FR2691542A1 - Determn. of uranium by liq. phase chromatography and UV spectrometry - using column filled with a polar stationary phase with mobile liq. phase contg. cetyl tri:methyl:ammonium ion and solvent favouring presence of uranyl tri;nitrate - Google Patents

Abstract

Process for determn. of uranium present in a soln. in the form of nitrate comprises (1) injecting a sample of the uranium-contg. into a chromatography column filled with an apolar stationary phase and run through with a mobile liq. phase contg. cetyltrimethylammonium ion, bisulphate anion, sulphate anion and a solvent favouring the presence of uranium in the form of uranyl trinitrate, and (2) determining the amt. of uranium leaving the column by UV spectrometry. In an alternative embodiment, (1) the sample is introduced into a preconcn. chromatography column filled with an apolar stationary phase and run through with a mobile liq. phase contg. cetyltrimethylammonium ion, sulphate anion and a solvent favouring the presence of uranium in the form of uranyl trinitrate, (2) the first column eluted with a second mobile liq. phase contg. cetyltrimethylammonium ion, bisulphate anion, sulphate anion and a solvent favouring the presence of uranium in the form of uranyl trinitrate, (3) the eluate is introduced into a second chromatography column filled with a stationary apolar phase and (4) the amt. of uranium leaving the column is determined by UV spectrometry. USE/ADVANTAGE - The procedure allows determn. of uranium at very low concns., e.g. of 1 microgram to 0.1 mg/1, with good accuracy and reproducibility, e.g. for determn. of U in various aq. or organic solns. obtd. during the treatment of used nuclear fuels, and in other solns. such as sulphochromic baths, tripolyphosphate solns., solns. of uranium minerals, river water, urine etc.

Description

Uranium dosing process by liquid chromatography
The present invention relates to a process for the determination of uranium present in the form of nitrate in aqueous or organic solutions.

 It applies in particular to the determination of uranium, even in trace amounts, in the various solutions of a facility for the reprocessing of spent nuclear fuels, such as solutions with a high uranium content (0.1 to 500g / l) as Solutions for dissolving irradiated fuels, Oxalic mother liquors, plutonium concentrates, Fission product concentrates and charged solvents, and solutions with low uranium content (lg to 0.1g / l) such as uranium standard solutions, fuel peeling solutions, plutonium dissolution solutions, effluent treatment station solutions, fission product concentrates, carbonate treatment solutions of organic solvents, purified organic solvents and solutions leaving the uranium extraction columns. It also applies to the determination of uranium in other types of solutions such as sulfochromic bath solutions, tripolyphosphates solutions, uranium ore solutions, river water, urine, etc.

 Until now, in plants for the reprocessing of spent nuclear fuels, uranium determinations have been carried out by potentiometric, spectrophotometric or isotopic dilution methods. However, these methods are not suitable for assaying small amounts of uranium in the presence of fission products. Also, new assay techniques have recently been developed, such as spectrofluorimetry, plasma emission spectrometry and liquid chromatography.

 A method for metering uranium by chroma tography in liquid phase is described for example by J.P. Muller et al in Analusis, 1987, vol.

15, no.5, p. 209-216. This process consists in retaining the uranium in the form of a pair of ions consisting of the uranyl trinitrate anion and the tetrahexylammonium cation on a stationary phase based on grafted silica, then in eluting the uranium by a phase appropriate mobile containing acetonitrile, water and a developing ion such as the sulfate or hydrogen sulfate anion, and finally detecting the uranium leaving the column by spectrometry in the ultraviolet.

 However, this method is not applicable to the determination of trace uranium, that is to say at concentrations lower than 0.1 mg / l, since it is impossible to obtain the fixation of these traces. of uranium on the stationary phase under the conditions described in this document.

 The present invention specifically relates to a process for metering uranium by liquid chromatography, which allows the metering of very small amounts of uranium, for example less than 0.1 mg / l, with good accuracy rates and reproducibility.

According to the invention, the method for determining the uranium present in a solution in the form of nitrate, comprises the following steps
10) take a sample of the solution containing uranium and inject it into a chromatography column filled with a stationary apolar phase and traversed by a mobile liquid phase containing the cetyl trimethylammonium ion, a hydrogen sulphate anion, a sulphate anion and a solvent promoting the presence of uranium in the form of uranyl trinitrate, and
20) detect and determine the quantity of uranium leaving the column by spectrometry in the ultraviolet.

 In this process, the use of a mobile phase comprising the cetyl trimethylammonium ion makes it possible to promote the fixation of uranium on the stationary apolar phase, even when the uranium is present in trace amounts.

 Furthermore, the use in the mobile phase of two anions constituted respectively by the hydrogen sulphate anion and the sulphate anion, makes it possible to obtain not only a good fixation of the uranium on the stationary phase, but also its elution in good conditions.

 Indeed, by choosing appropriate amounts of sulfate anion and hydrogen sulfate anion, it is possible to elute the uranium in a suitable time.

 In the mobile phase, the solvent used also helps to promote uranium uptake.

Generally, this solvent is a mixture of water and acetonitrile and the ratio by volume of water: acetonitrile is chosen to promote this fixation.

As an example, this ratio can be 75:25.

 In this mobile phase, the anions can be present in the form of alkali metal or ammonium salts and the cetyl trimethylammonium ion can be introduced in the form of bromide.

 As an example of a mobile phase which can be used, mention may be made of that comprising - from 1.10-4 to 1.10'2mol / l of cetyl tri methyl bromide ammonium um, - from O to 5.10 ~ 2mol / l of hydrogen sulphate sodium, - from O to 5.10'2mol / l of ammonium sulphate, in a solvent constituted by a mixture of 25% by volume of acetonitrile and 75% by volume of water.

 One can also add to this mobile phase trifluoroacetic acid, for example 1t3.10 ~ 2mol / l of this acid because the presence of this makes it possible to refine the peak of uranium, that is to say say decrease the width at the base of the peak, and thus give better precision and better reproducibility.

 In the first step of the process of the invention, it is possible to further dilute the sample of uranium to be assayed in an aqueous solution which may contain suitable additives, chosen according to the nature of the solution to be assayed.

Thus, in the case of solutions containing plutonium, it is advantageous to dilute the sample in an ascorbic acid solution to selectively reduce the plutonium to valence
IV because Pu (VI) disturbs the chromatograms.

In the case of solutions loaded with uranium
IV, it is preferable to add ferric nitrate to the dilution solution to oxidize the uranium
IV uranium VI, and then add if necessary ascorbic acid as above.

 The dilution rates can vary over a wide range and depend in particular on the uranium content of the solutions to be assayed.

 When the uranium content is low, the dilution rate is also low because it is necessary to introduce into the chromatography column a quantity of uranium ranging from 5.10-3 to 251U9.

 In addition, the dilution rate depends on the nitric acidity of the solution because it should not exceed 0.1mol / L.

 When the dosing solution contains very small amounts of uranium, an additional step of preconcentration of the uranium is preferably carried out on a preconcentration chromatographic column.

Also, according to a preferred embodiment of the method of the invention, it comprises the following steps
10) take a sample of the solution containing uranium and inject it into a first preconcentration chromatographic column filled with a stationary apolar phase and traversed by a first mobile liquid phase containing the cetyl trimethylammonium ion, a sulfate anion and a solvent favoring the presence of uranium in the form of uranyl trinitrate,
20) elute the first preconcentration column with a second mobile liquid phase containing the cetyl trimethylammonium ion, a hydrogen sulphate anion and a solvent favoring the presence of uranium in the form of uranyl trinitrate,
30) introducing the eluate from the first preconcentration column into a second chromatography column filled with a stationary apolar phase, and
40) detect and determine the quantity of uranium leaving the second column of chromatography by spectrometry in the ultraviolet.

 In this preferred embodiment of the process of the invention, the sample is preferably diluted in an appropriate solution and the composition of the dilution solution is chosen so as to promote the uranium fixation on the stationary phase of the preconcentration column, then its elution under good conditions.

 Generally, the dilution solution comprises the cetyl trimethylammonium ion, a hydrogen sulphate anion, a sodium acetate buffer and a solvent constituted by a mixture of water and acetonitrile.

Sometimes ascorbic acid is also added to the dilution solution to selectively reduce the plutonium that may be present in the sample because Plutonium at valence
VI disrupts the chromatograms.

 By way of example, the dilution solution can consist of a mixture of water and acetonitrile comprising cetyl trimethylammonium bromide, sodium hydrogen sulphate and sodium acetate. In this solution, the contents of sodium hydrogensulfate, of sodium acetate, of acetonitrile and of water are chosen in particular as a function of the uranium content of the solution to be assayed and of the nature of this solution.

 Thus, when the uranium contents of the solution to be assayed are low, the amounts of water and acetonitrile of the dilution solution are chosen so as to favor the fixing of the uranium on the preconcentration column and its separation from the other elements of the solution.

For example, a water / acetonitrile volume ratio of 1.5 makes it possible to promote this fixation.
On the other hand, when the metering solution contains more uranium, the water / acetonitrile volume ratio in the dilution solution can be higher and go for example up to 2.

 The compositions of the mobile phases are chosen according to the aim sought.

 Thus, the composition of the first mobile phase used in the first preconcentration chromatographic column is chosen so as to promote the concentration of uranium on this first column and its easy dilution by the second mobile phase.

 Advantageously, the solvent of the first mobile phase and the solvent of the second mobile phase consist of a mixture of acetonitrile and water but the volume ratio of water / acetonitrile is lower in the second mobile phase than in the first phase. mobile.

 The water / acetonitrile volume ratio is chosen as a function of the anions used in the mobile phase so that the uranium comes out sufficiently, but it also depends on the length of the column and the nature of the stationary phase.

 Generally, grafted silica having, for example, a particle size of 5 m, is used as the apolar stationary phase in the two columns.

 With such a stationary phase, a water / acetonitrile ratio of 4 can be used in the first mobile liquid phase and a water / acetonitrile ratio of 1.5 in the second mobile liquid phase.

 This preferred embodiment of the process of the invention can be used both for the determination of solutions containing traces of uranium and for the determination of solutions with higher uranium contents.

 Indeed, for solutions with a relatively high uranium content, the sample can be more diluted and thus reduce the quantities of other elements present in the sample such as plutonium and fission products.

 The solution to be assayed can be an organic solvent, a concentrated solution of fission products or a concentrated solution of plutonium, and the dilution rates can range in this case from 200 to 1000. The dilution rate is chosen so that the quantity of uranium injected into the column is in the range of 0.005 to 3ug.

 In the case of solutions with a low uranium content, the dilution rate can be very low, for example 2.

 In the preferred embodiment of the method of the invention, it is important to subject the first preconcentration chromatographic column to a preliminary treatment to promote the complete retention of uranium on this column.

 This treatment can be carried out by circulating in the column for twenty hours the dilution solutions mentioned above after having added to them an appropriate amount of 14M nitric acid.

 The solution used for this treatment of the preconcentration column must in no case pass over the second chromatographic column.

 The process of the invention can be implemented in conventional chromatography installations comprising: - one or more chromatographic pumps guaranteeing for example flow rates from 0.05 to 10 ml / min, - one or more chromatographic valves each comprising a loop 'sampling, for example from 1 to 100pu, - stainless steel chromatography columns, - a spectrophotometric detector working for example at a wavelength of 254nm, and - a calculator for controlling and determining the area of Chromatographic peaks corresponding to Uranium, which are representative of the quantities of uranium in the samples.

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 drawing in which
FIG. 1 represents a chromatographic installation with a single column for the determination of uranium present in solutions,
FIG. 2 represents a chromatography installation comprising two columns with the valves in position corresponding to the preconcentration step, and
- Figure 3 shows the installation of Figure 2 avoc I oe \ / annoe positioned so as to perform the metering of uranium.

 In FIG. 1, it can be seen that the chromatography installation which is more specifically intended for the determination of uranium at contents greater than 0.1 g / l, comprises a chromatography column (1) into which a sample is introduced by a three-way injection valve (3) provided with an internal loop (5) for sampling the desired volume; The valve (3) can be connected via a pump (7) to a tank (9) of mobile liquid phase.

 At the outlet of the column (1), uranium is detected by means of the spectrometric detector (11) fitted with a calculator integrator.

 By way of example, the use of this installation for dosing uranium in an aqueous solution containing more than 0.19 / l of uranium is described below.

 In this case, first dilute an ImI sample of the solution to be dosed with 19ml of a solution of ascorbic acid containing 5.10'2mol / l of ascorbic acid, then inject îtjl of the sample into the chromatography column (1) via the valve (3) with a mobile phase consisting of a solution comprising - 5.10'3mol / l of cetyL trimethylammonium bromide, - 2.10'2mol / l of sodium hydrogen sulphate, and - 0.18 mol / l of ammonium sulfate, in an acetonitrile-water mixture comprising 25% by volume of acetonitrile and 75% by volume of water.

 The mobile phase is circulated in the column at a flow rate of Iml / min. The column has a length of 13 cm and an internal diameter of 4.6 mm and it is filled with a stationary phase based on grafted silica of C1 type with a particle size of 5 μm.

 The peak corresponding to uranium is detected at the column outlet by means of the detector (11) and the area of this peak is measured using the integrator. The elution time is of the order of 8 minutes, the uranium retention time being 5 to 6 minutes.

 Reference is then made to the calibration curve which was obtained by carrying out the same operations on samples of which the uranium concentrations are known, and the uranium content of the sample analyzed is thus determined.

 The same assay can be carried out on a solution consisting of an organic solvent containing uranium, but in this case, after diluting the sample with the solution of ascorbic acid to 5.10'2mol / l of ascorbic acid, stirred vigorously and allow to settle, then inject 1 jul of the lower phase and carry out the rest of the operations in the same way.

 FIGS. 2 and 3 show a chromatography installation suitable for the determination of trace state uranium, which comprises a preconcentration column and an analysis column.

In these figures, the same references have been used as those in FIG. 1 to designate
The elements common to both installations. Thus, we see that the installation of Figures 2 and 3 further comprises a preconcentratio-n chromatographic column (13) supplied by a three-way valve (15) which can be connected, either to the three-way valve (3 ), or to the chromatography column (1) and to a pump (17) associated with a mobile phase reservoir (19).

 In FIG. 2, the valve (15) is shown in the position which corresponds to the preconcentration step, while in FIG. 3 the valve (15) is in the position which corresponds to the actual analysis step. .

 This installation can be used both to dose the uranium present in trace amounts in aqueous solutions as well as to dose the uranium at higher contents ranging for example up to 100 mg / l.

To carry out this assay, mobile phases having the following compositions are used
1. first mobile phase in the tank (9) consisting of a solution comprising: - 5.10'3mol / l of cetyl trimethylammonium bromide, and - 0.1mol of ammonium sulfate, in an acetonitrile-water mixture (20:80 in volume).

 2. second mobile phase in the tank (19) consisting of a solution comprising - 5.10'3mol / l of cetyl trimethylammonium bromide, and - 1.10 ~ 2mol / l of sodium hydrogen sulphate, in an acetonitrile-water mixture (40: 60 by volume).

 These mobile phases are degassed before being used.

 To carry out the assay, proceed as follows.

 First of all, the samples are diluted, using a different dilution rate and dilution solutions depending on whether they are samples whose uranium content is greater than 0.1 mg / l or less than 0 , 1mg / l.

 a) Samples containing more than 0.1 mg / L of uranium.

 In this case, dilution solution is used, solution n01 which comprises - 1.102moL / l of cetyl trimethylammonium bromide, - 4.10-2mol / l of sodium hydrogen sulphate, and - 0.15mol / l of acetate sodium, in an acetonitrile-water mixture (35:65 by volume).

 Dilution rates can range from 200 to 2000.

 To carry out the dilution with a rate of 200, 0.1mol of the sample to be analyzed is introduced into a 20ml volumetric flask, then 0.5mL of an ascorbic acid solution at 1mol / l and the volume is completed. to 20ml with dilution solution n01 having the composition given above.

 b) Samples containing less than 0.1 mg / l of uranium.

 In this case, the solution n02 which comprises - 1.102mol / l of cetyl trimethylammonium bromide, - 8.10'2mol / l of sodium hydrogen sulphate, and - 0.30mol / l of sodium acetate is used as dilution solution. , in an acetonitrile-water mixture (40:60 by volume).

 In this case, the dilution rate is generally 2.

 To carry out this dilution, 10 ml of the sample to be assayed are introduced into a 20 ml volumetric flask and the volume is made up to 20 ml with dilution solution No. 2 having the composition given above.

 After diluting the samples, the assays are carried out as follows.

 First put the preconcentration column (13) in the circuit of the mobile phase n01 as shown in Figure 2, then inject into this preconcentration column (13), 6ml of the diluted sample by circulating in the column the first mobile phase coming from the tank (9) at a flow rate of 2 ml / min. After Smin of circulation, the position of the valve (15) is changed to connect the preconcentration column (13) to the mobile phase reservoir (2) and to the chromatography column (1) as shown in Figure (3).

 The second mobile phase from the reservoir (19) is then circulated in the column (13) and the column (1) at a flow rate of 1 ml / min for 12 min, and the uranium leaving the column (1) is detected at detector means (11). The area of the uranium peak which is representative of the quantity of uranium present in the sample is determined by means of the associated integrator.

 As before, a calibration curve obtained from reference solutions having known concentrations of uranium makes it possible to obtain the correlation between peak areas and uranium concentrations.

 This preferred embodiment of the process of the invention can be used with numerous aqueous or organic solutions, with or without the addition of ascorbic acid to selectively reduce the plutonium present in the samples.

 In the case of organic solvents, dilution rates of 200 can be used so that these are completely miscible with the dilution solution.

 In the case of concentrated plutonium solutions, a dilution rate of 1000 can be used with dilution solution n01 and the addition of ascorbic acid. This can be done by introducing 0.02ml of the sample to be analyzed into a 20ml volumetric flask, then 0.5ml of an ascorbic acid solution with Imolil and completing the volume to 20ml with dilution solution n01 .

 The process of the invention is therefore very advantageous since it applies to all solutions, in particular those having high contents of plutonium or of fission products, and is suitable for a range of uranium concentrations ranging from 1 jugal to 5009 / l of uranium.

 In addition, it is advantageous with regard to the duration of the metering, the volume of effluents produced, the rate of precision and automation.

 In fact, the duration of the assay is very short since it is approximately 6 min for uranium concentrations greater than 0.1 g and 15 min for lower concentrations.

 The volume of sample used for each assay is also very small, less than 100 r, and the same is true of the volumes of effluents produced by these assays since these represent only 8 liters for 200 assays.

 The confidence interval of the analysis result has the relative amplitude of the 95% confidence level: - + 5.10'3 for uranium concentrations higher than lg / l, and - + 2.10-2 for lower uranium concentrations at lg / l.

 Finally, the analysis stations can be fully automated.

Claims (12)

 1. Method for determining the uranium present in a solution in the form of nitrate, characterized in that it comprises the following stages  10) take a sample of the solution containing uranium and inject it into a chromatography column filled with a stationary apolar phase and traversed by a mobile liquid phase containing the cetyl trimethylammonium ion, a hydrogen sulphate anion, a sulphate anion and a solvent favoring the presence of uranium in the form of uranyl trinitrate, and  20) detect and determine the quantity of uranium leaving the column by spectrometry in the ultraviolet.  2. Method for determining the uranium present in a solution in the form of nitrate, characterized in that it comprises the following steps  10) take a sample of the uranium-containing solution and inject it into a first preconcentration chromatographic column filled with a stationary apolar phase and traversed by a first mobile liquid phase containing the cetyl trimethylammonium ion, a sulfate anion, and a solvent promoting the presence of uranium in the form of uranyl trinitrate,  20) elute the first preconcentration column by a second mobile liquid phase containing the cetyl trimethylammonium ion, a hydrogen sulphate anion and a solvent promoting the presence of uranium in the form of uranyl trinitrate,  30) introducing the eluate from the first preconcentration column into a second chromatography column filled with a stationary apolar phase, and  40) detect and determine the quantity of uranium leaving the second column of chromatography by spectrometry in the ultraviolet.  3. Method according to claim 2, characterized in that the sample is diluted in an appropriate dilution solution before injecting it into the first preconcentration column.  4. Method according to claim 3, characterized in that the dilution solution comprises the cetyl trimethylammonium ion, a hydrogen sulphate anion, a sodium acetate buffer and a solvent constituted by a mixture of water and acetonitrile.  5. Method according to claim 4, characterized in that the dilution solution comprises cetyltrimethylammonium bromide, Sodium hydrogen sulfate and sodium acetate in a mixture of water and acetonitrile with a water / acetonitrile volume ratio ranging from 1.5 to 2.  6. Method according to any one of claims 3 to 5, characterized in that the dilution solution comprises more ascorbic acid to reduce the plutonium possibly present in the sample.  7. Method according to any one of claims 1 to 6, characterized in that the solvent of the mobile liquid phase (s) is a mixture of water and acetonitrile.  8. Method according to any one of claims 2 and 7, characterized in that the solvent of the first mobile liquid phase is a mixture of water and acetonitrile with a water / acetonitrile volume ratio of 4 and in that that the solvent of the second mobile liquid phase is a mixture of water and acetonitrile with a volume ratio of water / acetonitrile of 1.5.  9. Method according to any one of claims 1 and 2, characterized in that the (the) phase (s) mobile (s) further comprise trifluoroacetic acid.  10. A method according to any one of claims 1 and 2, characterized in that the stationary phase of the preconcentration column and / or the stationary phase of the chromatography column consist of grafted silica having a particle size of Sum.  11. Method according to claim 3, characterized in that the first chromatographic preconcentration column is first subjected to a pretreatment with the dilution solution containing more than 14M nitric acid.  12. Method according to any one of claims 2, 8 and 11, characterized in that the solution to be assayed is an aqueous solution containing ijjg to 0.1 mg / l of uranium.