Cobalt-based alloy hardfacing in moderator system valves

For the past 30 years, materials containing cobalt have been intentionally avoided in the design and construction of moderator systems in CANDU plants. The rationale behind this approach was based on the perceived risk of neutron activation of 59Co to 60Co (a hard gamma emitter), resulting in increased radiation fields and higher doses around moderator equipment. One consequence of this approach was to avoid valve hardfacings high in cobalt concentration, such as Stellite-6, and instead relying on nickel-based valve hardfacings. Operating experience throughout the CANDU industry has shown that the nickel-based hardfacings used in moderator valves corrode in the low pH of the moderator system. This often resulted in valves that would not provide adequate isolation for maintenance or operational requirements. This paper will review the performance of hardfacings in CANDU moderator systems, and compare the relative contributions of cobalt from various moderator system components. (author)