Decontamination is one of the important processes for dismantling nuclear power plants. The purpose of decontamination is to reduce the radiation levels of contaminated nuclear facilities, ensuring the safety of workers involved in decommissioning and minimizing the amount of radioactive waste. In this study, we investigate the reaction mechanisms and their thermodynamic energies of the HyBRID (Hydrazine-Based Reductive participated metal Ion Decontamination) process for decontamination of the primary coolant system of a nuclear power plant. We computed the thermodynamic properties of HyBRID dissolution mechanisms in which corrosion metal oxides accumulated in the primary coolant systems along with radionuclides are dissolved by HyBRID decontamination agents (H2SO4/N2H4/CuSO4). The HyBRID reaction mechanism has been studied using a commercial database (HSC Chemistry®), but Cu ions have been used instead of Cu-hydrazine complexes when calculating reactions due to the absence of thermodynamic properties for Cu-hydrazine complexes. To address this limitation, we supplemented the quantum calculations with Cu-hydrazine complexes using the density functional calculations. It is intended to simulate a more practical reactions by calculating the reactions considering Cu-hydrazine complexes, and to improve understanding of the HyBRID dissolution reactions by qualitatively and quantitatively comparing the reactions without considering the complex formation.