During the operation of Pressurized Heavy Water Reactor (PHWR), corrosion oxide layers are formed on the surface of carbon steel SA 106 Grade B (GR.B), primary coolant system material. These oxide layers can be effectively removed using the common chemical decontaminant, oxalic acid (OA). However, the base metal of the structural material may also undergo corrosion, increasing the concentration of metal ions, such as ferrous ions, in the decontamination solution. The increased concentration of metal ions leads to an increased use of cation exchange resins in wastewater treatment, thereby increasing the amount of secondary wastes. Therefore, minimizing the corrosion of the base metal during chemical decontamination is crucial. In this study, imidazole (IM) and 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) were selected for their effectiveness in reducing carbon steel corrosion in acidic environments. Their efficiency as corrosion inhibitors was evaluated under actual decontamination conditions in OA solution. When [BMIM]Cl was added to OA, the corrosion depth of carbon steel decreased from 0.641 μm to 0.406 μm, and the corrosion rate decreased from 1.924 μm/h to 1.218 μm/h, both representing a reduction of 36.7%. In conclusion, this study suggests that [BMIM]Cl is a good candidate as a corrosion inhibitor to be further evaluated under chemical decontamination process.

The feeder pipes of the primary cooling system in a pressurized heavy water reactor (PHWR) are composed of carbon steel SA 106 GR.B. On the surface of this structural material, corrosion oxide layers including radionuclides are formed due to the presence of active species from water decomposition products caused by radiation, as well as the high temperature and high-pressure environment. These oxide layers decrease the heat transfer efficiency of the primary cooling system and pose a risk of radiation exposure to workers and the environment during maintenance and decommissioning, making effective decontamination essential. In this study, we simulated the formation of the corrosion oxide layer on the surface of carbon steel SA 106 GR.B, characterized the formed corrosion oxide layer, and investigated the dissolution characteristics of the corrosion oxide layer using oxalic acid (OA), a commercial chemical decontamination agent. The corrosion oxide layer formed has a thickness of approximately 4 μm and consists of hematite ( Fe2O3) and magnetite ( Fe3O4). The carbon steel coupons with formed oxide layers were dissolved in 10 mM and 20 mM OA solutions, resulting in iron ion concentrations of 220 ppm and 276 ppm in the OA respectively. In 10 mM and 20 mM OA, the corrosion depths of the coupons were 8.93 μm and 10.22 μm, with corrosion rates of 0.39 mg/cm2·h and 0.45 mg/cm2·h, respectively. Thus, this demonstrates that higher OA concentrations lead to increased dissolution and corrosion of steel.