JP2589886B2 - Valence control method for neptunium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention can be applied to a technique for separating neptunium (Np) from uranium (U) and plutonium (Pu) in a nuclear fuel reprocessing step.

[0002]

2. Description of the Related Art In a nuclear fuel reprocessing process, most of the transuranium element (TRU) except for Pu is mixed with an organic solvent (tributyl phosphate (TBP) and its diluent) in a co-decontamination step like fission products. To the high-level waste liquid side without being extracted into the mixture. However, most of Np in the TRU is transferred to the organic phase together with hexavalent U and tetravalent Pu as hexavalent in the co-decontamination step. The hexavalent Np transferred to the organic phase is partially reduced by a Pu reducing agent such as uranas or hydroxylamine nitrate (HAN) in the U / Pu partitioning step to become pentavalent or even tetravalent. The pentavalent Np moves to the Pu purification step together with Pu, and the unreduced hexavalent and tetravalent Np moves to the U purification step. In the U refining process, NOx is blown into the storage tank in advance to reduce the valence of U to 6
After adjusting the valency and the Np to pentavalent, respectively, extraction and separation are performed. In the Pu purification step, Pu is adjusted to have a valence of 4 and Np is set to be pentavalent in a NOx oxidation tower or the like, and then extracted and separated. Thus, in the nuclear fuel reprocessing process, Np
Is separated and removed from U or Pu in the U purification step or Pu purification step.

Conventionally, Np valence control methods include (1) N
Indirect methods using nitrous acid generated by blowing Ox, (2) a method of adding uranas, and (3) a method of adding hydrazine are used. When the method of injecting NOx is applied to an extractor such as a mixer settle or pulse Lacam, it is necessary to perform the method in advance in a storage tank in consideration of a decrease in extraction efficiency due to bubbles. Further, since the solvent extracts nitrous acid supplied to the process, it is difficult to control the concentration of nitrous acid in the process. The method of adding uranas is to remove part of uranium hexavalent, a product of the reprocessing plant,
This is a method using uranas (tetravalent U) prepared by an electrolysis method or the like. This method has the disadvantage that uranium is added in an amount several times the stoichiometric amount required for the reduction of Np, resulting in loss of uranium products. Uranus is Np
Has the ability to reduce to four valences and cannot be controlled to a pentavalent valence state. The method of adding hydrazine is a method using a reagent capable of decomposing into N ₂ gas and water, and does not cause generation of secondary waste. However, decomposition products of the reagent include hydrogen azide and the like. Sufficient safety design is required due to the presence of unstable components.

[0004]

In the conventional Np valence control method as described above, (1) the concentration of nitrous acid necessary to maintain the valence of Np at pentavalent throughout the extractor is controlled. However, there are problems such as (2) loss of uranium products, and (3) the need to take measures to ensure the safety of the process due to generation of unstable decomposition products. In addition, these technologies are based on hexavalent U and tetravalent P
It lacks the selectivity of reducing only Np from hexavalent to pentavalent in a system coexisting with both or either of u.

[0005]

According to the present invention, hexavalent Np coexisting with hexavalent U and / or tetravalent Pu in a nitric acid aqueous solution is converted into a non-extractable 5
Excellent remote control as a method for selectively reducing
Use an electrolysis method that does not generate secondary waste. At that time, N
To selectively control the valence of p, it is necessary to control the reduction electrode potential.

[0006]

According to the present invention, hexavalent Np coexisting with hexavalent U and / or tetravalent Pu in a nitric acid aqueous solution is selectively reduced to pentavalent which is non-extractable with respect to TBP. be able to. By using this method as an Np separation step between the reprocessing co-decontamination step and the U / Pu distribution step, only Np is converted to a pentavalent state while maintaining U and Pu in the organic phase as hexavalent and tetravalent, respectively. By reducing, the aqueous phase is transferred to the aqueous phase, and the U, Pu group and the Np group can be separated.

[0007]

EXAMPLES A case where selective reduction and separation of Np in a U-Pu-Np coexisting system will be described.

[0008] 20 ml of 3M nitric acid and 100
mg / l, hexavalent U at 25 g / l and tetravalent Pu at 1.
Organic solvent (30% TBP) containing each at a concentration of 3 g / l
/ N-dodecane) (20 ml) was placed in a batch-type electrolytic reduction reaction tank (volume: 50 ml), and sufficiently stirred to reach equilibrium for each concentration in both phases. Next, electrolytic reduction was performed while maintaining the two-phase interface by reducing the stirring speed. At that time, the potential of the reduction electrode (made of Pt) was kept at +0.4 V with respect to the potential of the saturated calomel reference electrode. FIG. 1 shows the apparatus used, and FIG. 2 shows the percentage (%) of the back-extracted U, Pu and Np as a change with time after the start of electrolytic reduction. U and Pu are hardly back-extracted even after 3 hours from the start of electrolysis.
On the other hand, as for Np, the aqueous phase concentration increased immediately after the start of electrolysis, and in 3 hours, 80% or more of the organic phase concentration before electrolysis was back-extracted into the aqueous phase as pentavalent. This result shows that if the electrolytic reduction potential is kept at +0.4 V (vs. the saturated calomel reference electrode), the hexavalent N and the hexavalent N coexist in the coexistence system of the hexavalent U and the tetravalent Pu.
This shows that p alone can be selectively reduced to pentavalent.

[0009]

According to the present invention, hexavalent Np coexisting with hexavalent U and / or tetravalent Pu in a nitric acid aqueous solution can be converted into a non-extractable pentavalent compound to TBP (tributyl phosphate). Np can be selectively reduced to Np, so that while maintaining U and Pu in the organic phase as hexavalent and tetravalent, respectively, only Np is reduced to pentavalent and transferred to the aqueous phase, whereby U and U are reduced. Np can be extracted and separated from Pu. As a result, by performing the selective reduction step of the present invention between the co-decontamination step and the U / Pu distribution step in the nuclear fuel reprocessing step, the remote control is excellent, and secondary waste is not generated. The present invention produces a remarkable effect specific to the present invention described in the specification that it can be safely operated in the process.

[Brief description of the drawings]

FIG. 1 schematically shows an apparatus for performing selective reductive separation of Np in an Np-U-Pu coexisting system according to the present invention.

FIG. 2 is the result of the implementation shown in FIG. The vertical axis represents U, Pu and Np back-extracted from the organic phase into the aqueous phase after the start of electrolysis.
, The horizontal axis is the time after the start of electrolysis.

[Explanation of symbols]

1 batch type electrolytic reduction reaction tank 2 30% TBP / n-dodecane organic solvent containing Np, U and Pu 3 3M nitric acid aqueous solution containing Np, U and Pu 4 Reduction electrode (Pt; 10 mm wide x 10 mm vertical x 0 mm thick) .2 mm) 5 Counter electrode (Pt; side 5 mm × length 10 mm × thickness 0.2 mm)
6 Reference electrode (saturated calomel electrode) 7 Glass tube with diaphragm 8 Stirrer for stirring 9 Stirrer 10 Electrolysis controller (potentiometer)

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Mitsuru Maeda 2 Shirane, Shikata, Shikata, Tokai-mura, Naka-gun, Ibaraki Pref. Japan Atomic Energy Research Institute Tokai Research Institute (56) References JP-A-48-38297 (JP, A) JP-A-51-64411 (JP, A) JP-B-49-15549 (JP, B2)

Claims (1)

(57) [Claims] 1. In a nuclear fuel reprocessing step, hexavalent Np coexisting with at least one of hexavalent U and tetravalent Pu is selectively selected from only Np without changing the valence of U and Pu. In the method of extracting and separating Np from U and Pu by reducing to pentavalent, a nitric acid aqueous solution, hexavalent Np, hexavalent U and / or tetravalent Pu are contained in an electrolytic reduction reaction tank. Stir the organic solvent with tributyl phosphate, and reduce the potential of the reducing electrode by +0.4 with respect to the potential of the saturated calomel reference electrode.
V and electrolytic reduction to selectively convert hexavalent Np to 5
A method for extracting and separating Np from U and Pu, characterized in that Np is extracted from an organic phase into an aqueous phase by reducing to a monovalent value.