Patent Number: 048204730
Section: description

EMBODIMENTS OF THE INVENTION Example 1 JIS SUS 304 stainless steel having a chemical composition (wt. %) as shown in Table 1 was subjected to various oxidation treatment with pure water of 250.degree. C. (in a liquid state), and then immersed in heated water of 288.degree. C. having a cobalt ion concentration of 10 ppb and a dissolved oxygen concentration of 200 ppb for 500 hours. The rate of corrosion and the amounts of Co adherence were examined with each film formed. The results are shown in Table 2. It will be understood that the second-step treatments can suppress the rate of corrosion to as low a value as about 230 .mu.g/m.sup.2. month or less and the amount of Co adherence to 10 .mu.g/cm.sup.2. month or less. TABLE 1 ______________________________________ C Si Mn S Ni Cr Co P Fe ______________________________________ 0.06 0.76 1.12 0.023 9.11 18.07 0.22 0.029 balance ______________________________________ FIGS. 1a and 1b are model diagrams of growth of an oxide film according to the two-step treatments of the present invention. It is believed that a primary oxide film 15 as shown in FIG. 1a is formed on the surface of a structural member 17 made of metals according to the first-step oxidation treatment, and that a secondary oxide film 16 as shown in FIG. 16 is formed in the porous portions of the primary oxide film. As shown in FIG. 1b, the primary oxide film is densified by formation of the secondary oxide film to form a thick and dense film as a whole. Films having a porosity of about 60% were obtained with dissolved oxygen concentrations of 100 and 200 ppb in Table 2, and a film having a porosity of about 26% was formed with a dissolved oxygen concentration of 500 ppb. TABLE 2 __________________________________________________________________________ Oxidation treatment conditions (250.degree. C.) Environment of model reactor water (288.degree. C.) First-step treatment Second-step treatment Amount of Rate of corrosion Rate of Co adherence Environment Time (h) Environment Time (h) (.mu.g/cm.sup.2) (.mu.g/cm.sup.2 .multidot. month) (.mu.g/cm.sup.2 .multidot. month) __________________________________________________________________________ Comparative not treated -- not treated -- 0 650 28 Examples DO 200 ppb 200 -- -- 180 390 18 DO .ltoreq. 5 ppb 200 -- -- 250 520 21 DO 8 ppm 200 -- -- 140 320 14 DO 8 ppm 100 DO &lt; 5 ppb 100 180 340 18 Present DO .ltoreq. 5 ppb 100 DO: 8 ppm 100 190 195 8 Invention DO 100 ppb 100 DO: 500 ppb 100 220 228 10 DO .ltoreq. 5 ppb 100 DO: 8 ppm 100 240 195 7 N.sub.2 H.sub.4 1,000 ppm DO .ltoreq. 5 ppb 100 DO: 8 ppm 100 200 130 6 N.sub.2 H.sub.4 H.sub.2 O.sub.2 1,000 ppm 1,000 ppm DO .ltoreq. 5 ppb 100 DO: 30 ppm 100 220 160 7 __________________________________________________________________________ (DO: dissolved oxygen concentration) Example 2 A boiling water nuclear power plant was subjected to the two-step oxidation treatments of this invention by controlling dissolved oxygen concentration of pure water of primary reactor cooling water when starting the operation and completing decontamination with use of heat generated by the operation of pumps in various systems of the plant as a heat source and without use of nuclear heating. FIG. 2 is a systematic view of a boiling water nuclear power plant. As can be appreciated by one of ordinary skill in the art, 6 denotes the turbine for the nuclear power plant; 7, a condenser, e.g., for reactor water after passage through the turbine; 8, a condensate purification apparatus; 9, a feed water heater, for water fed to the reactor; 12, a valve; 13, a vacuum pump; and 14, an exhaust tower. The oxidation treatments of this invention can be carried out while circulating cooling reactor water through a route of a reactor 1--a recycling system 2--a reactor water purification system 5. First, the system is filled with pure water. A main steam separation valve 25 is closed, and a recycle pump 3 is run. This elevates the water temperature in the system at a rate of 3.degree. C./h. In the first-step treatment, heated water having a dissolved oxygen concentration of 40 to 100 ppb and a temperature of 260.degree. to 280.degree. C. is circulated for 100 to 500 hours. Reduction in the dissolved oxygen concentration to 40 to 100 ppb can be achieved by opening the main steam separation valve 25 to blow steam into a main steam system 11 or by passing a nitrogen gas to a gas phase in the upper portion of the reactor 1. The temperature control can be achieved by controlling the amount of heated water passing through the reactor purification system 4. After the first-step treatment is effected for a predetermined time, the dissolved oxygen concentration is raised for conducting the secondstep treatment. Heated water having a dissolved oxygen concentration of 400 to 1,000 ppb and the same temperature of 260.degree. to 280.degree. C. as in the first-step treatment is circulated for 100 to 500 hours. Increase in the dissolved oxygen concentration to 400 to 1,000 ppb is attained by introducing oxygen from a sampling line 12" of the recycling system. A nuclear power plant where adherence of radioactive substances is reduced can be materialized by practicing the two-step oxidation treatments as described above. Example 3 In the same system as in Example 2, a recycle pump 3 is run to introduce hydrazine from a sampling line 12" of a recycling system in the first-step treatment. The hydrazine concentraton is thus adjusted to 100 to 1,000 ppb. Heated water having the thus adjusted concentration and a temperature of 200.degree. to 280.degree. C. is circulated for 100 to 500 hours. Subsequently, in the second-step treatment, hydrogen peroxide is introduced into heated water to provide a hydrogen peroxide concentration of 100 to 1,000 ppb. Heated water having the above-mentioned concentration and a temperature of 200.degree. to 280.degree. C. is circulated for 100 to 500 hours. The two-step oxidation treatments thus practiced can materialize a nuclear power plant where adherence of radioactive substances to the structural materials is suppressed. Example 4 In construction of a boiling water nuclear power plant, a temporary circulation line is attached to a supply water heater 9 before the installed heater 9 is connected with the plant. The two-step treatments of this invention are carried out by circulating heated water having a dissolved oxygen concentration adjusted while using an in-plant or a temporary boiler as the heat source. Although this procedure is to suppress release of metallic ions or metallic oxides from the supply water heater 9, supply water is in contact with only the inner surface of a heater tube 26 of the supply water heater 9 as shown in FIG. 2. Accordingly, an oxidation treatment has only to be applied on the inner surface of the heater tube 26. A temporary circulation line 24 is attached to the inlet and outlet of the heater tube 26 in carrying out the oxidation treatment as shown in FIG. 3. The circulation line 24 is provided with a circulation pump 18, a blowing line 20 for heating steam from a boiler, a blow-off line 21 for steam, and an oxygen supply line 22. Heating steam is blown into the target system filled with pure water which is circulated. Heated water having a dissolved oxygen concentration of 40 to 100 ppb and a temperature of 200.degree. to 250.degree. C. is circulated for 100 to 500 hours in the first-step treatment. Adjustment of the dissolved oxygen concentration is effected by oepning the valve of the blow-off line 21 to blow off part of steam when the temperature of heated water exceeds 100.degree. C. Subsequently, heated water having a dissolved oxygen concentration of 400 to 1,000 ppb and a temperature of 200.degree. to 250.degree. C. is circulated for 100 to 500 hours in the second-step treatment. Adjustment of the dissolved oxygen concentration in this treatment is effected by introducing a small amount of oxygen from the oxygen supply line 22. Although the above-mentioned two-step oxidation treatments have only to be effected on only the inner surface of the heater tube 26, circulation of heated water only through the inside of the heater tube 26 provides a remarkably large pressure difference between the inside and outside of the tube, leading to inconvenience such as deformation of the tube. Therefore, part of circulation water must be bypassed out of the tube to reduce the pressure difference. Thus, bypass line 23, for bypassing part of the circulation out of the tube, is provided. By the two-step oxidation treatments as described above, release of metallic ions or metallic oxides due to corrosion of the heater tube of the supply water heater can be reduced to decrease the radioactivity of reactor water. Thus a nuclear power plant where adherence of radioactive substances is suppressed can be materialized. Example 5 The two-step oxidation treatments according to the present invention are effected by passing steam having an oxygen concentration adjusted therein and originated from steam coming out of the in-plant boiler through the same supply water heater as in Example 4. Since it is difficult to decrease the content of oxygen in the steam coming out of the in-plant boiler, the steam is passed as such through the heater tube of the supply water heater in the first-step treatment. The steam of a temperature of about 200.degree. C. is passed for 100 to 500 hours. Subsequently, in the second-step treatment, steam having an oxygen concentration of 100 to 1,000 ppm adjusted by introducing an oxygen gas thereinto is passed for 500 to 1,000 hours. By these treatments, release of metallic ions or metallic oxides due to corrosion of the heater tube of the supply water heater can be reduced to decrease the radioactivity of reactor water. Thus a nuclear power plant where adherence of radioactive substances is suppressed can be materialized. Example 6 The oxidation treatments according to the present invention were conducted by using a steel pipe having the same specifications as those of SUS 304 stainless steel used in Example 1 outside the system of a nuclear power plant. In a vessel, a first-step oxidation treatment was carried out in pure water having a dissolved oxygen concentration of 100 ppb and a temperature of 250.degree. C. for 100 hours. Subsequently, an oxidation treatment was carried out in pure water having a dissolved oxygen concentration of 400 ppb and a temperature of 250.degree. C. for 100 hours. The corrosion test and the measurement of amount of Co adherence were made in the same manner as in Example 1. The rate of corrosion of the treated pipe was 200 .mu.g/cm.sup.2. month. The rate of Co adherence was 9 .mu.g/cm.sup.2. month. Thus excellent effects of suppressing corrosion and Co adherence were recognized. A steel pipe as prepared in the above-mentioned manner can be used by connecting the same with a predetermined portion of a nuclear power plant by welding. Where oxidation treatments cannot be effected in a vessel, the oxidation treatments on only the inner surface of the steel pipe can be effected in an apparatus provided with a circulation line and a heating means as shown in FIG. 3 in the same manner as described above. In this case, the treatments are so effected as to keep the temperature of the steel pipe to be subjected to the oxidation treatments. The steel pipe thus treated is detached, and can be connected with a predetermined portion of a nuclear power plant by welding as described above to be placed in service.