RU2410681C2 - Method of determining content of chlorine in uranium - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to determination of elements in uranium materials. The method of determining content of chlorine in uranium hexafluoride by measuring intensity of an analytical signal in a sample under analysis involves hydrolysis of a uranium hexafluoride sample and dilution of the sample to uranium concentration of 1 g/l, where determination of chlorine in uranium hexafluoride is carried out using a direct method without pre-separation of chlorine from the analysed sample using an inductively coupled plasma mass-spectrometer having resolution m/Δm≤4000 relative units.

EFFECT: easy, reliable and rapid analysis.

4 tbl, 1 dwg

Description

The invention relates to analytical chemistry, and in particular to methods for determining the content of elements in uranium materials, in particular chlorine.

The invention can be used to determine the components whose content in uranium hexafluoride is subject to control.

Uranium hexafluoride, as a feedstock for enrichment using the uranium-235 isotope, has strict requirements for the purity of the product, in particular for the content of chlorine, an element that forms volatile fluorides, the presence of which negatively affects the work of separation cascades. The chlorine content of uranium hexafluoride according to ASTM 787-96 is limited to 100 ppm (100 · 10 ^-4 % of uranium).

Known techniques with preliminary separation of chlorine, described in the collection of standard methods ASTM. Among them, the method of titrimetric determination of chlorine in HFCs with preliminary separation of chlorine in the form of free chlorine (ASTM C 761-01) [1], the method of determination of total chlorine and fluorine in powders and tablets of uranium dioxide by the potentiometric method with preliminary separation of chlorine in the form of gaseous hydrogen chloride after pyrohydrolysis of a sample (ASTM C 1502-01) [2], X-ray fluorescence determination of bromine and chlorine in HFCs with preliminary precipitation of chlorine and bromine in the form of halides (ASTM C 1508-01) [3].

Known is the indirect (based on reaction products) method of determining chlorine in uranium hexafluoride [4], adopted as a prototype (widely used at the enterprise), which consists in the preliminary reduction of chlorates to chlorides, the oxidation of chloride ions with potassium permanganate to elemental chlorine, and the separation of gaseous chlorine from solution samples by a stream of nitrogen into an absorption solution prepared on the basis of sodium acetate and potassium iodide. The content of released iodine in the absorption solution is equivalent to the amount of chlorine absorbed. The iodine content is determined by spectrophotometric method.

 The above methods have the following disadvantages:

- the need for additional operations to separate chlorine from the analyzed sample, which leads to an increase in the complexity and duration of the analysis;

- additional operations lead to an increase in error.

There are direct methods of inductively coupled plasma mass spectrometric (ICP-MS) methods for determining the chlorine content in various objects, for example, a method for determining halogens, including chlorine, using a quadrupole mass analyzer with a resolution of 300-400 rel. [5].

The disadvantage of these methods is that, firstly, uranium materials are not among the analyzed objects, and secondly, the methods have low sensitivity for determining the chlorine content due to the low ionization efficiency in the plasma of the RF discharge, a high level of background, the suppressing effect of the matrix and distortion analytical signal of chlorine due to polyatomic isobar overlays of molecular ions ¹⁸ O- ¹⁶ O- ¹ H. As a result, the determination of chlorine at the level of 1000 ppm and below is impossible.

The objective of the invention is to provide such a method for the determination of chlorine in uranium, which, excluding the disadvantages inherent in the known methods, would provide a direct, reliable and rapid determination of the chlorine content in uranium hexafluoride at a level of 15 ppm to uranium.

The problem is solved in that in the claimed method for the determination of chlorine in uranium hexafluoride using a high resolution mass spectrometry method, including hydrolysis of a HFC sample, diluting a solution of hydrolyzed uranium hexafluoride to a uranium concentration of 1 g / l and establishing a calibration characteristic, measuring the intensity of the analytical signal of chlorine in the analyzed sample is carried out directly without preliminary separation of chlorine from the analyzed sample at the selected optimal operating parameters operation of the mass spectrometer in a mode of resolution m / Δm = 4000 RLU

The proposed method is implemented in the analysis of samples of production of uranium hexafluoride using a magnetospectral mass spectrometer with ICP Element-2. Hydrolysis of uranium hexafluoride samples was carried out to obtain a hydrolyzate with a uranium concentration of 200-300 g / l. Before measurements, the hydrolyzate was diluted with deionized water to a uranium concentration of 1 g / L.

As it was established during the development of the method, the effect of polyatomic isobar overlays on the chlorine isotope is completely eliminated when measured in the resolution mode m / Δm = 4000 rel. The drawing shows a plot of the mass spectrum obtained for the chlorine isotope in the resolution mode m / Δm = 4000 relative units As can be seen from it, the peaks corresponding to ³⁵ Cl and ¹⁸ O- ¹⁶ O- ¹ H are completely separated, which allows reliable measurements of the chlorine content.

As a result of research, the following patterns were identified.

1. The sensitivity of the determination of chlorine in uranium materials can be increased tenfold when selecting the operational parameters of inductively coupled plasma — the power of a high-frequency discharge and the flow rate of sample gas.

2. An increase in the signal integration time leads to a decrease in the detection limit.

3. Background levels during ICP-MS determination of chlorine are determined by the content of the real element being analyzed and come from the sample input system. Confident measurements of the analytical signal of chlorine when determining the chlorine content of the claimed method are possible when the background value of the chlorine ion is not more than 1000 imp / s, and to effectively reduce it (when this value is exceeded), the sample injection system should be washed with a 1% solution before measurements nitric acid, then a solution of hydrolyzed HFCs for at least 2 minutes.

4. The overwhelming effect of uranium is already observed at a concentration of 0.1 g / l, and at a concentration of uranium of 1 g / l, the chlorine signal intensity decreases by 3 times. At the same time, with an increase in the uranium concentration in the analyzed solution, the ratio of the “useful” signal to the “blank” signal remains almost unchanged, which indicates that the analytical characteristics of the method do not deteriorate.

5. To obtain well reproducible results for calibrating the mass spectrometer, it is necessary to prepare certified mixtures concentrated in the concentration of chlorine ion 20-100 times, and dilute them before measurement.

The research results are presented in tables 2 and 3.

As follows from table 2, the maximum value of the value of the analytical signal is achieved when the power of the RF generator, equal to 1300 watts, and the flow rate of the spray gas of 0.93 l / min

Table 3 The optimal parameters of the measurement program of the analytical signal of chlorine. Working parameter Value Cooling Gas flow rate, l / min 16 l / min Auxiliary Gas flow rate, l / min 0.8 l / min Sample gas consumption, l / min 0.96 l / min RF generator power 1300 watts Accelerating voltage 2000 V Atomizer type Concentric, Polyamide Detector Mode Dual mode Type Scan Mode Electrostatic scan Resolution Resolution average Δm / m = 4000 Solution Integration Time Total Time 70 s Flushing time of the sample injection system 120 s The concentration of uranium in solution 1 g / l

To determine the dependence of the analytical signal intensity on the chlorine content, certified mixtures of AC ₁ , AC ₁ , AC ₃ were prepared with a mass fraction of chlorine of 15, 50, 100 μg / g of uranium, respectively, which are solutions of the chloride ion in the hydrolyzate of uranium hexafluoride (uranium hexafluoride "heavy "Depleted in uranium-235 fraction of the dump of the separation plant, that is, uranium hexafluoride that has passed the separation cascades.) High purity (not containing chlorine) with a mass concentration of uranium of 200-300 g / l.

To establish the calibration dependence, a solution of uranium hexafluoride hydrolyzate (not containing chlorine) was introduced into the discharge plasma with a mass concentration of uranium of 1 g / l as a solution of a blank sample, as well as certified mixtures with a chlorine content of 15, 50, and 100 μg / g of uranium their ascending order. The counting rate of ³⁵ Cl isotope ions was measured.

Using the mass spectrometer software using the multiple regression method, we obtained the dependence of the analytical signal intensity (imp / s) on the mass fraction of chlorine (μg / g uranium) in the certified mixture, described by the equation: I = a · C, where

C is the chlorine content in the calibration sample, μg / g of uranium;

I is the intensity of the analytical signal, imp / s;

and - the calculated coefficient equal to 32.198.

Based on the data obtained, a calibration graph was constructed, according to which the chlorine content in the analyzed samples was determined.

An example of determining the chlorine content of uranium hexafluoride using an inductively coupled plasma high resolution mass spectrometry method.

A sample of uranium hexafluoride is hydrolyzed with deionized water.

Using a pipette of variable capacity 100-1000 mm ^{3, a} necessary aliquot of the hydrolyzate solution is measured in a polypropylene flask with a capacity of 50 cm ³ , calculated under the condition that the concentration of uranium in the solution is 1 g / l, adjusted to the mark with deionized water and mixed. For each sample, two parallel definitions are performed.

Sample solutions prepared for measurement are introduced into the discharge plasma. The intensity of the analytical signal of the chlorine ion in the sample solutions of hydrolyzed uranium hexafluoride is measured in accordance with the measurement parameters on the Element-2 mass spectrometer shown in Table 2.

The mass fraction of chlorine in μg / g of uranium in the solutions of samples of hydrolyzed uranium hexafluoride is determined using the established calibration characteristics.

The values of the characteristics of the errors in determining the mass fraction of chlorine to uranium in uranium hexafluoride by the present method and the prototype method are shown in table 4.

The proposed method has successfully passed tests that confirmed the high accuracy and rapidity of determining the mass fraction of chlorine in uranium hexafluoride. The method allows you to abandon the stage of preliminary separation of chlorine from the analyzed sample and the associated additional time-consuming operations that affect the accuracy and reproducibility of the analysis.

Information sources

1 Annual book of ASTM standards: C 761 Titrimetric Determination of chlorine;

2 Annual book of ASTM standards: C 1502-01 Standard Test Method for Determination of Total Chlorine and Fluorine in Uranium Dioxide and Gadolinium Oxide;

3 Annual book of ASTM standards: C 1508-01 Standard Test Method for Determination of Bromine and Chlorine in UF6 and Uranyl Nitrate by X-Ray Fluorescence (XRF) Spectroscopy;

4. OI 001.474-2005. Chlorine. Methods of spectrophotometric determination in HFCs. Inv. No. 16/10251;

5. Application of Inductively Coupled Plasma Mass Spectrometry to simultaneous Determination of Chlorine, Bromine, and Iodine in National Bureau of Standards Standard Reference Material 1648 Urban Particulate / A.R. Date, M.E. Stuart // J. Anal. At.Spectr., August, vol. 3, 1988, pp. 659-665;

Claims (1)

A method for determining the chlorine content in uranium hexafluoride by measuring the intensity of the analytical signal in the test sample, including hydrolysis of a sample of uranium hexafluoride and diluting the sample to a uranium concentration of 1 g / l, characterized in that the determination of chlorine in uranium hexafluoride is carried out directly without prior separation of chlorine from the analyzed sample using an inductively coupled plasma mass spectrometer having a resolution of m / Δm <4000 rel. units

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