Abstract:
A nuclear power plant using a structural material consisting of metal which comes into contact with a liquid in which radioactive substances are dissolved has a function to suppress the deposition of the radioactive substances. In this function, metal elements constituting the surface layer of the structural material which comes into contact with the liquid are oxidized by injecting oxidizing agents from outside, thereby preliminarily producing an oxide film. The oxidizing agents comprise at least one kind selected from the group consisting of oxygen, hydrogen peroxide, chromate, nitrite, molybdate, tungstate and ferrate. Particularly, by suppressing the deposition of radioactive substances on a stainless steel, the dose rate of exposure of workers is effectively reduced.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a nuclear power plant and, more particularly, to a nuclear power plant in which a treatment for suppressing deposition of radioactive substance is performed for structural material to be used while coming into contact with a liquid in which radioactive substance is dissolved, such as a piping of the primary cooling water system. 
     The piping, pumps, valves, or the like which are used in the primary cooling water system of a nuclear power station are made of stainless steel, Stellite, or the like. When these metals are used for a long period of time, they are subject to corrosion and damage, causing composite metal elements to be dissolved in the primary cooling water and brought into the nuclear reactor. Most of the dissolved metal elements are oxidized and deposited on the fuel rods, where they are irradiated by neutrons. Hence, the radioactive nuclides such as  60  Co,  58  Co,  51  Cr,  64  Mn, etc. are produced. These radioactive nuclides are redissolved in the primary cooling water and float as ions or insoluble solid components (hereinafter, referred to as clads). Portions of the floating clads are removed by a desalting device or the like for purification of the reactor water. However, the remaining clads are deposited on the surface of the structural material consisting mainly of stainless steel while they are circulating in the primary cooling water system. Therefore, a dose rate on the surface of the structural material increases and there is a problem on radiation exposure of workers who perform maintenance and inspection. 
     Thus, as a method of preventing an increase in the dose rate at the surface of the structural material, the methods of elimination of the radioactive substances deposited on the structural material are studied and carried out. Presently, the eliminating method includes the following three methods. 
     (1) Mechanical cleaning method 
     (2) Chemical cleaning method 
     (3) Cleaning method by electrolysis 
     The method (1) is mainly applied to parts, for example, the surface is cleaned by a high-pressure jet water. However, it is difficult to remove the radioactive substance having a large adhesive property by this method, and is impossible to systematically clean over a wide range. In fact, even if the dose rate were reduced temporarily by this method, the dose rate tends to increase again due to the subsequent use over a long period of time. 
     According to the method (2), the oxide film on the surface of the steel is dissolved by the chemical reaction using chemicals such as acid solution or the like, thereby removing the radioactive substance existing in the oxide film. This method has a problem of corrosive damage of the structural material due to the chemicals. That is to say, when the oxide film is dissolved, the structural material is also subject to corrosive damage and there is a fear of causing a stress corrosion cracking of the structural material due to the small amount of chemicals remaining after the cleaning. 
     The method (3) has also the same problem as the method (1). 
     Furthermore, to reduce the quantity of adhered radioactive substance, it is performed a method of suppressing elution of metal elements which are the sources of radioactive substance. Namely, oxygen or hydrogen peroxide is injected into the water supplying system to suppress the corrosion of the structural material, thereby reducing the amount of corrosive products entering into the nuclear reactor. 
     However, even if such a method is used, it is impossible to completely prevent the corrosion of the structural material of the primary cooling water system and to eliminate the radioactive substances in the primary cooling water; therefore, a problem on increase in dose rate at the surface due to the deposition of the radioactive substance into the structural material still remains. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to eliminate such problems and to provide a nuclear power plant which can suppress the deposition of radioactive substances on the structural material to be used in contact with the liquid in which radioactive substances are dissolved by preliminarily forming an oxide film onto the portion that will come into contact with the liquid for such a structural material. 
     According to the present invention, the above object is accomplished by preliminarily forming an oxide film on the structural material by oxidizing the metal elements constituting the surface layer of the structural material which will come into contact with the liquid in which radioactive substances are dissolved by injecting from outside oxidizing agents. 
     For this purpose, the present invention has such a feature that the above oxidizing agents comprise at least one kind selected from the group consisting of oxygen, hydrogen peroxide, chromate, nitrite, molybdate, tungstate and ferrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a relation between the amount of an oxide film of stainless steel and the time; 
     FIGS. 2 and 3 are diagrams showing a relation between the amount of deposition of  60  Co and the time; and 
     FIG. 4 is a diagram showing a relation between the amount of deposition of radioactive substance and the thickness of oxide film. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been found that the main radioactive nuclide which contributes to the exposure of workers concerning a nuclear power plant is  60  Co, from the results of various analyses (for example, G. Romeo, Proceedings of The 7th International Congress on Metallic Corrosion, P1456, 1978). The deca energy strength of Υ rays of this radioactive nuclide is so high as to be 1.7 and 1.33 MeV and its half life is so long as to be 5.26 years, so that once it has been deposited on the structural material it becomes a cause of an increase in surface dose rate over a long period of time. Therefore, how the deposition of this  60  Co, is suppressed is a key to reduce the dose rate. 
     On the other hand, although the structural materials of the pressure vessels, piping, pumps, valves, etc. around the plant reactor consist of stainles steel, Stellite, Inconel, and carbon steel, stainless steel occupies 97 percent of the water contact area. Thus, it is most effective for reduction of exposure to suppress the deposition of radioactive substances onto the stainless steel. 
     The radioactive nuclide dissolving in the reactor water enters into the oxide film formed on the surface of the stainless steel by corrosion thereof in the process of formation of the oxide film. By the way, according to the study by the present inventors, it has been assumed that suppression of the growth of the oxide film will contribute to a reduction of deposition since there is a relative relation between the depositing rate of the radioactive nuclide and the oxide film growing rate. 
     An increase in the amount (m) of oxide film of stainless steel under the circumstance of reactor water is represented by the logarithmic law of time (t) as shown in equation (1): 
     
         m=a log t+b                                                . . . (1) 
    
     wherein a and b are constants. 
     That is, with a growth oxide film, its growing rate decreases. Therefore, by preliminarily forming a suitable non-radioactive oxide film, it is possible to suppress the formation of new oxide film after the structural material has been dipped into the liquid in which radioactive substance is dissolved, so that it is possible to suppress the deposition of the radioactive substance which frequently occurs upon formation of oxide film. 
     By the way, the difficulty in diffusion of radioactive substance into oxide film depends upon the composition and form of the film. Since  60  Co is a bivalent atom, it relatively easily enters into the oxide film having a crystal structure with bivalent metal. For example, in case of Fe 2  O 4  having a spinel structure (M II  O.M III   2  O 3 ), the position of Fe II  is substituted by  60  Co II . 
     Therefore, it has been found that the deposition can be effectively suppressed by preliminarily forming the oxide film which consists mainly of α-Fe III   2  O 3 , without any bivalent atoms, and the present invention was made. 
     Examples of the present invention will be described hereinbelow; however, the present invention is not limited to them. 
     EXAMPLE 1 
     A stainless steel haviang a chemical composition shown in Table 1 was dipped in the reactor water flowing at a flow rate of 0.5 m/s for 25-1000 hours. The amount of the formed oxide film and the amount of  60  Co deposited were measured. 
     
                       TABLE 1______________________________________Chemical composition (%)JIS    C      Si     Mn    S    Ni   Cr    Co   P______________________________________SUS304 0.06   0.76   1.12  0.023                           9.11 18.07 0.22 0.029______________________________________ 
    
     Before the stainless steel is dipped, the surface thereof was machine-worked and thereafter it was degreased and cleaned. A concentration of  60  Co in the reactor water was 1×10 -4  μCi/ml and not lower than 90% of  60  Co existed as ions. The temperature was 230° C., dissolved oxygen concentration was 150-170 ppb, and pH was 6.9-7.2. 
     The principal metal element composition of the oxide film formed when it has been dipped for 1000 hours is shown in Table 2. About 90% is iron element. 
     
                       TABLE 2______________________________________weight μg/cm.sup.2 (metal element composition %)Fe       Co           Ni        Cr______________________________________293 (89.1)    0.940 (0.286)                 10.1 (3.07)                           25.0 (7.60)______________________________________ 
    
     FIG. 1 shows a change in the amount (total amount of iron, cobalt, nickel, and chrome) of the typical metal elements in the oxide film formed per unit area of the stainless steel as a function of time. 
     It can be seen that the amount of oxide film increases in accordance with the logarithmic law over 100 hours. 
     On the other hand, FIG. 2 shows a change in the amount of deposition of  60  Co as a function of time. Similarly to the amount of oxide film, the amount of deposition of  60  Co increases in accordance with the logarithmic law over 100 hours. 
     As a result, it will be appreciated from FIGS. 1 and 2 that the depositing rate of  60  Co complies with the growing rate of oxide film. The oxide film growing rate gradually reduces with its growth. 
     EXAMPLE 2 
     The stainless steel similar to that used in Example 1 was dipped in the water under the conditions shown below for 25-500 hours to preliminarily produce the non-radioactive oxide film on the surface. 
     Temperature: 285° C. 
     Dissolved oxygen concentration: 200 ppb 
     Electric conductivity: 0.1 μS/cm 
     Flow rate: 0.1 cm/sec 
     In this way, the stainless steel on which the oxide film consisting mainly of α-Fe 2  O 3  has been preliminarily formed by the pre-oxidation treatment was dipped in the reactor water under the same conditions as Example 1, and a change in the amount of deposition of  60  Co as a function of time was examined. The result is shown in FIG. 3 together with the non-treated stainless steel (curve 6). Curves 1 to 5 respectively show the relations in the cases where the times of pre-oxidation treatment are 25, 50, 100, 200, and 500 hours. When the stainless steel was dipped in the reactor water over 100 hours, the deposition of  60  Co onto the stainless steel having been subjected to the pre-oxidation treatment was remarkably suppressed. 
     EXAMPLE 3 
     FIG. 4 shows a relation between the thickness of oxide film and the amount of deposition of radioactive substance. It will be understood that the deposition can be effectively suppressed by forming an oxide film having a thickness of 300 Å or more by oxidation treatment. 
     EXAMPLE 4 
     When the piping, equipment or the like used in a nuclear power plant are cleaned by the chemical method or the like and are used again, the oxide film on the surface of the structural material has been dissolved and peeled off by the cleaning operation. Thus, the metal bare surface is exposed and the amount of deposition of radioactive substance upon reuse shows the time change similar to that of FIG. 2. Therefore, the deposition of radioactive substance can be suppressed by reusing them after the pre-oxidation treatment according to the present invention was performed. 
     EXAMPLE 5 
     Although there are many types of nuclear power plants, the present invention can be applied to any of them. For example, in a nuclear power plant of the boiling water reactor, the pressure vessel, piping of the recirculating system, piping of the primary coolant cleaning system or the like come into contact with the reactor water which includes radioactive substances. Further, in a nuclear power plant of the pressurized water reactor, the pressure vessel, structural material in the reactor, steam generator or the like come into contact with the similar reactor water. Therefore, the deposition of the radioactive substances can be suppressed by applying the structural materials having been subjected to the preoxidation treatment of the present invention to all or part of the structural material consisting of one or two or more metals selected from the group of stainless steel, Inconel, carbon steel and Stellite that comes into contact with the liquid containing these radioactive substances. Moreover, it is possible to provide a plant in which an exposure of workers is small. 
     EXAMPLE 6 
     The following methods are considered to apply the present invention to the nuclear power plant shown in Example 5. 
     (1) For the piping, equipment or the like of the small shape, the oxidation treatment can be performed on the structural material by the autoclave treatment in the water containing the oxidizing agents such as oxygen or the like, which were set forth in item (2) before they are installed in the plant. 
     (2) For the piping, equipment or the like which have been already installed in the plant, the oxidation treatment can be preliminarily performed on the surface which comes into contact with the water by a method of heating the water by introducing the steam at high temperature from outside, or by means of the heat generated by the operation of the pumps that have been already installed, or by a method of adding oxidizing agents such as oxygen, hydrogen peroxide, chromate, nitrite, molybdate, tungstate, ferrate, etc. into the water even at a low temperature. 
     As will be obvious from the above description, according to the present invention, the deposition of radioactive substances on the structural material consisting of metal can be suppressed by simple means and its applicable range is wide. Particularly, the present invention is suitable to reduce the exposure of workers since it suppresses an increase in dose rate when it is applied to the structural materials including stainless steel that are used in a nuclear power plant. The present invention has a high practical value and is industrially very worthwhile.