Split pin replacement at Surry Unit 1

During the past seven years of operating experience with the Westinghouse nuclear steam supply system, failures of the control rod guide tube (CRGT) support pins on the lower flange have been encountered. These support pins, commonly referred to as split pins, orient and hold the CRGT in proper alignment with the upper end fitting of the fuel assembly. The failures of this pin have been attributed principally to the material heat treatment of the pins during the original fabrication and installation. In November 1984, Babcock and Wilcox replaced the CRGTs at Surry power station unit 1 with a set of spare CRGTs that were originally assigned to North Anna station. The replacement split pins in these CRGTs had improved heat treatment and geometric configuration to reduce stresses during operation. Failures have also been reported in components called flexures located in the upper section of the CRGT. The failures in the flexures have also been attributed to material heat treatment. Replacement flexures were also installed incorporating an improved heat treatment and geometric configuration to reduce the stresses in the area of reported failures. The modifications on the split pin and the flexures were performed to improve component performance, reducing the potential for a failure causing loose parts to migrate to the steam generator and cause significant damage