Liliane Aparecida Silva et al. Metallurgy andMetalurgia materials e materiais Determination of U3O8 http://dx.doi.org/10.1590/0370-44672016700069 in UO2 by infrared spectroscopy Liliane Aparecida Silva Abstract Mestranda Centro de Desenvolvimento da The oxygen-uranium (O-U) system has various oxides, such as UO2, U4O9, U3O8, Tecnologia Nuclear – CDTN/CNEN and UO3. Uranium dioxide is the most important one because it is used as nuclear fuel Belo Horizonte - Minas Gerais – Brasil in nuclear power plants. UO2 can have a wide stoichiometric variation due to excess or [email protected] deficiency of oxygen in its crystal lattice, which can cause significant modifications of its proprieties. O/U relation determination by gravimetry cannot differentiate a stoi- Fernando Soares Lameiras chiometric deviation from contents of other uranium oxides in UO2. The presence of Pesquisador Titular other oxides in the manufacturing of UO2 powder or sintered pellets is a critical factor. Centro de Desenvolvimento da Fourier Transform Infrared Spectroscopy (FTIR) was used to identify U3O8 in samples -1 -1 Tecnologia Nuclear – CDTN/CNEN of UO2 powder. UO2 can be identified by bands at 340 cm and 470 cm , and U3O8 -1 -1 Belo Horizonte - Minas Gerais – Brasil and UO3 by bands at 735 cm , 910 cm , respectively. The methodology for sample [email protected] 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 Ana Maria Matildes dos Santos samples of UO2 powder was measured by FTIR with good agreement with X-rays dif- Pesquisadora Titular fractometry (XRD). Centro de Desenvolvimento da Tecnologia Nuclear – CDTN/CNEN Keywords: FTIR, uranium dioxide, triuranium octoxide. Belo Horizonte – Minas Gerais – Brasil [email protected] Wilmar Barbosa Ferraz Pesquisador Titular Centro de Desenvolvimento da Tecnologia Nuclear – CDTN/CNEN Belo Horizonte – Minas Gerais - Brasil [email protected] João Batista Santos Barbosa Pesquisador Titular Centro de Desenvolvimento da Tecnologia Nuclear - CDTN/CNEN Belo Horizonte – Minas Gerais – Brasil [email protected] 1. Introduction UO2 is the most used nuclear fuel in ence on mechanisms controlled by diffusion, The chemical and physical proper- nuclear power plants. It is manufactured in such as grain growth, creep, release of fission ties of uranium oxides have been studied the form of sintered cylindrical pellets by gases, and thermal conductivity, in addition for many years using various techniques powder metallurgy processes. It has been to chemical state and behavior of fission (Guéneau et al., 2002). Small changes in stoi- very well studied because it shows changes products. The O/U ratio increases with fuel chiometry produce considerable variation in its properties due to excess or deficiency of burnup. Moreover, the sintering kinetics in uranium oxides, where uranium atoms oxygen in wide stoichiometric ranges (O/U of UO2 pellets is controlled by diffusion can be in more than one oxidation state. relation). O-U system presents various ox- processes. Thus, the final characteristics During UO2 manufacturing process, both ides, which have good dimensional stability of manufactured UO2 pellets, which must as powder or pellets, all thermodynamically under radiation and are chemically stable in be kept within very narrow limits, also stable phases of O-U system may occur a nuclear reactor environment. depend on the O/U ratio of UO2 powder. (UO2, U4O9, UO3, and U3O8). The presence The stoichiometry is very important The O/U relation is an important parameter of different phases also changes properties for UO2 performance as nuclear fuel. Fuel to be controlled, both in UO2 powder and of UO2 pellets and their thermodynamic oxygen potential and O/U ratio have influ- sintered pellets. performance as nuclear fuel. REM: Int. Eng. J., Ouro Preto, 70(1),59-62, jan. mar. | 2017 59 Determination of U3O8 in UO2 by infrared spectroscopy Measurement of the O/U ratio is usu- observed between 1000 and 200 cm-1. The low 700 cm-1 compared to spectra obtained ally performed by gravimetric methods, conclusions of these authors were: with UO2 powder dispersed in the polymer which give partial information on UO2 • The UO2 infrared spectrum has a matrix. This difference was attributed to composition. It must be complemented composite absorption of two components: the effect of dispersion of the infrared beam by measures that identify the presence of a transverse optical phonon absorption, by particles of UO2, which have diameters other phases. X-rays diffraction is usually TO, at 340 cm-1 and a longitudinal optical comparable to the wavelengths of incident employed for phase identification and quan- absorption, LO, which causes a shoulder at infrared radiation. tification. Nevertheless, it is hard to differen- 470 cm-1; In order to improve the visualization of tiate the uranium oxide patterns for angles • When oxygen is incorporated into UO2 infrared spectrum, Allen et al., (1976), between 20 and 40 degrees, especially at low UO2 crystalline lattice, the intensity TO/LO acquired spectra at 5 K. No improvement levels. Thus, the use of X-ray diffraction to ratio increases; was observed, thus showing that the bands control the presence of other phases in UO2 • A solid solution of oxygen in UO2 are not due to thermodynamic effects. To -1 becomes impractical in an industrial process. can be distinguished from a mixture of U4O9 visualize the band at 500 cm of U4O9, There are indications that infrared and UO2 by infrared spectroscopy; Allen and Holmes, (1994), had dispersed 3 -1 spectroscopy is very sensitive to the pres- • U4O9 has a weak band at 740 cm . parts of oxide in 100 parts of KBr. In other ence of phases in the O-U system, even at This band may be assigned to asymmetric oxides samples the dispersion was 1 part of low levels. This technique is little explored stretching of continuous chains of uranium oxide in 100 parts of KBr. The band at 450 -1 for this purpose in the case of O-U system. and oxygen atoms with the same -O-U-O- cm in U4O9 spectrum was attributed to the The objective of this work was to exploit it U- bond length. These strings do not exist in fact this oxide is the only one to show mixed -1 -1 in order to complement measurements of UO2 fluorite lattice, such that no band is ob- valences. The bands at 425 cm and 510 cm the O/U ratios. Since this technique is rapid served in this region. α-UO3 and U3O8, which were assigned to U3O7, respectively β-U3O7 and inexpensive, it has the potential to be contain these chains, have intense bands at and γ-U3O7. Ohwada and Soga, (1973), -1 used in the line production of UO2 powder 740 cm . U4O9 has a faint band, possibly due reported a strong band of α-U3O8 at 735 and pellets. to early formation of these chains. cm-1. Hoekstra and Siegel, (1973), attributed Allen et al., (1976), noticed that Axe and Pettit, (1966), measured this band to possible hydration of the oxide, infrared spectroscopy may be used to iden- infrared reflectivity of UO2 single crystals. which would have the kind of deformation tify phases in O-U system based on bands They notice a big difference in the region be- U-O-H chain. 2. Materials and methods UO2 powder was provided by IPEN - essary to disperse the samples in KBr pow- of this band in the spectra of UO2/U3O8 Institute of Energy and Nuclear Research. der (Sigma Aldrich, spectroscopic grade, mixtures was used to estimate the content U3O8 powder was obtained at CDTN by 99.99% purity). For a given KBr mass 1% of U3O8. The regression of the calibration calcining UO2 powder in air for 3 hours mass of uranium oxide was diluted. curve was performed with Minitab 17. o at 400 C. Fourier Transform infrared The mixtures of UO2 and U3O8 pow- The XRD diffractograms were spectroscopy (FTIR) was performed with ders shown in Table 1 were prepared to obtained with a RIGAKU X-ray diffrac- a BOMEM spectrometer, model MB102, calibrate the FTIR spectra and to be used as tometer, model D/MAX-ULTIMA and the -1 in the 7000-400 cm range with resolution reference for the quantitative analysis with quantitative analysis of the UO2 and U3O8 of 8 cm-1 and 64 scans, at absorbance mode. XRD. Figure 1 shows the FTIR spectra of mixtures was carried out considering (001) Due to the intense absorption of infrared pure UO2 and pure U3O8. The band at 735 peak by the RIR (reference intensity ratio) -1 radiation by the uranium oxides, it was nec- cm is related only to U3O8. The intensity method using JADE 9.0 software. Table 1 UO2 (w/o) 99 97 95 93 90 80 60 40 20 Mixtures of UO2 and U3O8 U O (w/o) 1 3 5 7 10 20 40 60 80 3 8 used to calibrate the FTIR spectra. Figure 1 FTIR spectra of UO2 and U3O8 powder. 60 REM: Int. Eng. J., Ouro Preto, 70(1), 59-62, jan. mar. | 2017 Liliane Aparecida Silva et al. To obtain an unknown mixture air at 300, 310, 320, 330 and 340 °C for and RIR method using JADE 9.0 software of UO2 and U3O8 powders, 7.0 g of UO2 two hours with a 10 °C/min heating rate were used to estimate the U3O8 content in powder samples were partially calcined in in a muffle furnace. The calibrated FTIR these calcined UO2 samples. 3. Results and discussion Table 2 shows values (absolute and U3O8 mixtures as a function of correlating U3O8 content with relative ab- -1 and relative to 100% U3O8) of intensity U3O8 concentration.