Method for producing solidified bodies

Abstract

To provide a technology for increasing a content of radioactive aluminum in solidifying an alkali active material containing radioactive aluminum.SOLUTION: A production method of a solidified body includes a dissolving step (S1) of dissolving an aluminum alloy into a hydroxide solution of an alkali metal to produce an aluminum dissolution solution, a mixing step (S3) of mixing the aluminum dissolution solution, an active filler, and an alkali silica solution as raw materials to produce an alkali active material, and a solidifying step (S4) of filling a mold with the alkali active material and curing to produce a solidified body.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a solidified body containing aluminum as waste material. [Background technology]

[0002] The technical standards for the burial of radioactive miscellaneous solid waste stipulate that the drums must not contain any substances that may impair the waste's integrity and that the maximum radioactivity concentration must not be exceeded. Normal radioactive miscellaneous solid waste (e.g., ferrous metals, concrete, glass, etc.) can be solidified to meet the standards by storing the waste in drums and filling them with concrete or other fillers.

[0003] However, aluminum that has been activated or contaminated by radioactive materials (hereinafter referred to as "radioactive aluminum") is classified as difficult waste because, due to its chemical properties, when it reacts with alkaline materials such as concrete it generates hydrogen gas, which damages the solidified body. For this reason, radioactive aluminum is not solidified and is currently stored in controlled areas of nuclear facilities, etc.

[0004] In recent years, as described in Patent Documents 1 and 2, alkali-activated materials (so-called geopolymers) have been attracting attention as a substitute for concrete. Although geopolymers were developed primarily as building materials, they are also considered suitable as materials for solidifying radioactive waste due to their flexibility in composition, heat resistance, chemical stability, radiation resistance, and the like. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-239446 A [Patent Document 2] JP 2020-19681 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 2 only describes that radioactive materials can be solidified with geopolymers, and it is difficult to say that it discloses a specific method for solidifying radioactive aluminum with geopolymers. In particular, in order to reduce the total number of solidified bodies, it is important to increase the amount of radioactive aluminum contained in each solidified body. Note that this issue applies not only to radioactive aluminum, but also to aluminum as general waste.

[0007] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a technique for increasing the aluminum content as waste when solidifying an alkali activated material containing aluminum. [Means for solving the problem]

[0008] A method for producing a solidified body according to one embodiment of the present invention includes a dissolving step of producing an aluminum solution by dissolving an aluminum alloy in an alkali metal hydroxide solution, a kneading step of producing an alkali activated material by kneading the aluminum solution, an active filler, and an alkali silica solution as raw materials, and a solidification step of producing a solidified body by filling the alkali activated material into a mold and curing it. The molar ratio of silicon to aluminum in the solidified body is 1.8 or more, and the aluminum content in the solidified body is less than 8.0 [wt%]. It is characterized by: Effect of the Invention

[0009] According to the present invention, when an alkali activated material containing aluminum is solidified, the content of aluminum as waste can be increased. [Brief description of the drawings]

[0010] [Figure 1] 1A to 1C are diagrams showing steps of a method for producing a radioactive solidified body. [Diagram 2] FIG. 1 is a graph showing the relationship between the content ratio of silicon to aluminum and the average compressive strength of radioactive solidified bodies. [Diagram 3]FIG. 13 is a graph showing the relationship between the amount of aluminum filled in a radioactive solidified body and the average compressive strength of the radioactive solidified body. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the method for producing radioactive solidified bodies according to the present embodiment will be described with reference to the drawings. FIG. 1 is a diagram showing the steps of the method for producing radioactive solidified bodies. Note that the embodiment of the present invention described below shows an example of how the present invention is embodied, and does not limit the scope of the present invention to the scope of the embodiment. Therefore, the present invention can be practiced by making various modifications to the embodiment.

[0012] The method for producing a radioactive solidified body according to this embodiment is a method for producing a solidified body of an alkaline activated material (so-called geopolymer) using a radioactive aluminum alloy as a raw material. However, the present invention is not limited to a "method for producing a radioactive solidified body" using a radioactive aluminum alloy as a raw material, but can also be used as a "method for producing a solidified body" using aluminum alloy as general waste as a raw material.

[0013] The aluminum alloy of this embodiment is one in which additives (e.g., Fe, Mn, Cu, Mg, Si, etc.) are added to aluminum. Typically, the aluminum alloy is a 1000 series, 5000 series, or 6000 series aluminum alloy, but is not limited thereto. The content ratio of the additives in the aluminum alloy is about 0.1 to 5%, more typically about 0.5 to 2%.

[0014] Aluminum alloys are used, for example, as materials for containment vessels of research and test reactors, and are also used in commercial power reactors. The aluminum alloys that are the subject of this embodiment are those that are discarded as waste from these nuclear reactors. Radioactive aluminum is not limited to aluminum alloys that have been activated and are themselves radioactive, but also includes aluminum alloys to which radioactive materials are attached.

[0015] First, as shown in FIG. 1, a radioactive aluminum alloy is dissolved in a sodium hydroxide solution to generate an aluminum solution (sodium aluminate) (S1). In the aluminum solution, the aluminum of the aluminum alloy is dissolved and ionized, while the additives remain as solids without dissolving. This process is an example of a dissolving process. In the dissolving process, a hydroxide solution of an alkali metal other than sodium (typically potassium) can be used instead of the sodium hydroxide solution.

[0016] Next, the substances that are additives to the aluminum alloy and do not dissolve in the sodium hydroxide solution (hereinafter referred to as "impurities") are separated from the aluminum solution (S2). Any known method can be used to separate the impurities, and for example, suction filtration, filter press, etc. can be used. This step is an example of a separation step.

[0017] Next, the aluminum solution from which the impurities have been separated, an active filler, and an alkali silica solution are kneaded to produce an alkali active material (S3). The active filler is a substance that contains amorphous silica and does not contain aluminum, and for example, silica fume (silicon dioxide) can be used. The alkali silica solution is, for example, sodium silicate or potassium silicate, and contains the same alkali metal as the alkali metal in S1. This process is an example of a kneading process.

[0018] More specifically, first, silica fume is stimulated by alkali and becomes monomeric, as shown in Chemical Formula 1. Next, the monomeric silica fume and aluminum dissolving solution undergo dehydration condensation polymerization to generate an alkali-activated material, as shown in Chemical Formula 2. In addition, an alkali metal (K in Chemical Formula 2) is used to compensate for the tetravalent Si by the trivalent Al.

[0019] [ka]

[0020] [ka]

[0021] Here, since the alkaline active material requires aluminum as a raw material, it is common to use an active filler containing aluminum as the raw material. However, in this embodiment, an aluminum solution in which a radioactive aluminum alloy is dissolved, an active filler not containing aluminum, and an alkaline silica solution are used as raw materials. That is, the aluminum used as the raw material of the alkaline active material according to this embodiment is only the radioactive aluminum alloy.

[0022] Next, the alkaline active material is filled into the mold and cured to produce a solidified body containing radioactive aluminum (hereinafter referred to as "radioactive solidified body") (S4). More specifically, "curing" refers to a process in which the mold filled with the alkaline active material is sealed and heated at 20 to 80°C (e.g., 60°C) for a period of about half a day to about a week (e.g., four days) to promote a chemical reaction (dehydration condensation polymerization) that solidifies the radioactive aluminum. This process is an example of a solidification process.

[0023] Next, an experiment for confirming the effect of the method for producing radioactive solidified bodies according to this embodiment will be described with reference to Figures 2 and 3. Figure 2 is a diagram showing the relationship between the content ratio of silicon to aluminum (hereinafter referred to as "silicon / aluminum ratio") and the average compressive strength of radioactive solidified bodies. Figure 3 is a diagram showing the relationship between the amount of aluminum filled into the radioactive solidified bodies and the average compressive strength of the radioactive solidified bodies.

[0024] An experiment was conducted to measure the average compressive strength of multiple radioactive solidified bodies produced by the above method, varying the silicon to aluminum content ratio from 1.0 to 2.2 (Fig. 2) and varying the aluminum filling amount in the radioactive solidified bodies from 7.31 to 9.27 [wt%] (Fig. 3). The plot "●" indicates the experimental result when the sodium to aluminum content ratio (hereinafter referred to as "sodium / aluminum ratio") was set to 1.0. The plot "▲" indicates the experimental result when the sodium / aluminum ratio was set to 1.1.

[0025] Radioactive solidified bodies are required to have a certain degree of strength to prevent breakage during storage that would expose the radioactive aluminum, while the aluminum content in the radioactive solidified bodies needs to be increased in order to reduce the total amount of radioactive solidified bodies.

[0026] From the above viewpoint, referring to Fig. 2, it was confirmed that there is a large difference in average compressive strength between cases where the silicon / aluminum ratio is 1.8 or more and cases where it is less than 1.8. That is, in order to impart sufficient average compressive strength to radioactive solidified bodies, it is desirable to set the silicon / aluminum ratio to 1.8 or more. Also, in order to reduce the total amount of radioactive solidified bodies, it is desirable for the silicon / aluminum ratio to be close to 1.8.

[0027] Also, from the above viewpoint, referring to Fig. 3, it was confirmed that there is a large difference in average compressive strength between cases where the aluminum filling amount is less than 8.0 and cases where it is 8.0 or more. That is, in order to impart sufficient average compressive strength to the radioactive solidified body, it is desirable to set the aluminum content in the radioactive solidified body to less than 8.0 [wt%]. Also, in order to reduce the total amount of radioactive solidified body, it is desirable to set the aluminum content in the radioactive solidified body to close to 8.0 [wt%].

[0028] 2 and 3, with the sodium / aluminum ratio limited to 1.0, all plots "●" with a silicon / aluminum ratio of 1.6 or more have an average compressive strength exceeding 1.0 [MPa], and all plots "●" with an aluminum filling amount of 8.25 or more have an average compressive strength exceeding 1.0 [MPa]. In other words, by setting the sodium / aluminum ratio to 1.0, it is possible to achieve both an improvement in the average compressive strength and an increase in the aluminum content, compared to when the sodium / aluminum ratio is 1.1.

[0029] However, if the sodium / aluminum ratio is 1.0, there is a possibility that not all of the aluminum will be ionized during the melting process. Therefore, from the viewpoint of ionizing all of the aluminum during the melting process, it is desirable for the sodium / aluminum ratio to be higher than 1.0.

[0030] According to the above embodiment, for example, the following advantageous effects are achieved.

[0031] According to the above embodiment, the radioactive aluminum alloy is used as the raw material of the alkaline active material. This allows the content of radioactive aluminum in the alkaline active material to be increased compared to the case where the radioactive aluminum alloy is used as the solidified material of the alkaline active material and an active filler containing aluminum (e.g., fly ash, metakaolin) is used as the raw material of the alkaline active material. As a result, the total amount of the solidified body is reduced.

[0032] Furthermore, according to the above embodiment, by using an active filler containing amorphous silica, the reaction shown in Chemical Formula 1 can be smoothly carried out. Similarly, by using a liquid aluminum solution as the raw material of the alkaline active material, the reaction shown in Chemical Formula 2 can be smoothly carried out.

[0033] In addition, since the half-life of activated aluminum among the elements contained in the aluminum alloy is shorter than that of the main additives, most of the radioactive impurities are not dissolved in the hydroxide solution. Therefore, according to the above embodiment, the radioactivity concentration of the radioactive solidified body can be reduced by separating the impurities from the aluminum dissolving solution prior to the kneading process.

[0034] Furthermore, by setting the silicon / aluminum ratio to a value equal to or greater than 1.8 and close to 1.8, it is possible to achieve both an improvement in the average compressive strength and an increase in the aluminum content. Similarly, by setting the aluminum content in the radioactive solidified body to a value less than 8.0 wt% and close to 8.0 wt%, it is possible to achieve both an improvement in the average compressive strength and an increase in the aluminum content.

[0035] Although the embodiments of the present invention have been described above, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Claims

1. a dissolving step of dissolving an aluminum alloy in an alkali metal hydroxide solution to produce an aluminum solution; a kneading step of producing an alkaline active material by kneading the aluminum solution, an active filler, and an alkaline silica solution as raw materials; a solidification step of filling the alkali activated material into a mold and curing the mold to produce a solidified body, A method for producing a solidified body, characterized in that the molar ratio of silicon to aluminum in the solidified body is 1.8 or more, and the aluminum content in the solidified body is less than 8.0 [wt %].

2. The method for producing a solidified body according to claim 1, A method for producing a solidified body, characterized in that the aluminum used as a raw material for the alkaline activated material contains only the activated aluminum alloy or the aluminum alloy having a radioactive substance attached thereto.

3. The method for producing a solidified body according to claim 2, 2. A method for producing a solidified body, wherein the active filler contains amorphous silica.

4. The method for producing a solidified body according to claim 3, 2. A method for producing a solidified body, wherein the active filler is silica fume.

5. The method for producing a solidified body according to any one of claims 1 to 4, A separation step of separating impurities from the aluminum solution, A method for producing a solidified body, comprising the steps of: kneading the aluminum solution from which the impurities have been separated, the active filler, and the alkali silica solution in the kneading step.

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