Due to extensive deposits on the shell (i.e., natural water) side of the boiler of the Ontario Hydro Nuclear Power Demonstration Nuclear Generating Station (NPD NGS) unit, the gross power output dropped from 25 to ∼18 MW(electric). The deposits consisted mainly of iron and copper compounds with phosphate as a significant minor component. The boiler has Inconel 600 tubing; the other boiler and drum components are made from carbon steel. To remove the deposits, a shutdown chemical clean of the boiler was used. The cleaning process was a multistage process consisting of sequences of copper removal stages and iron removal stages. These were

  1. a copper removal stage using 10% ammonia solution at 25 to 35°C with air sparging to provide oxidizing conditions and assist solvent circulation
  2. iron removal with a solution containing 8% ethylenediaminetetraacetic acid, 2% citric acid, 0.4% hydrazine, and 0.2% inhibitor at pH 6.0 at 93°C. (Nitrogen sparging was used in this stage to maintain deaerated conditions and to circulate the solvent.)
An anti-foam agent was used to minimize foaming in all cleaning stages. The actual cleaning process consisted of a total of six copper removal stages and four iron removal stages followed by flushing with demineralized water and by passivation of carbon steel surfaces. The chemical clean resulted in 900 kg of deposits (magnetite, copper, and other metal and anionic species) being removed by dissolution. An unknown quantity of suspended solids was also removed by in-line filters and in the waste solvent. Corrosion of boiler materials during the “clean”was measured at 10 µm for carbon steel and 0.3 µm for Inconel 600. The work described shows that we now have a chemical cleaning process that could be used as a basis for cleaning all present and committed Ontario Hydro CANadian Deuterium Uranium Pressurized Heavy Water (CANDU-PHW) Inconel 600, Monel 400, and Incoloy 800 tubed boilers. The same process could be used as a basis to clean pressurized water reactor boilers