pump's discharge tubing resulted in a breach of Primary Containment. Leakage from Primary Containment is permitted, providing the leak rate does not compromise the ability of the barrier to mitigate the consequences of a Design Bases Loss of Coolant Accident (DBA LOCA). A Primary Containment maximum allowable accident leak rate of 1.5% Primary Containment air wt./day at 44 prig [Pa is the established analytical (accident) limit]. The Technical Specification Limit for the maximum allowable Primary Containment leak rate is 0.8% wt./day (La at Pa). The combined local leak rate test (Type B and C tests) acceptance criterion is less than or equal to 0.6 La, calculated on a minimum pathway basis, at all times when primary containment is required. A leak rate test was performed on the system. When the combined minimum pathway flow is summed with the restricted or the unrestricted calculated flows, the resultant flow exceeds all applicable limits, up to and including the analytical limit (1.5% wt./day).
CORRECTIVE ACTIONS:
1. The system was isolated in nineteen minutes after receiving the H2/02 Trouble Alarm, which restored Primary Containment.
2. The pump, pump to motor coupling, coupling spider and the fractured section of discharge tubing were replaced. The system was subsequently leak rate tested satisfactorily in accordance with the Primary Containment Leak Rate Test Program.
3. The discharge tubing was re-configured to reduce the effects of pump vibration on the system.
4. Preventative Maintenance practices and procedures for this equipment will be evaluated for improvement opportunities.
ADDITIONAL INFORMATION:
No similar events have occurred at VY within ten years.