Abstract
In 1975, Pressurized Water Reactor (PWR) nuclear power plant suppliers recommended switching from sodium phosphate to an All Volatile Treatment (AVT) using hydrazine and/or filming amines for secondary chemistry control. The change was implemented to prevent caustic corrosion and under-deposit attack (wastage) on Inconel 600* steam generator tubes. This change did eliminate corrosion problems associated with sodium phosphates, but a new corrosion problem developed in many operating PWRs. Acid chlorides and other impurities previously neutralized by phosphates concentrate in the crevice annuli between the Inconel 600 tubes and carbon steel support plates, causing accelerated corrosion of the support plates. The resulting corrosion deposit is essentially pure magnetite, and progressively increases in volume and density until the crevice annuli is impacted and the tube is deformed beyond yield (denting).
As a result of work sponsored by the Electric Power Research Institute (EPRI), UNC Nuclear Industries developed a two-solvent process for dissolving iron- and copper-bearing sludges from PWR steam generators, and has identified a suitable solvent for cleaning corrosion products from the support plate crevices to relieve denting. The process developed by UNC has undergone pilot scale testing using synthetic and actual steam generator sludges to qualify the process for use in operating nuclear power plants.
