JP2023105754A - Method for decontaminating contaminated metal member and method for manufacturing regenerated metal - Google Patents

Abstract

To easily and efficiently decontaminate a radionuclide-contaminated metal at low cost and a method for efficiently manufacturing a regenerated metal by easily decontaminating the contaminated metal.SOLUTION: An aspect of the present invention relates to a method for decontaminating a radionuclide-contaminated metal member, the method including a polishing step of making a first particle for polishing collide against the surface of the metal member, the polishing step being finished in a preset polishing time. Another aspect of the present invention relates to a method for manufacturing a regenerated metal including a step of melting a metal member of which radioactive nuclide has been removed by the decontamination method.SELECTED DRAWING: None

Description

The present invention relates to a method for decontaminating contaminated metal members and a method for producing recycled metal.

Radioactive waste generated at nuclear facilities may have radionuclides attached to its surface. Among such radioactive wastes, metal members can be easily reused by removing radionuclides from their surfaces. To remove radionuclides adhering to the surface of metal members, it is required to apply a decontamination method according to the state of adhesion.

There is known an apparatus that grinds and decontaminates the surface of an object to be treated by colliding abrasive particles with the object at high speed (Japanese Patent Application Laid-Open No. 2000-075095). This decontamination apparatus supplies abrasive particles to a supersonic steam jet formed by water ejected from a water nozzle and steam ejected from a steam nozzle, and blasts the abrasive particles onto the object to be processed at high speed. is collided with

JP-A-2000-075095

The decontamination apparatus described in the above publication uses ceramic particles as the abrasive particles, so that it has a high grinding ability, the abrasive particles can be reused, and efficient decontamination is possible. The decontamination device has a complicated nozzle structure for forming the supersonic steam jet, and furthermore, ceramic particles are used. .

The inventors of the present invention have investigated a low-cost and simple decontamination method. Radionuclides such as Co-60 are usually present as they are incorporated into crystals of oxides such as rust generated on metal surfaces, and the state of adhesion differs depending on the properties of these crystals. For example, crystals adhering to the surface of metal used in members such as pipes and equipment that make up the cooling system of a nuclear reactor are oxide films formed by corrosion of the metal surface. are evenly distributed and strongly adhered to each other. On the other hand, crystals deposited by corrosion products produced elsewhere, for example, carried by cooling water and adhering to the oxide film, are non-uniformly distributed and adhere weakly. That is, radionuclides are classified into adherents that are uniformly distributed and are relatively strongly bound to the metal surface and adherents that are non-uniformly distributed and are relatively weakly bound to the aforesaid metallic surface. can be done. In general, methods for decontaminating such adherents and deposits are classified into chemical methods and mechanical methods, but in decontamination of radioactive waste, it is advantageous to generate less waste by decontamination. It is said that In the mechanical method, the method of colliding particles such as dry blasting can be applied to the removal of both the solid matter and the adhering matter, and is widely used as a pretreatment method in metal processing, so equipment is easily available. can. Therefore, it was concluded that the particle collision method is highly applicable as a low-cost and simple decontamination method for radioactive metal waste.

That is, an object of the present invention is to provide a low-cost, simple, and efficient decontamination method for metal members contaminated with radionuclides, and a method for efficiently producing recycled metal.

One aspect of the present invention for solving the above problems is a method for decontaminating a metal member contaminated with a radionuclide, comprising a polishing step of colliding first particles for polishing against the surface of the metal member, The polishing process is finished at the specified polishing time.

Another aspect of the present invention is a method for producing recycled metal, comprising the step of melting a metal member from which radionuclides have been removed by the decontamination method.

The decontamination method of the present invention can easily and efficiently decontaminate metal members contaminated with radionuclides at low cost. The method for producing recycled metal of the present invention can efficiently produce recycled metal by using the metal member decontaminated by the decontamination method.

FIG. 1 is a flowchart showing a method for decontaminating contaminated metals according to one embodiment of the present invention. FIG. 2 is a graph showing the relationship between polishing time and radioactivity concentration for contaminated metal.

One aspect of the present invention is a method for decontaminating a metal member contaminated with a radionuclide, comprising a polishing step of colliding first particles for polishing against the surface of the metal member, wherein the polishing time is set in advance. The polishing step ends.

The decontamination method polishes a metal member contaminated with a radionuclide by colliding the first particles for polishing with the surface of the metal member. Therefore, it is possible to decontaminate contaminated metals simply and at low cost. Moreover, since the said decontamination method complete|finishes the said grinding|polishing process in predetermined time, it can decontaminate efficiently.

The polishing time is preferably 4 minutes or more. By performing polishing in a short time in this manner, more efficient decontamination can be achieved.

Further comprising a post-polishing measurement step of measuring the density of the radionuclides on the surface of the metal member after the polishing step, wherein if the measured value in the post-polishing measurement step is equal to or greater than a preset post-polishing threshold value, the polishing is performed. It is preferable to repeat the step and the post-polishing measurement step. By doing so, the certainty of decontamination can be improved.

A pre-polishing measurement step of measuring the density of the radionuclide on the surface of the metal member before the polishing step; It is preferable to further include a rough polishing step of colliding second particles for polishing before the polishing step. If the density of the radionuclides on the surface of the metal member before the polishing step is high, there is a possibility that sufficient decontamination cannot be achieved even by polishing for the predetermined time. The surface of the metal member having a high density of radionuclides before the polishing step is subjected to rough polishing as a pretreatment to reduce the density of the radionuclides, and then the polishing step is performed to ensure decontamination. It is possible to further improve the performance.

It is preferable to further include a cleaning step of spraying a cleaning fluid onto the surface of the metal member after the rough polishing step. Dust containing the radionuclide generated by polishing in the rough polishing step may reattach to the surface of the metal member. By performing the cleaning step after the rough polishing step, reattached dust can be removed from the surface of the metal member, and the efficiency of decontamination by the polishing step can be further improved.

The cleaning fluid may be air. By cleaning with air, the cleaning process can be performed efficiently at low cost.

Yet another aspect of the present invention is a method for producing recycled metal, comprising the step of melting the metal member from which the radionuclide has been removed by the decontamination method.

Since the method for manufacturing the recycled metal melts the decontaminated metal member simply and efficiently, the recycled metal can be manufactured efficiently.

[Details of the mode for carrying out the invention]
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail.

<Method for manufacturing recycled metal>
A method for manufacturing recycled metal, which is an embodiment of the present invention, comprises a step of melting a metal member from which radionuclides have been removed. The method of melting the metal member from which the radionuclide has been removed is not particularly limited, and a known method can be adopted. For example, the metal member from which the radionuclide has been removed is put into a heated furnace and melted. method. By simultaneously melting a plurality of metal members, it is possible to obtain homogeneous and large-volume materials, which can be processed into various metal products after cooling.

Preferably, the molten metal is sampled to measure the radioactivity concentration. By doing so, the metal member can be reused more safely.

[Method for decontamination of contaminated metals]
The method for decontaminating metal contaminated with the radionuclides includes a polishing step of colliding first particles for polishing against the surface of the metal member, and the polishing step is completed in a preset polishing time.

As shown in FIG. 1, the method for decontaminating contaminated metals in this embodiment comprises a pre-polishing measurement step of measuring the density of the radionuclides on the surface of the metal member before the polishing step, and the pre-polishing measurement step. a rough polishing step of colliding the second particles for polishing with the surface of the metal member whose measured value is equal to or higher than a preset pre-polishing threshold before the polishing step; and the surface of the metal after the rough polishing step. and a cleaning step of spraying a cleaning fluid on. Further, the method for decontamination of contaminated metals in the present embodiment further includes a post-polishing measurement step of measuring the density of the radionuclides on the surface of the metal member after the polishing step.

Metal members contaminated with radionuclides (contaminated metal members) in the present embodiment are, for example, pipes, equipment, and the like that are removed along with equipment renewal or decommissioning of nuclear facilities. Specifically, it means a metal that is difficult to reuse because it is a pipe, equipment, etc. that constitutes a cooling system of a nuclear reactor, and a radionuclide such as Co-60 adheres to its surface. Here, metals that are difficult to reuse refer to metals attached with radionuclides that exceed the permissible amount specified by the operator when reused within nuclear power plants, and are reused outside of nuclear power plants. In some cases, it is metal with attached radionuclides in amounts exceeding the permissible amount stipulated by law.

(Measurement process before polishing)
In the pre-polishing measurement step, the radionuclide density on the surface of the contaminated metal member before polishing is measured. The method for measuring the radionuclide density is not particularly limited, and examples thereof include a method using a known survey meter or the like.

In the pre-polishing measurement step, a pre-polishing threshold is set. The pre-polishing threshold value is set to an arbitrary value based on past performance, sampling, etc., and can be set to 100 [Bq/g] as the radioactivity concentration, for example. If the radioactivity concentration calculated from the measured value of the radionuclide density on the surface of the contaminated metal member is less than the pre-polishing threshold, the contaminated metal member is subjected to a polishing step, which will be described later. The contaminated metal member having the radioactivity concentration equal to or higher than the pre-polishing threshold is subjected to a rough polishing step, which will be described later. The radioactivity concentration [Bq/g] is obtained by multiplying the measured value of the radionuclide density [Bq/cm ² ] on the surface of the contaminated metal member by the thickness of the metal member and dividing by the density of the metal member. calculate.

In general, radionuclides adhere to the surface of a metal member (metal), are not taken into the interior thereof, and are not activated. Therefore, it is considered sufficient to measure the density of radionuclides on the surface of the metal member for decontamination of the metal member. Therefore, the pre-polishing threshold, the post-polishing threshold described later, and the post-rough polishing threshold described later may be set by the radioactivity concentration [Bq/g], or by the radionuclide density [Bq/cm ² ]. You may In the present embodiment, as an example, the pre-polishing threshold, the post-polishing threshold described later, and the post-rough-polishing threshold described later are set by the radioactivity concentration [Bq/g].

The pre-polishing threshold may be set as a ratio to the post-polishing threshold in the post-polishing measurement step described below. Specifically, the ratio of the pre-polishing threshold to the post-polishing threshold may be set to 10, for example. That is, when the post-polishing threshold value is 1 [Bq/g], the pre-polishing threshold value may be 10 [Bq/g], which is 10 times that value.

(rough polishing process)
In the rough polishing step, the surface of the metal member whose measured value in the pre-polishing measuring step is equal to or greater than the pre-polishing threshold value is collided with the second particles for polishing. The collision method is not particularly limited, but in the present embodiment, a method of dry blasting (also called shot blasting) using second particles on the surface of the contaminated metal member will be described. The rough polishing step is a pretreatment for the polishing step. Dry blasting is performed on the surface of the contaminated metal member having the pre-polishing threshold value or more to remove the radionuclide adhering to the surface of the metal member and reduce the radioactivity concentration of the contaminated metal member. For example, when the pre-polishing threshold value is set to 100 [Bq/g], the rough polishing process is performed on the contaminated metal member exceeding 100 [Bq/g].

The second particles as the projection material (media) used in the rough polishing step preferably have a larger particle size than the first particles as the projection material used in the later-described polishing step. By doing so, the radioactivity concentration of the contaminated metal member can be relatively easily reduced to a range less than the pre-polishing threshold. The second particles may have substantially the same particle size as the first particles and have a higher hardness than the first particles, or may be the same as the first particles.

The rough polishing step is not performed to reduce the radioactivity concentration of the contaminated metal to a reusable range (e.g., 10 [Bq/g] or less), but the contaminated metal member is subjected to a polishing step. is performed to reduce the radioactivity concentration (for example, 100 [Bq/g] or less).

The rough polishing step may be terminated after the rough polishing time has been set in advance, or the post-rough polishing measurement step of measuring the density of radionuclides on the surface of the metal member after rough polishing. may be provided, and the process may be terminated when this measured value becomes equal to or less than a preset post-rough polishing threshold value (for example, 100 [Bq/g]).

(Cleaning process)
In the cleaning step, a cleaning fluid is sprayed onto the surface of the metal member after the rough polishing step. In the rough polishing step, dust containing the radionuclide is generated by polishing, and this dust may reattach to the surface of the metal member. By performing the cleaning step, the reattached dust can be removed from the surface of the metal member. By removing the dust, it is possible to improve the certainty of reducing the radioactivity concentration by the rough polishing step.

As the cleaning fluid, a liquid such as water may be used, but it is preferable to use air. As a cleaning process using air, if the dry blasting uses a compressor, after the completion of the rough polishing process, the injection of the projection material (second particles) in the dry blasting is stopped, and the air of the compressor is removed. is preferably sprayed onto the surface of the metal member. If the dry blasting does not use a compressor, an air supply device such as a compressor may be separately prepared, and air may be blown onto the surface of the metal member by this air supply device.

(polishing process)
In the polishing step, first particles for polishing are caused to collide with the surface of the contaminated metal member subjected to the rough polishing step and the surface of the contaminated metal whose measured value in the pre-polishing measurement step is less than the pre-polishing threshold value. . As a method of colliding, for example, dry blasting is performed on the surface of the contaminated metal member. Dry blasting the surface of the contaminated metal member removes the radionuclide adhering to the surface and reduces the radioactivity concentration of the contaminated metal member to a reusable range.

The first particles are not particularly limited, and for example, iron-based, aluminum-based, glass-based, dry ice, etc. can be used, and among them, metal particles are preferable. The particle size of the first particles is not particularly limited, and, for example, particles having a size of 0.3 mm or more and 2.0 mm or less can be used.

The polishing process ends when a preset polishing time elapses. The polishing time can be arbitrarily set based on past performance.

The polishing time is preferably 4 minutes or longer. The lower limit of the polishing time is preferably 5 minutes, more preferably 6 minutes. The upper limit of the polishing time is not particularly limited, but is, for example, 12 minutes, preferably 10 minutes, and more preferably 8 minutes. If the polishing time is less than the lower limit, there is a possibility that sufficient decontamination cannot be achieved. If the polishing time exceeds the upper limit, the efficiency of decontamination may decrease.

The polishing step and the rough polishing step are performed on the contaminated metal member in a room such as a blast chamber. A particle recovery nozzle or the like may be provided in this chamber to recover the first particles and the second particles and reuse them in the polishing step and the rough polishing step. When polishing the inner surface of a reactor cooling heat transfer tube or the like, the first particles or the second particles are caused to collide from one end of the pipe, and the first particles or the second particles are applied from the other end of the pipe. The second particles may be recovered.

(Measurement process after polishing)
In the post-polishing measurement step, the density of radionuclides on the surface of the metal member after the polishing step is measured. The method for measuring the radionuclide density is not particularly limited, and examples thereof include a method using a known survey meter or the like.

In the post-polishing measurement step, a post-polishing threshold is set. The post-polishing threshold is set to a value at which the metal can be melted, and can be set to 10 [Bq/g], for example. If the radioactivity concentration calculated from the radionuclide density measurements on the surface of the metal member is less than the post-polishing threshold, the metal member is subjected to the melting step. When the radioactivity concentration is equal to or higher than the post-polishing threshold, the metal member is subjected to the polishing step again. After the metal member undergoes the polishing step again, the post-polishing measurement step is repeated until the radioactivity concentration of the metal member becomes less than the post-polishing threshold value. By doing so, the certainty of decontamination of the metal member can be improved.

The post-polishing threshold may be set as a ratio to the pre-polishing threshold in the pre-polishing measurement step. Specifically, the ratio of the post-polishing threshold to the pre-polishing threshold may be set as 1/10, for example. That is, when the pre-polishing threshold value is 10 [Bq/g], the post-polishing threshold value may be 1/10 of 1 [Bq/g].

<Advantages>
In the method for decontaminating contaminated metals, since the surface of the contaminated metal member is polished by dry blasting, decontamination can be easily performed at low cost. Contaminated metal members can be decontaminated. For example, when the target value (the post-polishing threshold value) is 0.1 [Bq/g], the target is achieved if the decontamination result is 0.1 [Bq/g] or less. Therefore, regardless of the level of the initial radioactivity concentration of the contaminated metal member, if it is positioned as a process to be systematically performed, it can be utilized as a planned decontamination method. Moreover, since the method for manufacturing the recycled metal uses the metal member decontaminated by the decontamination method, the recycled metal can be efficiently manufactured.

[Other embodiments]
The above-described embodiments do not limit the configuration of the present invention. Therefore, in the embodiment, the components of each part of the embodiment can be omitted, replaced, or added based on the description of the present specification and common general technical knowledge, and all of them are interpreted as belonging to the scope of the present invention. should.

The rough polishing step and the cleaning step are not essential components. That is, all contaminated metal members may be decontaminated only by the polishing process. In this case, the contaminated metal member whose measured value in the post-polishing measurement step is equal to or greater than the post-polishing threshold value is preferably subjected to the polishing step again.

Also, the post-polishing measurement step is not an essential configuration. That is, all of the contaminated metal members subjected to the polishing step may be subjected to melting without measuring the radionuclide density. In this case, it is preferable to perform the rough polishing step for those whose measured values in the pre-polishing measurement are equal to or greater than the pre-polishing threshold value.

[Example]
The effect of the decontamination method of the present invention was verified using the heat transfer tubes of the heat exchanger that were in contact with the cooling water of the nuclear reactor for a long period of time. Since there are a plurality of U-shaped heat transfer tubes with the same contamination state, it is possible to perform comparative evaluation under different decontamination conditions. Seven heat transfer tubes were decontaminated at different polishing times, and the results of the radioactivity concentration remaining in each U-shaped portion are shown in FIG.

The radioactivity concentration before decontamination of the U-shaped portions of the seven heat transfer tubes was approximately 100 [Bq/g]. The polishing time in the polishing process for each of the seven heat transfer tubes was 0, 7, 9, 12, 14, 16, and 18 minutes. The concentration of radioactivity remaining in the U-shaped portion of the six heat transfer tubes subjected to the polishing process converged to 10 [Bq/g]. It can be seen from FIG. 1 that the radioactivity concentration does not decrease even if the polishing process is performed for a certain period of time or more. The reason for this is thought to be that although the crystals adhering to the metal surface and the radionuclides in these crystals are removed by scraping off the convexities on the surface of the object by dry blasting, the radionuclides remain in the concave portions. be done. The metal surface after the polishing step is shiny and does not appear to have any crystals remaining, but the radionuclide has a significant amount of radioactivity even if the mass is very small. is 2.39×10 ⁻¹⁴ g and has a radioactivity of 1 Bq. For this reason, a certain level of radioactivity concentration remains due to the influence of the crystals remaining in the concave portions where the first particles (projection material) cannot reach in the polishing step, and the radioactivity concentration remains even if the polishing time is extended. cannot be reduced. From the above, the polishing time for the heat transfer tubes can be set at, for example, 5 minutes.

The decontamination method of the present invention can easily and efficiently remove radionuclides on metal surfaces at low cost, so it is suitable for reusing metal members that are removed due to equipment renewal or decommissioning of nuclear facilities. Available.

Claims (7)

A method for decontaminating a metal member contaminated with a radionuclide,
A polishing step of colliding the first particles for polishing against the surface of the metal member,
A method for decontaminating a contaminated metal member, wherein the polishing process is completed in a preset polishing time. 2. The method of decontaminating a contaminated metal member according to claim 1, wherein said polishing time is 4 minutes or longer. Further comprising a post-polishing measurement step of measuring the density of the radionuclides on the surface of the metal member after the polishing step,
3. The method of decontaminating a contaminated metal member according to claim 1, wherein the polishing step and the post-polishing measurement step are repeated when the measured value in the post-polishing measurement step is equal to or greater than a preset post-polishing threshold value. . A pre-polishing measurement step of measuring the density of the radionuclides on the surface of the metal member before the polishing step;
2. A rough polishing step of colliding second particles for polishing before the polishing step with the surface of the metal member whose measured value in the pre-polishing measuring step is equal to or greater than a preset pre-polishing threshold value. A method for decontaminating a contaminated metal member according to claim 2 or 3. 5. The method of decontaminating a contaminated metal member according to claim 4, further comprising a cleaning step of spraying a cleaning fluid onto the surface of said metal member after said rough polishing step. 6. The method of decontaminating a contaminated metal member according to claim 5, wherein said cleaning fluid is air. A method for producing recycled metal, comprising the step of melting the metal member from which the radionuclide has been removed by the decontamination method according to any one of claims 1 to 6.

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