05000331/LER-2013-002
| Duane Arnold Energy Center | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3312013002R01 - NRC Website | |
I. Description of Event:
On June 6, 2013, Reactor Core Isolation Cooling (RCIC) TS Surveillance STP 3.5.3-02 was successfully performed. On June 21, 2013, Operations observed the RCIC speed indicator,'SI2457, to be at approximately 1200 RPM while RCIC was in the standby condition. A Condition Report (CR) was initiated and screened as not affecting the operability of RCIC. The CR stated there was no safety significance; the instrument was for indication only. A degraded instrument sticker was placed on the tachometer and a Work Request was initiated to calibrate the tachometer. On the morning of August 22, 2013, the Work Order attempted to calibrate SI2457. The calibration was unsuccessful and the workers backed out of the calibration.
At 1340 on August 22, 2013, while operating at 100% power, during the performance of Surveillance Test Procedure (STP) 3.5.3-02, RCIC System Operability Test, the RCIC turbine tripped on electronic over- speed immediately upon startup. Subsequent troubleshooting found the RCIC governor controls dropping resistor in the power supply circuit to be partially open. This resulted in a lower than required power supply to the RCIC electronic governor which allowed the RCIC turbine to over-speed and trip on electronic over-speed.
At the start of this event, there were no other structures, systems or components inoperable that contributed to the event.
II. Assessment of Safety Consequences:
RCIC was inoperable from June 21, 2013 to August 23, 2013. Technical Specification Section 3.5.3, RCIC System, Action Statement A.2 for RCIC System inoperable, requires restoration of RCIC System to operable status in 14 days, or be in Mode 3 in the following 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> was exceeded. Therefore, this condition resulted in a Condition Prohibited by TS and is reportable to the NRC pursuant to 10 CFR 50.73(a)(2)(i)(B).
The safety significance for this event was very low based on fact that RCIC could have been operated in manual as directed by existing procedures.
This event did not result in a safety system functional failure.
III. Cause of Event:
The Root Causes of this event were determined to be the following:
1. The RCIC dropping resistor failed due to unknown material design changes of the resistor by the manufacturer that reduced performance margins in the given application. This resulted in over-heating and ultimate failure.
2. Screenings of the Condition Report written on the RCIC speed indicator failed to correctly assess RCIC operability due to process gaps.
IV. Corrective Actions:
The RCIC governor controls dropping resistor was replaced and RCIC was returned to service on August 24, 2013 at 0515.
The following corrective actions will be taken to preclude recurrence of this event:
1. Modifications will be made to the RCIC power supply to provide margin to thermal aging and to provide monitoring of output to detect any failures.
2. Guidance will be incorporated into site processes for how to conduct an independent review of the Shift Managers immediate operability determination on a risk significant Structure, System, or Component.
V. Additional Information:
Previous Similar Occurrences:
A review of License Event Reports from the past 5 years did not identify any similar occurrence.
EIIS System and Component Codes:
BN — Reactor Core Isolation Cooling System.
Reporting Requirements:
This event is being reported as an Operation or Condition Prohibited by TS, 10CFR50.73(a)(2)(i)(B).