Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

Ion-exchange properties are inherent in insoluble substances composed of acidic (cationite) or principal groups (anionite) that are firmly bound thereto. One example of cationite is Dowex-50 (Russian analog, KU-2), a styrenedivinylbenzoic polymer with grafted-SО3Н groups. An example of highly basic anionite is Dowex-1 (Russian analog, AV-17), a styrenedivinylbenzoic polymer with grafted charged– СН2N+(СН3)3 groups.

Uranium ions can be absorbed by way of cationic exchange from relatively dissolved sulfuric-acid solutions (<0.5 n). This also involves removal of unabsorbed impurities. Most of the uranium separation and purification processes are however based on desorption by flushing the column with a reagent solution that forms an anionic complex with uranium. Uranium sorption (VI) from a solution based on a cationite out of acidic solutions is intensified in the following series of acids: H3PO4<H2SO4<HCl<HNO3<HClO4. Analytical procedures normally use cationites to remove phosphate-type anions, arsenats and whatever else hindering analyses.

Use of anionites helps sorb uranium’s anion complexes with such anions as nitrate, chloride, fluoride, acetate, sulfate or carbonate.

Gravimetry

Gravimetry is the oldest and a highly accurate analytical chemistry technique. Essentially, gravimetric analysis consists in measuring (weighing) the mass of the analyzed substance. This method has a weighed portion of material, being roasted in the air, converted into black-and-green U3O8 with the resultant pure product subsequently weighed. The measured weight of U3O8 is adjustable given the presence of nonvolatile impurities identified otherwise, say, spectrometrically.

U3O8 is used as a weight form, e.g. as a stable compound with a constant uranium-oxygen relation. To arrive at an accurate result, one needs to control stoichiometry of the sample following the roasting (constancy of uranium-oxygen relation). This depends on initial chemical composition, surface/volume relation in the analyzed sample, temperature and combustion time. It has been found that obtaining homogeneous U3O8 from the whole of the sample requires slow combustion in a controlled environment.

The uranium concentration in the analyzed sample is calculated from the formula:

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where UA is the uranium concentration in the sample, g(U)/g(sample); W is the roasted oxide weight (uranium oxide and impurity weight), g; F is the nonstoichiometry factor (the relation of stoichiometric uranium oxide that contains impurity oxides to nonstoichiometric impurity-containing uranium oxide obtained by roasting), g/g; С is the quantity of impurity oxides, in grams, as contained in a gram of the roasted oxide, g/g; G is the gravimetric (stoichiometric) factor (U/U3O8) subject to adjustment given the uranium isotopic composition, g(U)/g(U3O8); and S is the charge of the sample taken for the analysis, g.

Gravimetry is employed also to analyze plutonium samples, more rarely though than for uranium. The weight form in the event of plutonium is PuO2. For a high accuracy attainable by gravimetry, one needs to know precisely and take into account the content of oxygen and impurities in the sample.

Titration by Davis-Grey method for quantitative analysis of uranium content

Redox potentiometry is a titration method in which a titrating solution of the known concentration is added from a burette to a certain amount of the inspected solution the concentration of which is not known. Titration involves emf measurements with the titration curve Е=f(VT), where VT is the titrant volume, plotted. The measuring system is made of two electrodes plunged into the analyzed solution and an emf recorder (Fig. 5.26). One of the electrodes in the circuit, the so-called comparison electrode, has a constant potential value, and the potential of the other, the indicator electrode, varies in the titration process, with an abrupt jump taking place at the equivalence point.

For a potentiometric curve, the added titrating solution volume VT is plotted on the abscissa axis and the emf value is plotted on the ordinate axis. To determine accurately the equivalence point, the differential

during a titration process when the number of gram-equivalents of the titrating substance is equal to the number of gram-equivalents of the titrated component.

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At the equivalence point, all of the uranium in the solution turns hexavalent with its quantity characterized by the volume of the oxidizer used:

U = (NV + 6W/M × A/100)×[atomic weight of uranium/2], (5.27) sample weight, g

where U is the uranium concentration in the analyzed sample; N is the normality of the titrant (potassium bichromate); V is the volume of the titrant used (in liters); W is the weight of the added solid potassium bichromate (in high-accuracy measurements or when a large quantity of U(IV) is titrated); A is the purity of potassium bichromate shown in the certificate (in %/100); and M is the molecular weight of potassium bichromate (294.19 g/mol).

Davis-Grey method (NBS) is applicable to analyses of different uranium solutions: nitro-, sulfateand perchlorate. The solutions to be analyzed are obtained by dissolving samples of uranium oxide, uranium metal, salts and alloys, as well as nuclear fuel fragments in aluminum, steel or zirconium cladding.

The accuracy data of titrimetric analysis by Davis-Grey method is given in Table 5.14.

Table 5.14

Errors in data of uranium content measurements by Davis-Grey method

Normality is the number of gram-equivalents of the dissolved material in 1 liter of a solution. For example, 2N-solution of N2SO4, each liter whereof contains 98 g of acid.

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Mass spectrometry

The mechanical charged particle trajectories in magnetic fields depend on the masses and velocities thereof. The equation that describes ion movements has the following form:

is the electric field intensity, and B is the magnetic field induction vector. If to direct a beam of identically charged monoenergy ions into a

transverse magnetic field, the trajectories thereof will be circular orbits with the radius proportional to the ion masses. So ions with a different mass have different trajectories, this making it possible to separate and identify them.

Not all ion sources generate strictly monoenergy ions. In a static magnetic field, ions are separated both by mass and energy, which makes it more difficult to analyze them. By choosing the appropriate combination of an electric and a magnetic field (including variable ones), one may compensate ion separation by energy. So acting instruments are called double-focusing spectrometers.

A mass spectrometer consists of an ion source (this generates and accelerates ions, as well as forms the ion beam or packets), a mass analyzer (separates ions by masses) and a detection system. The measurement gives an ion distribution by masses (mass spectrum).

The intensity of the mass spectrum lines relates to the content of

where Hi is the peak height (or area) in the mass spectrum, Pi0 is the weight or relative quantity of the i-th component in the sample, and ki is the proportionality factor.

The quantity ki depends on whether a portion of the analyzed sample component has got into the detector and has been detected. The factor ki is the instrument sensitivity characteristic and has its own value for each atom type. ki is required to be invariable throughout the measurement. For

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their mechanical trajectories changed, so that ions of a different mass get into the fixed detector. High-rate scanning, using variations of the electric field that accelerates the ions in the source, may be used instead of low-rate magnetic-field scanning. Electric-field scanning is employed more commonly when the range of the ion mass differences is small, while magnetic-field scanning is used when this is broad.

A variety of detectors and measurement modes are used to detect ions, including current-type ones to analyze large-size samples and pulse-type ones to analyze small-size samples. Often, electronic multipliers (VEU) with a signal amplification factor of 106 and Faraday cylinders are used for detectors.

Nowadays, multicollector detection systems are in common use. Each detector in these systems is tuned to a particular ion mass and the system measures simultaneously the content of several isotopes (up to 9) in the sample. For example, analyses of Pu give, at a time, the content of isotopes with the masses of 238 to 244. No scanning is involved in analysis. A multicollector system makes measurements several times as fast.

Mass spectrometry analysis of uranium isotopic composition

A pure uranium fraction is applied to the strip. Strips are placed inside the mass spectrometer’s ion source with air pumped out of the source. The sample evaporates as the strip is heated while the singly charged ions produced by thermal ionization are accelerated and focused with the aid of electrostatic ion lenses in the mass analyzer. By presetting respective variations of the magnetic field or fields and/or the accelerating potential, the beams of ions of a different mass are sequentially focused onto the detector.

Using automated scanning, one sample insertion lock, a high-rate evacuation system and digital data processing, two operators are capable of analyzing 12 to 16 samples per day, still it is more realistic to have 7 to 9 daily analyzed samples.

Measurement quality control requires at least one analysis of a reference standard with the enrichment close to that of the measured material to be done throughout the series of analyses.

Mass spectrometry is the preferred and broadly employed technique to control uranium isotopic composition. Depending on the instrument sensitivity, an analysis requires 10–8 to 10–5 g of U.

Surface ionization mass spectrometry is also commonly used for destructive analyses of Pu isotopic composition. A plutonium analysis

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requires 10–9 to 10–6 g of Pu. Random errors of the Pu sample isotopic composition analysis are given in Table 5.15.

Table 5.15

Random errors in a Pu sample isotopic composition analysis by mass spectrometry method

Isotope dilution method

Isotope dilution is a method used to find the quantity mx of the element Z in a sample of the analyzed material. One must know the isotopic composition of this element (хi) (measured in advance).

The technique is based on using a hanging indicator of the same element with however an isotopic composition differing from that of the element in the analyzed sample. The sample solution of the analyzed material has a

certain amount of the indicator m0 with the isotopic composition xi0 added thereto. After the mixing, a sample is taken. The isotopic composition of the element in the sample taken ( xi ) differs from that in the original sample

(xi) and in the indicator ( xi0 ):

where Ai is the atomic weight of the i-th isotope of the element Z.

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The relation of the contents of the isotopes i and j in the mixture is equal

to:

wherefrom the formula to determine mx is derived:

The sample and the indicator are mixed so that U (Pu) from the sample and from the indicator would acquire similar properties and behave in the same way in further transformations. This is done using multiple redox cycles.

Isotope dilution mass spectrometry (IDMS) is a commonly used technique to determine U and Pu (and isotopic composition thereof) in fuel processing.

Resin bead technique

This is a dedicated technique to determine U and Pu in high-level solutions of spent reactor fuel. It is used for cases where the sample is to be moved to a long distance between laboratories. This does not require shielding against sample radiation. The analytical procedure is as shown in Fig. 5.27.

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