KR102219518B1 - Method for removing rare earth from spent nuclear fuel and electrodeposition method comprising the same - Google Patents

Abstract

The present invention relates to a method for removing rare earths from spent nuclear fuel and an electrolysis method including the same, and more particularly, to a spent nuclear fuel obtained by electrolytic reduction of pyroprocessing, and casts free for reaction with rare earth metals. Placing it in a sacrificial crucible having an energy of -50 to -300 kJ mol ^-1; Melting the spent nuclear fuel by heating the sacrificial crucible at a temperature of 1200°C to 1400°C; And removing the rare earth in the form of a metal oxide and tapping the melt from which the rare earth has been removed, and to an electrolysis method including the same.

Description

Method for removing rare earth from spent nuclear fuel and electrodeposition method comprising the same}

The present invention relates to a method of removing rare earths from spent nuclear fuel and an electrolysis method including the same, and more particularly, by using a sacrificial crucible, the transuranium (TRU) metal is used in a spent fuel metal convertor before the electrolytic recovery process. It relates to a method for effectively extracting and removing only rare earth (RE) metals without loss, and an electrolytic method including the same.

The pyro process is a process for recovering U (uranium)/TRU (transuranium) metals that can be reused from spent nuclear fuel in the form of oxide. The pyro process is largely a pretreatment process that recovers oxide nuclear fuel from spent nuclear fuel rods, an electrolytic reduction process that converts the recovered oxide nuclear fuel into a metal form, and recovers metal U and U/TRU (transuranic) from the metal converter. It can be divided into electrolytic recovery process for The electrolytic recovery process is again an electrorefining process that selectively recovers metallic uranium from a metal converting body, an electrolytic refining process that recovers U/TRU, and a drawdown (DD, Drawdown) that additionally recovers TRU and rare earth (RE, Rear Earth). It can be subdivided into processes. In the electrolytic refining process, the spent nuclear fuel metal convertor is put in a molten salt at 500°C, and uranium is selectively recovered in the form of dendrite to the solid cathode using an electrochemical method. U/TRU is recovered through an electrochemical method by replacing the electrode with a carbon anode. Each recovered material is prepared as a metallic ingot after removing the salt in a high temperature vacuum atmosphere.

Currently, in the U/TRU recovery electrosmelting process, an inert electrode is used as an anode and a liquid cadmium electrode (LCC) is used as a cathode. After configuring the electrode cell, a sufficient current flows to recover U/TRU to the LCC, but when only U and TRU are recovered, the concentration of RE in the molten salt increases when the amount of spent fuel is increased, so the recovered U/TRU product is RE This is because some REs are co-electrodeposited with U/TRUs when U/TRU is electrochemically recovered by using a liquid cadmium electrode (LCC) as a cathode in the electrolytic smelting process. That is, when the concentration of RE in the molten salt rises above a certain level, a problem occurs in that a large amount of RE is co-electrodeposited to the extent that the U/TRU product does not meet the conditions for producing nuclear fuel at a high speed. In this case, since the LCC operation must be performed under the condition that a large amount of RE is not electrodeposited, an additional drawdown process is required because a large amount of U/TRU remains in the electrolyzer as well as a problem of lengthening the operation time.

Therefore, in order to keep the RE concentration below a certain level, RE must be removed from the molten salt and discarded. To remove RE from molten salt, a TRU drawdown (DD) process and a RE drawdown (DD) process are required. In this way, for the efficient recovery of U and TRU, which is the main purpose of pyroprocessing, U recovery electrorefining and U/TRU recovery electrosmelting processes, as well as TRU drawdown and RE drawdown processes, are used in parallel.

However, it is difficult to recover the entire U/TRU while maintaining a low RE content due to the distribution coefficient of the Cd liquid cathode even using the existing electrolytic smelting and drawdown processes. Therefore, in order to solve this problem, it is necessary to effectively extract/remove only RE metal without loss of TRU metal from the spent fuel metal converter before the electrolytic recovery process.

Existing technologies that can be applied to the RE drawdown process include a method of oxidizing RE chloride in molten salt by injecting chemical additives such as phosphate (Patent 10-1474146), and RE chloride by contacting molten salt with liquid metal such as bismuth. Reduction and dissolution into liquid metal (Patent 10-1693775). However, these methods require a new device for converting RE chloride, as well as subsequent steps such as distillation or centrifugation at a high temperature in order to collect and discard the converted RE, so the process is complicated and uneconomical. Therefore, there is an urgent need for a new concept of RE drawdown process to effectively remove chloride-type RE in molten salt.

Accordingly, one aspect of the present invention relates to a method of removing rare earths from spent nuclear fuel.

Another aspect of the present invention relates to an electrolysis method comprising a method of removing rare earths from the spent nuclear fuel.

According to one aspect of the present invention, melting the spent nuclear fuel by heating the spent nuclear fuel obtained by electrolytic reduction of the pyroprocessing at the temperature of the RE metal; And removing the rare earth in the form of metal oxide and tapping the melt from which the rare earth has been removed, and a method for removing rare earth from spent nuclear fuel is provided.

According to another aspect of the present invention, the spent nuclear fuel obtained by electrolytic reduction of pyroprocessing is disposed in a sacrificial crucible having ^{a Gibbs free energy for reaction with a rare earth metal of -50 to -300 kJ mol -1.} step; Melting the spent nuclear fuel by heating the sacrificial crucible at a temperature of 1200°C to 1400°C; Removing the rare earth in the form of a metal oxide, and tapping the melt from which the rare earth is removed; And recovering uranium and transuranium from the rare earth metal oxide.

According to the present invention, it is possible to remove the rare earths present in the electrolytic salt using only the electrolytic refining and smelting process without any additional subsequent processes, and recover all amounts of uranium and transuranium, and at the same time satisfy the quantity of manufacturers of sodium cooling fast reactor (SFR) nuclear fuel. have.

1 shows an exemplary process conceptual diagram of a method of removing rare earths from spent nuclear fuel of the present invention.
2 shows a photograph of a U/RE mixture during the process of performing Experimental Example 1.
3 shows a photograph of the lower U melt during the process of performing Experimental Example 1.
Figure 4 shows the LiCl-KCl salt and the residual U mixture separated by using a vacuum distillation method.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

According to the present invention, through fusion and smelting using a sacrificial crucible, the entire amount of uranium (U) and ultra-uranium (TRU) present in the electrolytic salt are recovered by using only the electrolytic refining and smelting process without additional subsequent processes, and at the same time, the sodium cooling furnace It is provided for a method of removing rare earths from spent nuclear fuel that can meet (SFR) nuclear fuel manufacturers.

The method of removing rare earths from spent nuclear fuel of the present invention uses spent nuclear fuel obtained by electrolytic reduction of pyroprocessing, and the Gibbs free energy for reaction with rare earth metals is -50 to -300 kJ·mol ^-1 . Placing it in the sacrificial crucible; Melting the spent nuclear fuel by heating the sacrificial crucible at a temperature of 1200°C to 1400°C; And removing the rare earth in the form of a metal oxide and tapping the melt from which the rare earth has been removed.

The sacrificial crucible that can be used in the present invention is not particularly limited as long as it is a Gibbs free energy for reaction with a rare earth metal such as Nd, Ce, La, Pr, etc. -50 to -300 kJ mol ^{-1, and is preferably} Is a Gibbs free energy of -150 to -200 kJ mol ^-1 . If the Gibbs free energy is ^{less than -200 kJ mol -1} , there is a problem of oxidizing not only rare earth metals but also U/TRU, and Gibbs free energy is -50 kJ · mol ^- if more than ¹ there is a problem that the reaction rate is very slow.

For example, the sacrificial crucible that can be used in the present invention is zirconium oxide (ZrO ₂ ), aluminum oxide (Al ₂ O ₃ ), magnesia (MgO), yttria (Y ₂ O ₃ ) and urania (UO ₂ ) It is made of at least one material selected from the group consisting of, and is preferably zirconium oxide (ZrO ₂ ).

Such sacrificial crucibles have high reactivity with rare earth metals and low reactivity with uranium, thereby forming rare earths in the form of oxides, thereby forming a melt from which the rare earths are removed. Therefore, when the molten material is tapped, rare earth oxides can be recovered in the form of dross and skull.

In this case, the method of tapping the melt is not particularly limited, and, for example, a lower trip, tilting, and the like may be applied. The tapped melt can be subsequently used in the electrolytic recovery process.

Further, according to the present invention, there is provided an electrolysis method including a method of removing rare earths from the spent nuclear fuel.

In the electrolysis method of the present invention, the Gibbs free energy for the reaction of spent nuclear fuel obtained by electrolytic reduction of pyroprocessing with rare earth metals (Nd, Ce, La, Pr, etc.) is -50 to -300 kJ·mol ⁻ Placing it in a sacrificial crucible for ^{one person;} Melting the spent nuclear fuel by heating the sacrificial crucible at a temperature of 1200°C to 1400°C; Removing the rare earth in the form of a metal oxide, and tapping the melt from which the rare earth is removed; And recovering uranium and transuranium from the rare earth metal oxide.

That is, the electrolytic method of the present invention comprises the steps of removing rare earths in the form of metal oxides and tapping the melt from which the rare earths have been removed, following the method of removing rare earths from the spent nuclear fuel of the present invention; And recovering uranium and transuranium from the rare earth metal oxide.

In relation to the method of removing rare earths from spent nuclear fuel, all the above descriptions can be applied equally.

On the other hand, in the electrolysis method of the present invention, the step of recovering uranium and transuranium from the rare earth metal oxide comprises dissolving the rare earth metal oxide in the form of a rare earth metal chloride by charging the rare earth metal oxide into a molten salt in an electrolytic device. step; And charging the anode and the cathode and applying a current to electrodeposit the rare earth to the cathode.

When the rare earth metal oxide is charged into the molten salt in the electrolytic device and the rare earth metal oxide is dissolved in the form of a rare earth metal chloride, the uranium/transuranium residue is finally present in the salt.

The molten salt is LiCl-KCl, NaCl-KCl, LiCl-NaCl-CaCl ₂ -BaCl ₂ , NaCl-KCl-BaCl ₂ and LiCl-KCl-UCl ₃ Any one of the chloride may be used, for example LiCl-KCl-UCl ₃ may be used. In the case of using LiCl-KCl-UCl ₃ , for example, the rare earth oxide is dissolved in the form of chloride by the following equation (1), and the U/TRU oxide residue is finally present in the _{LiCl-KCl-RECl 3 salt.} Is done.

2RE ₂ O ₃ + 4UCl ₃ → 4RECl ₃ + 3UO ₂ + U Equation (1)

However, at this time because it reliably measures the composition of the rare earth oxide it is difficult, it is possible to add a ₃ bit by bit to determine the dissolution UCl completion of rare earth by check whether the remaining UCl _3. That is, if a _{small amount of UCl 3 remains as UCl 3} is continuously added to the LiCl-KCl salt, a small amount of rare earth metal may be added to make only _{RECl 3 remain in the LiCl-KCl salt.}

The dissolving step may be performed at a temperature of 450 to 550 °C, preferably 490 to 510 °C, more preferably 500 °C It can be done at a nearby temperature. When the temperature is less than 450° C., there is a problem that the conductivity and solubility of ions in the LiCl-KCl salt is lowered, and when the temperature is higher than 550° C., salts having a high vapor pressure may be volatilized.

After converting the rare earth oxide to rare earth chloride as described above, the rare earth is electrodeposited on the negative electrode through an electrochemical method in the molten salt. Meanwhile, the U/TRU oxide mixed with the salt from which the rare earth has been removed can be separated from the salt using a vacuum distillation method and reused in a pretreatment or electrolytic reduction process.

The anode that can be charged into the electrolytic apparatus of the present invention is preferably an inert anode, and, for example, a group consisting of tungsten (W), graphite, platinum (Pt), gold (Au), and molybdenum (Mo). It is preferable that it is made of at least one material selected from.

Meanwhile, in this case, the negative electrode may be a liquid or solid negative electrode.

More specifically, a liquid cathode made of at least one material selected from the group consisting of metals present in a liquid state at a temperature of 450 to 550°C, such as cadmium (Cd), zinc (Zn), bismuth (Bi), and gallium (Ga) In this case, the cathode may be disposed on a cathode crucible formed of any one of alumina (Al2O3), aluminum nitride (AlN), zirconia (ZrO2), yttria (Y2O3), and beryllia (BeO). have.

Alternatively, the cathode may be a solid cathode made of at least one material selected from the group consisting of molybdenum (Mo), stainless steel (STS), aluminum (Al), and tungsten (W).

In the step of electrodepositing the rare earth to the negative electrode, it is preferable to terminate the electrodeposition when the electrodeposition potential of the negative electrode reaches the Li electrodeposition potential. At this time, the Li electrodeposition potential is-2.4 V (vs. Ag/AgCl) or less. If current is applied until the cathode electrodeposition potential changes to the Li electrodeposition potential, all rare earths can be electrodeposited to the cathode.

Electrodeposition of the rare earth to the negative electrode may be performed at a temperature of 450 to 550 ℃, preferably 490 to 510 ℃, more preferably 500 ℃ It can be done at a nearby temperature. When the temperature is less than 450°C, there is a problem that the conductivity and solubility of the rare earth metal ions in the LiCl-KCl salt is lowered, and when the temperature is higher than 550°C, salts having a high vapor pressure may be volatilized.

The present invention further includes the step of distilling the rare earth metal recovered from the negative electrode, such that the rare earth metal recovered from the negative electrode can be recovered and reused through a distillation process. At this time, the distillation method and apparatus that can be used to perform the distillation process is not particularly limited, but, for example, a vacuum distillation method and a vacuum distillation apparatus may be used.

As described above, according to the present invention, the rare earth present in the electrolytic salt is removed by using only the electrolytic refining and smelting process without any additional subsequent processes, and the total amount of uranium and ultra-uranium is recovered, and at the same time, the quantity of a sodium cooling fast reactor (SFR) nuclear fuel manufacturer is satisfied. I can make it.

Hereinafter, the present invention will be described in more detail through specific examples. The following examples are only examples to aid understanding of the present invention, and the scope of the present invention is not limited thereto.

Example

Example One

Rare earth was removed from the spent nuclear fuel in the form of metal oxide by the following process from the number of kg of spent nuclear fuel in metal form obtained after the electrolytic reduction process of pyroprocessing, and uranium and transuranium were recovered from the rare earth metal oxide. The process is schematically shown in FIG. 1.

① The spent nuclear fuel was put in a _{high-reactivity ZrO 2} ^{sacrificial crucible with rare earth and smelted at a high temperature of 1200 o} C or higher, and the molten material was tapped at the bottom to remove the rare earth in the form of oxide. As a result, the U/TRU/Zr metal mixture obtained from Dross and Skull was recovered U (electrolytic refining) and U/TRU (electrolytic refining) through the following electrolytic recovery process.

② First, recover Dross and Skull and LiCl-KCl-UCl ₃ 500 It was charged in a molten salt at °C, and the rare earth oxide was dissolved in the form of chloride as shown in the following reaction formula. As a result, the U/TRU oxide residue in the reactor is finally LiCl-KCl-RECl ₃ It will be present in the salt.

2RE ₂ O ₃ + 4UCl ₃ → 4RECl ₃ + 3UO ₂ + U Equation (1)

At this time, since it is difficult to reliably measure the composition of the dross and skull residues, UCl ₃ is added little by little and the presence _{of UCl 3} is checked by Cyclic Voltammetry (CV) to determine whether the rare earth is completely dissolved. can do. That is, when the UCl ₃ as the continuously added to a LiCl-KCl salt of UCl ₃ consumption amount reaction, a small amount of UCl ₃ again, UCl ₃ exits the RE and the reaction in the LiCl-KCl salt and a small amount UCl _{Only 3} can be left. At this time, a small amount of rare earth metal may be added so that only _{RECl 3 remains in the LiCl-KCl salt.}

③ UCl ₃ was added as above to convert all rare earth oxides to rare earth chloride (RECl ₃ ), and then LiCl-KCl-RECl ₃ Rare earth is electrodeposited on the cathode through an electrochemical method on the salt. At this time, graphite, which is an inert anode, was used as the anode, and a crucible for a Zn liquid cathode and a BeO liquid cathode was used as the cathode. Electric current was applied until the cathode electrodeposition potential changed from -1.4 to 1.5 V (vs. Ag/AgCl) to Li electrodeposition potential of -2.4 to -2.5 V (vs. Ag/AgCl) to deposit all rare earths on the cathode.

④ Rare earth metal recovered from the cathode can be recovered and reused through a distillation process at a temperature of about 1000℃ or higher and a pressure of 50 mTorr or less.

⑤ In addition, U/TRU oxide mixed with LiCl-KCl salt in the state of removing rare earth is separated from LiCl-KCl salt by using vacuum distillation method at a temperature of about 1000℃ or higher and pressure of 50 mTorr or less, and pretreatment or electrolytic reduction. Can be reused in the process.

Experimental example One

By simulating the spent nuclear fuel in metal form obtained after the electrolytic reduction process of pyroprocessing, rare earths were removed from about 2.2 kg of uranium and rare earth metals in the form of metal oxides by the following procedure, and uranium was recovered from the rare earth metal oxides.

① Put the rare earth and highly reactive Y ₂ O _{3 in a} sacrificial crucible and ^{smelt at a high temperature of 1200 o} C or higher to solidify the dissolved material inside the crucible.

② Since the device possessed is a structure that is impossible to tap the lower part, the lower part of the solidified melt was cut with a cutter and separated into the U part in the lower part and the U/RE mixture part in the upper part. Figure 2 shows a photograph of the U/RE mixture.

③ The content of RE contained in the U part of the lower part is about 1 wt%, ensuring the purity that can be used for nuclear fuel manufacturing. Figure 3 shows a picture of the lower U melt.

④ Only rare earths were selectively removed from the upper U/RE mixture obtained with Dross and Skull to recover uranium. For the convenience of the experiment, about 2 g of Dross and Skull were recovered and LiCl-KCl-UCl ₃ 500 It was charged in a molten salt at °C, and the rare earth oxide was dissolved in the form of chloride as shown in the following reaction formula. As a result, a U oxide residue is finally present in the LiCl-KCl-RECl ₃ salt in the reactor.

2RE ₂ O ₃ + 4UCl ₃ → 4RECl ₃ + 3UO ₂ + U Equation (1)

At this time, since it is difficult to reliably measure the composition of the dross and skull residues, UCl ₃ is added little by little and the presence _{of UCl 3} is checked by Cyclic Voltammetry (CV) to determine whether the rare earth is completely dissolved. can do. That is, UCl ₃ a when continuously as the addition of a LiCl-KCl salt is UCl ₃ total volume reaction, a small amount of UCl ₃ again, UCl ₃ exits the RE and the reaction in the LiCl-KCl salt UCl a small amount _{of 30,000} You can make it remain.

⑤ U oxide mixed with LiCl-KCl salt in the state where rare earth was removed was recovered by separating from LiCl-KCl salt using a vacuum distillation method at a pressure of about 1000° C. and 50 mTorr or less. This residue can be reused in pretreatment or electrolytic reduction processes. Figure 4 shows the LiCl-KCl salt and the residual U mixture separated by using a vacuum distillation method.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and variations are possible without departing from the technical spirit of the present invention described in the claims. It will be obvious to those of ordinary skill in the field.

Claims (13)

delete delete Disposing the spent nuclear fuel obtained by electrolytic reduction of pyroprocessing in a sacrificial crucible having a Gibbs free energy for reaction with a rare earth metal of -50 to -300 kJ·mol ^-1;
Melting the spent nuclear fuel by heating the sacrificial crucible at a temperature of 1200°C to 1400°C;
Tapping the melt from which the rare earth in the form of metal oxide has been removed; And
And recovering uranium and transuranium from the rare earth metal oxide,
In the step of recovering uranium and transuranium from the rare earth metal oxide, charging the rare earth metal oxide into a molten salt in an electrolytic apparatus to dissolve the rare earth metal oxide in the form of a rare earth metal chloride, and charging the anode and the cathode and applying an electric current. And electrodepositing the rare earth to the cathode by applying.
The method of claim 3, wherein the sacrificial crucible is from the group _{consisting of zirconium oxide (ZrO 2} ), aluminum oxide (Al ₂ O ₃ ), magnesia (MgO), yttria (Y ₂ O ₃ ), and urania (UO ₂ ). Made of at least one material selected, the electrolysis method.
delete The method of claim 3, wherein the molten salt is a chloride of any one of LiCl-KCl, NaCl-KCl, LiCl-NaCl-CaCl ₂ -BaCl ₂ , NaCl-KCl-BaCl ₂ and LiCl-KCl-UCl ₃ .
The method of claim 3, wherein the dissolving step is performed at a temperature of 450 to 550°C.
The method of claim 3, wherein the anode is made of at least one material selected from the group consisting of tungsten (W), graphite, platinum (Pt), gold (Au), and molybdenum (Mo).
The method of claim 3, wherein the negative electrode is a liquid negative electrode made of at least one material selected from the group consisting of metals present in a liquid state at a temperature of 450 to 550°C, and alumina (Al2O3), aluminum nitride (AlN), zirconia (ZrO2) ), yttria (Y2O3), beryllia (BeO) disposed on the cathode crucible formed of any one of the material, electrolysis method.
The method of claim 3, wherein the cathode is a solid cathode made of at least one material selected from the group consisting of molybdenum (Mo), stainless steel (STS), aluminum (Al), and tungsten (W).
The method of claim 3, wherein the step of electrodepositing the rare earth to the negative electrode terminates the electrodeposition when the electrodeposition potential of the negative electrode reaches a Li electrodeposition potential, which is a reference potential.
The method of claim 3, wherein the step of electrodepositing the rare earth to the negative electrode is performed at a temperature of 450 to 550°C.
The method of claim 3, further comprising distilling the rare earth metal recovered from the cathode.

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