05000458/LER-2012-004
| River Bend Station - Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4582012004R00 - NRC Website | |
that one of the safety-related fans (**FAN**) in the standby service water cooling tower (BS) would not start from the remote shutdown panel. The initial investigation determined that the failure was due to incorrect maintenance that had been performed on May 3, 2011. This condition caused the fan to have been inoperable with respect to the function of the Remote Shutdown System, as governed by Technical Specification (TS) 3.3.3.2. This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as operations prohibited by technical specifications.
BACKGROUND
The standby cooling tower is a 200 percent capacity cooling tower with a 100 percent capacity water storage basin. The basin is the source of water for the four standby service water (SSVV) pumps. Twenty 40-horsepower vaneaxial fans are mounted in the tower in an induced draft arrangement, with ten fans dedicated to each of the two divisions of SSW. The fans are powered by the respective emergency diesel generators (DG), and are normally started by the operator in the main control room. The circuit breakers (**52**) supplying the individual fans have relays (**2**) that start the fans in a timed sequence to control the loading of the respective DG.
The ten fans in each division are divided into two cells of five fans. TS 3.7.1, "Standby Service Water System and Ultimate Heat Sink," does not address the operability of individual fans.
However, the Bases for that TS specifies that an operable fan cell comprises five operable fans. Thus, the inoperable fan caused the affected cell to be inoperable.
The test being conducted at the time of discovery was for verification of the ability to start the fans with the system configured for operation from outside the main control room (i.e., the remote shutdown function). During the initial troubleshooting, it was confirmed that the fans started as designed with the control switches configured for operation from inside the main control room. The remote shutdown function for the one affected fan was the only inoperable function.
INVESTIGATION and CAUSAL ANALYSIS The maintenance performed in May 2011 was a scheduled task to calibrate the timing relays in the individual circuit breakers for each fan. The post-maintenance test for this work required only the successful operation of the relay during bench testing. No functional check of the fans was specified by the work package.
discovered that the subject fan would not start. The initial troubleshooting determined that two wires were not connected to the correct points on the timing relay. The last time these wires were removed was in May 2011 during the calibration procedure.
The investigation of this event found that verification and marking of the lifted leads during removal of the timing relay for bench testing was ineffective. This led to the wires being attached to incorrect terminals upon re-installation of the relay. The erroneous wiring configuration prevented the start function of one fan in the remote shutdown alignment.
This investigation also determined that the post-maintenance test specified by the timing relay calibration work package was not effective, in that a functional test of the fans should have been required.
CORRECTIVE ACTION TO PREVENT RECURRENCE
The general maintenance procedure that controls lifted leads and jumpers will be revised to strengthen the verification requirements for this type of activity. This action is being tracked in the station's corrective action program.
Since the May 2011 relay calibration, the work management procedure that defines post maintenance tests was revised to provide improved guidance on determining effective test requirements.
PREVIOUS OCCURRENCE EVALUATION
No similar events have been reported by River Bend Station in the last five years.
SAFETY SIGNIFICANCE
The mis-wired relay affected only the ability to start one fan in the remote shutdown configuration. The design safety function of Division 1 SSW was maintained since the fan was at all times capable of being started from the main control room. Thus, 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**).)