Why are alloys such as Alloy 690 used for steam generator tubing?

• A. They are cheaper than Alloy 600.
• B. They have higher thermal conductivity.
• C. They offer superior resistance to primary water stress corrosion cracking compared with older alloys like Alloy 600.
• D. They are easier to machine.

Answer: (C)

Alloy 690 is used for steam generator tubing because it resists primary water stress corrosion cracking in the reactor coolant environment much better than older alloys like Alloy 600. In the hot, high-pressure primary water of a PWR, sensitive materials can crack along grain boundaries under tensile stresses aided by hydrogen and oxidizing species. The higher chromium content in Alloy 690, along with low carbon, promotes a stable protective oxide on the surface and reduces susceptibility to hydrogen-assisted cracking, making intergranular and surface cracks much less likely. Proper heat treatment also helps relieve residual stresses that would otherwise drive cracking. So the main reason is its superior resistance to primary-water–induced SCC, which improves tube reliability; factors like cost, thermal conductivity, or machinability aren’t the primary drivers.