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REM - International Engineering Journal

On-line version ISSN 2448-167X

REM, Int. Eng. J. vol.70 no.1 Ouro Preto Jan./Mar. 2017

https://doi.org/10.1590/0370-44672016700069 

Metallurgy and materials

Determination of U3O8 in UO2 by infrared spectroscopy

Liliane Aparecida Silva1 

Fernando Soares Lameiras2 

Ana Maria Matildes dos Santos3 

Wilmar Barbosa Ferraz4 

João Batista Santos Barbosa5 

1Mestranda, Centro de Desenvolvimento da Tecnologia Nuclear - CDTN/CNEN, Belo Horizonte - Minas Gerais - Brasil. lasfisica@gmail.com

2Pesquisador Titular, Centro de Desenvolvimento da Tecnologia Nuclear - CDTN/CNEN, Belo Horizonte - Minas Gerais - Brasil. fsl@cdtn.br

3Pesquisadora Titular, Centro de Desenvolvimento da Tecnologia Nuclear - CDTN/CNEN, Belo Horizonte - Minas Gerais - Brasil. amms@cdtn.br

4Pesquisador Titular, Centro de Desenvolvimento da Tecnologia Nuclear - CDTN/CNEN, Belo Horizonte - Minas Gerais - Brasil. ferrazw@cdtn.br

5Pesquisador Titular, Centro de Desenvolvimento da Tecnologia Nuclear - CDTN/CNEN, Belo Horizonte - Minas Gerais - Brasil. jbsb@cdtn.br


Abstract

The oxygen-uranium (O-U) system has various oxides, such as UO2, U4O9, U3O8, and UO3. Uranium dioxide is the most important one because it is used as nuclear fuel in nuclear power plants. UO2 can have a wide stoichiometric variation due to excess or deficiency of oxygen in its crystal lattice, which can cause significant modifications of its proprieties. O/U relation determination by gravimetry cannot differentiate a stoichiometric deviation from contents of other uranium oxides in UO2. The presence of other oxides in the manufacturing of UO2 powder or sintered pellets is a critical factor. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify U3O8 in samples of UO2 powder. UO2 can be identified by bands at 340 cm-1 and 470 cm-1, and U3O8 and UO3 by bands at 735 cm-1, 910 cm-1, respectively. The methodology for sample preparation for FTIR spectra acquisition is presented, as well as the calibration for quantitative measurement of U3O8 in UO2. The content of U3O8 in partially calcined samples of UO2 powder was measured by FTIR with good agreement with X-rays diffractometry (XRD).

Keywords: FTIR; uranium dioxide; triuranium octoxide

1. Introduction

UO2 is the most used nuclear fuel in nuclear power plants. It is manufactured in the form of sintered cylindrical pellets by powder metallurgy processes. It has been very well studied because it shows changes in its properties due to excess or deficiency of oxygen in wide stoichiometric ranges (O/U relation). O-U system presents various oxides, which have good dimensional stability under radiation and are chemically stable in a nuclear reactor environment.

The stoichiometry is very important for UO2 performance as nuclear fuel. Fuel oxygen potential and O/U ratio have influence on mechanisms controlled by diffusion, such as grain growth, creep, release of fission gases, and thermal conductivity, in addition to chemical state and behavior of fission products. The O/U ratio increases with fuel burnup. Moreover, the sintering kinetics of UO2 pellets is controlled by diffusion processes. Thus, the final characteristics of manufactured UO2 pellets, which must be kept within very narrow limits, also depend on the O/U ratio of UO2 powder. The O/U relation is an important parameter to be controlled, both in UO2 powder and sintered pellets.

The chemical and physical properties of uranium oxides have been studied for many years using various techniques (Guéneau et al., 2002). Small changes in stoichiometry produce considerable variation in uranium oxides, where uranium atoms can be in more than one oxidation state. During UO2 manufacturing process, both as powder or pellets, all thermodynamically stable phases of O-U system may occur (UO2, U4O9, UO3, and U3O8). The presence of different phases also changes properties of UO2 pellets and their thermodynamic performance as nuclear fuel.

Measurement of the O/U ratio is usually performed by gravimetric methods, which give partial information on UO2 composition. It must be complemented by measures that identify the presence of other phases. X-rays diffraction is usually employed for phase identification and quantification. Nevertheless, it is hard to differentiate the uranium oxide patterns for angles between 20 and 40 degrees, especially at low levels. Thus, the use of X-ray diffraction to control the presence of other phases in UO2 becomes impractical in an industrial process.

There are indications that infrared spectroscopy is very sensitive to the presence of phases in the O-U system, even at low levels. This technique is little explored for this purpose in the case of O-U system. The objective of this work was to exploit it in order to complement measurements of the O/U ratios. Since this technique is rapid and inexpensive, it has the potential to be used in the line production of UO2 powder and pellets.

Allen et al., (1976), noticed that infrared spectroscopy may be used to identify phases in O-U system based on bands observed between 1000 and 200 cm-1. The conclusions of these authors were:

  • The UO2 infrared spectrum has a composite absorption of two components: a transverse optical phonon absorption, TO, at 340 cm-1 and a longitudinal optical absorption, LO, which causes a shoulder at 470 cm-1;

  • When oxygen is incorporated into UO2 crystalline lattice, the intensity TO/LO ratio increases;

  • A solid solution of oxygen in UO2 can be distinguished from a mixture of U4O9 and UO2 by infrared spectroscopy;

  • U4O9 has a weak band at 740 cm-1. This band may be assigned to asymmetric stretching of continuous chains of uranium and oxygen atoms with the same -O-U-O-U- bond length. These strings do not exist in UO2 fluorite lattice, such that no band is observed in this region. a-UO3 and U3O8, which contain these chains, have intense bands at 740 cm-1. U4O9 has a faint band, possibly due to early formation of these chains.

Axe and Pettit, (1966), measured infrared reflectivity of UO2 single crystals. They notice a big difference in the region below 700 cm-1 compared to spectra obtained with UO2 powder dispersed in the polymer matrix. This difference was attributed to the effect of dispersion of the infrared beam by particles of UO2, which have diameters comparable to the wavelengths of incident infrared radiation.

In order to improve the visualization of UO2 infrared spectrum, Allen et al., (1976), acquired spectra at 5 K. No improvement was observed, thus showing that the bands are not due to thermodynamic effects. To visualize the band at 500 cm-1 of U4O9, Allen and Holmes, (1994), had dispersed 3 parts of oxide in 100 parts of KBr. In other oxides samples the dispersion was 1 part of oxide in 100 parts of KBr. The band at 450 cm-1 in U4O9 spectrum was attributed to the fact this oxide is the only one to show mixed valences. The bands at 425 cm-1 and 510 cm-1 were assigned to U3O7, respectively b-U3O7 and g-U3O7. Ohwada and Soga, (1973), reported a strong band of a-U3O8 at 735 cm-1. Hoekstra and Siegel, (1973), attributed this band to possible hydration of the oxide, which would have the kind of deformation U-O-H chain.

2. Materials and methods

UO2 powder was provided by IPEN - Institute of Energy and Nuclear Research. U3O8 powder was obtained at CDTN by calcining UO2 powder in air for 3 hours at 400ºC. Fourier Transform infrared spectroscopy (FTIR) was performed with a BOMEM spectrometer, model MB102, in the 7000-400 cm-1 range with resolution of 8 cm-1 and 64 scans, at absorbance mode. Due to the intense absorption of infrared radiation by the uranium oxides, it was necessary to disperse the samples in KBr powder (Sigma Aldrich, spectroscopic grade, 99.99% purity). For a given KBr mass 1% mass of uranium oxide was diluted.

The mixtures of UO2 and U3O8 powders shown in Table 1 were prepared to calibrate the FTIR spectra and to be used as reference for the quantitative analysis with XRD. Figure 1 shows the FTIR spectra of pure UO2 and pure U3O8. The band at 735 cm-1 is related only to U3O8. The intensity of this band in the spectra of UO2/U3O8 mixtures was used to estimate the content of U3O8. The regression of the calibration curve was performed with Minitab 17.

Table 1 Mixtures of UO2 and U3O8 used to calibrate the FTIR spectra. 

UO2 (w/o) 99 97 95 93 90 80 60 40 20
U3O8 (w/o) 1 3 5 7 10 20 40 60 80

Figure 1 FTIR spectra of UO2 and U3O8 powder. 

The XRD diffractograms were obtained with a RIGAKU X-ray diffractometer, model D/MAX-ULTIMA and the quantitative analysis of the UO2 and U3O8 mixtures was carried out considering (001) peak by the RIR (reference intensity ratio) method using JADE 9.0 software.

To obtain an unknown mixture of UO2 and U3O8 powders, 7.0 g of UO2 powder samples were partially calcined in air at 300, 310, 320, 330 and 340 ºC for two hours with a 10 ºC/min heating rate in a muffle furnace. The calibrated FTIR and RIR method using JADE 9.0 software were used to estimate the U3O8 content in these calcined UO2 samples.

3. Results and discussion

Table 2 shows values (absolute and relative to 100% U3O8) of intensity of the band at 735 cm-1 of known UO2 and U3O8 mixtures as a function of U3O8 concentration.

Table 2 Intensity of the band at 735 cm-1 of mixtures of UO2 and U3O8 powders. 

U3O8 w/o Replica 735 cm-1
band
intensity
a.u.
735 cm-1
band
intensity
relative to
100 w/o
U3O8
U3O8 w/o Replica 735 cm-1
band
intensity
a.u.
735 cm-1
band
intensity
relative to
100 w/o
U3O8
0 1 0.080 0.032 10 1 0.392 0.158
2 0.078 0.031 2 0.393 0.159
3 0.078 0.031 20 1 0.638 0.258
4 0.077 0.031 2 0.633 0.256
1 1 0.141 0.057 40 1 1.038 0.420
2 0.144 0.058 2 1.040 0.420
3 1 0.169 0.068 60 1 1.439 0.582
2 0.171 0.069 2 1.440 0.582
5 1 0.230 0.093 80 1 1.685 0.681
2 0.229 0.093 2 1.686 0.681
7 1 0.326 0.132 100 1 2.474 1.000
2 0.311 0.126 2 2.475 1.000

Figure 2 shows the linear regression correlating U3O8 content with relative absorbance at 735 cm-1 (confidence level of 95%). The linear model can be expressed by:

rel.abs.735=(0.0494±0.0089)+(0.0089±0.0002)×w/o.U3O8 (1)

Figure 2 Linear correlation model for the relative absorbance at 735 cm-1 and the content of U3O8 (a = 0.95). 

U3O8 content in partially calcined UO2 powder was estimated by FTIR using Equation 1. The results are shown in Table 3 compared with determinations obtained by X-rays diffractometry (DRX). Figure 3 shows that the agreement is quite good.

Table 3 Estimation of U3O8 in partially calcined UO2 powder (FTIR versus DRX). 

Calcining temperature
(ºC)
Relative absorbance at
735 cm-1
Estimated U3O8 content (w/o)
FTIR DRX
300 0.365 35.46±2.07 43.6
310 0.543 55.46±2.52 57.2
320 0.697 72.76±2.91 80.3
330 0.826 87.26±3.24 93.1
340 0.923 98.16±3.48 99.3

Figure 3 Comparison of measurement of U3O8 concentration in partially calcined UO2 powder (DRX versus FTIR). 

This method can also be extended to other oxides of the O-U systems, like UO3 (band at 910 cm-1) or U4O9.

4. Conclusion

Fourier Transform Infrared Spectroscopy (FTIR) was used to measure contents of U3O8 in partially calcined UO2 powders with good agreement with X-rays diffratometry measurements. A methodology for preparing uranium oxide samples for FTIR and XRD measurement was presented. Due to its low cost, simplicity, and rapidity, FTIR may be considered to complement O/U ratio measurements by gravimetric methods in UO2 powder and pellet manufacturing. This technique may also be extended to other uranium oxides, like UO3 or U4O9.

Acknowledgments

To CDTN/CNEN, CAPES, CNPq, and National Institute of Science and Technology on Innovative Nuclear Reactors.

References

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AXE, J. D., PETTIT, G. D. Infrared dielectric dispersion and lattice dynamics of uranium dioxide and thorium dioxide. Physical Review, v. 151, n. 2, p. 676-680, 1966. [ Links ]

GUÉNEAU, C., BAICHI, M., LABROCHE, D., CHATILLON, C., SUNDMAN, B. Thermodynamics assessment of uranium-oxygen system. Journal of Nuclear Materials, v. 304, n. 2-3, p. 161-175, 2002. [ Links ]

HOEKSTRA, H. R., SIEGEL, S. The uranium trioxide-water system. Journal of Inorganic and Nuclear Chemistry, v. 35, n. 3, p. 761-779, 1973. [ Links ]

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Received: June 06, 2016; Accepted: September 27, 2016

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