Thermodynamic consideration on dissolution behavior of Zn-containing oxide film during chemical decontamination in light water reactors

Abstract

Occupational radiation exposure has been one of the most important issues in light water reactors. The radiation source is dominantly radioactive cobalt incorporated in oxide film on structural materials. To lower radiation intensity, zinc injection has been implemented widely in light water reactors. Application of the zinc injection leads to incorporation of zinc in the oxide film and changes in the oxide film. Chemical decontamination is applied after shutdown and removes the radioactive cobalt. When the chemical decontamination was applied to plants which experienced the zinc injection, decontamination factor was sometimes lower than before the zinc injection. Then, in the present work thermodynamic solubility of divalent cobalt and divalent zinc was calculated with a thermodynamic code, assuming typical conditions of oxidizing steps of the chemical decontamination. It has been concluded that the solubility limit of divalent cobalt is 22 to 26 times higher than that of divalent zinc under any oxidizing condition. These figures are consistent with the plant data observed during the application of the chemical decontamination. It has been concluded that the oxidizing step of the typical chemical decontamination methods are suitable condition for effective dissolution of cobalt and less effective dissolution of zinc.