United States Nuclear Regulatory Commission - Protecting People and the Environment

Morning Report for January 9, 2008

Headquarters Daily Report

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Emergency Diesel Generator Failure Due to Degraded Solder Joints

Licensee/Facility:

GENERIC


Dockets:

License No:
Notification:

MR Number: H-2008-0001
Date: 01/08/2008

Discussion:

On December 10, 2007, the Salem, Unit 1, C emergency diesel generator (EDG) tripped during a planned surveillance test. As part of their troubleshooting efforts, the licensee identified that the EDG had tripped on mechanical overspeed and that the problem involved a loss of generator field, which provides a speed input to the governor. Further troubleshooting identified cracks in solder joints on a 30 year old Basler brand SBSR automatic voltage regulator circuit card. The cracks were consistent with the solder joint cracking discussed in a September 21, 2007, Part 21 notification issued by MPR Associates (ML072750470) and Information Notice 2007-036, "Emergency Diesel Generator Voltage Regulator Problems" (ML072780394). A maintenance bulletin issued by MPR Associates in November 2007 describes inspection techniques that can be used to identify these cracks, and recommends using a minimum magnification of 2.5 times.

At Salem, the cracked solder joints were not discovered during the initial visual examination, but only after taking digital photographs of the solder connections and then using a computer to magnify the images. The solder joints had to be magnified up to 40 times to see the cracks. The licensee identified that six of the nine susceptible solder connections were cracked. All nine susceptible solder connections were repaired and the licensee performed post-maintenance testing with instrumentation installed to verify proper EDG operation, including a smooth governor response during startup. On December 13, 2007, following completion of the required testing, the licensee returned the 1C EDG to an operable status.

The purpose of this Morning Report is to make licensees, NRC staff, and other interested parties aware of the recent EDG failure at Salem. This event demonstrates that voltage regulator inspections performed in accordance with the vendor's maintenance bulletin may not be sufficient to identify cracks in solder connections which could impact EDG operability, and that additional magnification may be necessary to identify these kinds of defects.

Contacts:

Name Office Abbrev Phone No E-Mail
BOWMAN, GREGORY T NRR (301) 415-2939 GTB1@nrc.gov
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