JPH1121690A - Method for refining metallic uranium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of a radioactive waste liq. or waste solvent by eliminating the need for water and org. solvent and to reduce the refining cost by simplifying the refining process. SOLUTION: An anode and a cathode are provided in a fluoride molten salt in which the impurity elements and uranium in an electrolytic cell are dissolved, a voltage is impressed between the anode and cathode to electrolyze the molten salt, and the uranium dissolved in the molten salt is deposited on the anode surface. The voltage is controlled in two stages to electrolyze the molten salt, the impurity elements are recovered in the first stage, then the cathodes are exchanged, and metallic uranium is recovered in the second stage. The molten salt is preferably selected from the binary molten salts of LiF-NaF, LiF-KF, LIF-BEF2 , NaF-BeF2 and KF-BeF2 and the ternary molten salts of LiF-NaF-KF, LiF-NaF-BeF2 , NaF-KF-BeF2 , LiF-NaF-CaF2 and LiF-KF-BaF2 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying metallic uranium containing impurity elements generated in a metallic uranium production facility and a processing facility.

[0002]

2. Description of the Related Art In a metal uranium manufacturing facility and a processing facility,
Scrap uranium is generated with the processing, cutting, cutting, and the like of metal uranium. This scrap uranium contains impurities in the processing step. Conventionally, this scrap uranium is dissolved in nitric acid, and the uranium element is separated and purified from metal uranium containing impurity elements by a solvent extraction method using tributylphosphoric acid (TBP) to obtain purified metal uranium. That is, in the conventional solvent extraction method, as shown in FIG. 5, after scrap uranium is dissolved in nitric acid to form a uranyl nitrate solution containing an impurity element, a solvent extraction treatment is performed using a solvent, and a purified uranyl nitrate solution is obtained. Separate impurity elements. The uranyl nitrate solution is then subjected to denitration and reduction to obtain purified uranium dioxide, and then converted to purified uranium tetrafluoride by a hydrogen fluoride treatment. This purified uranium tetrafluoride further contains Mg
And a reduction treatment with a metal by thermite reduction method to obtain purified uranium metal.

[0003]

However, in any of the conventional solvent extraction methods, it is necessary to appropriately dilute uranium-containing nitrate with water and then to react with a liquid such as a solvent. Alternatively, there is a problem that a waste solvent is generated, and steps such as evaporation, concentration, and solidification for treating the radioactive waste liquid or the waste solvent are required. The purpose of the present invention is
An object of the present invention is to provide a method for purifying metallic uranium, which does not require water or an organic solvent, does not generate a radioactive waste liquid or a waste solvent, simplifies a purification step, and reduces purification costs.

[0004]

The invention according to claim 1 is
As shown in FIGS. 1, 3 and 4, anodes 17 a and 17 b and cathodes 16 and 18 are provided in a molten fluoride 13 in which an impurity element and uranium in an electrolytic cell 11 are dissolved. This is an improvement in a method for purifying metallic uranium in which uranium dissolved in the molten salt 13 is precipitated on the surface of the cathode 17 by applying a voltage between the cathode 17b and the cathodes 16 and 18 to perform electrolysis of the molten salt. The characteristic configuration is that the molten salt electrolysis is performed by controlling the voltage in two stages, and as shown in FIG. 3, after the impurity element is recovered in the cathode 16 by the first stage molten salt electrolysis, as shown in FIG. Then, the cathode 16 is replaced with another cathode 18 and the metal uranium is recovered in the other cathode 18 by the second stage molten salt electrolysis. Uranium is deposited on the cathode surface by molten salt electrolysis without using water or an organic solvent. After recovering the impurity element to the cathode 16 by the molten salt electrolysis in the first stage, metallic uranium is recovered to the cathode 18 exchanged by the molten salt electrolysis in the second stage. Thereby, the purification step can be simplified as compared with the solvent extraction method.

The invention according to claim 2 is the invention according to claim 1, wherein a metal uranium 12 containing an impurity element is loaded in a basket 14 as shown in FIGS.
This is a method for purifying metal uranium in which the metal uranium 12 containing an impurity element is dissolved in the fluoride molten salt 13 by dipping in the fluoride molten salt 13 inside 1. The invention according to claim 3 is the invention according to claim 2, wherein the basket 1
4 is immersed in the molten fluoride 13
This is a method for purifying metal uranium in which fluorine gas is blown into the metal uranium to promote the dissolution of metal uranium 12 containing an impurity element. By loading the metal uranium 12 containing the impurity element into the basket 14 and immersing and dissolving it in the fluoride molten salt 13, the metal uranium 12 can be easily and safely dissolved in the fluoride molten salt 13. If fluorine gas is blown into the molten salt 13, dissolution of the metal uranium 12 can be promoted.

[0006] The invention according to claim 4 is the invention according to claims 1 to 3.
The invention according to any one of the above, wherein the molten fluoride salt 13 is L
iF-NaF, LiF-KF, LiF-BeF 2, Na
_{F-BeF 2, KF-BeF} 2 binary molten salts and LiF
-NaF-KF, LiF-NaF- BeF 2, NaF-
_{KF-BeF 2, LiF-NaF} -CaF 2, LiF-K
Selected from ternary molten salt F-BaF ₂ is a method for purifying a uranium metal is any molten salt. By the fluoride molten salt 13 being any of the molten salts described above,
The melting point of the molten salt can be lowered, and the operating temperature in uranium metal purification can be relatively low. Specifically, Table 1 shows an example of a preferable composition ratio in the above-described molten salt and an example of a melting point in the composition.

[0007]

[Table 1]

[0008]

Embodiments of the present invention will now be described with reference to the drawings. As shown in FIGS. 1 and 2, a fluoride molten salt 13 in which an impurity element and uranium are dissolved is stored in an electrolytic cell 11 made of graphite. Although not shown, a heating coil for heating and maintaining the inside of the tank at a predetermined temperature is provided outside the electrolytic tank 11. The molten fluoride salt 13 is loaded into the electrolytic cell 11 and heated by a heating coil (not shown) to at least the melting point of the molten fluoride salt and not more than the melting point of metallic uranium to be melted. Fluoride molten salt of scrap metal uranium 12 which is uranium metal containing impurity element 1
Dissolution into 3 is carried out by loading the scrap metal uranium 12 into a graphite basket 14 and immersing it in a molten fluoride salt 13 inside the electrolytic cell 11.

In addition, a gas introduction pipe 15 is separately provided in the electrolytic cell 11 when the scrap metal uranium 12 is dissolved. The gas introduction pipe 15 extends downward from above the electrolytic cell 11 and the lower part is provided in the fluoride molten salt 13. The lower part of the gas introduction pipe 15 is bent along the bottom of the electrolytic cell 11, and a gas ejection part 15a is provided at the bent part. As shown by the arrows in FIG. 2, when the basket 14 is immersed in the molten fluoride salt 13, fluorine gas is supplied to the gas introduction pipe 15, and the fluorine gas is blown out from the gas jetting portion 15 a to cause the impurity element in the basket 14. To promote the dissolution of scrap metal uranium 12 containing

When the scrap metal uranium 12 is dissolved in the molten fluoride salt 13, the basket 14 and the gas introduction pipe 1
5 is taken out of the electrolytic cell 11, and subsequently, molten salt electrolysis is performed. Molten salt electrolysis is performed in two stages by controlling the voltage applied between the anode and the cathode. As shown in FIGS. 1 and 3, impurity elements are recovered in the first stage of molten salt electrolysis. In the molten salt electrolysis of the first stage, a rod-shaped anode 17a made of graphite and a cathode 16 also made of graphite are provided in the fluoride molten salt 13, respectively.
Electrolysis voltage is applied between 7a and the cathode 16 to perform constant voltage electrolysis. In the molten salt electrolysis in the first stage, the cathode 1
6 Impurity element 1 dissolved in fluoride molten salt 13 on the surface
6a is collected.

After recovering the impurity element 16a, a second stage of molten salt electrolysis is performed. As shown in FIGS. 1 and 4, the molten salt electrolysis in the second stage is performed after replacing the cathode 16 having the impurity element 16a deposited on the surface with a new rod-shaped cathode 18 made of uranium metal. A solid metal uranium 18a is deposited on the surface of the substrate. In the second stage of molten salt electrolysis, the cathode 16 in the first stage is used.
When the anode 17a is replaced with a new cathode 18, the molten salt 1
It is preferable to pull up from 3 and replace it with a new rod-shaped anode 17b made of graphite. In the second stage of molten salt electrolysis, the electrolysis temperature is the same as that of the first stage of molten salt electrolysis, and is equal to or lower than the melting point of metallic uranium. Collected.

After completion of the molten salt electrolysis in the second stage, metal uranium 18a electrolytically deposited on a new cathode 18
8 is removed from the bath and mechanically peeled off the cathode surface.
Since the supporting salt component is attached to the uranium metal thus obtained, the metal uranium is heated to 800 ° C. in a vacuum atmosphere to remove these fluoride salts. As a result, the fluoride salt evaporates, and high-purity metallic uranium is obtained. This high-purity metallic uranium has a melting point of uranium (1
By heating to a temperature of 132 ° C. or higher and performing melt casting, a high-purity metal uranium ingot can be obtained.

The voltage applied between the anode 17a and the cathode 16 in the first stage molten salt electrolysis and the voltage applied between the anode 17b and the cathode 18 in the second stage molten salt electrolysis are the composition of the molten salt, Depending on the electrolysis conditions such as the melting temperature and the resistance of the electrode material, the degree of difficulty of metal precipitation in molten salt electrolysis is such that impurity elements are most likely to precipitate, then uranium is precipitated, and molten salt constituent elements are most likely to precipitate. It is a difficult metal. Therefore, the voltage in the molten salt electrolysis in the first stage of the present invention is a voltage at which the impurity element is precipitated but uranium is not precipitated, and the voltage in the molten salt electrolysis in the second stage is that the uranium is precipitated but the molten salt constituent element Is the voltage at which no deposition occurs. The respective voltages are determined by the electrolytic conditions such as the composition of the molten salt, the melting temperature, and the resistance of the electrode material.

[0014]

Next, embodiments of the present invention will be described. As shown in FIG. 2, first, LiF was placed in a graphite electrolytic cell 11.
3 g, 59.1 g of NaF and 298.6 g of KF, and the temperature was raised by a heating coil (not shown) to a temperature of 500 ° C.
Was melted. On the other hand, 100 g of metallic uranium (scrap uranium) containing impurity elements in the basket 14
The basket 14 was immersed in the supplied and molten LiF, NaF and KF. When the basket 14 was immersed in the molten fluoride salt 13, fluorine gas was supplied from the upper end of the gas introduction pipe 15 and blown out from below the basket 14 to promote the dissolution of the scrap metal uranium 12. As shown in FIG. 3, a graphite rod 16 and an anode 17 a each having a round bar shape having a diameter of 20 mm were immersed in the molten salt by 60 mm, respectively, to perform a first-stage molten salt electrolysis. In this molten salt electrolysis, constant voltage electrolysis is performed at an electrolysis voltage of 3.5 V for 2 hours while stirring the fluoride molten salt 13, and impurities 16 a are formed on the surface of the cathode 16.
Was precipitated.

Next, as shown in FIG. 4, the cathode 16 and the anode 17a were replaced with a rod-shaped cathode 18 made of uranium metal and an anode 17b made of graphite, respectively, and a second stage of molten salt electrolysis was performed. In the second stage of molten salt electrolysis, constant voltage electrolysis is performed at an electrolysis voltage of 8.0 V for 3 hours while stirring the fluoride molten salt 13, and metal uranium 18 a is formed on the surface of the cathode 18.
Was precipitated. The metal uranium 18a deposited on the surface of the cathode 18 was removed from the bath after the completion of the second-stage molten salt electrolysis, and was mechanically peeled off from the cathode surface. The peeled metal uranium was heated at 800 ° C. in a vacuum atmosphere to evaporate the attached fluoride salt, thereby obtaining high-purity metal uranium.

<Evaluation and Discussion> Scrap uranium initially supplied into the basket 14, precipitates deposited on the surface of the cathode 16 by the first stage molten salt electrolysis (first precipitates), and second stage molten salt electrolysis The obtained high-purity metallic uranium, and LiF and NaF after completion of the second-stage molten salt electrolysis
And the concentration of impurities in the molten salt consisting of KF
It was measured by CP emission spectroscopy. Table 2 shows the results.

[0017]

[Table 2]

As is clear from Table 2, the impurity concentration in the precipitate deposited by the first stage molten salt electrolysis is higher than the impurity concentration in scrap uranium, while the impurity concentration in metallic uranium is extremely low. You can see that. From this, it became clear that impurities in scrap uranium were recovered by the molten salt electrolysis in the first stage and did not migrate into metallic uranium. Further, the impurity concentration in the molten salt composed of LiF, NaF and KF is at most 10 ppm, and both the impurity element and the metallic uranium hardly remain in the molten salt after the molten salt electrolysis in the second stage. I understand. Therefore, it was found that high-purity uranium metal can be effectively deposited from scrap uranium. The high-purity metallic uranium obtained by the molten salt electrolysis in the second stage is heated to a temperature equal to or higher than the melting point of uranium (1132 ° C.) in an Ar atmosphere.
It was heated to 00 ° C. and melt-cast to obtain a high-purity metal uranium ingot.

[0019]

As described above, according to the present invention, uranium is deposited on the cathode surface by applying a voltage to the anode and the cathode in the molten fluoride salt and subjecting the molten salt to electrolysis.
No water or organic solvent is used, and no radioactive waste liquid or waste solvent resulting from the use of water or organic solvent is generated. Further, since the molten salt electrolysis is performed by controlling the voltage in two stages, and the impurity element is recovered by the first stage molten salt electrolysis, the cathode is exchanged, and the metal uranium is recovered by the second stage molten salt electrolysis. The purification step is simplified as compared with the solvent extraction method using a solvent conventionally used,
The cost of purifying high-purity metallic uranium from scrap uranium can be reduced.

Further, by loading metal uranium containing an impurity element into a basket and immersing it in a molten fluoride salt to dissolve it, the metal uranium can be easily and safely dissolved in the molten fluoride salt. By blowing fluorine gas into the salt, the dissolution of metallic uranium can be promoted. Further, the molten fluoride salt is converted to LiF-NaF, LiF
_{-KF, LiF-BeF 2, NaF} -BeF 2, KF-B
eF ₂ binary molten salt and LiF—NaF—KF, Li
F-NaF-BeF ₂ , NaF-KF-BeF ₂ , LiF
By using either of the molten salt selected from -NaF-CaF _2, ternary molten salt LiF-KF-BaF _2, it is possible to lower the melting point of the molten salt, the operating temperature in the metal uranium purification Can be lower.

[Brief description of the drawings]

FIG. 1 is a process chart illustrating a method for purifying metallic uranium of the present invention.

FIG. 2 is a configuration diagram of an electrolytic cell showing a state in which metallic uranium containing an impurity element is dissolved in a molten fluoride salt.

FIG. 3 is a configuration diagram of an electrolytic cell showing the first stage of molten salt electrolysis.

FIG. 4 is a configuration diagram of an electrolytic cell showing the second-stage molten salt electrolysis.

FIG. 5 is a process chart illustrating a conventional method for purifying metallic uranium.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Electrolysis tank 12 Scrap metal uranium 13 Fluoride molten salt 14 Basket 16, 18 Cathode 17a, 17b Anode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naoki Teramae 1002, 6-headed, Mukaiyama, Naka-cho, Naka-gun, Naka-gun, Ibaraki Prefecture 14 Mitsubishi Materials Corporation Naka Energy Laboratory

Claims (4)

[Claims] An anode (17a, 17b) and a cathode (16, 18) are provided in a fluoride molten salt (13) in which an impurity element and uranium in an electrolytic cell (11) are dissolved, and the anode ( 17a, 17b) and the cathode (1
A method for purifying metallic uranium in which uranium dissolved in the molten salt (13) is precipitated on the surface of the cathode (18) by applying a voltage between 6, 18) to perform molten salt electrolysis, After the molten salt electrolysis is performed and the impurity element is collected on the cathode (16) by the molten salt electrolysis in the first stage, the cathode (16) is exchanged for another cathode (18) to perform the second electrolysis. A method for purifying metallic uranium, comprising recovering metallic uranium to said another cathode (18) by a step of molten salt electrolysis. 2. A metal uranium containing an impurity element is loaded in a basket (14) and immersed in a molten fluoride salt (13) inside an electrolytic cell (11) by loading the metal uranium (12) containing the impurity element. The method for purifying metallic uranium according to claim 1, wherein (12) is dissolved in the fluoride molten salt (13). 3. A method for promoting the dissolution of metal uranium (12) containing an impurity element by blowing fluorine gas into the molten fluoride salt (13) when the basket (14) is immersed in the molten fluoride salt (13). Item 4. The method for purifying metallic uranium according to Item 2. 4. The molten fluoride salt (13) is LiF—NaF,
_{LiF-KF, LiF-BeF 2} , NaF-BeF 2, K
F-BeF ₂ binary molten salts and LiF-NaF-K
F, LiF-NaF-BeF ₂ , NaF-KF-Be
_{F 2, LiF-NaF-CaF} 2, LiF-KF-BaF
_The method for purifying metallic uranium according to any one of claims 1 to 3, wherein the molten salt is any molten salt selected from the ternary molten salts of (2).