JPH1123784A - Method for separating actinoid and lanthanoid elements - Google Patents

Abstract

(57) [Abstract] [Problem] It does not require strict concentration control, has an extremely high separation efficiency (separation coefficient), can easily separate actinoid elements in the subsequent process, and generates waste that is difficult to treat Not provide actinide elements. SOLUTION: In a group separation method for separating a high-level waste liquid for each element group, a HDEHP solvent 1 is mixed with an An fraction 2 containing an actinoid element to extract a solvent 6 to be treated containing an actinoid element. An aqueous solution 3 containing hydrazine carbonate is mixed as a reverse extract into the mixture, and this mixture is separated into an aqueous phase 4 and an organic phase 5, and the actinoid element is recovered from the aqueous phase. By setting the concentration of hydrazine carbonate to preferably about 0.5 mol / l or more and about 0.7 mol / l or less, the distribution ratio between the actinoid element and the lanthanoid element can be optimized and very high. Separation efficiency can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a technique for reprocessing spent nuclear fuel, and more particularly to a method for separating actinoid elements and lanthanoid elements in a group separation method for separating high-level waste liquid into element groups.

[0002]

2. Description of the Related Art In the reprocessing of spent nuclear fuel generated in a nuclear power plant, a group separation method in which radionuclides contained in high-level waste liquid are separated (group separation) according to their half-life and radiotoxicity and processed. Is currently being researched and developed in various countries. In particular, actinoid elements are long-lived alpha nuclides that remain toxic for a long period of time, and it is desired to separate them from high-level waste liquids and fission them to make them harmless or short-lived nuclides.

[0003] Fig. 3 is a schematic flow chart of a four-group separation process, which is the latest group separation process, and includes a combination of extraction separation (or precipitation separation), precipitation separation (or adsorption separation), inorganic ion exchange separation, and the like. , High-level effluents are grouped into transuranium elements, technetium-platinum elements, strontium-cesium groups, and other fission products. In this drawing, in the extraction separation (or precipitation separation), it is necessary to separate the transuranium element and the rare earth element, but these elements are elements of the same family in the periodic table and are generally difficult to separate. Such a group separation method,
For example, it is disclosed in a research report by the inventors of the present invention (Japan Atomic Energy Research Institute, Research Report 95-011, "Method of Back Extraction of Uranium from DIDPA Solvent", March 1995).

Here, the actinoid element is Ac, T
Actinium-based elements such as h, Pa, U, Np, Pu, Am, and Cm, and those having an atomic number of U or more (U, Np, Pu, Am, Cm, etc.) are referred to as transuranium elements. The lanthanoid elements are La, Ce, Pr,
Lanthanum-based elements such as Nd, Pm, Sm, Eu, and Dy, and further referred to as rare earth elements including Sc and Y.

FIG. 4 shows an example of a process for separating actinide elements from a high-level waste liquid (referred to as a TRUEX process). In the back extraction 1 in this process, the An fraction back-extracted with dilute nitric acid contains both actinide elements and lanthanoid elements. A problem that is currently an issue in the group separation process is that a large amount of lanthanoid elements are simultaneously back-extracted into a solution (An fraction) containing the back-extracted actinoid elements as shown in FIG. That is, the actinoid element and the lanthanoid element are very similar in nature because they are homologous, and there is a problem that efficient separation cannot be performed by the conventional process.

FIG. 5 is a flowchart showing a conventional process for separating an actinoid element and a lanthanoid element. This process comprises an extraction step and a back-extraction step. In the back-extraction step, the actinoid element and the lanthanoid element extracted in the extraction solvent are added to the aqueous phase A by the actinoid element (A
m, Cm, etc.) to separate lanthanoid elements into the organic phase O. The extraction / back extraction in this figure is usually performed by mixing a liquid to be treated (high-level waste liquid, etc.) with an appropriate extraction solvent / back extraction solution, allowing the mixture to stand, and separating it into an organic phase O and an aqueous phase A. Done. Here, the molar ratio (the number of moles in the organic phase / the number of moles in the aqueous phase) when a specific component is dissolved in an equilibrium state in both the organic phase O and the aqueous phase A is referred to as “partition ratio”.
Call.

[0007] In the reverse extraction of FIG.
After extraction with HP (di (2-ethylhexyl) orthophosphoric acid) solvent, DTPA (Di-ethylene Tri
Actinoid elements (Am, Cm, etc.) are extracted and separated into the aqueous phase using -amine Pentaacetic Acid). As a method of separating and recovering this actinoid element, TALSPE
AK method, Reversed TALSPAK method, DI
The DPA method and the like have been conventionally known.

[0008]

FIG. 6 is a diagram showing the relationship between the pH value and the distribution ratio in a conventional method for separating and recovering actinoid elements ("Development of a patitioning p.
rocess for the management of high-level waste ", FU
TURE NUCLEAR SYSTEMS: EMERGING FUEL CYCLES AND WAS
TE DISPOSAL OPTIONS, GLOBAL '93, September 1993, A
American NuclearSociety, Inc. More excerpt).

In FIG. 6, Am and Nd are extracted by extracting Am into the aqueous phase A and extracting Nd into the organic phase O.
In order to separate Nd, the distribution ratio of Am is 1 or less and Nd
It is necessary to adjust the pH to a range in which the distribution ratio becomes 1 or more, for example, in this example, a range of about 3.4 to 3.8 indicated by an arrow in the figure. However, as in this example, p
The tolerance of H is very narrow, so that the conventional DTPA
Back extraction requires highly precise and extremely strict pH adjustment, and if this pH value changes even slightly, the separation coefficient (DLn
/ DAn) is very poor.

The separation coefficient (DLn / DAn) is the ratio of the distribution ratio DLn of the lanthanoid element to the distribution ratio Dan of the actinoid element. The larger the separation coefficient, the more efficient the separation. The number of stages can be reduced.

Further, a large amount of a reagent (such as lactic acid or H
Since COOH) is used, these reagents remain as impurities in a subsequent process, and there is a problem that the treatment becomes difficult. Further, the DTPA used for the back extraction is also recovered as waste in a later step, and there is a problem that it is difficult to treat the waste DTPA.

The present invention has been made to solve such a problem. That is, an object of the present invention is to provide an actinide element that does not require strict concentration control, has a high separation efficiency (separation coefficient), can easily separate actinide elements in a post-process, and does not generate waste that is difficult to treat. And a method for separating lanthanoid elements.

[0013]

According to the present invention, in a group separation method for separating a high-level waste liquid for each element group, a HDEHP solvent is mixed with an An fraction containing an actinoid element to form a solvent to be treated containing an actinoid element. Is extracted, an aqueous solution containing hydrazine carbonate is mixed into the solvent to be treated as a back extract, and the mixture is separated into an aqueous phase and an organic phase,
A method for separating an actinoid element and a lanthanoid element is provided, wherein the actinoid element is recovered from the aqueous phase.

In order to solve the above-mentioned new problem, the inventors of the present invention have focused on a salt-free reagent that is easily decomposed, and have used a hydrazine carbonate solution, which is one of such salt-free reagents, to determine the exact concentration. Various experiments have found conditions that have high separation efficiency (separation coefficient) without requiring control and do not generate an emulsion phase. The present invention is based on such a new finding.

That is, according to the present invention, salt-free
Hydrazine carbonate (N_TwoH _Four)_TwoCO_TwoWater-soluble
Since the liquid is used as a back-extract, nitric acid
Easily by solution, electrolysis, thermal decomposition, etc._TwoGas, H_Two
Gas, CO_TwoAm, C
actinoid elements such as m can be easily separated
You.

Further, by using an HDEHP solvent as an extraction solvent in combination with this, the relationship between the concentration of hydrazine carbonate and the distribution ratio is close to parallel between the actinoid element and the lanthanoid element through experiments, thereby stabilizing it over a wide range. And it was confirmed that a very large separation coefficient was obtained. Therefore, it does not require strict concentration control as compared with the pH adjustment in the conventional method, has a high separation efficiency (separation coefficient), can easily separate actinoid elements in a later process, and has a difficult-to-treat waste. A method for separating actinide elements and lanthanoid elements that do not occur can be provided.

According to a preferred embodiment of the present invention, the concentration of the hydrazine carbonate is set in a range where the distribution ratio of the actinoid element is smaller than 1 and the distribution ratio of the lanthanoid element is larger than 1. According to the experimental examples of the present inventors, this range is about 0.4 mol / l or more and about 0.8 mol / l.
Mol / l or less, more preferably
It is 0.5 mol / l or more and about 0.7 mol / l or less. With this configuration, the aqueous phase A and the organic phase O from which the liquid mixture has been separated can have the aqueous phase A contain more actinoid elements than the organic phase O, and the organic phase O can contain more lanthanoid elements than the aqueous phase A. Thus, the actinoid element and the lanthanoid element can be efficiently separated.

The aqueous solution or the solvent phase preferably contains an alcohol in order to suppress the emulsion phase. This alcohol is, for example, ethanol, and it is preferable to add at least 4% or more, and at most 20% or less in order to suppress the formation of an emulsion phase. This alcohol is also a salt-free reagent like hydrazine carbonate, and can be easily removed by evaporation, thermal decomposition, or the like.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a separation apparatus to which the separation method of the present invention is applied. In this figure, 1 is an extraction solvent composed of an HDEHP solvent, 2 is an An fraction containing an actinoid element and a lanthanoid element, 3 is a hydrazine carbonate solution for selectively back-extracting an actinoid element (Am, Cm, etc.), and 4 is an actinoid element Is a solvent containing a lanthanoid element which has not been extracted. That is, the HDEHP solvent 1 was added to the An fraction 2 by this separation device.
To extract a solvent 6 to be treated containing an actinoid element and a lanthanoid element. An aqueous solution 3 containing hydrazine carbonate is mixed into the solvent 6 to be treated as a back-extracting solution. And the actinoid element is recovered from the aqueous phase A. Note that the structures of the extractor and the reverse extractor in this figure are the same as those of the conventional well-known one, and may be a single stage or a plurality of stages.

[0020]

FIG. 2 shows the experimental results when the HDEHP solvent according to the present invention was used as the extraction medium, and shows the relationship between the concentration of hydrazine carbonate in the aqueous solution 3 as a back-extracting agent and the distribution ratio (D). In this figure, the horizontal axis indicates the hydrazine carbonate concentration (mol / l), and the vertical axis indicates the distribution ratio. Further, Am (americium) and Cm (curium) in the figure are actinoid elements, and other La (lanthanum), Ce
(Cerium), Nd (neodymium), Sm (samarius), Eu (europium), Dy (dysprosium)
Is a lanthanoid element.

FIG. 2 shows that the concentration of hydrazine carbonate is preferably about 0.4 mol / l or more and about 0.8 mol / l or less, more preferably about 0.5 mol / l or more and about 0 mol / l or more. By setting the concentration to 0.7 mol / l or less, the distribution ratio of the actinoid elements (Am, Cm) can be made smaller than 1, and the lanthanoid elements (La, Ce, Nd, Sm, Eu, D
It can be seen that the distribution ratio of y) can be made larger than 1.

Since these concentrations vary depending on the operating conditions and other reagent concentrations, the present invention is not limited to this. The distribution ratio of the actinoid element can be made smaller than 1, and the distribution ratio of the lanthanoid element can be made 1 or less. It is better to set the range so that it can be made larger. In this test, the HDEHP solvent was based on 0.3 mol / l HDEHP-0.2 mol / l TBP-n dodecane (normal dodecane), but the concentration range of HDEHP and TBP may be changed as appropriate. it can. Here, TBP is an abbreviation for Tributyl phosphate.

According to the present invention described above, since an aqueous solution of hydrazine carbonate (N ₂ H ₄ ) ₂ CO ₂ , which is a salt-free reagent, is used as the back extract, nitric acid decomposition is performed in a subsequent step.
N ₂ gas, H ₂ gas,
It can be decomposed and removed into CO ₂ gas, and actinide elements such as Am and Cm can be easily separated. Further, in FIG. 2, the relationship between the concentration of hydrazine carbonate and the distribution ratio is close to parallel between actinide element and lanthanoid element, and a stable separation coefficient can be obtained in a wide range. The method for separating actinide elements and lanthanoid elements has a high separation efficiency (separation coefficient), can easily separate actinide elements in a later step, and does not generate difficult-to-treat waste.

Table 1 shows the calculated separation coefficients of the actinoid element and the lanthanoid element from the results shown in FIG.

[0025]

[Table 1]

As is clear from Table 1, while the actinide elements Am and Cm are more back-extracted into the aqueous phase, the lanthanide elements La, Ce, Nd, Sm,
In order to separate Eu and Dy, the concentration of hydrazine carbonate is set to about 0.5.
By setting the molar ratio to 6 mol / l, a very high (at least 26 or more) separation coefficient can be obtained, whereby the separation can be performed efficiently, and the number of stages required in the extraction step can be reduced.

Table 2 is a comparative example using a DIDPA solvent as an extraction solvent.

[0028]

[Table 2]

As is clear from Table 2, the actinoid element and the lanthanoid element can be separated even when the DIDPA solvent is used, but the separation factor is much lower than that of the present invention (at least about 8). Therefore, the required number of stages in the extraction step is required to be three times or more as compared with the method of the present invention.

The aqueous solution of hydrazi carbonate used as the back extract in the present invention preferably contains an alcohol in order to suppress the emulsion phase. This alcohol is, for example, ethanol, and it is preferable to add at least 4% or more, and at most 20% or less in order to suppress the formation of an emulsion phase. This alcohol is also a salt-free reagent like hydrazine carbonate, and can be easily removed by evaporation, thermal decomposition, or the like.

It should be noted that the present invention is not limited to the above-described embodiment, but can be variously modified without departing from the gist of the present invention.

[0032]

As described above, according to the method for separating an actinide element and a lanthanoid element of the present invention, the hydrazine carbonate concentration is important in the operating conditions of the extraction operation. Since the distribution ratio graph of the element and the lanthanoid element is nearly parallel, the separation coefficient is stable. Since it is a salt-free reagent, it can be decomposed by nitric acid decomposition, electrolysis, thermal decomposition, etc., and actinoid elements (Am, Cm, etc.) can be easily prepared. Can be separated.

Further, by using the HDEHP solvent as the extraction medium, a very high (at least 26 or more) separation coefficient can be obtained, whereby the separation can be performed efficiently, and the number of required plates in the extraction step can be reduced. it can. That is, a large separation coefficient means a high selective extraction efficiency. Therefore, since the number of stages of the extractor can be reduced, a compact, simple and low-cost apparatus can be manufactured.

Therefore, the method for separating actinides and lanthanoids of the present invention does not require strict concentration control, has a very high separation efficiency (separation coefficient), and can easily separate actinides in the subsequent steps. It has excellent effects such as no generation of difficult-to-treat waste.

[Brief description of the drawings]

FIG. 1 is a block diagram of a separation apparatus to which a separation method according to the present invention is applied.

FIG. 2 is a diagram showing the relationship between hydrazine carbonate concentration and distribution ratio in the method of the present invention.

FIG. 3 is a schematic flow diagram of a four-group separation process, which is the latest group separation process.

FIG. 4 is a flow chart showing a process for separating actinide elements from high-level waste liquid.

FIG. 5 is a flowchart showing a conventional separation process of an actinoid element and a lanthanoid element.

FIG. 6 is a diagram showing a relationship between a pH value and a distribution ratio in a conventional actinoid element separation and recovery process.

[Explanation of symbols]

A Aqueous phase O Organic phase 1 Extraction solvent consisting of HDEHP solvent 2 An fraction containing actinoid and lanthanoid elements 3 Hydrazine carbonate solution 4 Hydrazine carbonate solution extracted with actinoid elements 5 Solvent containing lanthanoid elements 6 Including actinoid elements and lanthanoid elements Solvent to be treated

Claims (5)

[Claims] 1. A group separation method for separating a high-level waste liquid for each element group, wherein HDEHP is added to an An fraction containing an actinoid element.
A solvent to be treated is extracted by mixing a solvent, an aqueous solution containing hydrazine carbonate is mixed into the solvent to be treated as a back extraction solution, and the mixed solution is separated into an aqueous phase and an organic phase. A method for separating an actinoid element and a lanthanoid element, comprising recovering an actinoid element from an actinide element. 2. The separation according to claim 1, wherein the concentration of the hydrazine carbonate is set so that the distribution ratio of the actinoid element is smaller than 1 and the distribution ratio of the lanthanoid element is larger than 1. Method. 3. The concentration of the hydrazine carbonate is about 0.4.
The separation method according to claim 2, wherein the amount is not less than mol / l and not more than about 0.8 mol / l. 4. The concentration of the hydrazine carbonate is about 0.5
The separation method according to claim 3, wherein the amount is not less than mol / l and not more than about 0.7 mol / l. 5. The method according to claim 1, wherein the aqueous solution or the solvent phase contains an alcohol for suppressing an emulsion phase.