Which alloy families are typically used for primary system piping and components in PWRs, and why?

• A. Copper-based alloys are used for pressure boundaries due to high strength.
• B. Low-alloy steels for pressure boundaries; stainless steels and nickel-based alloys for regions needing corrosion resistance.
• C. Aluminum alloys are used for all primary piping.
• D. Titanium alloys are used for corrosion resistance.

Answer: (B)

In a PWR, the materials chosen for the primary system must stand up to high-temperature, high-pressure reactor coolant and the effects of neutron irradiation, while also allowing reliable fabrication and reasonable cost. For the pressure boundary and piping, low-alloy steels are used because they provide the needed strength and toughness after appropriate heat treatment and they weld well into complex piping networks. In regions that encounter more aggressive corrosion conditions or higher risk of corrosion-related degradation, stainless steels and nickel-based alloys are used. Stainless steels offer good oxidation and general corrosion resistance in the reactor coolant environment, while nickel-based alloys push corrosion resistance even further and perform well at high temperatures, making them suitable for components or areas most exposed to challenging water chemistry or stress corrosion concerns. Aluminum or titanium alloys aren’t typically used for the primary loop due to limitations in strength, irradiation behavior, and cost, and copper-based or other exotic alloys don’t provide the right balance of performance for the pressure boundary and corrosion resistance needed in the primary system.