05000219/LER-2015-001
| Oyster Creek, Unit 1 | |
| Event date: | 2-2-2015 |
|---|---|
| Report date: | 08-12-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 50916 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 2192015001R01 - NRC Website | |
Description of Event
On March 19, 2015 the EPA was in control of reactor pressure and the MPR was set as the backup regulator. At 2128, Oyster Creek received a ground on the DC-A bus (9XF-8-e). The DC ground caused the indicating light for the MPR to Illuminate while the EPR was in control. Operations validated the EPR was in control and had reasonable assurance the MPR was available as the backup regulator.
On March 20, 2015 at 0944, an AC ground was received which removed multiple Indications in the Main Control Room (MCR) including the MPR relay position and pressure set-point position. The last known MPR relay position was 10% from the EPR relay position.
On March 21, 2015, troubleshooting on the DC ground was in progress. At 2115, a lead was lifted and the MPR positon indication light bulb was removed, isolating the DC ground to the circuit for the MPR control" light. This circuit contains the MPR "in control" light bulb socket In the MCR and the associated mechanical limit switch, FRS-1, in the Front Standard.
On March 22, 2015, the execution of a troubleshooting action plan (TSAP) determined that the FRS-1 limit switch and associated wiring were the source of the ground. Additionally, it was identified that the limit switch compartment contained water, and wire insulation was degraded within the switch compartment. The source of this water was a local steam leak from the MPR sensing line that had been present since the 1R25 startup. At approximately 1300, replacement of the FRS-1 limit switch and its degraded wires was pursued.
On March 22, 2015 at 1414, while tracing out the FRS-1 wires to be replaced in a junction box containing various Turbine Controls wiring in the Front Standard, EPR wiring was disturbed and the EPR circuit failed. The MPR relay position was at -22% away from the EPR, which allowed reactor pressure to peak at 1036.6 psig and the reactor to SCRAM on APRM Hi-Hi flux.
Analysis of Event
Following the actuation, all systems responded as expected; therefore, this event is of low safety significance.
Cause of Event
Subsequent troubleshooting determined that the cause of the EPR failure was due to a loss of a control valve position feedback signal to the EPR circuitry from a linear variable differential transformer, DT-1. This signal was grounded, and as a result, the EPR circuitry defaulted to close the Turbine Control Valves which raised reactor pressure. Failure analysis of a grounded connector plug and wiring associated with DT-1, determined that insulation on individual wires within the wiring bundle was significantly degraded due to age (e.g., stiff, brittle insulation), and was internally grounded. This degraded wire was inside the Junction box, and in the immediate vicinity of the FRS-1 switch wires being traced out by the DC ground troubleshooting team at the time of the EPA failure. Interviews concluded that this wire was disturbed during this evolution and caused the EPR failure.
Corrective Actions
The corrective actions to address the Root Cause of the EPR failure and the probable cause of the MPR failure, associated with the sensing line steam leak, were completed during Forced Outage 1F36 following the SCRAM March 2015.
Additional Corrective Actions to be completed:
1. Complete the 1R26 Front Standard inspection for degraded wiring and evaluate and replace as appropriate.
2. Document EPR degraded wiring and DT-1 (Differential Transmitter) was replaced.
3. Schedule an inspection of wiring, insulation and terminations In the Front Standard for degradation and evaluate and replace as appropriate for the 1R26 Fall 2016 outage.
4. Document repair of MPR Sensing Line steam leak.
5. Document restoration of MPR Position Indication.
4111111011
- 111111111 6. Confidence Run — observe MPR indication and function through August 2015 to provide confidence it works correctly following repair in May 2015. Document it is working satisfactorily or document new IR number if there are any concerns.
7. Duty Station Management assigned the Contributing Causes to drive Corrective Actions using the Risk Management and Decision Making process.
Previous Occurrences
There were no previous occurrences of wiring degradation leading to a SCRAM at Oyster Creek as identified in the Root Cause Report.