Method of reducing radioactivity in nuclear plant

A method of reducing radioactivity in a nuclear plant by forming oxide films on the surfaces of metallic structural members which constitute a nuclear power plant which generates electric power by rotating a generator with a turbine driven by steam generated in a reactor, the oxide films being formed by circulating pure water of a temperature of at least 200.degree. C. from a pressure vessel of the reactor through a recycling system and partially through a reactor water purification system to said pressure vessel before said structural members are exposed to reactor water containing radioactive substances, said reactor water having an oxidizing capacity, comprising the steps of:

• controlling the oxidizing capacity of pure water to be lower than that of said reactor water,
• subjecting said structural members to a first-step oxidation treatment of heating by circulation of pure water of a temperature of at least 200.degree. C., the pure water being the pure water having the oxidizing capacity controlled to be lower than that of the reactor water,
• further controlling the controlled pure water to have a higher oxidizing capacity than that of said reactor water, and
• further subjecting the thus treated structural members to a second-step oxidation treatment of heating by circulation of the further controlled pure water, of a temperature of at least 200.degree. C., further controlled to have the higher oxidizing capacity than that of said reactor water, such that in the first-step oxidation treatment a relatively thick and porous oxide film, as compared to the oxide film formed in the second-step oxidation treatment, is formed, and in the second-step oxidation treatment a relatively denser and thinner oxide film than the oxide film obtained in said first-step oxidation treatment is formed.

A method of reducing radioactivity in a nuclear plant by forming oxide films on the surfaces of structural members made of steel to be in contact with high-temperature and high-pressure reactor water, comprising the steps of:

• preparing reactor water, to be used in the nuclear plant, the reactor water being substantially free from radioactive substances;
• effecting a first control of the dissolved oxygen concentration of said reactor water so as to be less in its oxidizing capacity than that of the reactor water used in the nuclear plant;
• subjecting said structural members to a first-step oxidation treatment of heating said structural members in contact with said reactor water controlled in said first control of the dissolved oxygen concentration at 200.degree. C. or higher for a predetermined period of time, thereby to provide thick and porous oxide films on the surfaces of the structural members;
• effecting a second control of the dissolved oxygen concentration of said reactor water so as to be higher in the oxidizing capacity than that of the reactor water used in the nuclear plant;
• further subjecting said structural members treated in said first-step oxidation treatment to a second-step oxidation treatment of heating said treated structural members in contact with said reactor water controlled in said second control of the dissolved oxygen concentration at 200.degree. C. or higher for a predetermined period of time, thereby to make said oxide films into thick and dense oxide films;
• and then
• operating the nuclear plant to produce electric power.