WO1994017530A1

WO1994017530A1 - Selective separation of caesium using crown ethers

Info

Publication number: WO1994017530A1
Application number: PCT/FR1994/000094
Authority: WO
Inventors: Alain Raymond; Patricia Paviet
Original assignee: Commissariat A L'energie Atomique; Compagnie Generale Des Matieres Nucleaires
Priority date: 1993-01-27
Filing date: 1994-01-26
Publication date: 1994-08-04
Also published as: FR2700709A1; FR2700709B1

Abstract

A method for selectively separating caesium using a crown ether and sodium tetraphenylborate, wherein the caesium is separated using an immiscible phase including crown ether tBB21C7 and sodium tetraphenylborate. The caesium may then be re-extracted in nitric acid 0.01-5N. The method may be used for the quantitative analysis of caesium 135 in aqueous solutions from the reprocessing of waste nuclear fuels.

Description

SELECTIVE SEPARATION OF CESIUM BY MEANS OF ETHERS-CROONNE

The present invention relates to a process for selectively separating cesium from aqueous solutions, usable in particular for the determination of ^'•• ' ^• -'Cs in aqueous solutions from nuclear installations.

Certain fission products emitting β ~ such as technetium 99, iodine 129 and cesium 135 whose periods are greater than 10 ^{* +} years are very difficult to dose routinely because not being gamma emitters, they are are not accessible by direct measurement and require prior separation.

In the case of cesium 135, this prior separation is difficult to achieve from r such matri¬ as complex as aqueous effluents from facilities for reprocessing spent nuclear fuel.

In addition, cesium 135 is present in very small quantities in these effluents, while cesium 137 is present in larger quantities ( ¹ 35 ( ^* _s / 137cs = about 10 ^"* ^); also, it is easier the to separate and measure the cesium 137 concentrations of such solutions.

Cesium 135 is a pure beta emitter of energy 0.2lMeV and of period 3.10 ⁶ years. It is produced by the disintegration of xenon or by the nuclear fission of uranium 235 and plutonium 239 with a yield of 6.66%. It is therefore not assayable by spectrometer and gamma as is possible for cesium 137. It is known that the separation of cesium from aqueous solutions can be carried out by solvent extraction, for example by means of crown crowns, as described by Blasius et al in Radiochim. Acta, 35, 173 (1984).

However, the distribution coefficients obtained remain relatively low, which makes it difficult to extract all of the cesium 135 for dosing.

The present invention specifically relates to a process for separating cesium from aqueous solutions by means of a system of ext ra ctants which makes it possible to obtain high extraction rates and selectivity.

According to the invention, the method for separating cesium from an aqueous solution comprises the following steps:

1 °) bringing the aqueous solution into contact with an immiscible phase comprising ether r-crown tBB21C7 of formula:

and sodium tetraphenylborate, to separate the cesium in this immiscible phase, and

2) separate the immiscible phase containing the cesium from the aqueous solution.

In this process, the use of a mixture of crown ether (tBB21C7) and sodium tetraphenylborate promotes the extraction of cesium compared to the other species present in the solution, in particularly those appearing in effluents and solutions from nuclear installations.

According to a first embodiment of the process of the invention, the extractant system is diluted in a liquid organic phase immiscible with the aqueous solution containing cesium. In this case, in the first step of the process of the invention, the aqueous solution is brought into contact with an immiscible liquid organic phase which comprises r-crown ether tBB21C7, sodium tetraphenylborate and at least one di organic glow, and the organic phase containing cesium can then be easily separated from the aqueous solution by simple techniques such as decantation.

The organic diluents used in the immiscible organic phase also have an influence on the extraction of cesium. By way of example of useful fluids, mention may be made of chlorinated fluids such as tét rach loroéthane, yl acetate and aromatic fluids such as n i t robenzene.

Preferably, ni t robenzene is used.

The concentration of extractants in the organic phase comprising such a diluent can vary over a wide range. Generally, use is made of crown ether and sodium tetraphenyl bo¬ spleen which vary from 0.01 to 1 mol / l with a concentration of crown ether / tet rapheny sodium Iborate concentration of 1 to 20, preferably a molar ratio of ether-crown / sodium tetraphenylborate of 10.

According to a second embodiment of the process of the invention, the extractant system is fixed on a solid phase such as a resin ion exchange or an inert substrate (if lice, resin), for example in a ch ro atographic column. In this case, in the first step, the aqueous solution is brought into contact with the solid support on which the extractant system is fixed.

After cesium has been fixed in the immiscible solid or liquid phase, it can be reextracted in an aqueous solution comprising, for example, from 0.01 to 5 mol / l of HNO3.

This re-extraction mode is easy to implement and allows high separation efficiency to be achieved.

A subject of the invention is also a method for dosing cesium 135 present in an aqueous solution originating from a nuclear installation, in particular from an installation for reprocessing spent nuclear fuels.

This process comprises the following steps: a) taking a sample of the aqueous solution, b) separating cesium 135 and cesium 137 present in the sample taken using an immiscible phase comprising crown ether tBB21C7 and sodium raphenylborate using the process described above, c) re-extracting cesium 135 and cesium 137 thus separated in an aqueous re-extraction solution comprising from 0.01 to 5 mol / l of HNO3, and d) determining the Cs 135 concentration of this solution.

To carry out step d), it is possible to measure the cesium-137 activity of the solution by gamma spectrometry, then to measure the isotopic ratio ^{es 1 um} —LΣ_L_ of the solution by thermoionization cesium 137 mass speet and deduce from these measurements the Cs 135 concentration.

Indeed, the activity of the ¹ ^ t 7ςs _are easily measured by gamma Speet Romet ry with measuring lines at 31,8keV, 32,2keV, 36,3keV and 661,6keV, the latter being the most intense line . Also, the activity in Cs 137 can be determined by gamma spectrometer spectroscopy using for example a counting time of 3 h so that the statistical error on each of the photonic emissions is clearly less than 1%. Then, we measure the atomic ratio ' ^• -' ^• -'Cs / '- ^ Cs per thermalized speet with thermionization mass, which gives the activity ratio ¹ * - ^* * - ^* Cs / ^{' •} - ' ^ Cs and makes it possible to determine from this report the concentration in ^' ' ^• '- ^• ' Cs of the solution.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of illustration and not limitation, with reference to the appended figure.

This figure is a diagram representing the procedure which can be used to carry out a dosage of ^ - * - "** ^* Cs by the method of the invention.

According to this operating mode, a volume V (0.5 to 20 ml) of the effluent to be dosed is taken, then 0.1V to 1V of the herbal crown tBB21C7 is added thereto at a concentration of 0, 01 to 1mol / l in nitrobenzene. Then 0.1V to 1V of sodium tetraphenylbo¬ rate is added at a concentration of 0.01 to 1mol / l in nitrobenzene, then the whole is stirred for about 30 minutes and the organic phase is separated. per hundred ri fugati on. We then re-extracts the cesium from the organic phase by bringing it into contact with an aqueous re-extraction solution comprising 0.01 to 5mo l / 1 of HNO3, then measurements are carried out on the aqueous solution containing the re-extracted cesium. Thus, on the one hand, cesium 137 is measured by gamma spectrometer and the Cs 135 / Cs 137 ratio by spe and mass thermionization. From these two measurements, the cesium 135 activity of the aqueous solution is determined.

The following examples illustrate the influence of certain parameters on the cesium extraction and the results obtained during the dosing of effluents from plants for reprocessing spent nuclear fuels.

Examples 1 to 7: Influence of the extractant and of the diluent on the cesium extraction.

In these examples, cesium is extracted from aqueous solutions by bringing them into contact with immiscible organic phases containing various extractants and diluents, operating as described above.

In each case, 2 ml of standard solution of ^ - ^ Cs at 185Bq / ml containing 1.5 mol / l of HNO3 are used as the aqueous phase. The organic phase consists of an extractant or a system of extractants in a diluent and its composition is given in Table 1 below.

To carry out the extraction, the 2 ml of aqueous phase are brought into contact with the volume of organic phase given in table 1 with stirring for 30 min, then the two phases are separated and the cesium 137 activity of the organic phase is measured. by scintillation counting liquid.

The distribution coefficient of cesium D ₍ * _{s) is then determined} , which corresponds to the ratio of the cesium concentration of the organic phase to the cesium concentration of the aqueous phase.

Table 1 gives the results obtained.

Table 1

Organic phase

Ex Volume Di luant Ext ractants Concentration ^D CS (ml) (mol / l)

1 4 DC24C8 acetate 5.10 ^-2 1.6.10-1 of any TBPNa 5.10 " ²

2 4 TBPNa acetate 10-1 1,8.10 ^-2 of anyl

3 5 Ni t robenzene tBB21C7 6.10 ^-2 1.31 TBPNa 4.10 ^-3

4 3 Tetrachloro- tBB21C7 1.10-1 0.38 ethane

5 3 Ni trobenzene tBB21C7 1.10-1 0.55

6 3 Nitrobenzene DC24C8 1.10-1 0.62

7 3 Tetrachloro- DC24C8 1.10-1 0.45 ethane

TBPNa = sodium tetraphenylborate um

In view of this table, it can be seen that example 3 which relates to the use of nitrobenzene as a diluent with the extractant system of the invention (tBB21C7 and TBPNa) gives the best results, namely a distribution coefficient cesium greater than 1.

Furthermore, the comparison of Examples 3 and 4 shows the synergistic effect due to the presence of TBPNa in the immiscible organic phase.

Example 8: Extraction of cesium from aqueous effluents.

In this example, cesium is extracted from 1 ml of real effluent containing, in addition to cesium, radioactive elements such as ruthenium 106. For this extraction, 1 ml of real effluent is brought into contact to which 1 ml of 3N nitric acid with 2 ml of organic phase consisting of nitrobenzene containing sodium tetraphenylborate, tBB21C7 or a mixture of the two. After 30 min of contact, the two phases are separated, then the cesium present in the organic phase is re-extracted by contacting with 2 ml of 0.5N HNO3.

The activity in ¹ ** '7cs of the aqueous re-extraction solution is then determined by counting by gamma spectrometer. The extraction yield of ^η * - 7cs is thus determined (in%) which corresponds to the ratio of the initial activity in Cs 137 of the effluent to the activity in Cs 137 of the re-extraction solution multiplied by 100, the activity in ¹³⁷ Cs of the starting effluent having also been determined by counting by gamma spectrometer.

The results obtained are given in Table 2 which follows. 4/17530

1 0

Table 2

Concent rati on of the ext ractant in La (TPBNa) = 10 ~ ² (tBB2U7) = 5.10 ~ ² ⁽ tBB21C7 ⁾ = 10 ~ ¹ phase ⁽ TPBNa ⁾ = 5.10 ~ ² organi

(moL / l)

Ext racti on yield 11 78 64

(%)

It is thus noted that the use of the extractant mixture of the invention makes it possible to re-extract 78% of the cesium, which corresponds to the best result. Examples 9 to 11: Determination of 135 _c in aqueous effluents.

In these examples, ¹ - ⁵⁵ Cs is measured in effluents from a plant for the reprocessing of spent nuclear fuels. _In each case, the assay is carried out from 5 ml of effluent using 5 ml of organic phase consisting of nitrobenzene containing 0.05 mol / l of tBB21C7 and 0.02 mol / L of sodium tetraphenylborate. After bringing the two phases into contact with stirring for 30 min, the two phases are separated by one hundred ri fugati on and the cesium is re-extracted from the organic phase in 5 ml of a 0.5N aqueous solution of nitric acid. The activity is then determined in ¹³³⁷ Cs of the solution by gamma-ray spectrometer 0 and the ratio ¹ -55 _Cs / ³ Cs by mass spectrometer with thermionization, and the activity is deduced therefrom. ¹³⁵ Cs of the effluent.

The results obtained are given in table 3.

Table 3

Activity (Bq / ml) Report

Ex. 135 _Cs 137 _Cs 135 _{Cs /} 137 _C s

9 2,62.10- ² 523 5.10 ^"5

10 2.04.10-2 2800 7.28.10 " ⁶

11 5.18.10- ² 2762 1.87.10- ⁵

These results are in agreement with the data available on this type of effluent.

The process of the invention is therefore very advantageous for the dosage of ^{13 5} cs since the total time for carrying out the extraction and re-extraction is only 1 hour, the extraction proper not exceeding 30 min for a series of 8 samples.

Moreover. The extraction yield is very good since it is 60 to 80% and the detection limit is approximately 5.10 ^-3 Bq / ml.

Finally, this process is easy to implement and inexpensive, the extraction being able to be carried out in Corning type tubes with simple contacting of a volume of aqueous phase and a volume of organic phase.

Claims

1. Process for separating cesium from an aqueous solution, characterized in that it comprises the following steps:

1 °) bringing the aqueous solution into contact with an immiscible phase comprising the crown ether tBB21 C7 of formula:

2. Method according to claim 1, characterized in that in the first step, the aqueous solution is brought into contact with an immiscible liquid organic phase comprising the crown ether tBB21C7, sodium tetraphenylborate and at least one organic diluent.

3. Method according to claim 2, characterized in that the organic diluent is ni t robenzene.

4. Method according to claim 2, characterized in that the organic phase consists of nitrobenzene comprising from 0.01 to 1mol / L of crown ether tBB21C7 and from 0.01 to 1mol / L of sodium tetraphenylborate.

5. Method according to any one of Claims 1 to 4, characterized in that the cesium is then separated from phase i miscible by re-extraction in an aqueous solution comprising 0.01 to 5 mol / l of HNO3.

6. Method according to any one of claims 1 to 5, characterized in that in Step 1, the aqueous solution is brought into contact with a solid support on which are fixed the crown ether and sodium tetraphenylborate.

7. Method for dosing cesium 135 present in an aqueous solution originating from a nuclear installation, characterized in that it comprises the following steps: a) taking a sample of the aqueous solution, b) separating cesium 135 and cesium-137 present in the échanti llon taken using an immiscible phase comprising éthei the crown TBB21C7 and sodium tetraphenylborate ^by implementing the method according to any one of claims 1 to 4, c) re-extract the cesium 135 and cesium 137 thus separated in an aqueous re-extraction solution comprising from 0.01 to 5moL / l of HNO3, ^and d) determining the Cs 135 concentration of this solution.

8. Method according to claim 7, characterized in that in step d), the activity in Cs 137 of the solution is measured by spectrometry and gamma spectra, the isotopic ratio Cs135 / Cs137 of the solution by spect is measured. thermionization mass rometri e, and the Cs 135 concentration of the solution is deduced therefrom.