Reduction device and method for producing reduced product

Abstract

To provide a reduction device having improved reduction efficiency compared to conventional devices. [Solution] The reduction device 100 comprises an electrolytic cell 3 containing molten salt S and a metal oxide 7 to be reduced in the molten salt S, a cathode 2 disposed inside the electrolytic cell 3, an anode 1 disposed inside the electrolytic cell 3 and above the cathode 2, and a partition member 5 disposed vertically between the anode 1 and the cathode 2, defining a flow path 53 connecting the anode 1 and the cathode 2, and having an upper opening 51 and a lower opening 52.

Description

[Technical Field]

[0001] The present invention relates to a reduction device and a method for producing a reduced product. [Background technology]

[0002] There is a known technology for recovering elemental metals by reducing spent oxide nuclear fuel, for example, from a fast reactor, through electrolysis in molten salt. Patent Document 1 describes "a method for converting uranium and / or plutonium oxide into metal by immersing the oxide in molten salt, characterized in that the molten salt contains lithium chloride and the oxide is reduced to metal by electrolysis using the oxide as a cathode." [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-142585 Summary of the Invention [Problem to be solved by the invention]

[0004] In the technology described in Patent Document 1, the product generated at the cathode by the reduction of oxides is transferred to the anode by rotating the carbon anode in a horizontal plane using an anode rotation device. However, the flow generated by the rotation of the anode is a rotating flow. Therefore, the time required for the product to reach the anode from the cathode is increased, and the overall reduction rate tends to slow down. This reduces the reduction efficiency. The problem to be solved by the present disclosure is to provide a reduction device and a method for producing a reduced product that have improved reduction efficiency compared to conventional devices. [Means for solving the problem]

[0005] The reduction device of the present disclosure includes an electrolytic cell that contains a molten salt and a metal oxide to be reduced in the molten salt, a cathode disposed inside the electrolytic cell, an anode disposed inside the electrolytic cell and above the cathode, and a partition member that is disposed vertically between the anode and the cathode, that defines a flow path connecting the anode and the cathode, and that has an upper opening and a lower opening. Other solutions will be described later in the description of the preferred embodiment of the invention. [Effects of the Invention]

[0006] According to the present disclosure, it is possible to provide a reduction device and a method for producing a reduced product that have improved reduction efficiency compared to conventional methods. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram showing a reduction device according to the present disclosure. [Figure 2A] FIG. 2 is a diagram illustrating the state of a metal oxide in the early stage of reduction. [Figure 2B] FIG. 1 is a diagram illustrating the state of a metal oxide in the final stage of reduction. [Figure 3] 1 is a flowchart showing a method for producing a reduced product according to the present disclosure. [Figure 4] FIG. 10 is a schematic diagram showing a reduction device according to another embodiment. [Figure 5] FIG. 10 is a schematic diagram showing a reduction device according to another embodiment. [Figure 6] FIG. 10 is a schematic diagram showing a reduction device according to another embodiment. [Figure 7] FIG. 10 is a schematic diagram showing a reduction device according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, modes for carrying out the present disclosure (hereinafter referred to as "embodiments") will be described with reference to the drawings. In the following description of one embodiment, other embodiments applicable to the one embodiment will also be described as appropriate. The present disclosure is not limited to the one embodiment described below, and different embodiments can be combined with each other or modified as desired without significantly impairing the effects of the present disclosure. Furthermore, the same components will be given the same reference numerals, and redundant descriptions will be omitted. Furthermore, components having the same functions will be given the same names. The contents shown are merely schematic, and for convenience of illustration, changes may be made from the actual configuration within the scope of not significantly impairing the effects of the present disclosure, and some components may be omitted or modified between drawings. Furthermore, the same embodiment does not necessarily have to include all of the configurations.

[0009] FIG. 1 is a schematic diagram showing a reduction apparatus 100 according to the present disclosure. The reduction apparatus 100 is an apparatus for reducing a metal oxide 7 in a molten salt S contained (stored) inside an electrolytic cell 3. Reduction of the metal oxide 7 typically produces elemental metals. In the example of the present disclosure, the metal oxide 7 is a spent oxide nuclear fuel, such as uranium oxide. Oxide nuclear fuel (metal oxide), such as uranium oxide or plutonium oxide, is used in a light-water reactor, for example, and the spent oxide nuclear fuel is discharged. The spent oxide nuclear fuel is reduced by the reduction apparatus 100 to produce elemental radioactive metals. The obtained elemental radioactive metals can be reused as nuclear fuel in a fast reactor. These processes can establish nuclear fuel cycle technology. However, the metal oxide 7 is not limited to these examples and may be, for example, an oxide of a non-radioactive metal.

[0010] The reduction device 100 includes an anode 1, a cathode 2, an electrolytic cell 3, and a partition member 5. For convenience, the electrolytic cell 3 will be described first. The electrolytic cell 3 is a reaction vessel that contains molten salt S and a metal oxide 7 to be reduced in the molten salt S. A lid 30 is attached to the electrolytic cell 3, making the interior of the electrolytic cell 3 airtight. The molten salt S contained in the electrolytic cell 3 can be determined depending on the type of metal oxide 7 to be reduced. In the example of the present disclosure, as described above, the metal oxide 7 is spent oxide fuel (specifically, uranium oxide), and the molten salt S is molten lithium chloride (an example of a salt). However, in the example of the present disclosure, the molten salt S further contains a predetermined concentration (e.g., 1% by mass) of lithium oxide as an oxygen ion generation source for passing an electric current.

[0011] Inside the electrolytic cell 3, there are a liquid phase 31 and a gas phase 34. The liquid phase 31 and the gas phase 34 are separated by the liquid level L of the molten salt S. The liquid phase 31 is a molten salt (lithium chloride), and the gas phase 34 is an inert gas such as argon or nitrogen. By configuring the gas phase 34 with an inert gas, oxidation of the molten salt S and the elemental metal produced by reduction can be suppressed. Preferably, the gas phase 34 is an inert gas that does not contain moisture.

[0012] However, the gas phase 34 may contain gases (e.g., oxygen molecules) generated by the reduction of the metal oxide in the liquid phase 31. Of the gases that may be contained, gases that affect the reduction (e.g., oxidizing gases such as oxygen molecules) are preferably removed from the gas phase 34. An embodiment for removing such gases will be described later.

[0013] The electrolytic cell 3 is made of a material, such as a nickel-based alloy, that has excellent corrosion resistance at high temperatures to the molten salt S. Although not shown, a ceramic layer, such as alumina or magnesia, may be formed on the inner surface of the electrolytic cell 3.

[0014] The anode 1 and the cathode 2 are both electrodes placed inside the electrolytic cell 3. The anode 1 and the cathode 2 are both placed below the liquid level L (i.e., placed in the liquid phase 31) and are immersed in the molten salt S. The anode 1 is placed higher than the cathode 2. Therefore, the anode 1 is placed relatively higher inside the electrolytic cell 3, and the cathode 2 is placed relatively lower inside the electrolytic cell 3. In the illustrated example, the anode 1 is placed directly above the cathode 2, but this does not have to be directly above, and it is sufficient that the anode 1 is above the cathode 2 in the height direction.

[0015] The anode 1 and the cathode 2 are connected to a power supply (not shown). The voltage applied between the anode 1 and the cathode 2 is a potential at which the metal oxide 7 can be reduced, for example, as shown in formula (2) below. The current value is determined so that a target amount of reduction is achieved in a target time. The reduction of the metal oxide 7 proceeds by measuring the potential of the cathode 2 relative to the molten salt S with a reference electrode 14 and controlling the potential applied between the anode 1 and the cathode 2. Specifically, the potential is controlled so that the potential applied between the metal oxide 7 and the molten salt S exceeds the redox potential of the metal oxide 7, thereby allowing the reduction to proceed.

[0016] The reference electrode 14 is Li / L + The reference electrode 14 is an electrode that generates a constant reference potential, such as the equilibrium potential of the molten salt S. When the molten salt S is molten lithium chloride, the reference electrode 14 can be constructed, for example, by packing a lithium alloy inside a ceramic (alumina, magnesia, etc.) and ensuring ionic conduction through a liquid junction with the molten salt S. However, the reference electrode 14 may also be an electrode that generates a constant potential by using, for example, an electrode packed with a Ni / NiO mixture. Furthermore, when the molten salt S has a melting point in the 500°C range, the reference electrode 14 may also be an electrode in which silver / silver chloride is packed into ion-conductive glass.

[0017] The anode 1 and the cathode 2 are arranged side by side in the up-down direction (vertical direction). A metal oxide 7 is arranged between the anode 1 and the cathode 2. The metal oxide 7 may be in the form of, for example, a block, pellet, granules, or powder. The metal oxide 7 is accommodated in a container 6 formed, for example, of a porous basket. The container 6 is a part of the cathode 2, and the metal oxide 7 is in contact with the cathode 2. Therefore, electrons are supplied to the metal oxide 7 through the surface of the container 6. On the other hand, the anode 1 is not in contact with the metal oxide 7 or the cathode 2.

[0018] The anode 1 is made of, for example, a material that is corrosion-resistant to the molten salt S and has a wide potential window. Examples of such materials include platinum and carbon. The cathode 2 is made of, for example, a material that is electrically conductive and corrosion-resistant to the molten salt S. Examples of such materials include stainless steel.

[0019] Electrolysis in the molten salt S in which the anode 1 and the cathode 2 are immersed is carried out as follows.

[0020] At the cathode 2, oxygen ions (O 2- ) is generated. The generated oxygen ions are attracted to the anode 1 by the electric field generated between the anode 1 and the cathode 2. In the example of the present disclosure, lithium oxide is present in the molten salt S as described above. Therefore, oxygen ions generated by the decomposition of lithium oxide are released into the molten salt S. This enhances electrical conductivity during electrolysis. However, even in the absence of lithium oxide, reduction proceeds due to oxygen ions derived from the metal oxide 7. The oxygen ions are reduced at the anode 1, and oxygen molecules (elemental oxygen; usually, gaseous oxygen; oxygen bubbles) are generated on the surface of the anode 1.

[0021] A series of reduction reactions including the above reactions are shown in the following formulas (1) to (5).

[0022] Reaction at anode 1 2O 2- → O2 (single) + 4e - …Formula (1) Reaction at cathode 2 UO2+4e - →U(single)+2O 2- …Formula (2) Li + +e - →Li (single unit) ...Equation (3) UO2 + 4Li (element) → U (element) + 2Li2O ... Equation (4) Reactions in molten salt S Li2O→2Li + +O 2- …Equation (5)

[0023] As shown in equations (1) to (5), in principle, under control of appropriate temperature and electrolysis conditions for promoting reduction, the reduction of metal oxide 7, such as uranium oxide (equations (2) to (4)), and the generation of oxygen molecules (equation (1)) proceed continuously. During this process, electrical conduction in the molten salt S is achieved by the movement of oxygen ions. Therefore, the generation of oxygen ions accompanying the reduction of metal oxide 7, as shown in equation (2), the movement of oxygen ions generated in the electrolytic cell 3 to the anode 1, and the generation of oxygen molecules by the reduction of oxygen ions, as shown in equation (1), proceed continuously. Of these, the reaction in equation (2) proceeds on the surface of the cathode 2, and the reaction in equation (1) proceeds on the surface of the anode 1. Therefore, the reactions proceed relatively quickly. However, the rate at which oxygen ions generated in the electrolytic cell 3 move to the anode 1 is relatively slow.

[0024] Therefore, the reduction device 100 of the present disclosure is provided with a partition member 5. By providing the partition member 5, it is possible to generate a flow that flows directly from the cathode 2 to the anode 1 by the elemental lithium generated by the above formula (3), as will be described in detail later. Because oxygen ions can reach the anode 1 on this direct flow (i.e., the migration speed of the oxygen ions can be accelerated), the time it takes for the oxygen ions to reach the anode 1 can be shortened. This enables the reduction reaction to be accelerated, and the reduction efficiency to be improved compared to conventional methods.

[0025] The partitioning member 5 is placed in the molten salt S, and is disposed along the vertical direction between the anode 1 and the cathode 2. Here, "disposed along the vertical direction" refers to a form in which the partitioning member 5 is disposed in the vertical direction (up and down direction) as shown in Fig. 1, as well as a form in which the partitioning member 5 is tilted at an angle of, for example, 30° or less with respect to the vertical direction.

[0026] The material of the partitioning member 5 is not particularly limited as long as it is a material that is resistant to the molten salt S, and examples thereof include a material that does not have electrical conductivity (e.g., ceramics such as alumina and magnesia) and a material that has electrical conductivity (e.g., stainless steel). However, when an electrically conductive material is used, it is preferable that the partitioning member 5 is not electrically connected to both the anode 1 and the cathode 2 (i.e., is insulated).

[0027] The partitioning member 5 defines a flow path 53 that connects the anode 1 and the cathode 2. The partitioning member 5 also has an upper opening 51 and a lower opening 52. The surface of the partitioning member 5 facing the flow path 53 (the inner surface) is a smooth surface, but the surface may be provided with, for example, irregularities (grooves or the like) between the upper opening 51 and the lower opening 52 to facilitate the flow of the molten salt S.

[0028] The flow path 53 is a flow path through which not only the molten salt S but also oxygen ions and elemental lithium (metallic lithium) flow, as will be described in detail later. However, for simplicity of explanation, the present disclosure will be described simply as the flow of molten salt S as appropriate. A flow of molten salt S is formed in the flow path 53 from the cathode 2 toward the anode 1, i.e., from bottom to top. The flow path 53 is formed between the upper opening 51 and the lower opening 52. Therefore, the flow path 53 is formed in the space 32 inside the partitioning member 5, and the molten salt S in the space 33 outside (around) the partitioning member 5 flows into the flow path 53 through the lower opening 52. On the other hand, the molten salt S in the flow path 53 flows out into the space 33 through the upper opening 51.

[0029] The partitioning member 5 has a tubular shape such as a cylinder or a rectangular tube by disposing side plates on the sides of the anode 1, the cathode 2, and the metal oxide 7. In the illustrated example, the partitioning member 5 has no openings other than the upper opening 51 and the lower opening 52. Therefore, the partitioning member 5 has a structure in which the molten salt S does not enter or exit at any part other than the upper opening 51 and the lower opening 52.

[0030] The anode 1 is disposed above the partitioning member 5. Here, "above" may refer to the inside of the partitioning member 5 as shown in FIG. 1 or the outside of the partitioning member 5. However, the anode 1 is preferably disposed inside the partitioning member 5. This allows the flow of molten salt S in the flow path 53 to come into contact with the anode 1 particularly easily. The anode 1 is also preferably disposed near the upper opening 51. This allows the molten salt S flowing out from the upper opening 51 to easily reach the anode 1.

[0031] The cathode 2 is disposed below the partitioning member 5. Here, "below" may be below the inside of the partitioning member 5 as shown in FIG. 1 or below the outside of the partitioning member 5. However, the cathode 2 is preferably disposed inside the partitioning member 5. This makes it particularly easy for the flow of molten salt S flowing through the flow path 53 to come into contact with the cathode 2. In addition, the cathode 2 is preferably disposed near the lower opening 52. This allows oxygen ions generated at the cathode 2 and the like to reach the anode 1 through the flow path 53. This improves the efficiency of use of oxygen ions and the reduction efficiency.

[0032] The metal oxide 7 is disposed between the anode 1 and the cathode 2. That is, the metal oxide 7 is sandwiched between the anode 1 and the cathode 2. The metal oxide 7 is also disposed inside the partition member 5. This arrangement makes it possible to prevent the product (first product, for example, oxygen ions) generated by the reduction of the metal oxide 7 from diffusing throughout the interior of the electrolytic cell 3. This makes it possible to promote the product reaching the anode 1 through the flow path 53.

[0033] As described above, the metal oxide 7 is accommodated in the container 6, which is part of the cathode 2. In the example of the present disclosure, the container 6 is disposed in the flow path 53 (inside the partition member 5). It is preferable that the gap between the outer wall surface of the container 6 and the inner wall surface of the partition member 5 is as small as possible. This makes it possible to suppress unintended downward convection in the flow path 53.

[0034] In the example of the present disclosure, as shown in the above formula (3), the molten salt S generates a metal component with a lower density than the molten salt S as the reduction of the metal oxide 7 progresses. Specifically, for example, lithium chloride, which is the molten salt S, generates elemental lithium, which has a lower density than lithium chloride, as the reduction of the metal oxide 7 progresses. Furthermore, at least a portion of the generated elemental lithium dissolves in the molten salt S. At the same temperature, the density of lithium is lower than the density of lithium chloride, which is the molten salt S. Therefore, a difference in density of the fluid occurs inside and outside the flow path 53 partitioned by the partition member 5 due to the generation of elemental lithium. Specifically, for example, when reduction is performed using molten salt S at 650°C, the density of elemental lithium is 0.5 g / cm 3 and the density of molten lithium chloride is 1.5 g / cm 3 Therefore, the density of elemental lithium is one-third of the density of molten lithium chloride.

[0035] By using such molten salt S, the density of the fluid inside the partitioning member 5 (molten lithium chloride and elemental lithium in the space 32) can be made lower than the density of the fluid outside the partitioning member 5 (mainly molten lithium chloride in the space 33). As a result, the molten salt S flows from the cathode 2 toward the anode 1 in the flow path 53, driven by the density difference between the inside and outside of the flow path 53. That is, the relatively light elemental lithium rises in the flow path 53, and the molten salt S around the partitioning member 5 flows into the flow path 53 from the lower opening 52, which is the inlet of the flow path 53. As a result, an upward flow of the molten salt S from the cathode 2 toward the anode 1 can be generated in the flow path 53, producing a so-called "chimney"-like effect. Moreover, the formation of such an upward flow is due to the density difference between the fluids in the space 32 and the space 33. This eliminates the need to consume electricity or the like to generate the upward flow.

[0036] FIG. 2A is a diagram illustrating the state of the metal oxide 7 in the early stages of reduction. Reduction of the metal oxide 7 begins in region 71. Region 71 is a part of the metal oxide 7, located near the portion in contact with the cathode 2. Reduction produces a metal 74 (usually an elemental substance, such as uranium). Reduction by electrolysis (electrolytic reduction) proceeds in a region where the metal produced by reduction, the molten salt S, and the metal oxide 7 coexist. This region (interface) is called a three-phase coexistence line. Reduction proceeds as the three-phase coexistence line advances into the interior of the metal oxide 7.

[0037] FIG. 2B is a diagram illustrating the state of metal oxide 7 at the final stage of reduction. At the final stage of reduction, metal 74, which was contained in metal oxide 7 at the start of reduction, is present in region 71. As reduction progresses, region 71 ceases to be a metal oxide. However, for convenience, region 74 is considered to be part of metal oxide 7 regardless of the degree of reduction. Although not shown, the chemical reaction represented by formula (3) above progresses on the outer surface of metal oxide 7, and elemental lithium adheres to metal oxide 7 so as to spread on the outer surface of metal oxide 7. Meanwhile, metal oxide 7 remains in region 73 inside metal oxide 7, surrounded by region 74. Molten salt S penetrates regions 73 and 74 through gaps, cracks, pores, and the like at grain boundaries.

[0038] In region 73, as the reduction represented by the above formula (2) progresses, oxygen ions migrate to the molten salt S outside region 74. The oxygen ions that have migrated to the external molten salt S migrate to the anode 1 as described above, thereby allowing the reduction to proceed continuously. The migration from region 73 to the external molten salt S via region 74 is mainly dominated by diffusion. For this reason, it is preferable to make the oxygen ion concentration around the metal oxide 7 lower than the oxygen ion concentration inside the metal oxide 7. This can promote diffusion, and thus the reduction in region 73.

[0039] As described above, an upward flow of the molten salt S occurs in the flow path 53 formed inside the partition member 5. That is, the molten salt S flows in the flow path 53 from the cathode 2 toward the anode 1, driven by the density difference between the inside and outside of the flow path 53. This promotes contact between the upward flow of the molten salt S and the metal oxide 7. This allows the oxygen ion concentration around the metal oxide 7 to be quickly reduced, improving the reduction efficiency.

[0040] Returning to FIG. 1 , at the anode 1, as shown in the above formula (1), oxygen ions generated at the cathode 2 or the like release electrons, generating oxygen molecules. Oxygen molecules are usually generated as gaseous oxygen, and the generated oxygen bubbles adhere to, for example, the surface of the anode 1. However, in the flow path 53 formed inside the partition member 5, a flow of molten salt S is formed from the cathode 2 toward the anode 1. Therefore, when this flow comes into contact with the anode 1, the oxygen bubbles adhering to the surface of the anode 1 are peeled off by physical force. This makes it easier for the surface of the anode 1 to be exposed to the molten salt S, accelerating the oxidation reaction of oxygen ions in the molten salt S that occurs on the surface (the reaction shown in the above formula (1)). Furthermore, the presence of the flow toward the anode 1 makes it easier for the peeled bubbles to flow (be released) into the gas phase 34 present above the anode 1. This can reduce the concentration of oxygen (oxygen ions, oxygen molecules) in the liquid phase 31, making it easier to promote the reactions represented by the above formulas (1), (2) and (5).

[0041] Application of a voltage between the anode 1 and the cathode 2 generates Joule heat. As a result, the temperature of the molten salt S rises in the flow path 53 in which the anode 1 and the cathode 2 are arranged. The rise in temperature reduces the density of the molten salt S. As a result, due to the temperature difference, the difference in density between the outside of the partitioning member 5, which has a relatively low temperature and low density, and the outside of the partitioning member 5, which has a relatively high temperature and high density, increases. This further increases the density difference caused by the generation of metallic lithium, and further enhances the "chimney" effect caused by the formation of a flow of molten salt S.

[0042] The anode 1 is positioned so that elemental lithium (product; first product) generated from the molten salt S at the cathode 2 as the reduction proceeds reacts with oxygen molecules generated at the anode 1. Specifically, the partition member 5 allows the elemental lithium generated at the cathode 2 to flow upward and reach the anode 1, where the elemental lithium reacts with at least some of the oxygen molecules generated at the anode 1. This generates oxygen ions and lithium ions. That is, near the anode 1, oxygen molecules generated from the oxygen ions react with metallic lithium generated at the cathode 2, thereby generating oxygen ions and lithium ions again. This positioning of the anode 1 allows the elemental lithium flowing to the anode 1 to convert oxygen molecules present near the surface of the anode 1 back into oxygen ions. This prevents the reduction of oxygen ions at the anode 1 from being hindered. Furthermore, the generated oxygen ions contribute to the electrical conductivity of the molten salt S, thereby accelerating electrolysis.

[0043] The generated oxygen ions and lithium ions are discharged from the flow channel 53 by the upward flow formed in the flow channel 53, pass through the space 33, and then flow back into the flow channel 53 from the lower opening 52. Therefore, they are unlikely to remain around the anode 1. This prevents oxygen molecules generated at the anode 1 from inhibiting the reduction of oxygen ions at the anode 1 or from migrating to the cathode 2 and re-oxidizing the generated elemental metal (such as metallic uranium) to an oxide. This improves the reduction efficiency.

[0044] Fig. 3 is a flowchart showing a method for producing a reduced product (usually a metal element) according to the present disclosure (hereinafter simply referred to as the "production method according to the present disclosure"). The production method according to the present disclosure can be carried out using, for example, the reduction apparatus 100 shown in Fig. 1 above. Therefore, Fig. 3 will be described with appropriate reference to Fig. 1 above. Furthermore, the matters described with reference to Figs. 1 and 2 above can be similarly applied to the description of Fig. 3.

[0045] The manufacturing method of the present disclosure is a method for producing a reduction product (e.g., elemental metal such as lithium element or metallic lithium) generated by the reduction of a metal compound. However, the final product does not have to be a reduction product of an elemental metal or the like. In other words, within the same electrolytic cell 3, for example, immediately after a reduction product of an elemental metal or the like is produced, a reaction may proceed in which another metal compound is produced from the reduction product of the elemental metal or the like. In such a case, although the final product is not a reduction product but a metal compound, because a metal compound is produced via the reduction product, it is included in the scope of the manufacturing method of the present disclosure.

[0046] The manufacturing method of the present disclosure includes a reduction step S1, a transfer step S2, and a removal step S3. While these steps are shown separately in Fig. 3 for convenience, in practice these steps are performed in parallel (i.e., simultaneously). That is, for example, while the reduction step S1 is being performed, the lithium element, oxygen ions, and the like produced by the reduction as described above are transferred to the anode 1 (transfer step S2), and the product (oxygen molecules) at the anode 1 is removed (removal step S3).

[0047] The reduction step S1 is a step of reducing the metal oxide 7 in the molten salt S by contact with the cathode 2 within the space 32 partitioned by the partition member 5. The metal oxide 7 is disposed in the molten salt S between the anode 1 disposed relatively above in the molten salt S and the cathode 2 disposed relatively below in the molten salt S.

[0048] The transfer step S2 is a step of transferring the first product to the anode 1 using the density difference between the density of the molten salt S in the space 32 inside the partitioning member 5 and the density of the molten salt S in the space 33 (another space) outside the partitioning member 5 as a driving force. The first product is a product generated in at least one of the metal oxide 7 or the cathode 2 in the reduction step S1, and in the example of the present disclosure, it is oxygen ions and elemental lithium, but it may be only one of these. The partitioning member 5 is a member that separates the space 32, in which a flow of the molten salt S is formed between the anode 1 and the cathode 2, from the space 33 (another space).

[0049] The removal step S3 is a step of removing the second product from the surface of the anode 1 by contacting the first product transferred to the anode 1 in the transfer step S2 with the second product on the anode 1 that has adhered to the anode 1. In the example of the present disclosure, the second product is oxygen molecules, but is not limited to this. Examples of the removal mode include a mode in which bubbles of oxygen molecules are physically stripped from the anode 1 by the flow of the molten salt S, a mode in which elemental lithium reacts with oxygen molecules to generate lithium ions, and the bubbles disappear from the anode 1, etc.

[0050] According to the manufacturing method of the present disclosure, the flow of the first product in the transfer step S2 can remove the second product adhering to the surface of the anode 1. This can promote the chemical reaction occurring on the surface of the anode 1, and improve the efficiency of the reduction.

[0051] Fig. 4 is a schematic diagram showing a reduction device 100 of another embodiment. Fig. 4 shows a simplified structure of the reduction device 100. The reduction device 100 shown in Fig. 4 further includes a reduction mechanism 9 that reduces the oxygen partial pressure in the liquid phase 31 in the electrolytic cell 3. The provision of the reduction mechanism 9 can promote the reaction (the above formula (1)) that produces oxygen molecules in the liquid phase 31, thereby improving the reduction efficiency.

[0052] The reduction mechanism 9 is, for example, a mechanism that transfers bubbles of oxygen molecules that have adhered to the anode 1 in the liquid phase 31 to the gas phase 34. As shown in the above formula (1), bubbles of oxygen molecules are mainly generated at the anode 1. Therefore, it can be said that most of the oxygen molecules in the liquid phase 31 are present at the anode 1. Therefore, by using the reduction mechanism 9 to transfer the bubbles of oxygen molecules that have adhered to the anode 1 to the gas phase 34, the activity of oxygen at the anode 1 can be reduced, and the oxygen partial pressure can be reduced efficiently.

[0053] The reduction mechanism 9 includes a degassing device 91 and an aeration device 92. The degassing device 91 is a device that removes oxygen molecules from the gas phase 34. The degassing device 91 is connected to the gas phase 34 via the exhaust pipe 8 and the air intake pipe 15. The degassing device 91 also includes oxidizable metal pellets, such as copper, and a heating mechanism that heats the metal pellets to a high temperature, both of which are not shown. However, the degassing device 91 may also include an adsorbent that selectively adsorbs oxygen molecules.

[0054] The air diffuser 92 is disposed near the anode 1 (for example, below the anode 1). The gas diffused from the air diffuser 92 is sprayed onto the anode 1. However, it is not necessary to spray the gas. For example, the air diffuser 92 can diffuse the molten salt S present near the anode 1 so as to agitate it, thereby causing oxygen molecule bubbles to detach from the surface of the anode 1. As a result, the oxygen molecule bubbles attached to the surface of the anode 1 are peeled off from the anode 1, and the oxygen molecule bubbles become mobile in the liquid phase 31. Furthermore, a vibration mechanism (not shown) that vibrates the anode 1 may be provided to peel off the bubbles attached to the anode 1. Because the bubbles are light, they rise in the liquid phase 31 and move through the liquid surface L into the gas phase 34.

[0055] The gas (e.g., containing oxygen and an inert gas) in the gas phase 34 supplied to the degasser 91 through the exhaust pipe 8 comes into contact with the high-temperature metal pellets. This oxidizes the metal pellets and removes the oxygen molecules from the gas. The gas from which the oxygen molecules have been removed is supplied to the air diffuser 92 through the air supply pipe 15. The supplied gas (e.g., the inert gas remaining after removing the oxygen) is blown onto the surface of the anode 1 through the air diffuser 92. This removes air bubbles on the surface of the anode 1.

[0056] FIG. 5 is a schematic diagram showing a reduction device 100 of another embodiment. FIG. 5 shows a simplified structure of the reduction device 100. In the reduction device 100 shown in FIG. 1 above, the partitioning member 5 and the flow path 53 are vertically long. Therefore, the horizontal dimension (left-right dimension) of each of the partitioning member 5 and the flow path 53 is shorter than the vertical dimension (top-bottom dimension). However, in the reduction device 100 shown in FIG. 5, the partitioning member 5 and the flow path 53 are horizontally long. Therefore, the horizontal dimension (left-right dimension) of each of the partitioning member 5 and the flow path 53 is longer than the vertical dimension (top-bottom dimension). This structure can prevent the distance between the cathode 2 and the metal oxide 7 from becoming excessively long, thereby increasing the amount of metal oxide 7 that can be reduced.

[0057] FIG. 6 is a schematic diagram showing a reduction device 100 of another embodiment. In FIG. 6, the structure of the reduction device 100 is shown in a simplified form. The reduction device 100 further includes an extrusion mechanism 12. The extrusion mechanism 12 is a mechanism that extrudes the metal oxide 7 into the flow path 53. The extrusion mechanism 12 includes, for example, a screw and a motor that rotates the screw, both of which are not shown. The screw is rotated by the motor, and the metal oxide 7 is extruded into the flow path 53 by the rotating screw.

[0058] The partitioning member 5 stands upright on the floor 35 of the electrolytic cell 3. That is, the partitioning member 5 is a member that comes into contact with the molten salt S inside the electrolytic cell 3. The partitioning member 5 is arranged in the vertical direction. A lower opening 52 that opens toward the side of the partitioning member 5 is formed below the partitioning member 5 (above the floor 35). As will be described in detail later, the metal oxide 7 rises through the flow path 53. For this reason, it is preferable to block the lower opening 52 with, for example, a mesh-like member (not shown) so that the metal oxide 7 rising next to the lower opening 52 does not leak out from the lower opening 52. This allows the molten salt S to flow into the flow path 53 through the lower opening 52, but prevents the metal oxide 7 from leaking out.

[0059] A pipe 13 extending outside the electrolytic cell 3 is connected to the lower end of the partition member 5 (at the same height as the floor surface 35). The pipe 13 is a pipe that connects the flow path 53 and the extrusion mechanism 12.

[0060] In the example of FIG. 5, the cathode 2 is disposed within the flow path 53. For example, the cathode 2 is plate-shaped and disposed vertically along the side wall of the flow path 53 (for example, parallel to the side wall). The cathode 2 is a member separate from the partition member 5. In this manner, the cathode 2 can be disposed at a desired position within the flow path 53, improving the degree of freedom in disposing the cathode 2.

[0061] When the extrusion mechanism 12 begins to extrude the metal oxide 7 into the horizontally disposed pipe 13, the metal oxide 7 moves horizontally within the pipe 13. The metal oxide 7 turns upward at the bent portion of the pipe 13 and then moves vertically (upward) within the pipe 13. As it moves further, the metal oxide 7 begins to enter the flow path 53 from the upper end of the pipe 13, i.e., the lower end of the partition member 5. As the extrusion continues, the metal oxide 7 rises within the flow path 53. Therefore, the extrusion mechanism 12 extrudes the metal oxide 7 from below to above within the flow path 53. However, if the partition member 5 extends in a direction other than vertical, the metal oxide 7 does not need to rise vertically but moves in the same direction as the extension direction of the partition member 5.

[0062] The metal oxide 7 extruded by the extrusion mechanism 12 moves (rises in the illustrated example) within the flow path 53 while contacting the cathode 2. In this manner, the reduction of the metal oxide 7 can be continuously performed. Furthermore, elemental lithium is produced at the cathode 2 as shown in the above formula (3). Therefore, elemental lithium may adhere to the cathode 2. However, because the metal oxide 7 moves while contacting the cathode 2, even if elemental lithium adheres to the cathode 2, the adhered elemental lithium can be scraped off. This makes it possible to avoid the cathode 2 being covered with elemental lithium, which would inhibit the reaction shown in formula (3).

[0063] The amount of metal oxide 7 processed can be controlled by controlling the extrusion speed of extrusion mechanism 12. For example, depending on the operating conditions of reduction device 100, the amount of elemental metal 74 produced by reduction may be predetermined. The reduction rate can be controlled to some extent by the current value, voltage value, etc., but the current value, voltage value, etc. are set to values ​​that allow reduction to proceed, so are limited to some extent, making it difficult to control the reduction rate significantly. Therefore, the amount of metal oxide 7 processed can be controlled by controlling the supply rate of metal oxide 7 and supplying it continuously.

[0064] The reduction rate is faster at the metal oxide 7 closer to the anode 1 in contact with the cathode 2. This is for the following reason. In the initial stage of reduction, no potential is applied to the metal oxide 7, but lithium ions are reduced on the cathode 2 to produce elemental lithium. The produced elemental lithium acts as the cathode 2, and the elemental lithium spreads over the metal oxide 7. Eventually, the entire surface of the metal oxide 7 is covered with elemental lithium. At this time, the greatest potential is concentrated between the part of the metal oxide 7 closest to the anode 1 and the anode 1. As a result, reduction of the interior of the metal oxide 7 proceeds preferentially from the upper part of the metal oxide 7, resulting in the production of elemental metal. Therefore, by extruding the metal oxide 7 from bottom to top over a sufficient period of time so that reduction can proceed to the interior, the metal oxide 7 can be reduced continuously and efficiently.

[0065] Thus, the reduction efficiency of the metal oxide 7 increases upward in the flow path 53. Therefore, the reduction efficiency can be improved by controlling the supply rate of the metal oxide 7 flowing from below to above and extending the time the metal oxide 7 remains in the upper part as much as possible. At this time, the flow path 53 also contains the upward flow of molten salt S as described above. However, the rising speed of the molten salt S is sufficiently faster than the upward movement speed of the metal oxide 7. Therefore, elemental lithium, oxygen ions, and the like produced by the reduction of the metal oxide 7 are quickly discharged from the flow path 53.

[0066] The elemental metal 74 (product; third product) generated from the metal oxide 7 upon contact with the cathode 2 is discharged from the upper opening 51 of the partitioning member 5 into the space 33 outside the partitioning member 5. In this manner, the metal 74 can be easily recovered. For example, the discharged metal 74 falls to the floor 35 of the electrolytic cell 3 due to its own weight. The metal 74 that has fallen to the floor 35 can be easily recovered outside the electrolytic cell 3 by, for example, a recovery mechanism (e.g., a conveyor) not shown. The metal 74 may be recovered by stopping the reduction device 100 when a desired amount of metal 74 has been produced, or may be recovered using a recovery mechanism while the reduction device 100 is continuing to operate.

[0067] FIG. 7 is a schematic diagram showing a reduction device 100 according to another embodiment. In FIG. 7, the structure of the reduction device 100 is shown in a simplified form. In the reduction device 100, unlike the reduction device 100 shown in FIG. 6 above, the partitioning member 5 also functions as the cathode 2. This makes it possible to reduce the installation space for the cathode 2 in the flow path 53 and facilitate the movement of the metal oxide 7 within the flow path 53. The material of the partitioning member 5 can be the same as the material of the cathode 2.

[0068] A hollow insulator 60 is disposed between the partitioning member 5 and the pipe 13. This electrically insulates the partitioning member 5 from the pipe 13. Meanwhile, the interior of the pipe 13 and the interior of the partitioning member 5 (flow path 53) are in communication with each other via the interior of the hollow insulator 60. Therefore, similar to the reduction device 100 shown in FIG. 6 above, the metal oxide 7 can be pushed out into the flow path 53 by the pushing mechanism 12.

[0069] The lower opening 10 is disposed on the side wall of the partitioning member 5, immediately above (in the vicinity of) the insulator 60. Because the partitioning member 5 also serves as the cathode 2, reduction occurs throughout the entire partitioning member 5. Therefore, by disposing the lower opening 10 on the partitioning member 5 immediately above the insulator 60, the molten salt S can come into contact with the entire partitioning member 5. This makes it easier to discharge products (oxygen ions, elemental lithium, etc.) generated by reduction throughout the entire partitioning member 5. [Explanation of symbols]

[0070] 1 Anode 10 Bottom opening 100 Reduction Device 12 Extrusion mechanism 13 Piping 14 Reference electrode 15 Air supply pipe 2 cathode 3 Electrolytic cell 31 Liquid phase 32 Space 33 Space 34 Gas Phase 35 Floor 5 Compartment members 51 Upper opening 52 Lower opening 53 Flow path 6. Storage container 60 Insulator 7. Metal oxides 71 areas 72 areas 73 areas 74 metal 8 exhaust pipe 9 Reduction mechanism 91 Degassing device 92 Air diffuser L liquid level S Molten salt S1 Reduction step S2 Moving process S3 removal process

Claims

1. an electrolytic cell containing a molten salt and a metal oxide to be reduced in the molten salt; a cathode disposed inside the electrolytic cell; an anode disposed inside the electrolytic cell and above the cathode; a partition member disposed vertically between the anode and the cathode, partitioning a flow path connecting the anode and the cathode, and having an upper opening and a lower opening. A reduction device characterized by:

2. The reduction device according to claim 1, The molten salt generates a metal component having a lower density than the molten salt as the reduction of the metal oxide progresses. A reduction device characterized by:

3. The reduction device according to claim 2, The molten salt flows through the flow channel from the cathode to the anode using the density difference between the inside and outside of the flow channel as a driving force. A reduction device characterized by:

4. The reduction device according to claim 1, The metal oxide is a spent oxide nuclear fuel. A reduction device characterized by:

5. The reduction device according to claim 1, The anode is disposed inside the partition member. A reduction device characterized by:

6. The reduction device according to claim 1, The cathode is disposed inside the partition member. A reduction device characterized by:

7. The reduction device according to claim 1, The metal oxide is disposed inside the partition member. A reduction device characterized by:

8. The reduction device according to claim 1, Furthermore, a reduction mechanism for reducing the oxygen partial pressure in the liquid phase in the electrolytic cell is provided. A reduction device characterized by:

9. The reduction device according to claim 8, The reduction mechanism is a mechanism for transferring bubbles of oxygen molecules attached to the anode in the liquid phase to the gas phase. A reduction device characterized by:

10. The reduction device according to claim 1, The anode is disposed so that a product produced from the molten salt at the cathode as the reduction proceeds reacts with oxygen molecules produced at the anode. A reduction device characterized by:

11. The reduction device according to claim 1, further comprising an extrusion mechanism for extruding the metal oxide into the flow path; the cathode is disposed within the flow path; The metal oxide extruded by the extrusion mechanism moves through the flow channel while contacting the cathode. A reduction device characterized by:

12. The reduction device according to claim 11, the partition member is disposed along a vertical direction, and the extrusion mechanism extrudes the metal oxide from below toward above in the flow path; A product generated from the metal oxide upon contact with the cathode is discharged to the outside of the partition member through the upper opening of the partition member. A reduction device characterized by:

13. The reduction device according to claim 11, The cathode is a separate member from the partition member. A reduction device characterized by:

14. The reduction device according to claim 11, The partition member also functions as the cathode. A reduction device characterized by:

15. a reduction step in which a metal oxide in the molten salt is reduced by contact with an anode disposed relatively above the cathode disposed relatively below the anode in the molten salt; a transferring step of transferring a first product generated at at least one of the metal oxide and the cathode in the reduction step to the anode using a density difference between the density of the molten salt in the space inside a partitioning member that separates a space in which a flow of molten salt is formed between the anode and the cathode from another space and the density of the molten salt in the other space outside the partitioning member as a driving force; a removal step of removing the second product from the surface of the anode by contacting the first product transferred to the anode in the transfer step with a second product attached to the anode. A method for producing a reduced product, comprising:

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