Patent Number: 048204730
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a novel method of reducing radioactivity, particularly to a method of suppressing adherence of radioactive substances to structural materials used in contact with a liquid containing the radioactive substances dissolved therein, for example, primary cooling water piping in a nuclear power plant, and to a method of suppressing release of metallic ions or metallic oxides released from the structural materials and activated in a reactor core. Piping, pumps, valves etc. used in a primary cooling water system of a nuclear power station are made of stainless steel, Stellite which is a Co based alloy, etc. (hereinafter briefly referred to as "structural members"). These metals are subject to corrosion or damages in the course of long-term services thereof. As a result, constituent metallic elements are dissolved in reactor cooling water (hereinafter briefly referred to as "cooling water") to be entrained into the reactor. Most of the dissolved metallic elements are converted into oxides thereof, which then adhere to fuel rods. In this state, the metallic elements are irradiated with neutrons. As a result, radioactive nuclides such as .sup.60 Co, .sup.58 Co, .sup.51 Cr, and .sup.54 Mn are formed. These radioactive nuclides are dissolved into primary cooling water again to suspend in the form of ions or insoluble solid components (hereinafter referred to as "crud"). Part of them is removed in a demineralizer, etc. for purification of reactor water, but the rest adheres to the surfaces of structural members in the course of circulation through the primary cooling water system. Therefore, the dose rate on the surfaces of the structural members increases, thus presenting a problem of radiation exposure of workers during the course of maintenance and inspection. Accordingly, there have been proposed methods of suppressing causative dissolution of the above-mentioned metallic elements for decreasing the amount of the adhering radioactive substances. These methods include, for example, a method of suppressing corrosion of the structural members by using a corrosion-resistant material, and a method of suppressing corrosion of the structural members by introducing oxygen into a water supply system. However, in either method, corrosion of the structural members in the primary cooling water system including the water supply system cannot be sufficiently suppressed, and hence the amount of the radioactive substances in primary cooling water cannot be sufficiently decreased. Therefore, the does rate on the surfaces of the structural members due to the adherence of the radioactive substances thereto is increased. On the other hand, methods of removing radioactive substances adhering to the structural members have been investigated and practiced. These methods include (1) mechanical washing, (2) electrolytic washing, and (3) chemical washing. The methods (1) and (2) encounter a difficulty in removing radioactive substances strongly adhering to the surfaces of the structural members, and are unable to decontaminate systematically over a wide area. The method (3) comprises dissolving an oxide film on the steel surface by a chemical reaction using a chemical such as an acid solution to remove radioactive substances present in the film. In this method, even if the dose rate is temporarily decreased, rapid recontamination occurs when the structural members are exposed to a liquid containing the radioactive substances dissolved therein at a high concentration again. A method of suppressing adherence of radioactive substances by preliminarily providing an oxide film on the surfaces of structural members is disclosed in, for example, Japanese Patent Laid-Open Nos. 121197/1980 and 37498/1984. In this method, however, the adherence behavior of the radioactive substances markedly varies depending on the properties of the oxide film preliminarily provided. For example, the behavior of radioactive ions varies depending on the charged state of the oxide film. Also the rate of growth of oxide films newly formed on the surfaces of structural members after immersion in a liquid containing radioactive substances dissolved therein varies depending on the properties of the existent film. Thus a satisfactory film is not always formed. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of reducing radioactivity in a nuclear plant having a contact with pure, high-temperature and high-pressure water containing radioactive substances. In accordance with the present invention, there is provided a method of reducing radioactivity by preliminarily forming oxide films on the surfaces of metallic structural members to be in contact with high-temperature and high-pressure reactor water containing radioactive substances before the metallic structural members are exposed to said reactor water, characterized in that, after a first-step oxidation treatment of heating said metallic structural members in a high-temperature environment, the metallic structural members thus treated are further subjected to a second-step oxidation treatment of heating them in an environment having a higher oxidizing capacity than that of the environment in said first-step oxidation treatment to form denser oxide films that those obtained in the first step oxidation treatment. That is, the method of suppressing adherence of radioactive substances to structural members to be in contact with reactor cooling water containing radioactive substances according to the present invention comprises formation of oxide films having a relatively high porosity but a sufficient film thickness in the first-step treatment and subsequent formation of thin but dense films in the second-step treatment. The formation of a thick and porous film in the firststep treatment may be attained by an oxidation treatment with heated water or steam having a low oxidizing capacity, while the formation of a thin but dense film in the secondstep treatment may be attained by an oxidation treatment with heated water or steam having a higher oxidizing capacity than that in the first-step treatment. The present invention is based on an idea that adherence of radioactive substances may be suppressed by reducing the rate of corrosion of structural members in view of the fact that the radioactive substances contained in high-temperature and high-pressure water are confined into the structural members in the course of formation of oxide films caused by corrosion of the structural members with high-temperature and high-pressure water. The film grows thick by the first-step treatment alone in heated water having a low oxidizing capacity but is insufficient in the corrosion-suppressing effect in an environment of reactor cooling water. Therefore, the film is not sufficient in the effect of suppressing adherence of radioactive substances. On the other hand, the second-step treaatment alone in heated water having a high oxidizing capacity provides a very thin but dense film, which is, however, subject to scratches and denaturation. Thus, in an environment of reactor cooling water, film breakage easily occurs because of the thinness of the film. Therefore the corrosion-suppressing effect and hence the effect of suppressing adherence of radioactive substances cannot be sufficiently exhibited. It has been found that, when a thick film treated in heated water having a low oxidizing capacity is treated in heated water having a high oxidizing capacity, the resultant corrosion-suppressing effect is very remarkable. The reason why a thick film is formed with a low oxidizing capacity may be that an iron oxide forming the oxide film is a little easily dissolved in an environment of reactor cooling water so that the film may become porous enough to promote the growth of the oxide film due to the progress of oxidation through the micropores. On the other hand, the reason why only a thin film is formed in heated water having a high oxidizing capacity may be that an iron oxide forming the film is scarcely dissolved with the high oxidizing capacity so that the film may become dense enough to suppress the subsequent growth of the film. The dense film has a high corrosion-suppressing effect but is so liable to be broken that no sufficient effect can be obtained in an environment of reactor cooling water. Accordingly, formation of a thick and dense film provides the effects of greatly suppressing the corrosion in an environment of reactor cooling water and, hence, greatly suppressing the adherence of radioactive substances. In view of the above, when a thick but porous film is first formed in an environment having a low oxidizing capacity and then treated in an environment having a high oxidizing capacity, micropores in an initial oxide film are filled with a dense oxide to form a dense and thick film as a whole, thus enhancing the effect of suppressing corrosion after contact with an environment of a reactor. As a result, the effect of suppressing the adherence of radioactive substances is enhanced. With an appropriate treatment, there is a possibility that as dense a film as the secondary oxide film may be formed on the primary oxide film. The oxidizing capacity of heated water used in the first-step treatment of the two-step process must be, in principle, lower than that of reactor cooling water, while that in the second-step treatment must be higher than that of reactor cooling water. These oxidation treatments can be effected by heated water, steam, and a heated non-oxidizing gas having a high purity. Ar, N.sub.2, He, etc. can be used as such gas. For example, cooling water for a boiling water reactor usually contains 200 ppb of dissolved oxygen. The oxidizing capacity of cooling water depends on the dissolved oxygen concentration of the cooling water. Accordingly, in short, the cooling water is preferably pure water of 200.degree. C. or higher having a dissolved oxygen concentration of less than 200 ppb, particularly preferably 40 to 100 ppb, in the first-step treatment, and a dissolved oxygen concentration of more than 200 ppb, more preferably 300 ppb to 8 ppm, particularly preferably 300 to 1,000 ppb, in the second-step treatment. The treatment time in each step is preferably 100 to 500 hours, more preferably 100 to 200 hours. Control of the dissolved oxygen concentration can be achieved by deaeration, introduction of oxygen, or the like. The thickness of a film in the first-step treatment is about 0.5 to 3 .mu.m, while that of a film in the second-step treatment is about 0.05 to 0.5 .mu.m. In the case of treatment of austenite stainless steel, the amount of an oxide film in the first-step treatment is preferably 100 to 200 .mu.g/cm.sup.2 with a porosity of 60 to 70%, while that of an oxide film in the second-step treatment is preferably 10 to 100 .mu.g/cm.sup.2 with a porosity of 20 to 40%. In order to reduce the oxidizing capacity in the firststep treatment, addition of a reducing substance such as hydrazine, hydrogen, or an organic chemical may be useful besides lowering of the dissolved oxygen concentration. The amount of the reducing substance that may be added is preferably 1,000 ppm or less. In order to increase the oxidizing capacity in the second-step treatment, besides increase of the dissolved oxygen concentration, addition of an oxidizing substance such as hydrogen peroxide, a permanganate, or a chromate may be useful. The amount of the oxidizing substance that may be added is preferably 1,000 ppm or less. A denser film can be formed with a weak alkalinity, too. A pH of 8 to 10 is preferred. These treatments may not necessarily be effected with complete separation of the first-step treatment and the second step treatment. For example, oxidation may be effected under conditions continuously variable from a low oxidizing capacity to a high oxidizing capacity. Instead of two steps, several steps of different oxidation conditions may be employed. A theoretical background which has led to the present invention will now be described in more detail. Radioactive substances dissolved in reactor water are confined into an oxide film formed on the surface of stainless steel due to corrosion of the stainless steel during the course of formation of the oxide film. The oxide film grows toward the inner side (side of the body metal) in the interface between the oxide film and the body metal in water of a high temperature. The radioactive substances diffuse and move through the film toward the inner side to be confined into the oxide film in the same interface. The flux (J.sub.0) of the radioactive substances can be expressed by formula (1): ##EQU1## wherein d: thickness of oxide film k.sub.0 : proportional constant PA1 D: diffusion constant PA1 C.sub.1 : concentration of radioactive substance in reactor water, and PA1 C.sub.2 : concentration of radioactive nuclide in the interfare between film and metal. PA1 k.sub.1 : proportional constant, and PA1 m: amount of oxide film. The thickness (d) of the oxide film can be expressed by the following formula: EQU d=k.sub.1 m (2) wherein Thus, J.sub.0 can be alternately expressed by formula (3): ##EQU2## On the other hand, the rate (J.sub.1) of confinement of radioactive substances into the film can be expressed by formula (4) using a rate (dm/dt) of growth of the oxide film: EQU J.sub.1 =k.sub.2 C.sub.2 (dm/dt) (4) wherein K.sub.2 : proportional constant. The rate (J) of accumulation of radioactive substances is expressed by the formula: J=J.sub.0 =J.sub.1. Thus, when C.sub.2 is eliminated from the formulae (3) and (4), the following formula is obtained. ##EQU3## On the other hand, when the rate of accumulation of radioactive substances is determined by the step of diffusion, the rate (J) can be expressed by formula (6): ##EQU4## The formula (6) demonstrates that the rate (J) of accumulation of radioactive subtances is proportional to the diffusion constant (D) and the radioactive substance concentration (C.sub.1) of reactor water, and inversely proportional to the amount of the film, namely the thickness of the film. Accordingly, formation of a dense and thick oxide film having a low diffusion constant is one measure useful for suppressing the rate of accumulation of radioactive substances. Another measure is reduction in the concentration of the radioactive substance in reactor water, namely suppression of release of ions and oxides of metals, such as, cobalt or nickel to be activated in a reactor core, and iron capable of promoting activation of these metals, into reactor water due to corrosion of structural members. The corrosion of these structural members can be suppressed by forming a dense and thick oxide film. As described above, in order to suppress the accumulation of radioactive substances, a dense and thick oxide film has only to be preliminarily formed on structural members to be in contact with reactor water before nuclear heating begins in a nuclear plant. For example, in the case of stainless steel used in the structural members, the rate of adherence of radioactive substances shows an interrelation with the rate of growth of the film according to the study of the present inventors. Thus, suppression of film growth is presumed to lead to reduction in adherence of them. The reason why the rate of adherence of radioactive substances shows an interrelation with the rate of film growth may be that the radioactive substances are confined in growth points of the film. Accordingly, as the film growth is suppressed, the frequency of confinement of the radioactive substances is decreased. An increase in the amount (m) of the film on the stainless steel in an environment of cooling water is expressed by formula (7) including the logarithm of time (t): EQU m=a log (bt+1) (7) wherein a and b are constants. Accordingly, the rate of film growth is initially high, but gets lower as the film grows further. Therefore, preliminary formation of an adequate non-radioactive oxide film particularly exerts effects of suppressing confinement of radioactive substances and dissolution of metal ions. In other words, renewed film formation after immersion in a liquid containing radioactive substances dissolved therein can be suppressed and, hence, adherence of radioactive substances frequently observed during film formation can be suppressed. As a result of investigations on conditions of film formation with attention paid to the fact that adherence of radioactive substances can be suppressed by preliminary formation of an adequate non-radioactive oxide film on metallic structural members to be used in contact with reactor cooling water containing radioactive substances dissolved therein, and particularly to the fact that the rate of adherence of radioactive substances depends on the thickness and density of the preliminarily formed oxide film, the inventors of the present invention have found that the abovementioned rate of adherence can be remarkably reduced when an additional oxidation treatment of the film under strongly oxidizing conditions is conducted after an oxidation treatment of the film under weakly oxidizing conditions. There are several types of nuclear plants and the method of the present invention can be applied to any of them. In a boiling water nuclear plant, a pressure vessel of a reactor, piping in a recycling system, piping in a primary coolant purification system, etc. are in contact with reactor water containing radioactive substances. In a pressurized water nuclear plant, a pressure vessel of a reactor, structural materials in the reactor, a steam generator, etc. are in contact with reactor water as described above. Therefore, when all or part of structural members made of stainless steel, Inconel, carbon steel and stellite and to be in contact with a liquid containing radioactive substances is subjected to the oxidation treatment of this invention, adherence of radioactive substances can be suppressed, leading to minimization of radiation exposure of workers. On the other hand, in the boiling water nuclear plant, since the concentration of radioactive substances in primary cooling water in contact with structural members of water supply and steam condensation systems is comparatively low, the adherence of the radioactive substances is low and, hence, a problem of an increase in dose rate does not substantially arise. However, metallic ions or metallic oxides released due to corrosion of structural members in these systems are entrained with supply water into a pressure vessel of a reactor, thus increasing the concentration of radioactive substances in reactor cooling water. Therefore, suppression of corrosion of structural members in these systems is an important problem, too. The method of this invention is basically aiming at suppression of corrosion of structural members. Prior to start of the operation of a nuclear plant, the above-mentioned systems, namely the water supply and steam condensation systems, are subjected to the first-step treatment under weakly oxidizing conditions and the second-step treatment under strongly oxidizing conditions to form oxide films highly protective against corrosion on the surfaces of the structural members. Thus, release of metallic ions or metallic oxides into primary cooling water can be reduced. As a result, the amount of radioactive substances adhering to the recycling system and the reactor water purification system can be decreased.