05000348/LER-2006-002

LER-2006-002, Main Steam Isolation Valve Failure to Close

Docket Number
Event date:	04-08-2006
Report date:	06-05-2006
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3482006002R00 - NRC Website

Westinghouse -- Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX]

Description of Event

On April 8, 2006, at 0105, with Unit 1 in Mode 3 prior to cooldown for a refueling outage, during Main Steam [SB] Isolation Valve (MSIV) and Bypass Valves Cold Shutdown Valves Inservice Test, the Unit 1 downstream MSIV's failed to fully stroke closed. The A valve closed approximately 70%, and valves B and C did not move from the open position. On April 8, 2006, at 0700, after some cooldown, the A valve was found 90% closed, and would not move open or closed with mechanical agitation or from the Main Control Room; the B valve was found 75% open, and was closed by repeated mechanical agitation; the C valve was found full open, and was closed by a single application of mechanical agitation. On April 9, 2006, at 1200, following completion of cooldown, all three valves would stroke both open and closed from the Main Control Room.

Cause of Event

Upon disassembly, the A valve shaft was found to have gouges under the packing follower location, and a crack initiated at a disc arm keyway. The B valve shaft was found to have wear marks under the packing follower location and an open circumferential crack that had initiated at a disc arm keyway. The keyway design results in a stress riser in the keyways. The C valve shaft was found to have deposition of packing material graphite bonded to the shaft in the packing region. The shafts and associated load bearing components of all three valves were found to have wear marks.

The shaft load bearing components had no defined criteria for replacement. Accumulation of wear allowed shaft clearances and roughness of components in contact with the shaft to increase over the years. This possibly allowed development of interference between the packing follower and shaft, as other shaft clearances increased while packing clearance remained tight due to packing replacement during each outage.

The thrust bearing on the shaft of downstream MSIV B was installed backwards. This coupled the shaft to the spacer on the opposite-actuator side of the valve disk assembly, causing increased wear rates in the bearing until axial clearances were lost, and thus increased drag resistance to shaft rotation. Relevant information (such as thrust bearing orientation and acceptance criteria for inspection of components subject to wear) was not specified in the procedure.

This event is unique to the downstream valves. The upstream valves are not subjected to high turbulence agitating the valve discs, which causes increased wear of load bearing components and cyclic tortional loads on the shaft. Due to less turbulence agitation, the upstream valves have not experienced the high-cycle fatigue that the downstream valves have experienced.

Safety Assessment The safety and health of the public were unaffected by this event.

The upstream valves were not affected by this event. Therefore, at least one train of Main Steam Line Isolation remained operable throughout this event. This event does not represent a Safety System Functional Failure.

Following cooldown, all three MSIV's stroked fully open and closed from the Main Control Room. All tests were conducted on the downstream valves without assistance of steam flow. It is the engineering judgment of the plant and the valve manufacturer that even though there may have been some binding in the full open position, had the valves been called upon to close under accident conditions with choke flow, they would have closed in the event of a steamline break downstream of the MSIV's.

The Unit 2 valves are operable at this time. The downstream Unit 2 MSIV's (farthest from containment) were inspected during the last Unit 2 outage, including penetrant testing. No cracks were identified in the shafts. The MSIV's were reassembled and satisfactorily tested and additionally were stroked in Hot Standby conditions. All of the Unit 2 MSIV's were partial stroke tested on April 24, 2006 and all stroked satisfactorily. In the event that a closure of the valves was necessary, movement into the steam flowpath would provide additional motive force to close the valves to assist the mechanical closure mechanisms.

Corrective Action The Maintenance procedure for MSIV overhaul will be revised by August 1, 2006. The Operations MSIV cold shutdown surveillance test procedure (STP) has been revised.

The quarterly partial stroke test procedure has been re-established for Units 1 and 2. The test cylinder used at Farley Nuclear Plant during partial stroke testing does not allow the disc to enter the flow stream, and therefore does not present the risks associated with partial stroke testing that the industry is currently experiencing.

All load-bearing components in contact with the shaft have been replaced on Unit 1 downstream valves including new shafts with radius design keyways.

Preventive maintenance (PM) for replacement of the shafts and load-bearing components in contact with the shafts will be established on Unit 1 and Unit 2 by August 1, 2006 Both upstream and downstream Unit 1 valves have been successfully tested at cold and hot shutdown conditions.

An engineering review to consider possible design changes to reduce the severity of the service conditions in the downstream valves will be completed by March 15, 2007.

Additional Information

The following LER's have been submitted in the past two years on Technical Specification violations:

Failure Troubleshooting