05000220/LER-2006-001
| Nine Mile Point | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| 2202006001R00 - NRC Website | |
I. Description of Event
On June 10, 2006, Nine Mile Point Unit 1 (NMP1) commenced a downpower to 12% power to perform a drywell entry to determine the cause of increased drywell leakage that began in January 2006. Packing on a Reactor Coolant Recirculation System (RCS) pump drain valve was found leaking. The drain valve is one of two normally closed drain valves on a line that is capped. The pump suction and discharge valves were then closed. The leaking drain valve was placed in a back seat position in an effort to stop the leak. Placing the drain valve in a back seat position reduced the leak rate, but a pin hole leak was discovered in a weld at the RCS piping to pump drain valve weld, on the downstream side of the valve. The drain valve was then closed to isolate the weld leak and permit power ascension, which began at 1420 on June 10, 2006, with a plan to perform necessary repairs in a near term planned outage. On June 11, 2006, at 0126 hours0.00146 days <br />0.035 hours <br />2.083333e-4 weeks <br />4.7943e-5 months <br /> (NMP1) entered Technical Specification (TS) 3.2.5, "Reactor Coolant System Leakage," required shutdown due to an increase in leakage exceeding 2 gallons per minute in a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period. NMP1 was placed in cold shutdown mode at 1417 hours0.0164 days <br />0.394 hours <br />0.00234 weeks <br />5.391685e-4 months <br /> on June 11, 2006.
II. Cause of Event
The source of the increased leakage was determined to be the drain valve packing. The cause of the packing leak was installation of incorrect packing in March 1997. The packing that was installed did not have the same diameter as the inside diameter of the stuffing box. Adequate performance standards for valve packing installation did not exist in the early 1990s. The NMP1 valve packing program was initially issued in November of 1998.
A contributing cause was a leak at the RCS piping to pump drain valve weld, on the downstream side of the valve. The cause of the leaking weld was determined to be outer diameter or inner diameter initiated mechanical fatigue that propagated from an original defect at the root of the weld.
III. Analysis of Event
This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(A), "The completion of any nuclear plant shutdown required by the plant's Technical Specifications...
There were no direct safety consequences associated with this event. There were no structures, systems or components that were inoperable that contributed to this event. All available plant safety systems operated as designed and the operators effectively stabilized reactor parameters. Based on the NMP1 probabilistic risk assessment, the risk increase was not significant.
Based on the above, the event did not pose a threat to the health and safety of the public or plant personnel.
IV. Corrective Actions
The RCS pump drain valve packing was replaced and the leaking weld downstream of the RCS pump drain valve was repaired. Replacement of incorrect packing on the redundant downstream valve, which was identified through extent of condition review, was entered in the corrective action program. The current NMP1 valve packing program provides adequate performance standards for packing installation.
V. Additional Information
A. Failed Components:
Reactor Coolant Recirculation System pump drain valve.
B. Previous similar events:
Two similar events occurred in 1997 where lack of a valve packing program resulted in incorrect packing installation and caused shutdowns due to increased drywell leakage. The NMP1 valve packing program was established in November 1998. The failed valve in this event was in scope for replacement during the refueling outage in 2005, but was chosen not to be replaced because it was assessed as a low risk due to its function, infrequent use and absence of known seat leakage.
C. Identification of component referred to in this Licensee Event Report:
Component� IEEE 805 System ID� IEEE 803.A Function Pump Drain Valve� AD� ISV Pump Dischage Valve� AD� ISV Pump Suction Valve� AD� ISV