N Reactor Steam Generator Performance

The N Reactor is equipped with 12 horizontal U-tube steam generators. Originally, these contained Type 304 stainless-steel tubes. From 1964 through 1971, ten were retubed with Inconel-600 tubes because severe stress corrosion cracking was experienced in stainless-steel tubes. Tube performance is monitored by eddy-current examinations, tube removals for destructive examination, and checking for radioactivity in the secondary system. The results of this program show that no significant corrosion has occurred in the Inconel-600 tubes in 11 yr of service. Five tube leaks have occurred in the two remaining units with Type 304 stainless-steel tubes over the same period.