JPH06324189A - Molten salt electrorefining method - Google Patents

Abstract

(57) [Summary] [Objective] The electrolytic refining process rate in the dry reprocessing of spent metal fuels is improved, and the useful nuclear fuel components with high purity can be produced without accompanying incidental precipitation of fission products. It is intended to provide a molten salt electrolytic refining method that can be purified and recovered. [Composition] The molten metal phase 24 having a metal fuel component concentration of 0.01% by weight or more is used as a common bottom surface, and the molten salt phase 25 having a metal fuel component concentration of 0.1% by weight or more forming an electrolytic bath on the molten metal phase 24 is electrically insulating. Are partitioned concentrically by the annular partition wall 27 of the above, and the cathode 28 and the molten salt phase inside are partitioned into the partitioned molten salt phase 25b on the outside.
The spent metal fuel holding means 29 which also serves as an anode is immersed in 25a, and the molten metal phase 24 is applied to the cathode 28 as an anode,
The molten metal phase 24 is used as a cathode for the spent metal fuel 31,
The present invention is characterized in that electricity is applied by one voltage application means 30 to simultaneously dissolve the anode of the spent nuclear fuel 31 and electrolytically deposit the refined metal fuel 34 on the surface of the cathode 28.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten salt electrorefining method used for electrolytic refining of metals, and in particular, it reprocesses spent metal fuel generated in a nuclear power plant to include it in the spent metal fuel. The present invention relates to a molten salt electrorefining method suitable for separating unnecessary fission products while purifying and recovering useful metals.

[0002]

2. Description of the Related Art Conventionally, spent metal fuel generated in a nuclear power plant is reprocessed to concentrate and recover useful metal components contained in the spent metal fuel, for example, fuel components such as uranium and plutonium. As a means for separating unnecessary fission products, an electrolytic cell for reprocessing plutonium nuclear fuel is known as shown in cross section in FIG. 3 (US Pat. No. 4,596,647). That is, a lower molten metal pool region that accommodates a molten metal phase 1 in which spent metal fuel is dissolved and contained, and a floating molten salt electrolyte (molten salt) formed on the lower molten metal pool region (molten metal phase 1). )
A metal container (pot) 4 provided with a partition plate 3 for apparently partitioning an intermediate region containing the phase 2, the lower molten metal pool region and the intermediate region (floating molten salt electrolyte phase 2); Anode 5 that is also expandable and contractable to the lower molten metal pool region 1, and also serves as a holding basket for spent metal fuel, cathode 6 that is expandable and contractable to the intermediate region 2 of the metal container 4, and the anode 5 and the cathode. Electrode mounting portions 7 and 8 having expansion / contraction means for expanding / contracting 6 into desired areas 1 and 2, and voltage supply means 9 and 9'for supplying required electrolytic power to the anode 5 and the cathode 6, respectively. An electrolytic cell 10 for reprocessing which is formed by the above is known. In FIG. 3, 11 is a lid that seals the metal container 4 while inserting the anode 5 and the cathode 6, 12 is a reinforcing member that reinforces the metal container 4 from the outer peripheral surface, and 13 is a reinforcing member 12. A heating means for heating the metal container 4 arranged on the outer peripheral surface, and 14 are outer casings of the electrolytic cell 10 for reprocessing plutonium nuclear fuel.

According to the electrolytic cell 10 for reprocessing plutonium nuclear fuel of the above construction, the spent metallic fuel is
It is reprocessed, purified and recovered as follows. That is, as a molten metal pool in the lower molten metal pool region 1 of the metal container 4, for example, molten cadmium 1, and as a molten salt electrolyte in the intermediate region 2 of the metal container 4, such as potassium chloride (KCl) -lithium chloride (LiCl). Each of the molten salts 2 is stored. On the other hand, the spent metal fuel holding basket that also serves as the anode 5 stores and holds the used metal fuel containing useful nuclear fuel components and unnecessary fission electrolyte, and inserts the holding basket 5 into the molten metal phase 1. Place and
Spent metal fuel is dissolved and contained in the molten metal phase 1.

When the spent metal fuel is dissolved / contained in the molten metal phase 1, the molten cadmium phase among the material components of the cladding and the unnecessary fission products which are incidentally mixed in the spent metal fuel. Components having a low solubility with respect to 1, such as iron, chromium, and molybdenum, do not dissolve but remain as a residue. Of the useful nuclear fuel components dissolved in the molten cadmium phase 1 and unnecessary fission products, substances having a relatively large absolute value of free energy for chloride formation (in other words, substances relatively easily becoming chloride), for example, Barium, potassium, sodium, cerium, neodymium,
Curium, plutonium, uranium, zirconium, etc. are oxidized and melted from the molten cadmium phase 1,
It moves into the molten salt phase 2 and becomes chlorinated. On the contrary, of the useful nuclear fuel components dissolved in the molten cadmium phase 1 and unnecessary fission products, substances having a small absolute value of free energy for chloride formation (in other words, substances which are less likely to become chloride), for example, rhodium, Palladium and the like are not oxidized and remain in the molten cadmium (anode) phase 1.

On the other hand, when the required electrolysis power is supplied in such a state, the useful nuclear fuel component which is melted from the molten cadmium phase 1 and moves into the molten salt phase 2 to become chloride, and unnecessary fission formation Among the substances, for example, curium, plutonium, uranium, zirconium, etc. are reduced and deposited on the surface of the cathode 6.
Chlorides of potassium, sodium, cerium, and neodymium are not reduced and remain as chlorides in the molten salt phase 2 as they are. That is, it is possible to select and control the type of substance (component) deposited on the surface of the cathode 6 in the molten salt phase 2 by applying a potential corresponding to the chloride formation free energy. Based on such a phenomenon, useful nuclear fuel components such as plutonium, uranium, and zirconium are selectively concentrated and recovered from a spent metal fuel containing useful nuclear fuel components and unnecessary fission products in a mixed state. To be done.

By the way, in the method for selectively concentrating and recovering the useful nuclear fuel component, the useful nuclear fuel component and unnecessary fission products contained in the spent metal fuel are dissolved in the molten cadmium phase 1. However, its dissolution rate is similar to that of the dissolution of solids in general liquids,
It is determined as a function of the diffusion coefficient and concentration gradient of the useful nuclear fuel components and unwanted fission products contained in the spent metallic fuel in molten cadmium phase 1. Therefore, the molten cadmium phase 1 should be sufficiently agitated to reduce the concentration of useful nuclear fuel components and unnecessary fission products around the holding basket (anode) 5 of the spent metallic fuel that has been immersed, and use it. Measures have been taken such as increasing the concentration gradient on the surface of spent metal fuel and increasing the diffusion coefficient by raising the operating temperature.

[0007]

However, there is a limit to the concentration gradient of the molten cadmium phase 1 by stirring and the improvement of the diffusion coefficient depending on the operating temperature, and there is a limit to the deposition of useful nuclear fuel components on the cathode 6 surface. The reality is that growth cannot be done efficiently, and further improvement and ingenuity is desired. As a countermeasure against this, the application of the so-called anodic dissolution method is drawing attention.
FIG. 4 is a cross-sectional view showing the essential structure of a molten salt electrolytic refining apparatus (electrolytic refining means) to which the anodic dissolution method is applied. 15 is an electrolytic cell, 16 is a solid cathode immersed in a molten salt phase described later. Reference numeral 17 indicates a spent metal fuel holding basket (anode) immersed in a molten salt phase described later, 18 indicates a stirrer for stirring the molten metal phase and the molten salt phase described below, and 19 indicates a molten metal phase composed of, for example, molten cadmium. , 20 is a molten salt phase composed of, for example, a potassium chloride-lithium chloride mixed melt, and 21a and 21b are solid-state cathodes 21a and 21b at one end via switches 22a and 22b, respectively.
6. An electrolysis voltage application device that is connected to a holding basket (anode) 17 for spent metal fuel and has the other end connected to the bottom wall surface of the electrolysis cell 15 to apply a required electrolysis voltage.

The electrolytic refining by the molten salt electrorefining apparatus is performed by immersing the holding basket (anode) 17 containing the solid electrode 16 and the spent metal fuel 23 in the molten salt phase 20 while closing the switch 22a. A current is applied between the holding basket (anode) 17 containing the spent metal fuel 23 and the molten cadmium phase 19 to apply a positive potential to the spent metal fuel 23 and a negative potential to the molten cadmium phase 19. At this time, since the switch 22b is opened and the solid cathode 16 is electrically neutral, the useful nuclear fuel components and unnecessary fission products contained in the spent metal fuel 23 are electrically separated. And a portion of the useful nuclear fuel components and unnecessary fission products contained in the spent metal fuel 23 as well as being oxidised to dissolve in the molten salt phase 20 as chloride.
Dissolves and disperses in the molten cadmium phase 19 acting as the cathode. Incidentally, in the molten salt phase 20, in order to increase the deposition rate of the useful nuclear fuel component described below on the surface of the solid cathode 16 is added chloride of the useful nuclear fuel component contained in the spent metal fuel 23. It is mixed and melted in advance.

Next, the switch 22a is closed, and electricity is passed between the solid cathode 16 and the molten cadmium phase 19 (functioning as an anode) to dissolve the useful nuclear fuel components dissolved in the molten cadmium phase 19 and unnecessary components. Various nuclear fission products into the molten salt phase 20 for incorporation and useful nuclear fuel components as a solid cathode
It is deposited on 16 surfaces, and useful nuclear fuel components are selectively recovered by repeating this deposition.

The molten salt electrolytic refining method to which the above-mentioned anodic dissolution method is applied has an advantage that the dissolution rate can be increased because the spent metal fuel 23 is electrically forcibly dissolved. However, this purification method serves as a cathode by further reducing the useful nuclear fuel components and unnecessary fission products once dissolved in the molten salt (electrolyte) phase 20 from the spent metal fuel 23 by electrooxidation. That is dissolved in the molten cadmium phase 19, the useful nuclear fuel components and unwanted fission products dissolved in the molten cadmium phase 19 are oxidized in the molten salt phase 20, and the useful nuclear fuel components are selectively In addition, the step of depositing on the surface of the solid cathode 6 needs to be performed at different times, which causes a problem that the operation time becomes long. Moreover, useful nuclear fuel components and unnecessary fission products contained in the spent metal fuel 23 are not reduced at the stage of being dissolved in the molten cadmium phase 19 functioning as a cathode, and are not reduced in the molten salt phase 20. Since it migrates and remains, there is a problem that unnecessary fission products, such as cesium, accompany the useful nuclear fuel component and are deposited on the surface of the solid cathode 16, thus purifying the useful nuclear fuel component that is practically satisfactory.・ Not a recovery method.

Furthermore, as a measure for improving the molten salt electrolytic refining method to which the above-mentioned anodic dissolution method is applied, the molten salt phase is divided by electrically insulating partition walls, one cathode of the molten salt phase region and the other molten salt phase region. A method has also been proposed in which spent metal fuel is immersed in each phase region to carry out molten salt electrolysis (JP-A-3
-75597 publication). However, in the molten salt electrolytic refining method proposed here, it is difficult to appropriately control the voltage of anodic dissolution and the voltage of cathodic deposition when simultaneously performing anodic dissolution and cathodic deposition using two power sources. There is a problem in practicality. That is, the useful nuclear fuel components and a part of unnecessary fission products that have been melted out of the anode basket by anodic dissolution are dissolved and dispersed in the molten cadmium phase. Inconveniences such as unnecessary accumulation in the cadmium phase, dissolution of some useful nuclear fuel components and unnecessary fission products in the molten cadmium phase, and deposition of molten cadmium on the cathode surface are observed.

The present invention has been made in consideration of the above circumstances, and improves the electrolytic refining process rate in dry reprocessing of spent metal fuels, while avoiding incidental deposition of unnecessary fission products. An object is to provide a molten salt electrolytic refining method capable of purifying and recovering a highly pure and useful nuclear fuel component.

[0013]

In the molten salt electrolytic refining method according to the present invention, a molten metal phase is used as a common bottom surface, and the molten salt phase forming an electrolytic bath on the molten metal phase is partitioned by electrically insulating partition walls. Then, immerse the cathode in one of the divided molten salt phases and the spent metal fuel that also serves as the anode in the other molten salt phase, and apply the required electrolysis voltage to dissolve the spent nuclear fuel into the anode. And a molten salt electrolytic refining method for electrolytically depositing a refined metal fuel on the cathode surface, wherein the concentration of the metal fuel component in the molten metal phase is 0.01% by weight or more, and the concentration of the metal fuel component in the molten salt phase is 0.1% by weight. Each of the above is set, and the molten salt phase is concentrically partitioned by an electrically insulating annular partition, and a cathode is used in the partitioned molten salt phase on the outer side and an anode is used in the molten salt phase on the inner side. Dipped the spent metal fuel,
The molten metal phase is used as an anode for the cathode, the molten metal phase is used as a cathode for the spent metal fuel, and electricity is applied to the anode surface of the spent nuclear fuel and the cathode surface of the refined metal fuel by energizing with one voltage application means. The feature is that analysis is performed simultaneously.

That is, in the molten salt electrolytic refining method according to the present invention, the molten salt phase is concentrically divided by an electrically insulating annular partition, while the concentration of the metal fuel component in the molten metal phase and the molten salt phase are It is characterized in that the concentration of the metal fuel component is controlled to a predetermined concentration, and that the anode dissolution of the spent metal fuel and the electrolytic deposition on the solid cathode surface are simultaneously performed by one voltage application means.

[0015]

In the molten salt electrorefining method and the molten salt electrorefining apparatus according to the present invention, first, useful nuclear fuel components and unnecessary fission products contained in the spent metal fuel are electrically oxidized, It is leached out as chloride in the inner molten salt phase region concentrically defined by the electrically insulating annular partition. Then, the chlorides of the dissolved useful nuclear fuel components and unnecessary fission products are reduced in the molten metal phase that acts (functions) as a cathode with respect to the outer molten salt phase region, and in the metallic state. Dissolves and disperses in the molten metal phase. Thus, of the useful nuclear fuel components dissolved and dispersed in the molten metal phase and unnecessary fission products, substances with a relatively large absolute value of chloride formation free energy (substances that are easily chlorinated) are oxidized. It melts out of the molten metal phase and becomes chlorinated in the inner molten salt phase region in which the cathode is immersed. Among useful nuclear fuel components dissolved and dispersed in the molten metal phase and unnecessary fission products, substances with a relatively small absolute free energy for chloride formation (substances that are difficult to be chloridized) are not oxidized. Remains in the molten metal phase. On the other hand, among the useful nuclear fuel components chlorinated in the inner molten salt phase region and unnecessary fission products, substances having a relatively small absolute value of free energy for chloride formation (substances which are relatively difficult to be chloridized). ) Is reduced and electrolytically deposited (recovered) on the cathode surface, but substances with a relatively large absolute free energy for chloride formation (substances that are more likely to become chloride) are not reduced, and It remains as chloride in the molten salt phase region. In this way, anodic dissolution is performed in the partitioned outer molten salt phase region, while electrolytic deposition on the cathode surface in the inner molten salt phase region is independently and simultaneously performed by one voltage application means. Since it is performed, the time required for the electrolytic refining operation can be shortened, and a useful atomic fuel component containing no impurities can be efficiently electrolytically deposited on the cathode surface.

[0016]

Embodiments of the present invention will be described below with reference to FIGS.

FIG. 1 schematically shows an embodiment of the molten salt electrolytic refining method according to the present invention, in which an apparatus configured as follows is prepared. That is, a molten electrolysis tank 26 that contains the molten metal phase 24 and a molten salt phase 25 that forms an electrolytic bath, and a molten salt phase that forms an electrolytic bath in a region in the molten electrolysis tank 26 where the molten metal phase 24 is contained. An electrically insulating annular partition wall 27 that concentrically partitions 25 into an inner (first) molten salt phase area 25a and an outer (second) molten salt phase area 25b, and is partitioned by the electrically insulating partition wall 27. For example, a solid cathode 28 housed in the outer molten salt phase region 25b and an anode function housed in the inner molten salt phase region 25a partitioned / separated by the electrically insulating annular partition wall 27 are provided. The spent metal fuel holding means (for example, basket) 29 and the only voltage applying means 30 for applying a required electrolytic voltage to the solid cathode 28 and the used metal fuel holding means 29 are provided. 31 anodic dissolution and solid electrolytic 28 surface electrolytic deposition, Serial adopts a simultaneous configuration by a voltage application means 30. In FIG. 1, 32a and 32b are changeover switches for using the molten metal phase 24 as an anode or a cathode, and 33a and 33b are electrolytic current detecting devices.

Here, the molten metal phase 24 is formed of a molten material such as cadmium, bismuth, lead, tin or zinc, and the molten salt phase 25 (25a, 25b) is, for example, potassium chloride-lithium chloride, Chloride-based melting such as potassium chloride-sodium chloride, cesium chloride-sodium chloride, potassium chloride-lithium chloride-sodium chloride, calcium chloride-barium chloride-lithium chloride-potassium chloride, calcium chloride-barium chloride-lithium chloride-sodium chloride Examples thereof include salts, and it is particularly preferable to use a potassium chloride-lithium chloride eutectic mixture (59:41 molar ratio).

Further, the means 29 for holding the spent metal fuel having an anode function is preferably a basket shape having holes such that the pieces of the spent metal fuel 31 do not fall off, and the solid cathode 28 is a useful nuclear fuel. Electrically conductive low-carbon steel capable of achieving stable cathodic deposition of the components is preferable, but molybdenum, tantalum, tungsten, gold, platinum, graphite, etc. may be used. For example, a molten metal is contained in an electrically insulating container. In any case, in order to prevent an electrical short circuit with the molten metal phase 24, an insulating board 28a is attached to the lower end surface.
Is preferably attached. Furthermore, the electrically insulating partition wall 27 for partitioning / separating the molten salt phase 25 into the first molten salt phase region 25a and the second molten salt phase region 25b is, for example, alumina, zirconia, nitride ceramics, carbide ceramics. What is formed by the etc. is desirable.

FIG. 2 is a sectional view showing in more detail an apparatus suitable for carrying out the molten salt electrolytic refining method according to the present invention.
In the basic (schematic) apparatus shown in FIG. 1,
The molten salt phase 25 is an electrically insulating annular partition wall 27, which is the first (inner side),
The second (outer) molten salt phase regions 25a, 25b are concentrically partitioned, and the partitioned outer molten salt phase accommodation region 25 is formed.
A solid cathode 28 and a stirrer 35 for stirring the molten salt phase are arranged and mounted in b, and a spent metal fuel holding means (for example, basket) 29 having an anode function is arranged and mounted in the molten salt phase accommodating region 25a inside. The lid 36 of the electrolytic cell 15 includes
A transfer container capable of raising and lowering the solid cathode 28, the spent metal fuel holding means 29, and the stirrer 35, respectively.
37a, 37b, 37c are detachably mounted integrally with the solid cathode 28 and the like. And the transfer container
37a, 37b and 37c are supported by a load cell 38 so that the weight of useful nuclear fuel component 34 deposited on the surface of the solid cathode 28 during the electrolytic refining operation can be constantly measured. Further, the electrolytic cell 15 is formed into a conical surface in the center of the bottom so as to collect the residue, and in order to keep the electrolytic cell 15 warm, a heat source 39 and a heat insulating material (heat insulating material) 40 are arranged outside. The electrolytic cell 15 having such a configuration is generally set on the base 41 for practical use.

In the above description, the used metal fuel holding means 29 is a basket made of low carbon steel, the cathode 28 is a low carbon steel cathode, and the distance between the cathode 28 and the molten metal phase 24 is 30 mm (10 mm). The ratio of the surface area of the molten metal phase 24 to the surface area of the solid cathode 28 is set to 5.4 / 1 (1/1 or more is acceptable), and the basket 29 made of low carbon steel and the solid cathode are used. 28
When a required electrolysis voltage is applied between the used metal fuel 31 and the spent metal fuel 31 stored and held in the low carbon steel basket 29.
From this, the nuclear fuel component and the like are eluted into the molten salt phase 25a inside, and the nuclear fuel component 34 can be deposited / collected on the surface of the solid cathode 28.

An example of a molten salt electrolytic refining method using the molten salt electrolytic refining apparatus shown in FIG. 2 will be described.
The spent metal fuel 31 is stored in a basket serving as a holding means for the spent metal fuel that also serves as 29, and is immersed in the inner molten salt phase region 25a while being pre-immersed in the outer molten salt phase region 25b. When a desired voltage is applied between the solid cathode 28 and the solid cathode 28, the molten metal phase 24 acts as a cathode for the spent metal fuel 31 in the basket 29, and the solid cathode 28
Is energized as an anode. At this time, in the inner molten salt phase region 25a, chloride of a useful nuclear fuel component contained in the spent metal fuel 31 is dissolved and contained in advance, and the molten metal phase 24 is stirred by the stirrer 35. To do. By applying the above voltage, the spent metal fuel in the basket 29
Useful nuclear fuel components and unwanted fission products in 31 are oxidized by so-called anodic dissolution and leach out as chlorides in the inner molten salt phase region 25a. That is, when an appropriate voltage is applied, for example, chromium, molybdenum, ruthenium, rhodium, palladium, platinum, among the material components of the cladding tube and unnecessary fission products contained in the spent metal fuel 31, Technesium or the like is left as a residue in the basket 29. On the one hand, among the useful nuclear fuel components and unwanted fission products, for example, the redox potential of barium, potassium, sodium, cesium, strontium, cerium, neodymium, uranium, neptunium, americium, curium, plutonium, zirconium, etc. Such a substance dissolves out as chloride in the inner molten salt phase region 25a.

The useful nuclear fuel components and unwanted fission products dissolved in the inner molten salt phase region 25a are then reduced in the molten metal (eg, molten cadmium) phase 24 functioning as the cathode at this time. , Dissolved and dispersed in the molten metal phase 24 in a metallic state. In other words, substances with extremely low redox potentials such as barium, potassium, sodium, cesium, and strontium are used as the molten salt phase region 25a inside.
Remains as chlorides, and nuclear fuel components such as cerium, neodymium, uranium, neptunium, americium, curium, plutonium, and zirconium, and unnecessary fission product components are dissolved in the molten metal phase 24 to a saturated state. Spread.

At the same time, of the solid fuel 28 in the outer molten salt phase region 25b, a chloride of the nuclear fuel component dissolved and dispersed in the molten metal phase 24 which acts (functions) as an anode and unnecessary fission products A substance with a large absolute value of free energy of formation of a substance (prone to chloride formation), such as cerium,
Neodymium, uranium, neptunium, americium, curium, plutonium, zirconium, etc. are oxidized and dissolved out of the molten metal phase 24, transferred to the first molten salt phase 25a, and chlorinated again. Thus, the inner molten salt phase
Of the components that move to 25a and become chlorinated again, substances (components) with a relatively small absolute free energy of formation of chloride, such as useful nuclear fuel components (metals) such as uranium and zirconium, are selectively reduced. Then, the solid cathode 28 is deposited / grown on the surface of the solid cathode 28 and collected. On the other hand, among the nuclear fuel components dissolved and dispersed in the molten metal phase 24 and unnecessary fission products, substances having a small absolute free energy of formation of chloride (hard to be chloridized), for example, ruthenium, rhodium, technesium, etc. Remains in the molten metal phase 24 without being oxidized.

In the molten salt electrorefining, when the initial anode current density is set to 0.8 A / cm ² or less, the current efficiency is also about 100% and the nuclear fuel component 34 is deposited and collected on the surface of the solid cathode 28. Was possible. For comparison, FIG.
In the molten salt electrolytic refining method using the apparatus having the configuration shown in FIG.
Table 1 also shows the current efficiency when the initial anode current density is set to 0.8 A / cm ² or more.

Table-1 Initial Cathode Current Density Current Efficiency Used for Fuel Dissolution Example 1 (0.6A / cm ² ) 100% Comparative Example 1 (0.9A / cm ² ) 78.2% As can be seen from Table-1 In the case of the embodiment, the current efficiency is about 2-3 times higher than that in the case of the comparative example. In other words, in the case of the example, since the iron that composes the anode basket has not reached the melting potential, the anodic dissolution is efficiently performed, and the initial anode current density should be set to 0.801 A / cm ² or less. is important.

In the molten salt electrolytic refining method according to the present invention and the conventional molten salt electrolytic refining method, the concentration of the nuclear fuel component dissolved in the anode side molten salt phase 25a is
When the efficiency of precipitation in the molten salt phase 25a was similarly compared in the case of 0.1% by weight or more or less (Comparative Example), it was as shown in Table 2.

[0028] As can be seen from Table-2, in Comparative Example 2, almost no precipitation occurs, whereas in Example 2, precipitation occurs with high current efficiency. That is, in the case of Example 2, since the nuclear fuel component dissolved in the molten salt phase 25a is sufficient, decomposition of the molten salt does not occur, and the nuclear fuel component is efficiently deposited in the molten salt phase. .

Further, in the molten salt electrorefining, when the initial cathode current density was set to 0.3 A / cm ² , the current efficiency was also about 100% and the nuclear fuel component 34 was deposited on the surface of the solid cathode 28.
It was possible to recover. For comparison, in the molten salt electrorefining method using the apparatus shown in FIG. 4, the initial cathode current density was 0.005 A / cm ² or 1.5 A /
Table 3 also shows the current efficiency when set to cm ² .

Table 3 Initial Cathode Current Density Current Efficiency of Precipitated Metal Deposited Example 3 (0.3A / cm ² ) 100% Comparative Example 3 (0.005A / cm ² ) 52.4% Comparative Example 4 (1.5A / cm ² ). ² ) 37.3% As can be seen from Table-3, the current efficiency of the example is about 2-3 times higher than that of the comparative example. That is, in the case of the example, since the applied voltage has not reached such a level as to cause a reaction such as decomposition of the molten salt, the cathodic deposition is efficiently performed, and the initial cathodic current density is 0.01 A / cm ² to
It is important to set within the range of 1.0A / cm ² .

In the molten salt electrorefining method according to the present invention and the conventional molten salt electrorefining method, the concentration of the nuclear fuel component dissolved in the molten metal phase or the molten salt phase is 0.01% by weight or more. 0.1 wt% (Example 4), 0.01 wt% or less 0.009 wt% (Comparative Example 5)
In the case of, similarly, the initial cathode current density was 0.05A / cm
^The current efficiency when electrolytic purification treatment is set to ²
4 and Table-5.

[0032] As can be seen from Tables 4,5, in the case of the comparative example, the atomic fuel component was hardly deposited on the cathode surface, whereas in the case of the example, the atomic fuel component was present on the surface of the cathode 28 with high current efficiency. It has been deposited. That is, in the case of the example, the molten metal phase
Since the nuclear fuel component is sufficiently present in 24 or the molten salt 25, the decomposition reaction of the molten metal phase 24 or the molten salt phase 25 does not occur, and the nuclear fuel component serves as the refined metal on the cathode 28 surface. It is thought that it will be deposited on. In this electrolysis / refining process, the nuclear fuel component eluted in the inner molten salt phase 25a is further transferred to and melted in the molten metal phase 24. The concentration must be 0.1% by weight or more, and the concentration of the nuclear fuel component in the molten salt phase 25b that is transferred from the molten metal phase 24 and dissolved is required to be 0.01% by weight or more.

In the above case, when the cathode formed by containing the molten metal in the electrically insulating container is used in place of the solid cathode 28, the nuclear fuel component dissolved and dispersed in the first molten salt phase 25a and unnecessary Among the fission products, neptunium, curium, and plutonium, which have a small absolute value of chloride free energy (are difficult to be chloridized), are deposited and recovered on the molten metal surface in the insulating container. On the other hand, cerium, neodymium, etc., which have a large absolute free energy of formation of chloride (prone to chloride formation), are not reduced and
Remains as chloride in the molten salt phase 25b of. That is, it is possible to control the kind of useful nuclear fuel component to be collected and purified on the surface of the solid cathode 28 by applying the potential corresponding to the free energy of chloride generation as the electrolysis voltage.

By the molten salt electrolytic refining method using the apparatus having the configuration shown in FIG. 2, the anode dissolution of the spent fuel and the precipitation / recovery of useful nuclear fuel components can be carried out simultaneously, so that the spent fuel is regenerated. The time is greatly reduced.
For example, if 1 kg of spent fuel is refined and recovered (regenerated) by the molten salt electrolytic refining method,
Compared with 10.2 hours, it takes 27.5 hours to perform purification / recovery (regeneration) by the molten salt electrolytic refining method applying the conventional chemical dissolution method.
It was shortened to 2.7, and it was possible to refine and recover the nuclear fuel component (metal) with better current efficiency. Here, when the conventional chemical dissolution method is applied, after the spent fuel is dissolved and dispersed in the molten metal phase, the electrolytic voltage is applied between the molten metal phase as an anode and the solid cathode in the molten salt phase. It is a method of applying and depositing / collecting on the surface of the solid cathode.

Furthermore, when anodic dissolution and cathodic deposition are simultaneously performed using the above two electrolysis voltage power sources, useful nuclear fuel components and a part of unnecessary fission products dissolved from the anode basket due to anodic dissolution are removed. It dissolves and disperses in the molten cadmium phase, but if the cathodic deposition voltage is not suitable at this time, in other words, if the anodic melting voltage is higher than the cathodic deposition voltage, the useful nuclear fuel components that have melted and unnecessary fission products If a part of the unnecessarily accumulates in the molten cadmium phase, and conversely the voltage of anodic dissolution is lower than the voltage of cathodic deposition, useful nuclear fuel components and unnecessary fission products are no longer dissolved in the molten cadmium phase, and Inconveniences such as the deposition of molten cadmium on the surface were observed, and even if the operation could be shortened, it is difficult to perform purification and recovery with high purity with good reproducibility. Was compared to the method the present invention were those provides many advantages in practical use.

According to the molten salt electrolytic refining method of the present invention, not only the refining / recovery processing time can be significantly shortened as described above, but also the amount of used salt waste generated can be reduced. That is, in the outer molten salt phase 25b, unnecessary fission products, such as barium, sodium, cesium, and strontium, do not migrate, and only cerium and neodymium, etc. are chlorinated after the reduction and recovery of uranium. Remains as a thing. When a reducing molten metal phase such as a cadmium-uranium alloy is brought into contact in this state, fission products such as cerium and neodymium are extracted into the reducing molten metal phase, so If the molten metal is evaporated and removed, cerium and neodymium can be treated as a small amount of waste. In other words, the outer molten salt phase 25b does not become waste and the inner molten salt phase 2b
Since only 5a becomes waste, the amount of waste is charged (uranium) 1 t
The amount of waste was 69 kg when the purification and recovery process of on was performed, whereas the conventional molten salt electrolytic refining means (Fig. 3, 4
The amount of waste is 160 kg, and the amount of exhaust is about 1 /
It was reduced to 2.3. Note that barium, sodium, cesium, strontium, etc. remaining in the molten salt phase 25a on the inner side can be selectively removed by an electrophoresis method or the like, and thus the salt can be regenerated and reused. .

[0037]

As can be seen from the above description, according to the molten salt electrolytic refining method of the present invention, the process of anodic dissolution of spent nuclear fuel and the process of electrodeposition and recovery of useful nuclear fuel components proceed simultaneously. Therefore, the operation time of the regeneration / purification / recovery process can be significantly shortened, and the electrolytic refining voltage in the process is simultaneously performed by one power source.
Moreover, since the voltages for anodic dissolution and cathode deposition are also appropriately controlled and applied, high-purity purification / recovery can be performed more efficiently and reliably. In addition, of the unnecessary fission products, cesium, strontium, etc. are removed from the electrolysis / deposition system to generate a relatively large amount and reduce the life of the molten salt phase, or to prevent the separation / recovery of useful nuclear fuel components. Therefore, it is possible to efficiently deposit and collect useful nuclear fuel components. Further, cerium and the like remaining in the molten salt phase in the region where useful nuclear fuel components are deposited and recovered can be extracted and removed by the alloy melt, so that the amount of salt waste generated can be reduced.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an embodiment example of a molten salt electrolytic refining method according to the present invention.

FIG. 2 is a cross-sectional view showing a configuration example of a main part of an apparatus suitable for carrying out a molten salt electrolytic refining method according to the present invention.

FIG. 3 is a cross-sectional view showing the main configuration of an apparatus used for carrying out a conventional molten salt electrolytic refining method.

FIG. 4 is a cross-sectional view showing a schematic configuration of a molten salt electrolytic refining apparatus used for carrying out a conventional molten salt electrolytic refining method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Molten metal phase (lower molten metal pool area) 2 ... Molten salt phase (intermediate area) 3 ... Partition plate 4 ... Metal container 5,17,29 ... Anode (spent metal fuel holding means) 6 ... Cathode 7,8
... Electrode mounting part 9,9 '... Voltage supply means 10,15,
26 ... Electrolytic cell 11, 36 ... Lid 12 ... Reinforcement 13 ... Heating means 14 ... Exterior body 16, 28 ... Solid electrode 28a ...
Insulating board 18, 35 ... Stirrer 19, 24 ... Molten metal phase
20, 25 ... Molten salt phase 25a, 25a ... Inner (first) molten salt phase region 25b, 25b ... Outer (second) molten salt phase region 21a, 21
b, 30 ... Electrolytic voltage applying device 22a, 22b, 32a, 32b
… Switch 23,31… Spent metallic fuel 27,27… Insulating annular partition wall 33a, 33b… Electrolytic current detection device 34… Refining / recovering component (cathode deposit) 37
a, 37b, 37c ... Transfer container 38 ... Load cell 39 ... Heating source 40 ... Heat insulation material (insulation material) 41 ... Base

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsuguyuki Kobayashi 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Stock company Toshiba Yokohama office

Claims (9)

[Claims] 1. A molten metal phase is used as a common bottom surface, a molten salt phase forming an electrolytic bath on the molten metal phase is partitioned by an electrically insulating partition, and a cathode is provided in one of the partitioned molten salt phases. , Melting by immersing the spent metal fuel also serving as an anode in the other molten salt phase and applying the required electrolysis voltage to perform the anodic dissolution of the spent nuclear fuel and the electrolytic deposition of the refined metal fuel on the cathode surface. In the salt electrolytic refining method, the metal fuel component concentration in the molten metal phase is set to 0.01 wt% or more, the metal fuel component concentration in the molten salt phase is set to 0.1 wt% or more, and the molten salt phase is electrically insulating. Are divided into concentric circles by a circular partition wall, the cathode is immersed in the divided molten salt phase on the outside, and the spent metal fuel also serving as the anode is immersed in the molten salt phase on the inside. Anode phase, molten metal for spent metal fuel A molten salt electrolytic refining method characterized in that a phase is used as a cathode, and current is applied by one voltage applying means to simultaneously dissolve an anode of a spent metal fuel and electrolytically deposit a refined metal fuel on a cathode surface. 2. The molten salt electrorefining method according to claim 1, wherein the molten salt phase is formed of chloride, fluoride or a mixture thereof. 3. The molten salt electrorefining method according to claim 1, wherein the anode is contained in a spent metal fuel rod or a basket made of a noble metal, alloy or oxide having a standard oxidation-reduction potential equal to or higher than that of iron. A method for electrolytic refining of molten salt, characterized in that it is composed of cut pieces of spent metal fuel rods. 4. The molten salt electrolytic refining method according to claim 1, wherein the cathode is iron, molybdenum, tantalum, tungsten,
A molten salt electrolytic refining method characterized by being composed of either gold, platinum or graphite. 5. The molten salt electrorefining method according to claim 1, wherein the initial current density of the anode is set to 0.8 A / cm ² or less. 6. The molten salt electrorefining method according to claim 1, wherein the concentration of the nuclear fuel component leached from the spent metal fuel in the anode side molten salt is set to 0.01% by weight or more. Salt electrorefining method. 7. The molten salt electrorefining method according to claim 1, wherein the initial current density of the cathode is set to 0.01 A / cm ² to 1.0 A / cm ² . 8. The molten salt electrorefining method according to claim 1, wherein the electrode distance between the molten metal phase and the cathode is set to 10 mm or more. 9. The molten salt electrorefining method according to claim 1, wherein the ratio of the surface area of the molten metal phase to the surface area of the cathode is 1/1.
A molten salt electrolytic refining method characterized by the above settings.