Colloid particle behaviour in CAN-DECON decontamination

The redeposition of colloidal magnetite particles can reduce the effectiveness of a CAN-DECON decontamination of reactors. A small negative charge on the magnetite surfaces is the source of this problem. Sodium salts of the following anionic polyelectrolytes were evaluated as surface charge modifiers: polyacrylic acid, polymethacrylic acid, poly (methyl vinyl ether/maleic anhydride), sulfonated polymers. A cationic polyelectrolyte, a polyamine, was also evaluated. An active and an inactive oxidized carbon steel sample were treated in the same beaker with 0.1 % CAN-DECON reagent and the polyelectrolyte. Activity pick-up by the inactive sample was measured. When no polyelectrolyte was added, 15 % of the Co-60 activity was redeposited. With polyelectrolyte addition in the 5-450 mg/kg range the Co-60 activity redeposition ranged from 8.5 % down to 0.8 %. Polyacrylic acid was the most effective reagent. The transfer of the magnetite outer oxide crystals from the active to the inactive surfaces is identified on SEM micrographs. The polyelectrolytes also promoted the removal of the innermost crystals of the outer oxide film