05000263/LER-2005-004

LER-2005-004,

Docket Number
Event date:	03-09-2005
Report date:	05-12-2005
2632005004R00 - NRC Website

Description On March 5, 2005 with the Monticello Nuclear Generating plant in a refueling outage, during a scheduled primary containment under vessel [RPV] walk down, a small water leak was noted on a control rod drive (CRD) [AA] flange. On March 9, Nuclear Management Company (NMC) determined that the leakage was from the top surface of the flange adjacent to the CRD insert line [PSP]. The leakage was determined to be active and the result of a flaw in the insert line to flange weld. The CRD was isolated to prevent movement and the drive was considered inoperable until the leak was repaired.

A portion of the insert piping was removed for examination and replaced under a station modification.

Following repairs, the CRD was tested and restored to service.

Event Analysis

The event was not reportable in accordance with 10 CFR 50.72 or 10 CFR 50.73, however in accordance with NUREG-1022 revision 2, a voluntary Licensee Event Report is being made for this event. The event is not classified as a safety system functional failure.

Safety Significance

The safety significance of this issue was low since the CRD was always capable of inserting into the reactor. In the event that the insert line severed at reactor power, the CRD could be inserted by using reactor pressure alone. The safety function of the CRD system would not be affected. The CRD insert lines are not part of the reactor coolant pressure boundary. A risk analysis determined that the effect on the Core Damage Frequency is less than 1 X 10-7 per year. This event had no adverse affect on the health and safety of the pubic.

Cause

Metallurgical examination of the pipe determined that the leak was the result of a small weld void in the weld between the insert line and the CRD housing flange created during original construction.

Corrective Action A short section of pipe was removed and replaced under a station modification. A visual inspection of all CRD insert and withdrawal piping connections at the CRD flange was performed and no other leakage was observed.

Failed Component Identification Pipe Manufacturer: Greenville Tubes, Inc (Greenville, PA)

Previous Similar Events

A review of the station corrective action program and industry databases found the following similar events: Monticello (LER 1998-03), Brunswick (LER 2-88-007), and Duane Arnold (LER 88-019). Each of these events involved leakage from either the CRD insert or withdrawal lines, however in each case, the leakage was attributed to a corrosion failure mechanism of the pipe. The event discussed in LER 2005-04 was due to a weld void created during original construction.