JP2565032B2 - U / Pu distribution method in Purex method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has been improved in order to efficiently carry out the U (uranium) / Pu (plutonium) partitioning step in the Purex method, which is one of the solvent extraction methods in the reprocessing of spent nuclear fuel. The present invention relates to a U / Pu distribution method.

[0002]

2. Description of the Related Art The Purex method has hitherto been widely adopted as one of solvent extraction methods for recovering U and Pu in spent nuclear fuel. Fig. 2 is a flow sheet for explaining the outline of the conventional Purex method. After adjusting the nitric acid concentration in the nitric acid solution of the spent nuclear fuel in the adjusting step, such as mixer / settler, centrifugal extractor or pulse column. Contact with 30% TBP organic solvent phase in a multi-stage extractor. As a result, U and Pu contained in the solution are extracted into the organic solvent phase, and fission products (F
P) is back-extracted into the nitric acid aqueous phase and is highly radioactive waste liquid (HAW)
Is removed as. This process is called a co-decontamination process.

The organic solvent from the co-decontamination process loaded with U and Pu is then sent to the U / Pu distribution process where Pu is made up of dilute nitric acid in a multistage extractor such as a mixer-settler, centrifugal extractor or pulse column. U and Pu are separated by contacting with a back extraction liquid to back-extract Pu into the nitric acid aqueous phase and leaving U in the organic solvent phase. Such a distribution process is conventionally performed by a reduction distribution method. That is, a relatively high temperature of 40 ° C. or higher is adopted as the distribution temperature, and U is left as it is.
Since the partition coefficient ratio between Pu and Pu is small and separation is impossible, a reducing agent such as hydroxylamine nitrate (HAN) or uranus nitrate (U (IV)) is added to the nitric acid aqueous phase which is the Pu back-extracting solution, The tetravalent Pu is completely extracted back to the trivalent aqueous phase by reducing it to the trivalent state.

On the other hand, as an alternative method to the above reduction distribution method,
It has also been proposed to use the acid partition method in the U / Pu partition process. This acid partitioning method does not use a reducing agent in the U / Pu partitioning step and therefore P
By adjusting the nitric acid concentration without reducing u, U and Pu are distributed to the organic solvent phase and the nitric acid aqueous phase, respectively. In this acid distribution method, it is necessary to use an extremely low distribution temperature of about 5 ° C. in order to expand the distribution coefficient ratio of U and Pu. Therefore, a cooling facility is installed in the entire distribution process, and the supply pipe to the distribution process is in the middle. It is necessary to install a cooler in.

[0005]

When the reduction / distribution method as described above is used in the U / Pu distribution step, Pu
It is necessary to add about 3 times the amount of reducing agent. Increasing the amount of reducing agent added not only increases the burden on the reagent preparation step, but also increases the amount of residual reducing agent in the Pu product. The Pu product from the partitioning process is sent to the Pu refining process, where Pu is reoxidized, but at the same time, the residual reducing agent in the Pu product must be completely decomposed. Therefore, as the amount of reducing agent added in the U / Pu distribution step increases, the burden on Pu reoxidation in the Pu purification step increases.

On the other hand, when the acid distribution method is adopted in the U / Pu distribution step, the distribution temperature must be as low as about 5 ° C. as described above, which requires an excessive cooling facility. Further, since there is a possibility that the third phase of Pu is formed in the low temperature region, strict temperature control and Pu concentration control are necessary to prevent this. Further, in the case of this acid distribution method, in order to increase the product specification, particularly the decontamination coefficient of the U product, there is no choice but to increase the number of distribution stages of the multistage extraction device, and there is also the problem that the extraction operation takes a long time.

Therefore, according to the present invention, when the U / Pu distribution step in the Purex method is carried out, a predetermined product specification can be achieved even if the addition amount of the reducing agent is reduced.
Moreover, the object of the present invention is to provide an improved U / Pu distribution method that does not require an excessive cooling facility and can reduce the number of distribution stages of the multistage extraction device.

[0008]

[Means for Solving the Problems] That is, the present invention comprises a co-decontamination step of contacting a nitric acid solution of a spent nuclear fuel with a 30% TBP organic phase to extract U and Pu into the organic phase, and a U-Pu A PUREX process partitioning process comprising a U / Pu partitioning process in which the loaded organic phase is contacted with a Pu back-extracting solution consisting of dilute nitric acid to back-extract Pu into a nitric acid aqueous phase and U is left in the organic phase. When implementing, Pu
The ratio of reducing agent to dilute nitric acid, which is the back-extracting solution, is reduced (reducing agent amount /
The amount of Pu is 0.02 to 1 and the distribution temperature is 10 to 30 ° C.

[0009]

In the present invention, since the distribution temperature is lower than that in the conventional reducing and partitioning method, the distribution coefficient ratio of U and Pu can be expanded, and most of Pu is back-extracted into the nitric acid aqueous phase in the non-reduced state. It can be separated from U. As a result, the amount of reducing agent added can be significantly reduced.

Further, most of Pu is back-extracted into the nitric acid aqueous phase in a non-reduced state, and thereafter, P remaining in the organic phase.
u is trivalently reduced by the action of the reducing agent added in a relatively small amount range and is almost completely back-extracted into the nitric acid aqueous phase. This makes it possible to improve the specifications of U products and Pu products.

Further, in the acid distribution method which does not use a reducing agent, the number of distribution stages of the multi-stage extraction device must be increased in order to increase the product specifications. The Pu remaining in the phase can be rapidly back-extracted with a short number of distribution stages.

Furthermore, since Pu remaining in the organic phase can be rapidly back-extracted by adding a small amount of reducing agent, extremely low temperature conditions and strict temperature control as in the conventional acid distribution method can be relaxed. The load on the cooling equipment can be reduced. Generally, the degree of isolation of the desired product,
That is, the Pu / U ratio in the U product is higher than the reducing agent ratio.
The higher the temperature is, and the lower the distribution temperature is, the more the tendency is improved.
In the direction. However, increasing the ratio of reducing agents causes waste
Increased volume, lower distribution temperature burdens cooling equipment
Since it leads to an increase in
Operation at a distribution temperature close to room temperature (30-40 ° C) is desired
Good For the above reason, in the present invention, the reducing agent ratio is 0.02 to
1 and the distribution temperature is in the range of 10 to 30 ° C.
It

[0013]

The present invention will be described in more detail with reference to the following examples. FIG. 1 is a flow sheet showing an example of a U / Pu distribution process using the distribution method of the present invention. A multi-stage centrifugal extractor with 16 distribution stages was used as an extractor, and an organic solvent (30% TBP / n-dodecane) was added from the organic phase inlet side to the extractor.
It is supplied at a flow rate of 0 l / h, and U from the co-decontamination process,
Pu loading solvent (U 66 g / l; Pu 10 g / l;
Nitric acid (0.05 N) was supplied at a flow rate of 124 l / h.
On the other hand, from the outlet side of the extractor, the Pu back extract is 0.2
N nitric acid at a flow rate of 80 l / h, reducing agent (hydroxylamine nitrate (HAN)) and stabilizer (hydrazine (HDZ))
3 g / l) at a flow rate of 0.25 l / h and further adjusted acid (9N nitric acid) at a flow rate of 5 l / h, respectively, and brought into alternating contact with the organic solvent phase in the extractor. The distribution temperature in the extraction device at this time was 10 ° C., and a cooling device was installed in each supply pipe to the extraction device to maintain the distribution temperature. The Pu product back-extracted in the nitric acid aqueous phase is sent to the Pu refining process, and the U product left in the organic solvent phase is sent to the U refining process.

Table 1 shows the results of calculation of the above conditions using the extraction process simulation code "Revised MIXSET". For comparison, under the same extraction apparatus and feed conditions as above, the conventional reduction partitioning method (reducing agent ratio 3.0; partitioning temperature 40 ° C.) and acid partitioning method (no reducing agent added; partitioning temperature 5 Table 1 also shows the calculation results for the case of using (° C.).

Further, the product specification set here, that is, Pu / U ratio in U product; 1 ppm or less, U / Pu ratio in Pu product, 30% or less, maximum Pu concentration in organic phase; 25 g / l or less, are achieved. For the method of the present invention, the reduction distribution method and the acid distribution method, the number of distribution stages of the extraction device required for
The results calculated using d MIXSET "are shown in Table 2.

[0016]

[0017]

As can be seen from Table 1, the reducing agent ratio required to achieve the product specification set value is 3.0 in the case of using HAN as the reducing agent in the conventional reducing and partitioning method.
On the other hand, according to the method of the present invention, it is 0.02, and the addition amount of the reducing agent is significantly reduced. Further, when comparing the method of the present invention with the conventional acid partitioning method in terms of product specifications, the Pu / U ratio in the U product is 1.0 p of the method of the present invention.
The acid partitioning method was 5.7 ppm with respect to pm, and the U / Pu ratio in the Pu product was 22.1% in the present invention method, whereas it was 24.5% in the acid partitioning method. It can be seen that the product specifications are improved compared to the distribution method.

Further, as can be seen from Table 2, the number of distribution stages required to achieve the product specification set value is 20 in the conventional reduction distribution method and 18 in the acid distribution method, whereas the method of the present invention. According to the method, the number of distribution stages is 16 and the number of distribution stages can be shortened. Also, the distribution temperature in the conventional acid distribution method is 5 ° C.
Since the product specification set value can be achieved even when the temperature is raised to 10 ° C. as in the method of the present invention, it is understood that the method of the present invention can reduce the burden on the cooling equipment as compared with the conventional acid distribution method.

[0020]

As described above, according to the present invention, when the U / Pu partitioning step in the Purex method is carried out, the reducing agent is added to the dilute nitric acid as the Pu back-extracting solution at a reducing agent ratio of 0.02 to 0.02. By adding in the range of 1 and performing the distribution temperature in the range of 10 to 30 ° C., the addition amount of the reducing agent can be reduced as compared with the conventional reduction and distribution method, and a predetermined product specification can be achieved. Further, as compared with the conventional acid distribution method, an excessively large cooling facility is not required and the number of distribution stages of the multistage extraction device can be shortened.

[Brief description of drawings]

FIG. 1 is a flow sheet showing an example of a U / Pu distribution process using the distribution method of the present invention.

FIG. 2 is a flow sheet showing the Purex method when a conventional reduction and distribution method is used in the U / Pu distribution step.

FIG. 3 is a flow sheet showing a U / Pu distribution process using a conventional acid distribution method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Tokai Kawata, Tokai Village, Naka-gun, Naka-gun, Ibaraki Prefecture, 4-4 Muramatsu, Tokai Plant, Power Reactor and Nuclear Fuel Development Corporation (72) Shotaro Hayashi, Tokai-mura, Naka-gun, Ibaraki Prefecture Muramatsu 4-33 33 Power Reactor / Nuclear Fuel Development Corporation Tokai Works

Claims (1)

(57) [Claims] 1. A 30% TB solution of a nitric acid solution of spent nuclear fuel
A co-decontamination step of contacting the organic phase with P to extract U and Pu into the organic phase, and contacting the organic phase loaded with U and Pu with a Pu back extract containing dilute nitric acid to convert Pu into a nitric acid aqueous phase. U / Pu that causes back extraction and U remains in the organic phase
When carrying out the distribution step of the Purex method consisting of the distribution step, the reducing agent is added to dilute nitric acid which is the Pu back extract in a reducing agent ratio (reducing agent amount / Pu amount) in the range of 0.02 to 1. In addition, the U / Pu distribution method in the Purex method is characterized in that the distribution temperature is in the range of 10 to 30 ° C.