JPH0314155B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for suppressing radioactive ion adhesion,
In particular, the present invention relates to a method for suppressing radioactive ion adhesion suitable for suppressing the adhesion of ⁶⁰ Co ions to the surface of structural materials of boiling water reactors. Nuclear power plants, such as piping and pumps used in the primary cooling water system of boiling water nuclear power plants,
When structural materials such as valves and equipment are used for a long period of time, metal impurities are leached from these structural materials.
The metal impurities are eventually carried into the nuclear reactor, where they are activated by neutron irradiation and generate radioactive corrosion products. Radioactive corrosion products include components dissolved in cooling water (hereinafter referred to as ions) and insoluble solid components (hereinafter referred to as cladding). In this way, ⁶⁰ Co and ⁵⁴ Mn were produced.
Radioactive corrosion products with long half-lives, such as those with long half-lives, adhere to the surfaces of structural materials while circulating in the primary cooling water system of the nuclear reactor. This causes an increase in the radiation dose rate on the surface of the structural material. Therefore, it becomes difficult to carry out maintenance and inspection of the nuclear reactor in a short period of time in order to prevent workers from being exposed to radiation. An object of the present invention is to eliminate the drawbacks of the prior art described above and to suppress the adhesion of radioactive ions to structural materials. The features of the present invention are Mg, Cr, V, Ti, Cu, Ni,
The purpose is to add metal ions of at least one substance selected from the group consisting of Zn and Cd to the coolant. In order to prevent an increase in the surface dose rate of the structural materials of the primary cooling system, mechanical removal of radioactive corrosion products adhering to the structural materials has been carried out. Mechanical cleaning methods make it possible to temporarily reduce the surface dose rate of structural materials. However, some of the attached radioactive corrosion products are very difficult to remove using mechanical cleaning methods. Due to the gradual accumulation of such deposits over many years, the surface dose rate of structural materials tends to increase gradually year by year. The present invention provides that radioactive corrosion products that adhere firmly to the surface of structural materials and are difficult to remove by mechanical cleaning methods are based on radioactive ions in the primary cooling water, and that the radioactive ions are When a ferrite layer is formed on the surface of a structural material together with non-radioactive ions, they are incorporated into the ferrite layer as a solid solution, and the rate of attachment of radioactive ions in the ferrite layer is inversely proportional to the concentration of metal ions in the coolant. This was done with a focus on That is, by increasing the concentration of metal ions in the coolant in which radioactive ions are present, the rate of adhesion of radioactive ions to the structural material ferrite layer is reduced. Various experiments were conducted to elucidate the mechanism by which radioactive corrosion products adhere to piping, pumps, valves, etc. in the recirculation system of boiling water nuclear power plants. These experiments revealed that corrosion products adhere as shown in the model schematically shown in FIG. In Fig. 1, 1 is the structural material of the primary cooling system;
2 indicates a ferrite layer, 3 indicates a cladding, and 4 indicates a radioactive ion. The ferrite layer 2 formed on the surface of the structural material 1 that comes into contact with the cooling water has α-type diiron trioxide (α-Fe ₂ O ₃ ) as its main component, and Ni,
Consists of solid solution of Co and Cr. The radioactive ions 4 dissolved in the cooling water are simultaneously taken into the ferrite layer 2 when the ferrite layer 2 is formed on the surface of the structural material 1;
Diffusion into the already formed ferrite layer 2, and by isotope exchange reaction etc.
There are some that replace Ni, Co, Mn, Cr, etc. and are firmly fixed. When forming the ferrite layer 2, radioactive ions diffuse into the oxide film formed on the surface of the structural material 1, and as the oxide film grows and oxidation of ferrous oxide (FeO) progresses, the radioactive ions become spinel. incorporated into the structure. The cladding 3 adheres to the surface of the ferrite layer 2 in a relatively loose manner. As mentioned above, radioactive corrosion products in the cooling water of a nuclear reactor pressure vessel can be broadly classified into radioactive ions and cladding. In boiling water nuclear power plants where measures are taken to suppress corrosion of structural materials, such as by injecting oxygen into the water condensation system, the weight concentration of radioactive corrosion products in the cooling water in the reactor pressure vessel is below several ppb. ing. In such plants, the radioactivity concentration of ions in the cooling water in the reactor pressure vessel is much greater than the radioactivity concentration in the cladding. In other words, the radioactivity concentration of the former is about 10 times higher than that of the latter. ⁵⁸ According to pipe adhesion experiments using tracers such as Co, it has been shown that when the radioactivity concentration of ions in the cooling water and the crud are equal, the contribution of ions to pipe adhesion is more than 15 times that of the crud. It was confirmed. Therefore, it is clear that prevention of ion adhesion is important in order to reduce the dose on the pipe surface. The inventors used the experimental apparatus shown in Figure 2 to
The adhesion of radioactive ions to the inner surface of the pipe was investigated. The structure of this experimental device will be briefly explained. Temperature rising section 11
In this example, a heater 11B is attached to the outside of a container 11A. The temperature raising section 11, the piping test section 12, and the pump 17 are connected by a piping 18, which is a closed loop. Piping 19A connected to tracer tank 13 and metal ion tank 14, respectively
and 19B are attached to the pipe 18. Pumps 15 and 16 are provided in piping 19A and 19B. The tracer tank 13 contains ⁵⁸ Co ions, and the metal ion tank 14 contains Ni ions. The experiment was conducted as follows. pump 1
7 to circulate cooling water (pure water was used in this experiment) within the temperature raising section 11, piping test section 12, and piping 18. The cooling water is heated in the temperature raising section 11 to 285° C., which is the temperature condition for cooling water in a boiling water reactor. The valve is opened and the pump 16 is driven to inject the Ni ions in the metal ion tank 14 into the cooling water in the pipe 18 so that the metal ion concentration in the cooling water flowing in the pipe 18 is about 1 ppb. When the specified concentration is reached, close the valve. This metal ion concentration is detected by an ion concentration meter attached to the pipe 18. Next, open the valve and drive the pump 15 to fill the tracer tank 13.
⁵⁸ Co ions are injected into the pipe 18. As a result, ^{the 58} Co ion concentration in the cooling water flowing through the pipe 18 is set to approximately 1×10 ⁻⁶ μCi/ml. in this way
Even if ⁵⁸ Co ions are added, the metal ion concentration in the cooling water flowing through the pipe 18 is about 1 ppb. While circulating the cooling water, ⁵⁸ Co ions are
It adheres to the inner surface of the piping of the piping test section 12. After a predetermined period of time had elapsed, the radioactivity of ⁵⁸ Co adhering to the piping test section 12 was measured using a NaI (Tl) scintillation detector. Next, a similar experiment was conducted by further injecting Ni ions into the cooling water to increase the metal ion concentration of the cooling water flowing through the pipe 18 to about 15 ppb. The ⁵⁸ Co ion concentration in the cooling water at this time is approximately 1×
10 ^-6 μCi/ml. From the measurement results of ⁵⁸ Co radioactivity in the piping test section 12 for each condition,
The pipe adhesion coefficient of ⁵⁸ Co was determined. The pipe adhesion coefficient δ (cm/h) of ⁵⁸ Co can be calculated using the following formula. dΓ/dt=δ _R … (1) Here, Γ is the amount of ⁵⁸ Co deposited per unit area,
(μCi/cm ² ), t is time (hr), and R is the concentration of ⁵⁸ Co in the circulating cooling water (μCi/ml). The adhesion coefficient of ⁵⁸ Co to piping is the latter (when the metal ion concentration is 15 ppb) and the former (when the metal ion concentration is 15 ppb).
1 ppb). This results in
It was found that increasing the Ni ion concentration in the cooling water can reduce the adhesion of radioactive ions. Similar experiments were conducted by adding Mg, Cr, V, Ti, Cu, Zn, and Cd ions to the cooling water instead of Ni, but as the concentration of these ions increased, the attachment coefficient of radioactive ions decreased. did. These metals are divalent or trivalent metals and are substances that are easily incorporated into the spinel structure of the ferrite layer. The adhesion coefficient of radioactive ions can also be reduced by increasing the concentration of Co, Mn, and Fe ions in the cooling water. However, Co and Mn are activated in the reactor, resulting in ⁶⁰ Co and ⁵⁴ Mn.
It is undesirable to add them to the coolant because they produce radionuclides with long half-lives. Also,
It is undesirable to add substances such as Fe, which become particulate oxides and adhere to the surface of fuel rods, become a source of Co ions, and become a factor in the formation of ⁶⁰ Co, into the cooling water. Mg, Cr, V, Ti, Cu, Ni,
Zn and Cd produce radionuclides as shown in Table 1 through nuclear reactions, but their half-lives are
It is significantly shorter than ⁶⁰ Co. Furthermore, even if these radioactive ions adhere to piping, their half-life is extremely short, so the radioactivity will decrease within a short period of time. Therefore, when applied to a nuclear power plant, the surface dose rate of the piping drops rapidly after the nuclear power plant shuts down, so maintenance and inspection of the piping becomes easy if the method is waited for a predetermined period of time after the nuclear power plant shuts down.

【table】

[Table] A preferred embodiment of the present invention applied to a boiling water nuclear power plant will be described below with reference to FIG. As the cooling water passes through the reactor core 32 within the reactor pressure vessel 21, the fuel rods within the reactor core 32 are cooled. Cooling water, on the other hand, is heated and turns into steam. The generated steam is sent from the reactor pressure vessel 21 to the turbine 22 through the main steam pipe 28 and condensed in the condenser 23. The condensed water in the condenser 23 is purified by a demineralizer 24 and further heated to a predetermined temperature by a feed water heater 25, and then supplied to the reactor as cooling water for cooling the reactor core 32 through a feed water pipe 29. It is returned into the pressure vessel 21. Cooling water within the reactor pressure vessel 21 flows through the recirculation piping 30 and is supplied into the reactor core 32 by driving the recirculation pump 33 . A portion of the cooling water flowing through recirculation piping 30 flows into piping 34 of the furnace purification system. This cooling water is
By driving the pump 36, the heat is cooled through the regenerative heat exchanger 35 and the non-regenerative heat exchanger 27, and then guided to the desalination filter 26 where it is purified. Radioactive clades and ions contained in the cooling water are removed by a desalination filter 26. The purified cooling water is heated in the regenerative heat exchanger 35 and then passed through the water supply pipe 29.
It is returned to the reactor pressure vessel 21 through the process. The tank 31 is filled with Cr ions. The conductivity meter 37 measures the metal ion concentration in the cooling water flowing inside the pipe 34. This measured value of metal ion concentration is sent to the controller 39. When the measured metal ion concentration falls below the set value (10PPb in this example), the valve 38 is opened based on a signal from the controller 39, and a gas is discharged from the tank 31 into the pipe 34.
Cr ions are implanted. reactor pressure vessel 21,
The metal ion concentration in the cooling water present in the pipes 30, 34, etc. is adjusted to 10PPb. Since the metal ion concentration in the cooling water in the reactor pressure vessel and piping of a conventional boiling water nuclear power plant is approximately 1 PPb, the metal ion concentration in the cooling water in this example was increased approximately 10 times. Become. Therefore,
In this example, it is possible to suppress the adhesion of radioactive ions, especially ⁶⁰ Co ions, to the surface of the structural material of the plant that comes into contact with the cooling water. It is now easier to handle radioactive solid waste such as used pipes generated by replacing pipes during repairs, parts replaced during maintenance of pumps and other equipment, and the waste disposal work is also easier. It becomes easier. Furthermore, as described above, maintenance and inspection of piping and the like becomes easier. When metal ions are injected into the cooling water in the piping 34 of the furnace purification system, it is necessary to do so on the downstream side of the desalination filter 26. Since the desalination filter 26 handles crud and ions in the cooling water, if metal ions are injected into the piping 34 upstream of the desalination filter 26, they will be removed by the desalination filter 26, and the effect of metal ion injection will be reduced. significantly reduced. Even if metal ions are injected into the pipe 34 on the downstream side of the desalination filter 26, the metal ions are gradually removed by the desalination filter 26 because the cooling water is circulated. However, the removed metal ions are replenished from the tank 31. On the other hand, the desalination filter 26
There is a concern that the service life of tank 31 will be significantly shortened.
Since the amount of metal ions injected from the filter is small, the life of the desalination filter 26 is not significantly shortened. The present invention can also be applied to the primary system of a pressurized water reactor. According to the present invention, the adhesion of radioactive ions to the surface of the structural material of a nuclear power plant that comes into contact with the coolant can be prevented.
can be significantly suppressed.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the adhesion mechanism of radioactive ions and crud to the surface of structural materials, Figure 2 is a system diagram of the experimental equipment that confirmed the functionality of the present invention, and Figure 3 is applied to a boiling water nuclear power plant. 1 is a system diagram of a preferred embodiment of the present invention. 21... Reactor pressure vessel, 26... Desalination filter, 30, 34... Piping, 31... Tank, 37
...Conductivity meter, 38...Valve.

Claims (1)

[Claims] 1. In a method for suppressing the adhesion of radioactive ions to the structural materials of a nuclear power plant that come into contact with a coolant containing radioactive ions, the amount of metal ions in the coolant is measured during the operation of a nuclear power plant, and the amount of metal ions in the coolant is measured. Mg, Cr, V,
A method for suppressing adhesion of radioactive ions, comprising injecting metal ions of at least one substance selected from the group consisting of Ti, Cu, Ni, Zn, and Cd into the coolant.