Nuclear Steam Generator Tube-to-Tubesheet Expansion

Abstract

Nuclear steam generators built for the CANDU program by Babcock and Wilcox from the NPD horizontal shell demonstration unit onward have incorporated a closed crevice configuration. Crevice closure was initially achieved by mechanical roller expansion. Roller expansion was qualified to requirements of an allowable wall thickness reduction, a minimum pull-out strength and leak tightness. A subsequent requirement for a process which would reduce the cold work and residual stress on the tubing, lead to implementation of high pressure hydraulic tube expansion. This allowed tubing which did not otherwise require post weld heat treatment to be put into service in the expanded condition. Recent developments for PWR replacement steam generators provide for hydraulic expansion through the full tubesheet thickness. In the industry generally, SG tube expansion transition regions have proven to be susceptible to cracking where an unfavourable combination of susceptible materials, water chemistry and high residual stress exists. For the subject steam generators, there have been essentially no tube expansion region failures. Those with materials which may have been susceptible had received a pressure vessel post weld heat treatment which reduced their susceptibility to cracking. Units with less susceptible materials have received hydraulic expansion but no PWHT cycle. The paper describes the development and application of tube expansion processes for the Babcock and Wilcox recirculating steam generators and their experience in operation.