Alloy optimization for PWR steam generator heat-transfer tubing

Several materials have been used for heat-transfer tubing in light water re actor steam generators, including Type 316 stainless steel (a few early uni ts in the late 1960s), Alloy 600MA (mill annealed), Alloy 600TT (thermally treated), Alloy 800Mod, and the current preference, Alloy 690TT. The suscep tibility of Alloy 600MA (Ni-16Cr-9Fe) to stress-corrosion cracking led to e xtensive corrosion studies to select and qualify a new material. Alloy 690T T (Ni-30Cr-10Fe) was chosen in the mid-1980s as the best heat-transfer tubi ng material, although Alloy 800Mod (Fe-33Ni-22Cr) remains an alternate choi ce. The basis for selecting Alloy 690TT is briefly summarized, along with a description of efforts to optimize tube manufacturing practices for corros ion resistance. Efforts at further optimization are described, including pr econditioning of the tubing by some form of in ner-diameter surface passiva tion in order to minimize corrosion product release rates upon initial serv ice.