JPH03246496A - Sticking suppressing method for radioactive material of piping or equipment for nuclear power plant - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

[00013

[Industrial Application Field] The present invention relates to a method for suppressing the adhesion of radioactive substances to new nuclear power plant piping or equipment used in contact with reactor water in which radioactive substances are dissolved, such as secondary cooling water system piping. . [0002] [0002] Piping, pumps, valves, etc. used in the primary cooling water system of a nuclear power plant are made of stainless steel, Stellite, etc. When these metals are used for a long period of time, they are subject to corrosion damage, and their constituent metal elements are leached into the primary cooling water and brought into the reactor. Most of the eluted metal elements become oxides and adhere to the fuel rods, where they are exposed to neutron irradiation. As a result, 6
Radioactive nuclides such as 0Co, 5Co, 5'Cr, and 64Mn are generated. These radionuclides are re-eluted into the primary cooling water and suspended as ions or insoluble solid components (hereinafter referred to as cladding). Some of the floating water is removed by demineralizers used to purify reactor water, but the rest adheres to the surfaces of structural materials, mainly made of stainless steel, while circulating through the primary cooling water system. For this reason, the dose rate on the surface of the structural material is high, and radiation exposure of workers when performing nine maintenance inspections has become a problem. [0003] Therefore, as a method for preventing an increase in the surface dose rate of structural materials, methods of removing radioactive substances attached to structural materials have been studied and implemented. There are currently three removal methods. [0004] (1) Mechanical cleaning method (2) Chemical cleaning method (3) Electrolytic cleaning method (1) Method is mainly applied to parts, for example, cleaning the surface with high-pressure jet water. . However, with this method, it is difficult to remove highly adhesive radioactive substances.
Furthermore, it is not possible to systematically decontaminate a wide area. In fact, even if the dose rate is temporarily reduced by this method, the dose rate tends to increase again with subsequent long-term use. [0005] The method (2) involves dissolving the oxide film on the steel surface through a chemical reaction using a chemical such as an acid solution, and removing radioactive substances present in the film. The problem with this method is the corrosion damage to the structural materials caused by the chemicals. In other words, when the coating is dissolved, the structural material may also be damaged by corrosion, and there is a risk that the small amount of chemicals remaining after decontamination may cause stress corrosion cracking of the structural material. [0006] The method (3) also has the same problems as (1). [0007] Furthermore, in order to reduce the amount of adhesion of radioactive substances, a method of suppressing the elution of metal elements, which are the source thereof, has also been implemented. That is, oxygen or hydrogen peroxide is injected into the water supply system to suppress corrosion of structural materials, thereby reducing the amount of corrosion products brought into the reactor. [0008] However, even if such a method is used, it is not possible to completely prevent corrosion of the structural materials of the sub-cooling water system, including the water supply system, and it is impossible to eliminate radioactive materials in the sub-cooling water. Therefore, an increase in surface dose rate due to adhesion of radioactive substances to structural materials remains a problem. [0009]

[Problem to be solved by the invention]

The purpose of the present invention is to eliminate these problems and eliminate radioactive substances from nuclear power plant piping or equipment by forming an oxide film in advance on the surface of piping or equipment used in contact with reactor water in which radioactive materials are dissolved. [00101

[Means for Solving the Problems] The present invention provides a method for suppressing the adhesion of radioactive substances to nuclear power plant piping or equipment made of metal that is used in contact with reactor water, in which the , outside the nuclear power plant system, by bringing the surface of the piping or equipment in contact with the reactor water into contact with degassed high-temperature, high-pressure pure water containing a trace amount of dissolved oxygen, and causing the pure water to flow, thereby forming an oxide film. A method for suppressing the adhesion of radioactive substances on piping or equipment for a nuclear power plant, characterized by forming a [0011] [Operation] As a result of various analyzes (for example, G
, Romeo, Proceedings of Th.
e 7th International Congress
ss on Metallic Corrosion,
P1456.1978) is being rejected. This radionuclide has a γ-ray decay energy intensity of 1.17 and 1.
．． Since it is as high as 33 MeV and has a long half-life of 5.26 years, once it adheres to structural materials, it causes an increase in the surface dose rate over a long period of time. Therefore, in order to reduce the dose rate, the key is how to suppress the adhesion of 60CO. [0012] On the other hand, structural materials such as pressure vessels, piping, pumps, and valves at the mouth of the plant are made of stainless steel, Stellite, Inconel, and carbon steel, and stainless steel accounts for 97% of the area in contact with water. Therefore, suppressing the adhesion of radioactive substances to stainless steel is the most effective way to reduce exposure. [0013] Radionuclides dissolved in reactor water are incorporated into an oxide film formed on the surface of stainless steel during its formation process. By the way, according to the inventor's research, since the adhesion rate of radionuclides shows a correlation with the film growth rate, it was presumed that suppressing the film growth would lead to a reduction in adhesion. [0014] The increase in the amount of coating on stainless steel in a reactor water environment is expressed by the logarithmic law of time. That is, as the film grows, its growth rate decreases. Therefore, if a suitable non-radioactive oxide film is formed in advance, it is possible to suppress the formation of a new film after immersion in a solution containing dissolved radioactive substances, and in turn, the radioactive Adhesion of substances can be suppressed. [0015] Incidentally, the difficulty of diffusing radioactive substances into the film depends on the film composition and form. Since 60CO is a divalent atom, it is effective to form a film mainly composed of α-Fe203 in an oxide film with a crystal structure containing divalent metals in order to suppress the adhesion of Co. This discovery led to the present invention. [0016] When forming an oxide film, the above-mentioned strong oxide film cannot be obtained on the surface of piping or equipment unless oxidation treatment is performed using degassed high-temperature, high-pressure pure water containing a trace amount of dissolved oxygen. In addition, the formation of an oxide film is carried out by the above-mentioned pure water,
The heating is performed outside the nuclear plant system by flowing pure water. [0017]

【Example】

Experimental Example 1 Stainless steel having the chemical composition shown in Table 1 was immersed in reactor water flowing at a flow rate of 0.5 m/s for 25 to 1000 hours to determine the amount of oxide film formed and the amount of 60C attached. The amount was measured. 1 size soil d″″d’lb’l;jL)% Reno [0018
]

[Table 1] (Table 1) Before immersion, the surface of the stainless steel was machined and then degreased and cleaned. The 60Co concentration in the reactor water is 1 x 10-4 μCi/
More than 90% of the ions existed in the form of ions. In addition, the temperature was 230°C, and the dissolved oxygen concentration was 150 to 17 op.
pb, pH was 6.9 to 7.2. [00203 Table 2 shows the main metal element composition of the oxide film formed when immersed for 1000 hours. Approximately 90% is iron element. [0021]

[Table 2] [0022] Figure 1 shows changes over time in the amounts of representative metal elements (total of iron, cobalt, nickel, and chromium) in the oxide film formed per unit area of stainless steel. It shows an increase according to the logarithmic side after 100 hours or more. [0023] On the other hand, FIG. 2 shows the change over time in the amount of attached 60CO. Similar to the amount of oxide film, it increased logarithmically over 100 hours. [0024] Therefore, it can be seen from FIGS. 11 and 2 that the deposition rate of 60Co is determined by the growth rate of the oxide film. Furthermore, the growth rate of the film gradually decreases as it grows. [0025] Experimental Example 2 The stainless steel described above was immersed for 25 to 500 hours in pure water at a high temperature and high pressure similar to the operating conditions of a nuclear power plant shown below, to form a non-radioactive oxide film on the surface in advance. [0026] Temperature: 285°C Dissolved oxygen concentration: 200 ppb Electrical conductivity: 0.1 μS/cm Flow rate: 0, 1 cm/sec Pressure crab 73 kg/cm2 Stainless steel was immersed in the above-mentioned reactor water, and the amount of 60Co deposited was We investigated changes over time. The results are shown in Figure 3 together with the results for untreated stainless steel. Curves 1 to 5 indicate preoxidation treatment times of 25 hours, 50 hours, 100 hours, and 200 hours, respectively.
The case of time and 500 hours is shown. Immersion in reactor water 1
The adhesion of 60Co to the pre-oxidized stainless steel was significantly suppressed over 00 hours. [0027] Oxide film thickness obtained by each of the above-mentioned treatment times and 60
The relationship with the amount of Co adhesion is shown in FIG. 60C is capable of forming a film with a thickness of 300A or more through oxidation treatment. It is extremely effective in suppressing the adhesion of [0028] Experimental Example 3 A comparison was made between the pre-oxidation treatment of the 5US304 steel shown in Experimental Example 1 using the steam of the comparative example and the method of the present invention using high-temperature, high-pressure pure water. Table 3 shows the treatment conditions and treated samples in reactor water under the same conditions as Experiment 1.
The amount of 60CO deposited after immersion for 0 hours is shown. [0030]

[Table 3] (Table 3) [0031] As shown in the table, compared to the case of using water vapor in the comparative example,
This shows that the amount of 60CO deposited can be significantly suppressed by using high-temperature, high-pressure pure water as in the present invention. The reason for this is that an oxide film mainly composed of Fe(II) is formed in steam, and in reactor water, this Fe(II) position is 6
This seems to be because 0Co (II) is less likely to be substituted and precipitated. On the other hand, in high-temperature, high-pressure pure water, a stable film mainly composed of Fe(II) is formed. As the oxide film on the surface of the structural material is dissolved and peeled off during the dyeing process, the metal base is exposed, and the amount of radioactive substances deposited upon reuse is as shown in Figure 2.
It shows a similar change over time. Therefore, by performing the pre-oxidation treatment of the present invention and then reusing it, it is possible to suppress the adhesion of radioactive substances. [0032] After chemical decontamination was applied to two pieces of 5US304 steel piping used in an actual nuclear power plant, electrical conductivity was reduced to 0.00% without oxidation film treatment and as pre-oxidation treatment on the surface in contact with reactor water. 1 pS/cm, temperature 275-285°C, dissolved oxygen concentration 350-38 oppb. Pressure cuff Using high temperature, high pressure pure water of 3 kg/cm2,
Oxidation treatment was carried out using running water at a flow rate of 0.1 cm/sec as described above for 0 hours, and then immersed in the above-mentioned reactor water for 500 hours at 60C. The amount of adhesion was compared. As a result, the amount of 60Co deposited was 0.453 μCi/cm2 for the product that was only washed with water after decontamination treatment, while it was 0.11 μC1/cm2 for the product that was pre-oxidized, and the amount of 60Co attached was reduced to about one-fifth by the pre-oxidation treatment. Reduced. [0033]

【Effect of the invention】

As described above, the present invention suppresses the adhesion of radioactive substances by forming an oxide film on the surfaces of nine nuclear power plant piping equipment, etc. that come into contact with reactor water by the above-mentioned autoclave treatment before they are installed in the plant. It is suitable for suppressing the increase in the dose rate of those piping and equipment and reducing the radiation exposure of workers, and has high practical value and is extremely meaningful industrially.

[Brief explanation of drawings]

[Figure 1] A diagram showing the relationship between the amount of stainless steel oxide film and time.

[Figure 2]

[Figure 3] A diagram showing the relationship between 60CO adhesion amount and time.

FIG. 4 is a diagram showing the relationship between the amount of radioactive material deposited and the thickness of the oxide film.

【Document name】

[Same month drawing figure Time while (h) [Figure 2] 'I: Open 10d''''Go4b'l;3t)%1<ノ

[Figure 3
] Figure time (h)

[Figure 4]

Claims (1)

[Claims] 1. In a method for suppressing the adhesion of radioactive substances to nuclear power plant piping or equipment made of metal that is used in contact with reactor water, the piping or equipment is preliminarily removed outside the nuclear power plant system before coming into contact with the reactor water. For use in a nuclear power plant, characterized in that an oxide film is formed by bringing the surface of the equipment in contact with the reactor water into contact with degassed high-temperature, high-pressure pure water containing a trace amount of dissolved oxygen and flowing the pure water. Method for suppressing adhesion of radioactive substances to piping or equipment.