[en] In 1989 and 1991, the Japanese first and second campaigns of full system decontamination were performed successfully in each loop of two independent core cooling systems of FUGEN as an operating nuclear power station. Dilute chelate reagent, Kuridecon-203 (KD-203) was used in 0.1% from 0.05% for the decontaminations at 120℃ for 24h. “Purifying decontamination” method, in which injection of reagent and purification of decontaminant are carried out simultaneously, was applied to control KD-203 concentration low and to restrain dose rate of the system during the decontaminations. 6.6 and 7.8man-Sv were saved and 7 to 8TBq of ⁶⁰Co were removed through the decontaminations. No negative influence for materials composed of the cooling circuits was observed in the visual inspection and in-situ materials tests carried out during the decontaminations. There is no incident derived from the campaigns for more than five year's operation. Dose rate of the primary cooling system has not reached to the pre-decontamination level yet after four year's operation. (author)

[en] A new low corrosive decontamination method was developed which uses both oxalic acid and hydrazine as the reducing agent and potassium permanganate as the oxidizing agent. Less corrosion of structural materials during the decontamination is realized by pH control of the reducing agent. The pH of 2.5, attained by adding hydrazine to oxalic acid, was the optimum pH for maintaining a high decontamination effect and lowering the corrosion at the same time. As this reducing agent can be decomposed into carbon dioxide, nitrogen and water by using a catalyst column with hydrogen peroxide, the amount of secondary radioactive waste is small. These good features were demonstrated through actual plant decontamination tasks. (author)