What is a dissimilar metal weld (DMW) and what degradation risk is associated with it in LWRs?

• A. A weld joining identical alloys; risk of uniform corrosion
• B. A weld joining different alloys; risk of PWSCC, brittle cracking, and residual stress issues
• C. A weld joining ferritic steel to aluminum; risk of galvanic corrosion
• D. A weld made with non-metallic filler; risk of debonding

Answer: (B)

Dissimilar metal welds are joints that connect different alloys. In light-water reactors, such welds are common when piping or components with distinct materials are joined, often using a nickel-based filler metal to bridge a stainless steel or carbon steel base. The mix of different chemistries, differences in mechanical properties across the weld and heat-affected zones, and the residual stresses left from welding create conditions where the primary coolant environment can drive cracking at the joint. Primary water stress corrosion cracking is the major degradation mechanism in these joints, and the combination of residual stresses and material brittleness can also lead to brittle cracking. These factors together make PWSCC, brittle cracking, and residual-stress issues the primary degradation risks for dissimilar metal welds in LWRs.