Recycled metal manufacturing method

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

[Technical Field]

[0001] The present invention relates to a method for decontaminating contaminated metal components and a method for producing recycled metals. [Background technology]

[0002] Radioactive waste generated at nuclear facilities may have radionuclides attached to its surface. For metal components of such radioactive waste, removing the radionuclides from the surface may make them easier to reuse. To remove radionuclides attached to the surface of metal components, it is necessary to apply a decontamination method appropriate to the degree of attachment.

[0003] A device is known that grinds and decontaminates the surface of a workpiece by colliding abrasive particles with the workpiece at high speed (Japanese Patent Laid-Open Publication No. 2000-075095). This decontamination device supplies abrasive particles to a supersonic steam jet formed by water sprayed from a water nozzle and steam sprayed from a steam nozzle, and causes the abrasive particles to collide with the workpiece at high speed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-075095 Summary of the Invention [Problem to be solved by the invention]

[0005] The decontamination device described in the publication is said to have high grinding ability and be able to reuse the abrasive particles by using ceramic particles as abrasive particles, thereby enabling efficient decontamination. However, the decontamination device has a complicated nozzle structure for forming the supersonic steam jet, and furthermore, it uses ceramic particles, so there is a risk that it will not be able to perform decontamination easily and at low cost.

[0006] The inventors of the present invention have investigated a low-cost, easy-to-use decontamination method. Radioactive nuclides, such as Co-60, typically exist trapped in oxide crystals, such as rust, that form on metal surfaces. The adhesion state varies depending on the nature of the crystals. For example, crystals adhering to metal surfaces used in components 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, and are evenly distributed and firmly adhered to the metal surface. On the other hand, crystals deposited by corrosion products, for example, carried by cooling water and adhering to the oxide film, are unevenly distributed and weakly adhered. In other words, radioactive nuclides can be classified into deposits that are evenly distributed and relatively firmly bonded to the metal surface, and deposits that are unevenly distributed and relatively weakly bonded to the deposits. Generally, methods for decontaminating such deposits are classified into chemical and mechanical methods. However, in the decontamination of radioactive waste, it is considered advantageous to generate less waste during decontamination. Among the mechanical methods, particle collision methods such as dry blasting can be applied to remove both the adhered and attached materials, and the equipment is readily available as it is widely used as a pretreatment method in metal processing. Therefore, we concluded that particle collision methods are highly applicable as a low-cost, simple method for decontaminating radioactive metal waste.

[0007] That is, an object of the present invention is to provide a method for decontaminating metal members contaminated with radionuclides easily and efficiently at low cost, and a method for efficiently producing recycled metals. [Means for solving the problem]

[0008] One aspect of the present invention that solves the above problem is a method for decontaminating metal components contaminated with radioactive nuclides, which includes a polishing step in which first polishing particles are collided with the surface of the metal component, and the polishing step is terminated after a preset polishing time.

[0009] Another aspect of the present invention is a method for producing recycled metals, comprising the step of melting metal members from which radioactive nuclides have been removed by the decontamination method. [Effects of the Invention]

[0010] The decontamination method of the present invention can decontaminate metal components contaminated with radioactive nuclides easily and efficiently at low cost. The method for producing recycled metal of the present invention can efficiently produce recycled metal by using metal components decontaminated by the decontamination method. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a flow diagram showing a method for decontaminating contaminant metals according to one embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing the relationship between polishing time and radioactivity concentration in contaminated metals. DETAILED DESCRIPTION OF THE INVENTION

[0012] One aspect of the present invention is a method for decontaminating a metal component contaminated with a radioactive nuclide, comprising a polishing step of colliding first particles with the surface of the metal component, and completing the polishing step after a preset polishing time.

[0013] This decontamination method polishes a metal member contaminated with radionuclides by colliding first particles with the surface of the metal member. This allows for simple and low-cost decontamination of contaminated metals. Furthermore, this decontamination method completes the polishing process within a predetermined time, allowing for efficient decontamination.

[0014] The polishing time is preferably 4 minutes or more. By polishing in such a short time, more efficient decontamination can be achieved.

[0015] The method may further include a post-polishing measurement step of measuring the density of the radioactive nuclides on the surface of the metal member after the polishing step, and if the measured value in the post-polishing measurement step is equal to or greater than a predetermined post-polishing threshold value, the polishing step and the post-polishing measurement step may be repeated. In this way, the reliability of decontamination can be improved.

[0016] The method may further include a pre-polishing measurement step of measuring the density of the radioactive nuclides on the surface of the metal component before the polishing step, and a rough polishing step of colliding second polishing particles with the surface of the metal component where the measured value in the pre-polishing measurement step is equal to or greater than a predetermined pre-polishing threshold value before the polishing step. If the density of the radioactive nuclides on the surface of the metal component before the polishing step is high, there is a risk that sufficient decontamination will not be achieved even by polishing for the specified time. By performing rough polishing as a pretreatment on the surface of the metal component where the density of radioactive nuclides is high before the polishing step, the density of the radioactive nuclides can be reduced, and then performing the polishing step, the reliability of decontamination can be further improved.

[0017] 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 radioactive nuclides generated by polishing in the rough polishing step may re-adhere to the surface of the metal member. By performing the cleaning step after the rough polishing step, the re-adhered dust can be removed from the surface of the metal member, and the efficiency of decontamination by the polishing step can be further improved.

[0018] Preferably, the cleaning fluid is air. By using air for cleaning, the cleaning process can be carried out efficiently at low cost.

[0019] Yet another aspect of the present invention is a method for producing recycled metals, comprising the step of melting metal members from which the radioactive nuclides have been removed by the decontamination method.

[0020] The method for producing recycled metals allows for the efficient production of recycled metals by melting decontaminated metal members simply and efficiently.

[0021] [Details of the Mode for Carrying Out the Invention] Hereinafter, embodiments of the present invention will be described in detail.

[0022] <Method of manufacturing recycled metals> A method for producing recycled metals according to one embodiment of the present invention includes a step of melting metal members from which radionuclides have been removed. The method for melting metal members from which radionuclides have been removed is not particularly limited, and known methods can be used, such as a method in which metal members from which radionuclides have been removed are placed in a heated furnace for melting. By melting multiple metal members simultaneously, a large amount of homogeneous material can be produced, which can be processed into various metal products after cooling.

[0023] It is preferable to sample the molten metal and measure the radioactivity concentration, which allows the metal components to be reused more safely.

[0024] [Method for decontaminating contaminated metals] The method for decontaminating metal contaminated with radioactive nuclides includes a polishing step of colliding first particles with the surface of the metal member, and the polishing step is completed after a preset polishing time.

[0025] 1, the decontamination method for contaminated metals in this embodiment further comprises a pre-polishing measurement step of measuring the density of the radioactive nuclides on the surface of the metal member before the polishing step, a rough polishing step of colliding second polishing particles with the surface of the metal member where the measured value in the pre-polishing measurement step is equal to or greater than a predetermined pre-polishing threshold value before the polishing step, and a cleaning step of spraying a cleaning fluid onto the metal surface after the rough polishing step.The decontamination method for contaminated metals in this embodiment further comprises a post-polishing measurement step of measuring the density of the radioactive nuclides on the surface of the metal member after the polishing step.

[0026] In this embodiment, a metal member contaminated with radionuclides (contaminated metal member) refers to, for example, piping, equipment, etc. that are removed due to equipment renewal or decommissioning of a nuclear facility. Specifically, this refers to metals that constitute the cooling system of a nuclear reactor, and that have radionuclides such as Co-60 attached to their surfaces, making them difficult to reuse. Here, metals that are difficult to reuse refer to metals that have radionuclides attached in an amount that exceeds the allowable amount specified by the operator when reused within a nuclear power plant, and metals that have radionuclides attached in an amount that exceeds the allowable amount specified by laws and regulations when reused outside a nuclear power plant.

[0027] (Pre-polishing measurement process) In the pre-polishing measurement step, the density of radioactive nuclides on the surface of the contaminated metal component before polishing is measured. The method for measuring the density of radioactive nuclides is not particularly limited, and examples thereof include a method using a known survey meter or the like.

[0028] In the pre-polishing measurement step, a pre-polishing threshold is set. The pre-polishing threshold is set to an arbitrary value based on past performance, sample collection, etc., and can be set to, for example, 100 [Bq / g] as a radioactivity concentration. If the radioactivity concentration calculated from the measured value of the density of radioactive nuclides on the surface of the contaminated metal component is less than the pre-polishing threshold, the contaminated metal component is subjected to a polishing step described later. If the radioactivity concentration is equal to or greater than the pre-polishing threshold, the contaminated metal component is subjected to a rough polishing step described later. The radioactivity concentration [Bq / g] is calculated based on the measured value of the density of radioactive nuclides on the surface of the contaminated metal component [Bq / cm 2 ] is multiplied by the plate thickness of the metal member and divided by the density of the metal member.

[0029] Generally, radionuclides adhere to the surface of a metal member (metal) and are neither incorporated into the metal nor activated. Therefore, it is considered sufficient to measure the density of radionuclides on the surface of the metal member in order to decontaminate the metal member. Therefore, the pre-polishing threshold, the post-polishing threshold (to be described later), and the post-rough polishing threshold (to be described later) may be set in terms of radioactivity concentration [Bq / g], and the radionuclide density [Bq / cm2 In this embodiment, as an example, the pre-polishing threshold value, the post-polishing threshold value (described later), and the post-rough polishing threshold value (described later) are set in terms of radioactivity concentration [Bq / g].

[0030] The pre-polishing threshold may be set as a ratio to a post-polishing threshold in a post-polishing measurement step described later. Specifically, the ratio of the pre-polishing threshold to the post-polishing threshold may be set to, for example, 10. In other words, if the post-polishing threshold is set to 1 [Bq / g], the pre-polishing threshold may be set to 10 times that, that is, 10 [Bq / g].

[0031] (rough polishing process) In the rough polishing process, second polishing particles are collided with the surface of the metal component whose measurement value in the pre-polishing measurement process is equal to or greater than the pre-polishing threshold. The method of collision is not particularly limited, but in this embodiment, a method of dry blasting (also called shot blasting) the surface of the contaminated metal component using second particles is described. The rough polishing process is a pretreatment for the polishing process. By dry blasting the surface of the contaminated metal component whose measurement value is equal to or greater than the pre-polishing threshold, radionuclides attached to the surface of the metal component are removed, thereby reducing the radioactivity concentration of the contaminated metal component. For example, if the pre-polishing threshold is set to 100 [Bq / g], the rough polishing process is performed on a contaminated metal component whose measurement value exceeds 100 [Bq / g].

[0032] The second particles used as the shot material (media) in the rough polishing step preferably have a larger particle size than the first particles used as the shot material in the polishing step described below. By doing so, the radioactivity concentration of the contaminated metal component can be relatively easily reduced to a range below the pre-polishing threshold. The second particles may have substantially the same particle size as the first particles and a higher hardness than the first particles, or may be the same as the first particles.

[0033] The rough polishing process is not performed to reduce the radioactivity concentration of the contaminated metal to a range where it can be reused (e.g., 10 [Bq / g] or less), but to reduce the radioactivity concentration to a level where the polishing process can be performed on the contaminated metal component (e.g., 100 [Bq / g] or less).

[0034] The rough polishing process may be terminated after a predetermined rough polishing time has elapsed, or a post-rough polishing measurement process may be provided in which the density of radioactive nuclides on the surface of the metal component is measured after rough polishing, and the process may be terminated when the measured value falls below a predetermined post-rough polishing threshold value (e.g., 100 [Bq / g]).

[0035] (Cleaning process) In the cleaning process, a cleaning fluid is sprayed onto the surface of the metal member after the rough polishing process. In the rough polishing process, dust containing the radioactive nuclides is generated by polishing, and this dust may re-adhere to the surface of the metal member. By performing the cleaning process, the re-adhered dust can be removed from the surface of the metal member. By removing the dust, it is possible to improve the reliability of reducing the radioactivity concentration by the rough polishing process.

[0036] The cleaning fluid may be a liquid such as water, but air is preferably used. In the cleaning process using air, if the dry blasting uses a compressor, the introduction of blasting material (second particles) in the dry blasting may be stopped after the rough polishing process is completed, and air from the compressor may be blown onto the surface of the metal component. If the dry blasting does not use a compressor, a separate air supply device such as a compressor may be prepared, and air may be blown onto the surface of the metal component using this air supply device.

[0037] (polishing process) In the polishing step, first polishing particles are collided with the surface of the contaminated metal component that has been subjected to the rough polishing step and the surface of the contaminated metal component whose measurement value in the pre-polishing measurement step is less than the pre-polishing threshold. As a method of collision, for example, dry blasting is performed on the surface of the contaminated metal component. By dry blasting the surface of the contaminated metal component, radionuclides adhering to the surface are removed, and the radioactivity concentration of the contaminated metal component is reduced to a range that allows reuse.

[0038] The primary particles are not particularly limited, and may be, for example, iron-based, aluminum-based, glass-based, dry ice-based, etc., with metal particles being preferred. The particle size of the primary particles is not particularly limited, and may be, for example, particles of 0.3 mm or more and 2.0 mm or less.

[0039] The polishing process is completed when a preset polishing time has elapsed, which can be set arbitrarily based on past performance, etc.

[0040] The polishing time is preferably 4 minutes or more. 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, more preferably 8 minutes. If the polishing time is less than the lower limit, sufficient decontamination may not be achieved. If the polishing time exceeds the upper limit, the efficiency of decontamination may decrease.

[0041] The polishing step and the rough polishing step are performed on the contaminated metal member in a chamber such as a blast chamber. A particle recovery nozzle or the like is provided in this chamber, and the first particles and the second particles are recovered and reused in the polishing step and the rough polishing step. When polishing the inner surface of a heat transfer tube for cooling a nuclear reactor, the first particles or the second particles are collided from one end of the tube, and the first particles or the second particles are recovered from the other end of the tube.

[0042] (Measurement process after polishing) In the post-polishing measurement step, the density of radioactive nuclides on the surface of the metal component after the polishing step is measured. The method for measuring the density of radioactive nuclides is not particularly limited, and examples thereof include methods using a known survey meter or the like.

[0043] In the post-polishing measurement process, a post-polishing threshold is set. The post-polishing threshold is set to a value that can melt the metal, and can be, for example, 10 [Bq / g]. If the radioactivity concentration calculated from the measured value of the density of radioactive nuclides on the surface of the metal component is less than the post-polishing threshold, the metal component is subjected to the melting process. If the radioactivity concentration is equal to or greater than the post-polishing threshold, the metal component is subjected to the polishing process again. After the metal component is subjected to the polishing process again, the post-polishing measurement process is again performed, and this is repeated until the radioactivity concentration of the metal component becomes less than the post-polishing threshold. This can improve the reliability of decontamination of the metal component.

[0044] 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 to, for example, 1 / 10. In other words, if the pre-polishing threshold is 10 [Bq / g], the post-polishing threshold may be set to 1 [Bq / g], which is 1 / 10 of that.

[0045] <Advantages> This decontamination method for contaminated metals involves polishing the surface of contaminated metal components using dry blasting, allowing for low-cost, simple decontamination. By completing the polishing process within a predetermined time, the contaminated metal components can be efficiently decontaminated. For example, if the target value (the post-polishing threshold) is 0.1 Bq / g, the target is achieved if the decontamination result is 0.1 Bq / g or less. Therefore, regardless of the initial radioactivity concentration of the contaminated metal components, this method can be used as a systematic decontamination method. Furthermore, this method for producing recycled metals uses metal components decontaminated by this decontamination method, allowing for efficient production of recycled metals.

[0046] [Other embodiments] The above-described embodiments do not limit the configuration of the present invention. Therefore, the above-described embodiments may include omissions, substitutions, or additions of components based on the description in this specification and common general technical knowledge, and all of these should be construed as falling within the scope of the present invention.

[0047] The rough polishing step and the cleaning step are not essential components. That is, all contaminated metal components may be decontaminated only by the polishing step. In this case, contaminated metal components whose measured values ​​in the post-polishing measurement step are equal to or greater than the post-polishing threshold value may be subjected to the polishing step again.

[0048] Furthermore, the post-polishing measurement step is not an essential step. That is, all of the contaminated metal components that have been subjected to the polishing step may be subjected to melting without measuring the density of radionuclides. In this case, it is advisable to perform the rough polishing step for those components whose measured value in the pre-polishing measurement is equal to or greater than the pre-polishing threshold value.

[0049] [Example] The effectiveness of the decontamination method of the present invention was verified using heat transfer tubes from a heat exchanger that had been in contact with reactor cooling water for a long period of time. Because there were multiple U-shaped tubes with the same contamination status, comparative evaluations under different decontamination conditions were possible. Seven of the heat transfer tubes were decontaminated using different polishing times, and the results of the radioactivity concentration remaining in each U-shaped section are shown in Figure 2.

[0050] The radioactivity concentration in the U-shaped portions of the seven heat transfer tubes before decontamination was approximately 100 Bq / g. The polishing time for each of the seven heat transfer tubes was 0, 7, 9, 12, 14, 16, and 18 minutes. The radioactivity concentration remaining in the U-shaped portions of the six heat transfer tubes that underwent the polishing process converged to 10 Bq / g. Figure 2 shows that the radioactivity concentration did not decrease even after the polishing process was performed for a certain period of time. This is thought to be because the dry blasting process removes the convex portions of the surface irregularities, removing the crystals and radionuclides adhering to the metal surface, but leaving the radionuclides in the concave portions. After the polishing process, the metal surface was shiny and did not appear to have any residual crystals. However, even a small amount of radionuclides can have significant radioactivity. For example, in the case of Co-60, the mass is 2.39 × 10 -14 g has a radioactivity of 1 Bq. Therefore, it is thought that a certain level of radioactivity concentration remains due to the influence of crystals remaining in recesses that cannot be reached by the first particles (projection material) in the polishing process, and that the radioactivity concentration cannot be reduced even if the polishing time is extended. For these reasons, the polishing time for the heat transfer tube can be set to, for example, 5 minutes. [Industrial Applicability]

[0051] The decontamination method of the present invention can remove radioactive nuclides from metal surfaces easily and efficiently at low cost, and therefore can be suitably used for reusing metal members removed due to equipment renewal or decommissioning of nuclear facilities.

Claims

1. A method for decontaminating metal components contaminated with radionuclides to produce reusable recycled metal, comprising: a polishing step of colliding first particles for polishing on the surface of the metal member; melting the metal member from which the radioactive nuclides have been removed without measuring the density of the radioactive nuclides; Equipped with A method for producing recycled metals, in which the polishing step is completed within a polishing time that is preset to be between 4 and 12 minutes.

2. a rough polishing step of causing second particles for polishing to collide with the surface of the metal member before the polishing step; The method for producing recycled metals according to claim 1, further comprising a cleaning step of spraying a cleaning fluid onto the surface of the metal member after the rough polishing step.

3. 3. The method for producing recycled metals according to claim 2, wherein the cleaning fluid is air.

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