JPH11142585A - Method for converting oxide into metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To convert an oxide into a metal by performing cathode reduction with the oxide as a cathode in place of the reduction of an oxide by metal lithium. SOLUTION: Li2 O is added to molten salt by a charge machine 7. Voltage is applied to a carbon anode and basket by an electrolysis power source 8, while an anode rotation device 6 rotates a carbon anode. When voltage is applied so as to make the basket negative potential lower than the carbon anode, oxide acts as a cathode, and the oxide is converted into a metal by chemical reaction to produce Li2 O. Since the carbon anode is rotated by the anode rotation device 6, convection produces toward the carbon anode from the bottom center of an electrolytic cell, and produced Li2 O is carried to an anode. On the anode, Li2 O is decomposed by chemical reaction and concentration of Li2 O is held constant. Reduction reaction of PuO2 +C→Pu+CO2 is electro- chemically performed, and conversion to the metal of the oxide is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for recovering uranium and plutonium after converting spent oxide nuclear fuel to metal, and more particularly to a method suitable for converting spent nuclear fuel to metal. .

[0002]

2. Description of the Related Art A method of reprocessing nuclear fuel for recovering uranium and / or plutonium from spent oxide nuclear fuel by electrolysis in a molten salt is described in JP-A-8-54493. In this method, uranium and transuranium elements contained in spent oxide nuclear fuel are reduced to a metal in a molten salt, and then this metal is used as an anode for electrolysis to convert uranium metal to a solid cathode and to a liquid cadmium cathode. Transuranium elemental metals such as uranium and plutonium are deposited to recover uranium and plutonium.

In this method, a spent oxide atomic fuel is immersed in a molten salt of lithium chloride or a molten salt of lithium chloride and potassium chloride containing lithium metal in a saturated concentration or more to oxidize lithium metal and uranium and / or transuranium elements. Reacts with the substance to convert uranium and / or transuranium elements into metals. This chemical reaction

[0004]

Embedded image UO ₂ + Li = U + Li ₂ O (1)

[0005]

Embedded image PuO ₂ + Li = Pu + Li ₂ O (2) In this method of converting to metal using lithium metal, the lithium metal is consumed and a lithium oxide Li ₂ O that hinders the reduction reaction is generated.

Therefore, the molten salt is electrolyzed periodically to obtain L
Reconvert i ₂ O to lithium metal. At this time, carbon is preferably used as the anode. The electrode reaction is

[0007]

Embedded image Li ₂ O = 2Li ⁺ + O ^2- (3)

[0008]

Embedded image Li ⁺ + e ⁻ = Li (cathode) (4)

[0009]

Embedded image 2O ²⁻ + C ＝ CO ₂ + 4e ⁻ (anode) (5)

[0010]

According to the above invention, uranium and / or plutonium can be recovered from spent nuclear fuel.

However, the above-mentioned invention does not consider the following two points. First, since the metal lithium is contained at a saturation concentration or more, the excess metal lithium is separated from the molten salt and floats on the molten salt. Exposure of the easily combustible metallic lithium onto the molten salt requires measures to avoid ignition and heat generation, complicates the apparatus and increases the amount of maintenance work. The second point is that the molten salt, Li ₂ O, must be electrolyzed periodically, which lowers the operation rate of the apparatus.

An object of the present invention is to provide a method for simultaneously using a molten salt having a lithium metal concentration of not more than the saturation solubility and simultaneously performing metal conversion of uranium and plutonium and electrolysis of Li ₂ O. It is to provide a conversion method.

[0013]

According to the present invention, in order to solve the above-mentioned problems, an oxide is converted to a metal by performing a cathode reduction using an oxide as a cathode instead of reducing the oxide with metallic lithium. . At this time, carbon is preferably used for the anode. When uranium and / or plutonium oxide is immersed in a molten salt containing lithium chloride, the oxide is used as a cathode, and carbon is used as an anode to pass an electric current.

[0014]

Embedded image UO ₂ + 4Li ⁺ + 4e ⁻ → U + 2Li ₂ O (6)

[0015]

The oxide is converted to a metal by the reaction of PuO ₂ + 4Li ⁺ + 4e ⁻ → U + 2Li ₂ O (7) and Li ₂
O 2 is produced.

On the other hand, at the anode

[0017]

Embedded image Li ₂ O is decomposed by the reaction of 2Li ₂ O + C → 4Li ⁺ + CO ₂ (8) As the voltage between the cathode and anode gradually increases, the current initially increases, but saturates at a certain value and reaches an upper limit. Since the upper limit of the current flowing to the anode is governed by the rate at which the Li ₂ O reaches the anode by diffusion, the current flowing through the anode higher the concentration of Li ₂ O is large.

However, since the currents flowing through the cathode and the anode have the same value, the upper limit of the current flowing through the cathode increases as the concentration of Li ₂ O increases. The conversion to the metal of the reaction formulas (6) and (7) is performed by the current flowing through the cathode, and therefore, the upper limit of the conversion speed to the metal increases as the concentration of Li ₂ O increases. That is, when the applied voltage between the cathode and the anode is increased, the conversion speed to metal gradually increases,
Eventually, the upper limit is reached, and the upper limit increases as the concentration of Li ₂ O increases. Therefore, to perform metal conversion quickly, L
The concentration of i ₂ O may be increased, and is preferably a saturation concentration.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) A preferred embodiment of the present invention, in which an apparatus for converting uranium and plutonium oxide to metal is described with reference to FIG. This metal conversion device is composed of an electrolytic cell 1, a molten salt 2 for electrolysis stuck in the electrolytic cell 2, an electrically conductive basket 4 holding uranium and plutonium oxide 3, a carbon anode 5, an anode rotating device 6, Li _{It comprises} a charging machine 7 for adding ₂ O to the molten salt, a power supply 8 for electrolysis, and a heater 9 for electrolyzer. The molten salt contains lithium chloride, and is maintained at a temperature of 500 ° C. or more by the electrolytic bath heater 9 to improve the electrical conductivity of the oxide. Uranium and plutonium oxide 3 is loaded into an electrically conductive basket 4 and held at the bottom of the electrolytic cell. The upper limit of the conversion rate of oxide to metal by electrolysis is L
Since the higher the concentration of i ₂ O is, the larger the concentration is, the more the Li ₂ O is added to the molten salt by the charging machine 7. A voltage is applied to the carbon anode and the basket by the electrolytic power source 8 while rotating the carbon anode by the anode rotating device 6. When a voltage is applied so that the basket has a negative potential from the carbon anode, the oxide acts as a cathode,

[0020]

Embedded image UO ₂ + 4Li ⁺ + 4e ⁻ → U + 2Li ₂ O (9)

[0021]

Embedded image _{^{PuO 2 + 4Li + + 4e -}} → Li 2 together with Pu + 2Li oxide by reaction of ₂ O ... (10) is converted to the metal
O 2 is produced. Since the carbon anode is rotated by the anode rotating device 6, convection occurs from the bottom center of the electrolytic cell toward the carbon anode, and the generated Li ₂ O is carried to the anode. At the anode

[0022]

[Image Omitted] Li ₂ O is decomposed by the reaction of 2Li ₂ O + C → 4Li ⁺ + CO ₂ (11), and the Li ₂ O concentration is kept constant. Cathode and anode reactions as a whole

[0023]

Embedded image UO ₂ + C → U + CO ₂ (12)

[0024]

Embedded image The reduction reaction of PuO ₂ + C → Pu + CO ₂ (13) is performed electrochemically, and the oxide is converted to a metal.

[0025]

As compared with the conventional method, the present invention does not contain metallic lithium, so that flammable metallic lithium is not exposed on the molten salt, and it is not necessary to take measures to avoid ignition or heat generation. This has the effect of simplifying the apparatus and reducing the amount of maintenance work. Further, since the decomposition of Li ₂ O is carried out simultaneously with the metal conversion, periodic electrolysis of operating the Li ₂ O as in the conventional methods can be omitted, there is a high becomes effective utilization.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an apparatus for converting an oxide of uranium and plutonium to a metal according to a preferred embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electrolysis tank, 2 ... Molten salt, 3 ... Oxide, 4 ... Basket, 5 ... Carbon anode, 6 ... Anode rotating device, 7 ... Injector, 8 ... Electrolysis power supply, 9 ... Electrolyzer heating heater.

Claims (2)

[Claims] A method for converting an oxide of uranium and / or plutonium into a metal by dipping the oxide of uranium and / or plutonium in a molten salt, wherein the molten salt contains lithium chloride and the oxide is electrolyzed using the oxide as a cathode. A metal conversion method of an oxide, comprising reducing the metal to a metal. 2. The method according to claim 1, wherein the metal conversion rate is increased by adding Li ₂ O to the molten salt in advance.
Metal oxide conversion method.