Decontamination of corrosion oxides in the heat transport system of a pressurized heavy water reactor using chelate-free inorganic acid

Abstract

Thick corrosion oxide (Fe3O4) layers containing radioactive metals are generated in a carbon steel heat transport system (HTS) of pressurized heavy water reactors during operation. It is necessary to decontaminate the oxide layers to reduce radiation exposure to workers before maintenance and decommissioning. Since the oxide layer thickness in the HTS is about 75 mu m, a considerable amounts of secondary wastes can be generated when the existing decontamination processes are applied. The aim of this study is to design a chemical decontamination process using chelate-free inorganic acid that can be effectively applied to remove the oxide layers in the HTS and to minimize the secondary waste generation. Therefore, Fe3O4 dissolution behaviors were evaluated in a Hydrazine Based Reductive metal Ion Decontamination (HyBRID) process using the chelate-free inorganic acid. From this result, the decontamination process of oxide layers in the HTS using recycling of decontamination process solution and the process control method using pH monitoring were constructed. In particular, process control based on pH monitoring can be easily used because decontamination performance can be controlled according to the goal; (1) effective removing the oxide layer at decommissioning, (2) avoiding corrosion of base metal and removing the oxide layer at maintenance.