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.

Chemical dissolution of oxide layer on carbon steel SA 106 GR.B-based oxalic acid
    Abstract
    1 Introduction
    2 Experimental section
        2.1 Materials
    3 Fabrication of oxide layer on carbon steel SA 106 GR.B
    4 Dissolution test of oxidized carbon steel SA 106 GR.B
    5 Characterization
    6 Results and discussion
        6.1 Characterization of oxidized carbon steel SA 106 GR.B
    7 Dissolution of oxidized carbon steel and characterization of dissolved carbon steel
    8 Conclusions
    Acknowledgements 
    References

• Changhyun Roh(Decommissioning Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, Korea) Corresponding author
• Bum Kyoung Seo(Decommissioning Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea)
• Miguta Faustine Ngulimi(Decommissioning Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, Korea)
• Kamal Asghar(Decommissioning Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, Korea)
• Sion Kim(Decommissioning Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, Korea)