05000219/LER-2004-005

LER-2004-005, 1 OF 4

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2192004005R00 - NRC Website

Description of Event

On Saturday, September 11, 2004, with the plant in the Run Mode at 100% power, the outboard Main Steam Isolation Valve (MSIV) (EDS SB-ISV) NSO4 failed to produce a half scram signal as expected during the quarterly 10% MSIV closure test. A 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> shutdown TS LCO was entered at 0930. To verify operability of the valve, plant power was reduced to 40% power and the MSIV full closure test was performed at 1747. This test also failed and NSO4A was declared inoperable. At 1823 the inboard MSIV NSO3A was isolated and electronically locked in place due to the NSO4A being inoperable per Technical Specifications action requirements.

With power maintained at approximately 40%, a complex troubleshooting plan was performed including the use of strain gauges that concluded the valve was stuck open due to an internal problem. A plant shutdown was performed on September 14, 2004 to repair the MSIV.

Internal inspection of the valve determined the failure to close was caused by excessive rib and poppet guide wear resulting from poppet vibration induced by steam flow. The MSIV was repaired and the GE SIL 568 recommended poppet backseat modification was also performed to prevent recurrence of this event on NSO4A.

This event is reportable per 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by Technical Specifications.

Analysis of Event

This event was significant in that it required shutdown of the plant to verify the cause and repair the failed MSIV. The failure mode was also determined to be a common mode that could have affected redundant safety system components.

The MSIVs are containment isolation valves designed to minimize coolant loss from the reactor vessel, and the resulting offsite dose, in the event of a Main Steam Line Break Accident. Two isolation valves are installed in each of the two 24 inch main steam lines, in parallel horizontal runs that penetrate the drywell through 36-inch diameter openings.

This event caused the loss of the containment isolation safety function for one of two redundant components, the outboard MSIV in the "A" Main Steam Line, however that release pathway would have been successfully isolated by the full closure of the inboard MSIV NSO3A on a reactor isolation signal.

The MSIV safety function of reactor and primary containment isolation were not lost during this event since NSO3A, the redundant MSIV was capable of performing the steam line isolation function, as required.

Therefore the safety significance of this event is minimal.

Cause of Event

The common mode failure for this event was damage to the lower rib guide and main poppet guide ring caused by poppet vibration induced by steam flow.

Further investigation has determined that the root cause of this event was untimely implementation of all of the GE SIL 568 recommendations to reduce damage to the MSIVs due to steam flow induced vibration.

Corrective Actions:

Immediate and Interim Corrective Actions 1. Installed poppet back seating modification in NSO4A during 1F07. (Complete) 2. Perform repairs to the guide ribs and poppet of NSO4A prior to startup from F07. Reference:

(Complete) 3. Develop an interim trending technique for,detecting rib guide wear for use during 1F07 to verify MSIV operability prior to startup. (Complete) 4. Complete interim trend testing (strain gauge) on all MSIVs and verify acceptability for restart.

(Complete) 5. Ensure that the poppet backseat modification for NSO4B is scheduled as a 1R20 contingency and installed if the valve is disassembled. (prior to restart from 1R20)

Corrective Actions to Prevent Recurrence

1. Schedule and install the poppet backseat modification on all remaining MSIVs. (NSO3B was completed in 1998 (17R), NSO4A was completed prior to restart from 1F07, NSO3A is scheduled for 1R20, and NSO4B is scheduled for 1R21). Date: 11/30/2006 2. Fully develop the stem strain gauge testing technique for MSIV testing to monitor rib wear.

Institutionalize the testing in the form of a preventive maintenance or surveillance test program. Date:

3/30/2005

A. Failed Components:

Main Steam Isolation Valve (NSO4A) was manufactured by Atwood and Morrill (A&M)

B. Previous similar events:

No other MSIV failures caused by rib wear were identified. LER 88-013, MSIV (NSO3A) stem failure caused by poppet vibration.

C. Identification of components referred to in this Licensee Event Report:

Components IEEE 805 System ID IEEE 803A Function