A steam generator tube of a nuclear power plant (hereinafter referred to as NPP) is a heat exchanger which transfers heat from a primary coolant loop to a secondary one to produce steam in the latter. At the early stage of nuclear industry, Alloy 600 was mainly used in steam generator tubes, but as plant operation times increased, Alloy 600 came to be known for its high susceptibility to primary water stress corrosion cracking (PWSCC).
To solve this problem, Alloy 690 containing a higher content of Cr than Alloy 600 has recently been used as steam generator tubes instead of Alloy 600 due to its higher resistance to PWSCC.
Alloy 600 is a Ni-based alloy with a composition, in weight percent, of 14-17% Cr, 6-10% Fe, 0.15% C max., 1% Mn max., 0.5% Si max., and 0.015% S max., and Alloy 690 is a Ni-base alloy with a composition, in weight percent, of 27-31% Cr, 7-11% Fe, 0.05% C max., 0.5% Mn max., 0.5% Si max., 0.5% Cu max., and 0.015% S max.
As described above, Alloy 690 is a material with a higher Cr concentration than Alloy 600, and was originally called “Inconel Alloy 690” after the name of its developer, or Inco Alloys International. Inc., but is now called “Alloy 690” since their patent has expired.
Since Alloy 690 has a lower thermal conductivity than Alloy 600 by 11%, a replaced steam generator made of Alloy 690 should contain a higher number of Alloy 690 tubes by 11%, or about 2,000 more, to compensate the loss of thermal heat transfer caused by a lower thermal conductivity of Alloy 690, leading to an increase in the size and manufacturing cost of a steam generator tube of Alloy 690.