JPH07207249A - Diluent for extractant for use in nuclear fuel reprocessing - Google Patents

Abstract

PURPOSE:To improve the efficiency of operation in the extraction of tri-, tetra-, and hexavalent metals from a highly radioactive liquid waste with a solvent containing CMPO by using a specific diluent to prevent the solvent from separating into two phases without reducing extraction efficiency. CONSTITUTION:2-Methylundecane, 3-methylundecane, or 2,2,4,6,6- pentamethylheptane is used as a diluent for inhibiting a solvent containing CMPO from separating into two phases upon contact with a high-concentration aqueous solution of metals. Of these compounds, 2-methylundecane has a remarkable effect. An extractant comprising a mixture of the solvent with the diluent is contacted with a highly radioactive liquid waste to extract tri-, tetra-, and hexavalent metals, and the extract is subjected to back extraction with, e.g. diluted nitric acid to separate the intended metal elements from the extractant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diluent that suppresses the two-phase separation phenomenon of a solvent that occurs when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration.

[0002]

[Prior art]

{Background of the Invention} [Reprocessing and Purex method] At present, the reprocessing of spent nuclear fuel is performed using the Purex method. The 30% TBP / dodecane solution used in this decontamination step can extract the tetravalent or hexavalent actinoids uranium and plutonium, and can efficiently recover them. However, most of the elements other than uranium and plutonium (which include radioisotopes) remain in the effluent without being extracted and the effluent is still highly radioactive.

[Mixed extraction solvent containing CMPO] The highly radioactive waste liquid generated from this reprocessing step contains radioactive isotopes with a long half-life, and the burden on waste management if these are not removed. Will be enormous. In particular, many isotopes of actinides such as americium and curium have long half-lives, and since these are trivalent metals, they cannot be removed by a conventional TBP / dodecane solution. Therefore, these trivalent metals are currently extracted using an extractant called CMPO (octyl (phenyl) -N, N-diisobutylcarbamoylmethylphosphine oxide) represented by the following chemical formula. That is, using a mixed extraction solvent in which CMPO, TBP (tributyl phosphate) and dodecane are mixed,
By removing high-valued actinides, the burden on waste management is reduced.

[0004]

[Chemical 1] [Two-Phase Separation Phenomenon of Organic Phase in Extraction Step] When extraction is carried out using this mixed extraction solvent, as shown in FIG. The tetravalent and hexavalent metal elements are extracted (extraction step), and then the metal extracted by dilute nitric acid or the like is back-extracted to separate the target element (back extraction step).

However, in this extraction step, when the excess metal is extracted in the mixed extraction solvent, the organic phase becomes 2%.
The phenomenon of phase separation may occur.

The occurrence of this phenomenon will be described with reference to FIG. When excess metal is extracted into a mixed extraction solvent, when it exceeds a certain concentration (hereinafter, this concentration is referred to as "separation concentration"), the distribution ratio, which was almost constant until then, begins to sharply decrease. , The organic phase is separated into two phases (hereinafter, this phenomenon is referred to as “two-phase separation phenomenon of organic phase”). This two-phase separation phenomenon of the organic phase is due to the fact that a portion where the metal concentration is high and a portion where the metal concentration is low occur in the organic phase due to the extraction of excess metal.

When the phenomenon of separation of the organic phase into two phases occurs, not only the distribution ratio is lowered and the extraction efficiency is deteriorated, but also the operation management becomes difficult. That is, since the extraction operation is generally performed using a counter-current multistage extractor such as a mixer-settler, when the phenomenon of separating into two phases occurs, the normal extraction / separation operation cannot be performed, which is a problem in operation management. Of.

For this reason, it is necessary to take measures to prevent the two-phase separation phenomenon of the organic phase. However, conventionally, as shown in FIG. 2, a dilution treatment is performed before the extraction step, By reducing the metal concentration of the treatment solution,
The occurrence of the two-phase separation phenomenon was prevented.

{Prior Art for Preventing Two-Phase Separation Phenomenon of Organic Phase} However, when this diluting treatment is performed, not only the number of steps is increased, but also the volume of the treatment solution is increased. Therefore, some research has been done to omit this.

[General Method] For example, in order to extract more metal without separating the extraction solvent into two phases, the operating temperature should be raised and TBP in the extraction solvent should be increased.
It has been reported that increasing the concentration and using a highly polar diluent are effective. Therefore, it is likely that the dilution process can be omitted by appropriately setting these conditions. However, although this method can be appropriately used for each individual case, it is not generalized and used in a plant.

[Method of changing the diluent] In another study, it was reported that when a branched hydrocarbon is used as the diluent, the two-phase separation phenomenon of the organic phase can be effectively prevented. (Z.Kolarik, EPHorw
itz, Solvent Extraction and Ion Exchange, 6 (1), 61-9
1,1988). That is, in this method, a straight chain hydrocarbon such as dodecane, which is widely used as a diluent in the Purex method, is changed to a hydrocarbon having many branches. According to this method, since only the diluent is changed, it can be used as it is in the nuclear industry, like the conventional Purex method.

[0012]

However, as shown in FIG. 4, generally, when the number of branched hydrocarbons increases, the two-phase separation phenomenon of the organic phase can be suppressed, Along with that, there is a problem that the distribution ratio of the organic phase tends to decrease. As described above, even if it becomes possible to prevent the two-phase separation phenomenon of the organic phase, it is not possible to solve the conventional problems if the distribution ratio is lowered. That is, even if the work efficiency is improved, if the extraction efficiency is deteriorated at the same time, the overall efficiency is not increased.

The present invention has been made in view of the above problems, and an object thereof is to suppress a two-phase separation phenomenon of a solvent that occurs when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration. The present invention is to provide a diluent that suppresses the two-phase separation phenomenon without causing a reduction in distribution ratio.

[0014]

In order to solve the above problems, in the diluent according to claim 1 of the present invention, when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration. It is a diluent that suppresses the two-phase separation phenomenon of a solvent that occurs, and is characterized by containing 2-methylundecane.

Further, in the diluent according to claim 2,
A diluent that suppresses the two-phase separation phenomenon of a solvent that occurs when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration, and is characterized by containing 3-methylundecane.

In the diluent according to claim 3, CMP is used.
A diluent that suppresses the two-phase separation phenomenon of a solvent that occurs when a solvent mixed with O is brought into contact with an aqueous solution having a high metal concentration, and contains 2,2,4,6,6-pentamethylheptane. Characterize.

[0017]

In the diluent according to the present invention as described above,
While maintaining a distribution ratio that allows trivalent actinides to be extracted,
It is possible to suppress the two-phase separation phenomenon of the solvent.

In the diluent containing 2-methylundecane according to the first aspect, it is possible to suppress the two-phase separation phenomenon of the solvent while maintaining the distribution ratio of the mixed solvent of dodecane and CMPO. As described above, in the diluent according to the first aspect, even if the two-phase separation phenomenon of the solvent is suppressed, the distribution ratio does not decrease, so that it is possible to improve the work efficiency without deteriorating the extraction efficiency. it can.

Further, in the diluent according to claim 2 or claim 3, the distribution ratio is lower than that of the dodecane-CMPO mixed solvent, but since the decrease in the distribution ratio is small, the trivalent actinide can be extracted. Is secured,
It is possible to suppress the two-phase separation phenomenon of the solvent.

[0020]

The preferred embodiments of the present invention will be described below.

In this embodiment, 0.2M CMP
2-methylundecane, 3-methylundecane or 2, represented by the following formula:
The mixed solvent is prepared by diluting with 2,4,6,6-pentamethylheptane. That is, in this embodiment, the mixed solvent is CMPO 0.2M and TBP 1.0.
It was formed by diluting it with a diluent to give M. This is because the conventional CMPO mixed solvent is 0.2M CMPO, 1.0
Corresponding to being prepared by diluting ~ 1.4 M TBP with dodecane.

[0022]

[Chemical 2] As the diluent, other than this, halogenated hydrocarbon-based ones and aromatic-based ones having a benzene ring as a structure are also considered, but these (especially the former) are Therefore, it is not suitable because the application of the actual process is limited. However, when the above-mentioned aliphatic hydrocarbon compound having a methyl group as a branch is used as a diluent, such a problem does not occur. Among the compounds having an aliphatic hydrocarbon group and having a methyl group as a branch, 2-methylundecane, or 3-methylundecane and 2,2,4,6,6-pentamethylheptane are particularly described below. Most suitable as a diluent.

In this example, when the temperature and the TBP concentration of the solvent were constant, the nitric acid solution of uranium was brought into contact with the extraction solvent, the concentration at which the organic phase was separated into two phases was determined, and diluted with dodecane. Compared with. The results are shown in Table 1 below.
Is shown in.

[0024]

[Table 1] As shown in Table 1, the diluent according to this example can improve work efficiency while ensuring a sufficient distribution rate for metal extraction. In particular, with 2-methylundecane, the work efficiency can be improved while maintaining the extraction efficiency of dodecane. This is irreversible, and unlike other diluents, 2-methylundecane does not lower the distribution ratio even if the two-phase separation phenomenon of the solvent is suppressed, and the work efficiency without deterioration of extraction efficiency. It means that you can measure the improvement of.

Incidentally, 3-methylundecane and 2,2
Even with 4,6,6-pentamethylheptane, the practical distribution ratio is maintained and there is no problem in use.

The compounds constituting the diluent according to this example are structural isomers, and the total number of carbon atoms is 1 due to the structure.
2 and is common in having a methyl group as a branch. However, the reason why such a remarkable effect is obtained especially in 2-methylundecane is not clear at present.

When extraction is performed using the mixed extraction solvent containing the diluent according to this embodiment, first, as shown in FIG. 1, the mixed extraction solvent and the highly radioactive waste liquid are first contacted with each other, as in the conventional case. Then, the trivalent, tetravalent, and hexavalent metal elements are extracted (extraction step), and then the metal extracted with dilute nitric acid or the like is back-extracted to separate the target element (reverse extraction step).
The mixed extraction solvent after use is washed (solvent washing step)
To be reused. Here, when the diluent according to the present embodiment is used, unlike the conventional case, the step of diluting the highly radioactive waste liquid can be deleted before the extraction step, or the burden of the dilution treatment operation can be reduced. be able to. this is,
When the diluent of this example is used, more metal can be extracted without separating the extraction solvent into two phases, so that it is possible to process a solution having a higher metal concentration. Is.

Further, even when the extraction operation is carried out by using a countercurrent multistage extractor such as a mixer-settler in the extraction step, the two-phase separation phenomenon does not occur, and the normal extraction / separation operation can be performed, which results in operational management. No trouble will occur. Furthermore, there is no need to increase operating temperature or TBP concentration in order to be able to extract more metal.

As described above, according to this embodiment, the working efficiency can be improved while maintaining the extraction efficiency, so that the overall efficiency of the reprocessing step is increased. Further, according to this, since only the diluent is changed, it can be used as it is in the nuclear industry as in the conventional Purex method and the like.

[0030]

As described above, in the diluent according to the present invention, the two-phase separation phenomenon of the solvent can be suppressed while maintaining the distribution ratio at which the trivalent actinide can be extracted. Therefore, when the diluent according to the present invention is used, more metal can be extracted without separating the extraction solvent into two phases, and it is possible to process a solution having a higher metal concentration. Become. This means that the feed concentration (concentration of the treatment liquid) in the reprocessing process can be increased, which leads to improvement of the treatment capacity of the waste liquid and omission of the diluting operation of the waste liquid to be treated. , Economically effective.

The present invention is an effective method for suppressing the phase separation of the organic phase that may occur in the solvent extraction process used in the spent nuclear fuel reprocessing facility. It can be used in chemical processes using an extraction solvent containing CMPO used for the separation of

[Brief description of drawings]

FIG. 1 is a flowchart showing a decontamination process when a diluent according to this example is used.

FIG. 2 is a flowchart showing a conventional decontamination process.

FIG. 3 is a diagram for explaining a two-phase separation phenomenon and a separation concentration of an organic phase.

FIG. 4 is a diagram showing a general relationship between a separation concentration and a distribution ratio when a branched hydrocarbon compound is used as a diluent.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Togashi 4-33 Muramatsu, Tokaimura, Naka-gun, Ibaraki Prefecture Power Reactor and Nuclear Fuel Development Corporation Tokai Office

Claims (3)

[Claims] 1. A diluent, which suppresses the two-phase separation phenomenon of a solvent that occurs when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration, wherein the diluent contains 2-methylundecane. Agent. 2. A diluent which suppresses the two-phase separation phenomenon of a solvent that occurs when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration, the diluent comprising 3-methylundecane. Agent. 3. A diluent for suppressing the two-phase separation phenomenon of a solvent, which occurs when a solvent mixed with CMPO is brought into contact with an aqueous solution having a high metal concentration, which is 2,2,4,6,6-pentamethyl. A diluent comprising heptane.