05000341/LER-2013-004
| Docket Number | |
| Event date: | 12-14-2013 |
|---|---|
| Report date: | 06-20-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3412013004R00 - NRC Website | |
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Initial Plant Conditions:
Mode 1 Reactor Power 100 percent
Description of the Event:
On April 21, 2014, an Engineering past operability evaluation of the electrical 72E Bus Voltage Regulator determined that the AC power distribution bus would not have performed its safety function under degraded grid voltage conditions during a design basis accident. The failure to enter the Limiting Condition for Operation 3.8.7 for the loss of one 480V bus within the required completion time is reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition which was prohibited by the plant's Technical Specifications.
During Refueling Outage 16, with the plant in Mode 5, on February 16, 2014, voltage on the 72E bus [BU] was found to be above 128 Volts AC. The expected value and set point for the 72E voltage regulator [RG] is approximately 120 Volts AC. Attempts to manually lower the voltage by Operations and Electrical Maintenance were unsuccessful. Engineering analysis was performed and determined the high voltage condition of the 72E Bus was satisfactory to support the loads necessary for Mode 5 and the addition of loads required for Mode 5 were not expected to draw the voltage down to invalidate the analysis. The regulator was returned to automatic control and an emergent issues team was formed to resolve the problem.
The emergent issues team inspected the 72E Bus Voltage Regulator, drive motors and gear assemblies. The team gathered all necessary parts to replace any failed or worn components, and reassembled accordingly. After being reassembled, the 72E Bus Voltage Regulator performed satisfactorily in Post Maintenance Testing.
A review of the weekly voltage readings taken in December 2013 suggests the 72E voltage regulator had stopped functioning properly between December 14, 2013 and December 21, 2013, prior to Refueling Outage 16 while in Mode 1. Therefore, between December 14, 2013, and February 10, 2014, the required actions and completion times for Limiting Condition for Operations 3.8.7, Electrical Distribution Systems while Operating [EC] while in Mode 1, were not met.
Under degraded voltage conditions, voltage requirements demand the 72E voltage regulator should be in its maximum position to mitigate a design-basis accident. With the regulator held at the as-found nominal position, the 72E Bus would have an unacceptable reduction in voltage to its supported loads. Therefore, the as-found position of the regulator in its nominal position would not sufficiently support the safety related electrical equipment (motor operated valves, battery chargers, etc.) under degraded voltage conditions during design basis accident mitigation.
The timeliness in discovery of the failed condition of the 72E regulator can be attributed to the configuration of the upstream step down transformers and voltage acceptance criteria. During Engineering analysis, it was demonstrated that the voltages coming from Transformer 65 and the Bus 72E transformer [XFMR] can mask a 72E regulator failure.
Significant Safety Consequences and Implications:
The purpose of Technical Specification 3.8.7 is to ensure the availability of AC and DC electrical power for the systems required to shut down the reactor and maintain it in a safe condition after an anticipated operational occurrence or a postulated Design Basis Accident. AC and DC electrical power is required for Engineered Safety Features to function during any analyzed accident with a loss of off site power, neither of which occurred coincident with this event. Maintaining the required Division 1 and 2 AC and DC electrical power distribution subsystems Operable ensures redundancy of the Engineered Safety Features is not defeated such that a single failure within any system or within the electrical power distribution subsystems will not prevent the safe shutdown of the reactor.
While the 72E Bus was in a degraded condition that would not support the required Division 2 switchgear [SWGR] loads during a Mode 1 Design Basis Accident concurrent with a degraded grid condition, buses 65F/72F, 64B/72B and 64C/72C were fully functional to operate the necessary required loads, mitigating a risk to Nuclear Safety.
Cause of the Event:
During the inspection of the 72E voltage regulator, the gear boxes and worm assemblies were disassembled and inspected. Significant wear was found on the worm gear shafts and on the worm bushings in the keyed/slotted area that engages with the worm. This wear was found on all three worm assemblies and contributed to looseness and excess movement causing binding to occur. Binding caused the drive motors to draw a higher current and overheat, leading to motor failure.
Corrective Actions:
The failed drive motors were replaced, worn worm assembly parts were either replaced with new parts from stock, or fabricated off site to satisfactorily complete the refurbishment and reinstatement of the 72E Bus Voltage Regulator.
Each of the other three General Electric Inductrol type Voltage Regulators of the same design as the 72E Bus Regulator did not experience high bus voltages, and are in proper working order.
Additional Information:
A. Failed Component: Bus Voltage Regulator Function: Regulate the voltage at the 480 Volt AC Bus by automatically adjusting upon system fluctuations.
Manufacturer: General Electric Inductrol Model Number: 5KCP16KG21A Failure Cause: Overheated drive motors 05000 341