05000298/LER-2014-003
| Cooper Nuclear Station | |
| Event date: | 02-25-2014 |
|---|---|
| Report date: | 06-26-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982014003R00 - NRC Website | |
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Cooper Nuclear Station
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1 (Power Operation) at 100 percent power, at the time of the event.
BACKGROUND
The Reactor Core Isolation Cooling (RCIC) [EIIS: BN] system provides makeup water to the reactor pressure vessel (RPV) [EIIS: RPV] following an RPV isolation in order to prevent the release of radioactive materials to the environs as a result of inadequate core cooling.
The RCIC system consists of a steam driven turbine-pump unit [EllS: TRB, P], piping, and valves [EIIS: V] to provide steam to the turbine, as well as piping and valves to transfer water from the suction source to the core via the Feedwater [EIIS: SJ] system line, where the coolant is distributed within the RPV. The RCIC system pump supply is normally aligned to the Emergency Condensate Storage Tanks. The pump discharge is normally isolated and valve operation is required to align in either RPV injection mode or test return mode.
Upon receipt of an initiation signal, the RCIC turbine accelerates to provide a specified pump flow. As the RCIC flow increases, the turbine control valve [EIIS: FCV] is automatically adjusted to maintain the required design flow.
RCIC is required to be operable during Modes 1, 2 and 3 with reactor steam dome pressure > 150 psig, since RCIC is the primary non-emergency core cooling system water source for core cooling when the RPV is isolated and pressurized.
EVENT DESCRIPTION
At 14:44 on February 25, 2014, a linkage pin in the RCIC trip and throttle valve, RCIC-MOV-M014, was found out of position and not properly retained. At the time of the event, the RCIC system was already inoperable for system maintenance activities. The pin is part of the linkage assembly that ensures a successful valve reset following normal RCIC turbine trip situations. For RCIC to automatically maintain RPV level, the valve must reset following turbine trips to allow the turbine to restart.
Maintenance personnel investigated the condition and found both set screws loose with no thread locker applied.
Although a component failure was not experienced, the linkage pin, in the as-found condition, may have become disengaged during future RCIC system operation and prevented RCIC-MOV-MO14 to automatically reset, thus causing a system failure. Later in the shift on February 25, 2014, maintenance personnel re-installed the pin and applied thread locker to both set screws. Operation of RCIC-MOV-MO14 was tested satisfactorily at 21:41.
Additionally, inspection was performed on all RCIC-MOV-M014 linkages, pins, and set screws. All were found in the correct position.
Previous maintenance history of RCIC-MOV-M014 was reviewed. In September 2010, preventive maintenance was performed that would have removed and re-installed the linkage pin and setscrews. At that time, maintenance personnel did not identify any adverse as-found conditions with either the linkage pin or set screws during disassembly of the valve.
A photograph taken of the linkage assembly in August 2012 was compared against a photograph taken following the repair of the linkage assembly on February 25, 2014. The photographic evidence showed that although the linkage pin was in the proper location in August of 2012, the set screws were not sufficiently engaged to properly retain it in the correct position. The linkage pin has flats ground for the set screws to contact. This should lock the pin in place and prevent rotation. It is unknown if the pin was installed with the flats rotated slightly during the September 2010 preventive maintenance or if the set screws were simply improperly tightened.
CNS has a unique configuration of RCIC-MOV-M014 in that it has an oil trip relay. This feature is not covered in vendor manual instructions. As such, in September 2010, personnel would have relied on skill-of-the-craft and standard mechanical practices for removal and re-installation of the linkage pin and set screws.
BASIS FOR REPORT
The cause evaluation performed for the event was completed on April 28, 2014, and established firm evidence that the condition that caused the out-of-position linkage pin existed since August 2012. Therefore, the event is being reported as a condition prohibited by Technical Specifications (TS) per 10 CFR 50.73(a)(2)(i)(B) in that the condition existed for a period of time longer than permitted by TS. This report is due 60 days from the date of discovery, i.e., June 27, 2014.
SAFETY SIGNIFICANCE
The safety significance associated with the discovery of an out-of-position linkage pin for the RCIC turbine trip automatic reset function is very low based on the following:
- RCIC's ability to provide reactor inventory control immediately following plant events was not impacted.
- Other high pressure reactor inventory make up systems (e.g., High Pressure Coolant Injection, Control Rod Drive, Reactor Feedwater) were not impacted.
- The as-found condition of the out-of-position linkage pin would have guaranteed the ability to automatically reset and use RCIC following at least one RCIC turbine trip.
- Continued RCIC operation following initial initiation of RCIC as an RPV injection source would have been achieved through procedural directed operator actions that do not result in RCIC turbine trips, and the need for automatic resets following multiple RCIC turbine trips would not be required.
Based on the above, this event resulted in a negligible increase to the core damage probability reflected in the internal events base model of the CNS Probabilistic Risk Assessment.
CAUSE
The cause of the linkage pin being out of position is inadequate work instructions during the valve overhaul in September 2010. The inadequate work instructions led to the set screws being out of position, which eventually let the linkage pin move out of position.
CORRECTIVE ACTION
As noted above, immediate corrective action was taken to properly re-install the linkage pin.
An additional corrective action was completed to revise the associated maintenance plan for RCIC-MOV-M014 to include guidance for installation of the linkage pin and set screws.
PREVIOUS EVENTS
There have been no events reported in the last three years related to RCIC.