JP3099019B2 - Selective back-extraction separation method for neptunium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for separating neptunium in a process for reprocessing spent nuclear fuel, and more particularly to a method for selectively separating neptunium from an organic phase containing either or both of plutonium and uranium.

[0002]

2. Description of the Related Art Spent nuclear fuel of a nuclear reactor contains unreacted uranium and plutonium produced by a nuclear reaction, and these are recovered and reused as nuclear fuel. Processing is taking place.

At present, the only technology established for reprocessing on a commercial scale is the purex (PUREX).
Only the law. In this method, spent nuclear fuel is dissolved in an aqueous nitric acid solution, and uranium, plutonium, and other fission products (FP) in the aqueous solution are separated by a solvent extraction operation. As a solvent extractant, tributyl phosphate (TBP) is used. ) Is diluted with an organic solvent such as dodecane.

FIG. 6 shows a schematic diagram of this step. In the co-decontamination step, plutonium and uranium are extracted into TBP / organic solvent and separated from other FP. Next, in the partitioning step, plutonium is reduced to change its valence from IV valence to III valence and made non-extractable in TBP / organic solvent, thereby extracting it into an aqueous solution and separating it from uranium.
When TBP / organic solvent and aqueous solution are mixed, each ion is distributed to each phase according to the distribution coefficient. Here, the organic solvent non-extractability means that the partition coefficient is mostly distributed to the aqueous phase at the time of this distribution, and the organic solvent extractability is mostly distributed to the organic solvent side. Distribution coefficient. After this, each solution was concentrated,
Purification yields uranium and plutonium. For the reduction of plutonium, Uranus nitrate U (NO ₃ ) ₄ , hydroxylamine nitrate NH ₂ OH · HNO ₃ , hydrazine N
₂ H ₄ , NO _x gas and the like are used as reagents.

[0005]

However, this conventional P
The UREX method is basically a technique for separating and recovering uranium and plutonium, and does not consider separation and recovery of other actinide elements. In particular, neptunium, which accounts for the majority of other actinide elements contained in spent nuclear fuel, has a valency that is easily changed according to various chemical operations and process conditions, and it is difficult to adjust with the reagents. In this case, despite being accompanied by plutonium and uranium, it is distributed to both aqueous solution and organic solvent in the following steps, and is finally removed as impurities. As a result, neptunium is contained in the high-level radioactive liquid waste. Neptunium ( ²³⁷ Np) has a half-life of 1
0 ⁶ years order and very long compared to the other FP, therefore high radioactivity daughter, it has been desired a technique for separating a radioactive liquid waste from top of the management of radioactive waste.

[0006] Neptunium is a serious neutron poison in LWR fuel and must be removed. However, it is difficult to adjust valence and it is difficult to separate plutonium and uranium. was there.

As one of the techniques for solving these problems, “Basic research on photochemical separation of actinide element (VII)” by the present inventors (Atomic Energy Society of Japan, 1996)
Spring Annual Meeting ”). This is a technique for photochemically separating plutonium and neptunium in a nitric acid aqueous solution containing plutonium and neptunium. However, in order to separate neptunium from other FP, uranium and plutonium in the reprocessing step, it is necessary to separate neptunium from the organic solvent obtained after the co-decontamination step containing 99% or more of the neptunium to be treated. Most effective. This is because in the subsequent steps, neptunium is distributed to each of the nitric acid solution and the organic solvent containing uranium and plutonium, so that a neptunium separation step is required for each. However, a technique for separating neptunium from an organic solvent has not been established.

In view of the above problems, an object of the present invention is to establish a method for selectively back-extracting and separating only neptunium from an organic solvent containing plutonium or uranium.

[0009]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems, and as a result, by irradiating ultraviolet rays to n-valent neptunium in an organic phase containing TBP, only neptunium is converted to V It has been found that the present invention can be reduced to a single value, and the present invention has been completed. That is, the method for selective back-extraction and separation of neptunium according to the present invention employs plutonium (IV and / or VI) (or uranium (IV and / or
Or VI)) and the organic phase containing neptunium (VI) and TBP is irradiated with ultraviolet light while mixing the aqueous solution of nitric acid to reduce the neptunium (VI) to neptunium (V). And a step of extracting and separating.

[0010] Even if an organic phase containing plutonium (IV and / or VI), uranium (IV and / or VI), and neptunium (VI) is mixed with an aqueous nitric acid solution, these elements remain in a state where the valence is not changed. Are mostly present in the organic phase and do not migrate very much to the aqueous nitric acid solution. However, when the mixed solution is irradiated with ultraviolet light, neptunium (VI) is photoexcited to be easily reduced, or the organic phase is photoexcited to exhibit a reducing action, or both effects reduce neptunium (VI). Then, it changes to neptunium (V). This neptunium (V) is neptunium (VI)
In contrast, most migrate to the aqueous nitric acid solution and are hardly partitioned into the organic phase. On the other hand, among the oxidation and reduction reactions of uranium (IV, VI) and plutonium (IV, VI) due to ultraviolet irradiation, the reduction reaction to plutonium (III), which is easily extracted into the aqueous nitric acid solution, does not occur. Even if there is a reaction, most of the products remain in the organic phase due to the valency that hardly transfers to the aqueous nitric acid solution. As a result, only neptunium (VI) contained in the organic phase can be selectively extracted and separated.

Further, prior to the extraction / separation step,
(1) By irradiating an aqueous nitric acid solution containing plutonium (III, IV and / or VI) (or uranium (IV and / or VI)) and neptunium (V and / or VI) with ultraviolet light, plutonium (III) Plutonium (I)
V and / or VI) (or uranium (IV) to uranium (VI)) and neptunium (V) to neptunium (V
The plutonium (IV and / or VI) contained in the aqueous nitric acid solution is added to I) by adding an organic phase containing tributyl phosphate to the aqueous nitric acid solution treated in the oxidizing step and mixing. (Or uranium (IV and / or VI)) and neptunium (VI) by extraction and separation into an organic phase.

These steps (1) and (2) are applicable to all steps in which the main elements in the solution are plutonium and / or uranium and neptunium after the above-mentioned co-decontamination step. It is important to adjust the valence of each element having various valences in the state to the organic solvent extractability. Where plutonium (IV) and uranium (V
The partition coefficient between the organic phase and the aqueous phase of I) is about 10 and about 20, respectively, while the partition coefficients of plutonium (VI) and uranium (IV) are about 3 and about 2, respectively. ) And uranium (VI) have higher partition coefficients and are more readily extracted into the organic phase than aqueous solutions. According to this pretreatment step, plutonium and uranium contained in the organic phase after the extraction and separation step (2) have larger IV and VI valences and distribution coefficients, respectively, as compared with the chemical treatment using reagents and the like. The proportion of those having a valence becomes large, and the proportion of neptunium extracted into the aqueous solution side in the subsequent neptunium back extraction / separation step is small, and effective selective back extraction / separation of neptunium becomes possible.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail.

First, a preferred embodiment of the neptunium back-extraction separation method of the present invention will be described.

In general reprocessing, uranium (U), plutonium (Pu), and neptunium (Np) are produced by an oxidizing atmosphere such as heated nitric acid during fuel dissolution.
99% or more of each of them is U-valent, Pu is hexavalent (partly tetravalent), and Np is hexavalent. At this time, the concentration of each element in the aqueous nitric acid solution is such that U is about 2 M / l and Pu is about 2 ×
10 ⁻² M / l and Np are about 2 × 10 ⁻³ M / l. Each of these elements in the valence state is TB
Solvent extraction with the organic phase containing P separates from FP.

The organic phase containing TBP includes TBP
Can be used as a solvent diluted with a linear hydrocarbon, for example, n-dodecane.
An organic phase containing 30% by weight of is used.

U contained in the extracted organic solvent,
The total concentration of Pu and Np is about 5 × 10 ⁻¹ M / l in a general reprocessing step.

In practicing the present invention, it is necessary that almost all FPs which may undergo a photochemical reaction by light irradiation have been removed in order to surely carry out the photoreaction of Np and effect effective Np separation. And, since the present invention can be applied to any organic phase after the co-decontamination step, TBP containing one or both of Pu and U and Np
Is contained in the above co-decontamination step.
And the organic phase extracted from Np, the organic phase containing U and Np prepared after separating U and Pu, or Pu
And an organic phase containing Np. At this time, P
It is preferable that Pu has four valences and U has six valences in order to increase the valence of u and U in the organic solvent at the time of back extraction.

In the present invention, an aqueous nitric acid solution having a concentration of 1 M to 10 M can be used as the aqueous nitric acid solution for the back extraction separation of Np, and 2 M to 4 M is particularly preferable. In particular, a 3M nitric acid aqueous solution generally used in the reprocessing step can be suitably used. If a relatively high-concentration nitric acid aqueous solution is used, the rate of U and Pu being back-extracted into the aqueous solution is reduced, and effective back-extraction separation of Np can be performed.

The apparatus used for irradiating the ultraviolet rays according to the present invention includes a nitric acid absorption wavelength region of 250 nm.
What is necessary is just to be able to irradiate light having a short wavelength of up to 350 nm, and a general mercury lamp or the like can be used. The irradiation intensity of the ultraviolet irradiation is preferably set to a relatively high irradiation intensity of 1 to 1.5 W / cm ² . The irradiation time is 5
About 20 minutes is preferable. While the reduction reaction of Np proceeds in a relatively short time, when irradiated with light for a long time, the reduced Np (V) is re-oxidized or disproportionated by 5%.
This is because the ratio of the valence of valence decreases, and conversely, the transfer from the aqueous solution to the organic phase occurs.

On the other hand, in the pretreatment step of the extraction / separation step of the present invention, the concentration of the nitric acid aqueous solution is preferably about the same as the concentration of the above-mentioned neptunium back-extraction / separation step. It is preferable to use a nitric acid aqueous solution. It is preferable to add urea to the nitric acid aqueous solution to remove nitrous acid generated by light irradiation. The amount of urea added is 10 times the maximum value of the amount of nitrous acid generated.
About twice is appropriate. In the above-mentioned extraction separation step,
It is not always necessary to add urea, but it may remain in the solution.

In this case, it is preferable to irradiate the light at a relatively high irradiation intensity, which is almost the same as that in the extraction and separation step described above. However, the irradiation time is preferably 30 minutes or more. This is because, in the present oxidation step, the reverse reduction reaction does not occur due to light irradiation, and it is necessary to take a sufficient reaction time in order to ensure the reaction.

FIG. 1 is a schematic diagram of a reprocessing step utilizing the present invention. Here, after a dissolving step 1 for dissolving spent nuclear fuel in an aqueous nitric acid solution and a co-decontamination step 2 for separating U, Pu, and Np from the aqueous nitric acid solution, an Np separation step 3 utilizing the present invention is performed. The organic phase containing U and Pu from which Np has been removed is led to the partitioning step 4, where U and Pu are separated and formed. As a result, high-purity U, Pu, and Np are obtained, so that the fuel can be easily and reliably adjusted when reused as nuclear fuel.

The reprocessing step utilizing the present invention is not limited to this, and without inserting the Np separation step 3, the Np from the organic phase containing Np and U after the distribution step 4 is removed.
The separation includes Np separation from an aqueous phase containing Np and Pu, and Np separation from an organic phase containing Np and Pu in a Pu purification step (not shown).

FIG. 2 is a diagram showing a configuration of a photochemical valence adjusting apparatus according to a preferred embodiment of the present invention. This photochemical valence adjusting device 11 includes 12 photochemical units.
It is created by arranging pieces. FIG. 3 shows the structure of each unit 13. A supply pipe 14 for supplying a liquid is connected to an upper portion of each of the units 13, and a discharge pipe 15 for discharging a processing liquid is connected to a lower portion of each of the units 13. A sample excel 17 made of a donut-shaped glass cell is arranged inside the unit 13, and a plurality of mercury lamps 18 are arranged inside and around the excel 17. Further, a cooling fan 19 is arranged below the unit 13 for cooling the processing liquid inside the excel 17 and the mercury lamp 18.

As an example, the inner diameter of the excel 17 is about 30 cm, and the width of the strip portion is 3 cm and the height is 10 cm. The mercury lamp 18 uses one with a power of 350 W at the center and four at the periphery, for a total of five. In this example, the processing capacity of the processing liquid (Np content: 1.0 × 10 ⁻² M / l) is about 150 l / hr in total.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described. Note that the present invention is not limited to this.

First, Np and Pu are each set to 1 × 10 ^−3.
A 3M nitric acid aqueous solution containing M / l and urea at 1 × 10 ^-2 M / l was prepared. In the initial state, this Np is almost 100% Np
(V), and Pu was almost 100% Pu (IV). 3 ml of this aqueous solution was irradiated with light at an irradiation intensity of 1.3 W / cm ² for about 20 minutes using a mercury lamp (BNO-250DI manufactured by WACOM) whose wavelength characteristics are shown in FIG. By this light irradiation, 17% of Pu ⁴⁺ becomes Pu ⁶⁺ , Np
More than 99% of ⁵⁺ was photooxidized to Np ⁶⁺ and adjusted to an organic solvent extractable valence. After stopping the light irradiation, T
3 ml of an n-dodecane solution containing 30% by weight of BP was added, and Pu and Np were extracted into this organic solvent.

The solvent after extraction contains Pu at 8.9 × 10 ^-4.
M / l and Np were contained at 8.8 × 10 ⁻⁴ M / l. The valence existence ratio of each element is such that Pu is PU ⁴⁺ 86
%, Pu ⁶⁺ 14%, and Np were 99% or more Np ⁵⁺ .
A fresh 3M nitric acid aqueous solution was added to 1 ml of the extracted organic solvent.
The mixture was irradiated with ultraviolet light at the same irradiation intensity of 1.3 W / cm ² using the same mercury lamp as described above. At this time, Np and P distributed to the aqueous phase and the organic phase
The change over time of u is shown in FIGS. 5A and 5B, respectively.

By light irradiation, Np ⁶⁺ in the solvent quickly becomes N
The reduced Np ⁵⁺ is reduced to p ^5+, and the reduced Np ⁵⁺ is transferred to the aqueous phase, and in about 10 minutes, about 97% of Np is transferred to the aqueous phase, and back extraction, which is extraction into the aqueous phase, is achieved. You. When light irradiation is continued, Np ⁵⁺ in the aqueous phase is oxidized to Np ⁶⁺ and changes to extract with an organic solvent, and the organic phase is transferred again. On the other hand, with respect to Pu, no oxidation or reduction reaction accompanying light irradiation in the solvent is observed, and only about 15% is back-extracted into the aqueous phase according to the partition coefficient.

Therefore, at this irradiation intensity, about 1
By irradiating for about 0 minutes, most of Np can be separated into the aqueous phase. That is, it was confirmed that back extraction and separation can be performed in a short time without using any special reagent.

[0032]

As described above, according to the present invention,
Since the valence of neptunium can be adjusted to V valence by light irradiation, uranium,
Only neptunium mixed with plutonium can be reliably back-extracted into the aqueous phase.

In this case, since a general mercury lamp can be used for light irradiation, the system can be simplified. Also,
Since the reaction occurs in a short time, efficient separation can be performed in a short time. Also, since no reagent is needed to adjust the valence,
No extra radioactive liquid waste is generated, and there is little effect on post-treatment.

Further, neptunium, a fissile material,
Since uranium and plutonium can be produced separately, it is possible to reliably adjust the mixing ratio when producing a reused fuel.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an example of a reprocessing process system using the present invention.

FIG. 2 is a schematic view of an example of a photochemical valence adjusting device for performing an Np separation step of the present invention.

FIG. 3 is a schematic diagram of the photochemical unit of FIG. 2;

FIG. 4 is a diagram showing wavelength characteristics of a mercury lamp that can be used for ultraviolet irradiation according to the present invention.

FIG. 5 is a diagram showing the result of neptunium separation according to Example 1.

FIG. 6 is a schematic view of a conventional reprocessing step.

[Explanation of symbols]

1 ... dissolution step, 2 ... co-decontamination step, 3 ... Np separation step, 4
... Distribution process, 11 ... Photochemical valence adjusting device, 13 ... Photochemical unit, 14 ... Supply tube, 15 ... Exhaust tube, 17 ... Excel for sample, 18 ... Mercury lamp, 19 ... Cooling fan.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Hiroshi Tomiyasu 2-12-1 Ookayama, Meguro-ku, Tokyo Inside the Institute of Reactor Engineering, Tokyo Institute of Technology (56) References Atomic Energy Society of Japan 1997 (35th) Spring Annual Meeting Abstracts III-IV Volume 514 (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 31/00 B01D 11/04 G01N 1/10 G21C 19/46

Claims (4)

(57) [Claims] An ultraviolet ray is applied to an organic phase containing plutonium (IV and / or VI) and neptunium (VI) and tributyl phosphate while mixing an aqueous nitric acid solution, thereby converting the neptunium (VI) to neptunium (V). And selectively extracting and separating neptunium from the organic phase into the aqueous nitric acid solution. 2. Prior to the extraction separation step, the plutonium (III) is irradiated by irradiating an aqueous nitric acid solution containing plutonium (III, IV and / or VI) and neptunium (V and / or VI) with ultraviolet light. ) To plutonium (IV and / or VI) and to oxidize the neptunium (V) to neptunium (VI), respectively, and adding and mixing an organic phase containing tributyl phosphate to the aqueous nitric acid solution treated in the oxidizing step. Extracting and separating plutonium (IV and / or VI) and neptunium (VI) contained in the nitric acid aqueous solution into the organic phase. Selective back-extraction separation method. 3. Irradiating the organic phase containing uranium (IV and / or VI) and neptunium (VI) and tributyl phosphate with ultraviolet light while mixing an aqueous nitric acid solution,
A method for selectively back-extracting and separating neptunium, comprising reducing the neptunium (VI) to neptunium (V) and extracting and separating the neptunium (V) from the organic phase into the aqueous nitric acid solution. 4. Prior to the extraction and separation step, the uranium (IV) is irradiated by irradiating a nitric acid aqueous solution containing uranium (IV and / or VI) and neptunium (V and / or VI) with ultraviolet light. The uranium (VI) is replaced with the neptunium (V).
I), an oxidizing step, and adding and mixing an organic phase containing tributyl phosphate to the aqueous nitric acid solution treated in the oxidizing step, thereby mixing uranium (IV and / or VI) contained in the aqueous nitric acid solution and 4. The method for selective back-extraction and separation of neptunium according to claim 3, further comprising: extracting and separating neptunium (VI) into the organic phase.