Dissimilar metal welds in light water reactors are prone to which degradation risks?

• A. PWSCC only
• B. Brittle cracking only
• C. PWSCC, brittle cracking, and residual stress issues
• D. No significant degradation risk

Answer: (A)

Dissimilar metal welds in light water reactor primary circuits are most at risk for primary water stress corrosion cracking. This crack mechanism occurs in susceptible nickel-base weld metals and their associated weld heat-affected zones when exposed to the reactor’s high-temperature, high-purity primary coolant. Tensile residual stresses from welding, combined with the service stresses and the corrosive environment, drive crack initiation at grain boundaries and slow, progressive crack growth characteristic of PWSCC. While residual stresses contribute to driving PWSCC, they are not a separate, dominant degradation mode on their own in this context. Brittle cracking is not the primary degradation mode for these welds under typical primary-loop conditions, and saying there is no significant degradation risk would overlook the well-established concern with PWSCC in dissimilar metal welds.