Test procedures used in the UK Alloy X-750 evaluation program

As a result of an increased incidence of stress corrosion cracking (SCC) of alloy X-750 control rod guide tube support pins installed in the primary circuit of operating pressurized water reactors, UKAEA and CEGB have developed a program to investigate the effectiveness of changes in pin design and material condition required to minimize the occurrence of stress corrosion cracking. Three modifications that are most likely to be beneficial in reducing the susceptibility of SCC are as follows: 1. A high-temperature solution anneal (1093 degree C, 2000 degree F) prior to the single aging treatment (704 degree C, 1300 degree F). 2. Reducing the pin maximum nominal shank stress by lowered installation torque. 3. Revising the pin design to reduce the stress concentration factor. Reducing the maximum shank stress and the bolt head/shank concentration factor has lowered the maximum peak stress at the shoulder by approximately 25 percent. Material/heat treatment modifications will be evaluated using U-bends, bent beam, and dynamic strain tests. Design changes and material/heat treatment conditions will be evaluated using notched bar tensile tests. 5 figs