Raman Spectroscopic Investigation of Thorium Dioxide– Uranium Dioxide (ThO2–UO2) Fuel Materials Rekha Rao,a,* R.K. Bhagat,b Nilesh P. Salke,a Arun Kumarb a b
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400 085, India Radiometallurgy Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
Raman spectroscopic investigations were carried out on proposed nuclear fuel thorium dioxide–uranium dioxide (ThO2–UO2) solid solutions and simulated fuels based on ThO2–UO2. Raman spectra of ThO2–UO2 solid solutions exhibited two-mode behavior in the entire composition range. Variations in mode frequencies and relative intensities of Raman modes enabled estimation of composition, defects, and oxygen stoichiometry in these compounds that are essential for their application. The present study shows that Raman spectroscopy is a simple, promising analytical tool for nondestructive characterization of this important class of nuclear fuel materials. Index Headings: Raman spectroscopy; Thorium dioxide (ThO2); Uranium dioxide (UO2); Solid solution.
INTRODUCTION There has been continued interest in thorium fuels and fuel cycles in India because of large deposits of thorium (nearly one third of the world resources) compared with very modest reserves of low-grade uranium.1 India has planned a nuclear power program based on its domestic reserves of uranium and thorium.2 Considering the large thorium reserves in India, the future nuclear power program will be based on Th–233U fuel cycle with the objective of providing long-term energy security to the country. The Advanced Heavy Water Reactor (AHWR) has been designed in the Bhabha Atomic Research Centre in Mumbai, India, for development of thoriumbased technologies.2 Thorium dioxide (ThO2) containing 18–22.5 wt% low-enriched uranium dioxide (UO2, with 19.75% enriched uranium) is being considered as a fuel for AHWR.3 Hence, a database relating to the ThO2–UO2 system is very important for India’s nuclear power program. Although ThO2 and UO2 form a homogeneous solid solution, the quantitative composition analysis of the ThO2–UO2 solid solution by wet chemical analysis involves dissolution of the solid solution, a process that is difficult due to the highly stable nature of ThO2. Whereas other methods of analysis require extensive sample preparation, Raman spectroscopy is an easy, quick, nondestructive technique that can help in quantitative analysis. Raman spectroscopy has been used for characterization and quantitative composition analyses in many mixed oxides, including HfxZr1-xO24 and Th 1-x Ln x O 2-x/2 systems. 5 Because the fluorite Received 5 June 2013; accepted 5 September 2013. * Author to whom correspondence should be sent. E-mail: rekhar@ barc.gov.in. DOI: 10.1366/13-07172
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structure of UO2 can accommodate large amounts of interstitial oxygen, it can form hyper-stoichiometric UO2þx or other higher oxides of uranium such as triuranium octoxide or uranium(IV,V) oxide (U4O9) that have different structures, with accompanying disorder only in the anion sublattice; the cation sublattice remains mostly unaffected. Identifying the phase of the oxide as well as oxygen stoichiometry is a challenging field where Raman spectroscopy is now contributing significantly. Recently, Raman spectroscopy has been used extensively to characterize nuclear fuel materials, especially to identify different phases of uranium oxides; to characterize defects and oxygen stoichiometry in unirradiated uranium oxides; 6 to provide supportive structural information about the surface;7,8 to study phase transitions due to oxidation;9 and to investigate phonon behavior at high pressures.10 Raman spectroscopy also has been used for identification of corrosion products formed on UO211 and to probe ion-beam, irradiation-induced damage in UO2.12 There are reports of the use of Raman spectroscopy to characterize plutonium dioxide (PuO2) and (U1-yPuy)O2 fuels, as well as to identify the nature of defects.13,14 The present studies are aimed at providing a sensitive, alternate characterization tool for ThO2–UO2 compounds. In general, mixed systems can show two different types of phonon behavior: (i) one-mode behavior in which the zone center optical phonon frequency varies continuously with composition from one end member to that of the other, for example, as in Ca1-xSrxF2 and Sr1-xBaxF215; and (ii) two-mode behavior in which zone center optical phonon frequencies of both the end members appear in all the intermediate compositions, for example, as in Cd1-xMnxSe.16 It is of interest to investigate the Raman spectra of a ThO2–UO2 mixed system to understand its phonon behavior. Two sets of nuclear fuels were investigated by Raman spectroscopy in the present study. The first set consists of ThO2–UO2 solid solutions with a wide range of UO2 contents. The second set of samples consists of simulated fuel (SIMFUEL), an unirradiated analog of spent nuclear fuel, made by doping ThO2–UO2 fuel with nonradioactive oxides in appropriate proportions to mimic the chemical effects of a nuclear reactor. SIMFUEL replicates the chemical composition of spent fuel so that detailed experiments to estimate the extent of degradation in the properties of the fuel due to the presence of fission products can be undertaken in the absence of radiation. Here, we report the results of Raman spectroscopic investigations carried out on ThO2–UO2 fuels and SIMFUELs based on ThO2–UO2 for
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FIG. 2. Raman spectra of ThO2–UO2 solid solution for different UO2 contents. (a) Region of F2g mode. (b) Region of F1u (LO) and higher order modes. Arrow in panel a indicates asymmetry in F2g due to UO2. The curves in panel (b) have been multiplied by a factor of five for clarity. FIG. 1. Variation of the lattice parameter with composition for the ThO2–UO2 system, indicating a linear relationship obeying the Vegard’s law.
composition analyses, estimation of extent of damage, and oxygen stoichiometry. Experimental Details. Thorium dioxide and UO2 form an ideal, homogeneous solid solution in the entire composition range17 and crystallize in the fluorite structure, space group Fm3m.18 ThO2–UO2 pellets of various compositions (0–100 wt% of UO2) were synthesized by a conventional powder metallurgical method, starting from UO2 and ThO2 powders, followed by comilling, compaction, and sintering at around 1700 8C, in a reducing atmosphere (92% argon and 8% hydrogen) to avoid formation of other oxides of uranium. The solid solutions of different compositions were characterized by X-ray diffraction. The diffraction patterns indicated that the samples were single-phase solid solutions and that the overall structure was a fluorite structure. Figure 1 shows the lattice parameter for each composition, obtained by Rietveld refinement of the powder diffraction data that varied linearly with composition, following Vegard’s law. The SIMFUELs corresponding to an average burn-up of 28 000 and 43 000 MWd/t (megawatt day per tonne) also were synthesized by the powder metallurgical route by adding high-purity compounds such as strontium oxide (SrO), yttrium oxide, zirconium dioxide, molybdenum trioxide, barium carbonate, lanthanum oxide,
cerium(IV) oxide (CeO2), neodymium(III) oxide, ruthenium(IV) oxide, rhodium(III) oxide, cesium carbonate, praseodymium(III, IV) oxide, samarium(III) oxide, and palladium to ThO2–3.45 wt% UO2 powders. Powder X-ray diffraction showed that all three compounds: ThO2-3.45 wt% UO2, SIMFUEL corresponding to 28 000 MWd/t, and SIMFUEL 43 000 MWd/t have fluorite structure.19 Raman spectroscopic measurements were carried out from polycrystalline pellets using an ;15 mW 532 nm laser. The focused spot size on the sample was about 20 lm, and the power density was low enough to avoid surface oxidation of the samples due to local heating by the laser beam, an effect commonly observed in this class of oxides.14 Raman spectra recorded from different regions of the pellet were reproducible, confirming homogeneity of the sample. Scattered light was analyzed using a home-built 0.9 m single monochromator,20 coupled with an edge filter, and was detected by a Peltier cooled, charge-coupled device. The spectrograph was calibrated using the Raman spectra of indene. Entrance slit was kept at 50 lm, thereby giving a spectral band pass of 3 cm1. Raman spectra could be reproduced with an accuracy of 60.2 cm1.
RESULTS AND DISCUSSION Raman Spectroscopy of ThO2–UO2 Solid Solutions. As mentioned, both ThO2 and UO2 crystallize in fluorite structure. Group theory predicts six optic phonon
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FIG. 3. Fitting of the observed Raman spectra of ThO2–UO2 solid solution for 20 wt% UO2 (a) in the region of first order modes of ThO2, and (b) in the region of higher order modes of ThO2. Continuous lines represent the experimental data, and the dotted and the dashed lines denote the individual Lorentzian and fitted curves respectively. The broad bands at around 1335 and 1575 cm1 are due to carbon.
branches, a triply degenerate Raman active mode F2g, and an infrared (IR) active mode F1u for fluorite structure. Figures 2a and 2b show Raman spectra of different compositions of the ThO2–UO2 solid solution. Pure ThO2 exhibits first-order Raman mode at 467 cm1 due to the symmetric breathing mode of the oxygen atoms around each thorium ion. Raman spectrum of stoichiometric UO2 has the corresponding F2g vibrational mode at around 448 cm1. The presence of defects distorts the translational symmetry and relaxes the selection rules, making several normally dipole-forbidden optical transitions observable. A weak feature observed in the Raman spectra of UO2 around 575 cm1, assigned to be the IR active F1u longitudinal optic (1LO) mode, appears to be due to the breakdown of selection rules as a result of presence of defects.21,22 A broad band observed in UO2 around 1150 cm1, a band that was initially assigned to an electronic crystal field transition, is now attributed to the second-order longitudinal optic (2LO) phonon based on the similarity of 1150 cm1 mode with 575 cm1 in terms of resonance profile, pressure dependence of intensity, and frequency.10 Its intensity is highly sensitive to minor changes in oxygen stoichiometry; it is more intense in stoichiometric UO2 samples,9 hence it is interesting to monitor this band as an indicator of the extent of UO2 oxidation. Although pure ThO2 has only one intense peak in the Raman spectrum, several new features are observed with the addition of UO2. Apart from the appearance of 1LO and 2LO modes, a clear asymmetry is observed in the F2g peak of ThO2, the position and intensity of which change with UO2 content (Figs. 2a and 2b). The relative intensity of 1LO and 2LO modes with regard to the F2g mode increases exponentially with addition of UO2. All the new features seen in the Raman spectra of a typical solid solution ThO2–20 wt% UO2 are shown more clearly in Figs. 3a and 3b. After appropriate background
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FIG. 4. Variation of Raman mode frequency with composition of ThO2– UO2.
subtraction, all the Raman bands are fitted to Lorentzians. The region of F2g peak with asymmetry toward low frequency is fitted to two Lorentzians. Although the peak due to ThO 2 remains at the same position, the asymmetry due to UO2 shifts toward lower frequency with and increase in UO2 content. The intensity of the asymmetry due to UO2 increases with addition of UO2, at the cost of intensity of the ThO2 peak. The region of the F2g mode shows a clear two-mode behavior in the entire composition. There is a weak broad mode at around 270 cm1 that is assigned to be F1u transverse optic (TO).9 In addition to overtones of 1LO, a Raman band is observed at around 1335 cm1, the position of which is found to be highly sensitive to the composition. To ascertain that it is not a luminescence band, we also have recorded Raman spectra of these compounds with a wavelength of 473 nm, and we found the presence of this band. This band was observed in UO2 by previous research when recorded with 785 nm laser23,24 and was attributed to the presence of carbon in the samples. However, carbon content in our samples was below the detection limit of the combustion analysis carried out to estimate carbon. Variation of peak positions and intensity in Raman spectra of ThO2–UO2 solid solutions can form the basis of quantitative analysis of the solid solution. Figure 4 shows the peak positions of all the features in the Raman spectra for different compositions of the solid solution.
FIG. 5. Raman spectra of ThO2–3.45 wt% UO2 (a), and SIMFUELs for 28 000 MWd/t (b) and 43 000 MWd/t (c).
All the peak positions vary smoothly and linearly with composition, and there is a considerable change in the position of the Raman modes with composition that may be useful in quantitative analysis of ThO2–UO2 solid solutions. The position of the asymmetry in the F2g mode shows a variation of 0.17 cm1 per percentage of UO2. From the present results, composition of any solid solution of ThO2–UO2 can be estimated. The composition of a ThO2–UO2 solid solution estimated using the position of asymmetry in F2g mode agrees within 3 wt% with that measured by powder X-ray diffraction (XRD) in the composition of interest for nuclear applications (18– 22.5 wt% UO2). Two-mode behavior of the mixed system in the Raman spectra enables us to qualitatively detect any higher oxide of uranium and hence the oxidation state of uranium in the mixed system. Raman Spectroscopy of SIMFUELs. Figure 5 shows the Raman spectrum of ThO2–3.45 wt% UO2 (Fig. 5a) and that of the SIMFUELs corresponding to burn-up 28 000 MWd/t (Fig. 5b) and 43 000 MWd/t (Fig. 5c) under ambient conditions. The strong narrow mode at 467 cm1 corresponds to the F2g symmetry in ThO2–3.45 wt% UO2 and has a full width half-maximum (FWHM) of about 6.5 cm1. In the SIMFUELs samples, this mode is slightly shifted by about 2 cm1 toward the higher frequency and is broader (FWHM of ;9 cm1). No significant decrease in the intensity of the F2g mode was observed. Higher frequency and broadening of the F2g mode indicates higher oxidation of the compound.6,9 In the region 520– 700 cm1, bands are seen in the spectra of SIMFUELs. The band at around 575 cm1 is assigned to F1u(LO), as already mentioned. The second band at around 620 cm1 also has been observed in UO2 samples due to oxidation by laser heating, when a higher laser power was used.14
As mentioned, we recorded all the Raman spectra at power levels below the threshold required to induce oxidation of the surface. A comparison with the report on irradiated fuels indicated the band at around 620 cm1 is attributed to oxygen sublattice damage either by the formation of Frenkel pairs (vacancy and interstitial oxygen atom) or hyperstoichiometry12,14 due to simulated radiation damage. This band appears to be due to oxygen ions placed in the interstitial sites of fluorite structure, leading to breakdown of translational symmetry and resulting in new disorder-induced bands assigned as an LO(X) band.10 The relative intensity of this band with respect to that of the F1u band is reported to be a measure of hyperstoichiometry x in UO2þx samples.9 Desgranges et al. 25 carried out polarized Raman measurements on a single crystallite of U4O 9, a superstructure of UO2, and they identified a Raman mode at 630 cm1 to be an Ag mode, along with another mode at 160 cm1 due to zone folding. Because we do not have polarized Raman data, it is not possible to comment on whether there is formation of an U4O9 type of phase in SIMFUELs. However, in the absence of the peak around 160 cm1, we assign the band at 620 cm1 to be due to oxygen sublattice damage. The presence of both 575 and 620 cm1 bands in the Raman spectra of SIMFUEL indicates presence of point defects as well as oxygen sublattice damage due to doping in the fluorite matrix. It may be noted that the oxygen sublattice damage is observed only in SIMFUEL samples and not in the first set of ThO2–UO2 samples where the defects are point defects. A very weak band around 1150 cm1 due to 2LO is observed in all the three samples (not shown in Fig. 5). Both of the SIMFUEL samples showed similar relative intensity of the defect peaks, indicating no significant difference in the extent of damage for the two different burn-ups. The present Raman results show that the major matrix in SIMFUEL is a fluorite-type, face-centered cubic. The presence of peak around 620 cm1 could be due to (i) the formation of U4O9 or (ii) the replacement of Th4þ sites by dopant cations. When Th4þ ions are substituted by dopants ions of different radii, the oxygen sublattice undergoes distortion to accommodate the dopant. The second possibility is based on the appearance of a defect peak in doped (with Zr4þ/Y3þ/La3þ) fluorite-structured systems, such as CeO2,22 and is the more probable reason for the appearance of the peak around 620 cm1; because sintering of SIMFUEL pellets was carried out in a reducing atmosphere, the possibility of formation of the U4O9 phase is less. It may be noted that no such clues about any structural changes were obtained from the corresponding XRD data.19 The present study of SIMFUELs indicates that the damage due to doping is minimal, unlike the loss of cubic symmetry observed in previous studies on SIMFUELs, due to the higher level of doping corresponding to higher burn-up and resulting in ordering of vacancies and structural changes.26
SUMMARY AND CONCLUSIONS A detailed Raman spectroscopic investigation was carried out on ThO2–UO2 solid solutions and SIMFUELs based on ThO2–UO2. The results suggest that Raman
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spectroscopy is feasible as a simple, nondestructive technique to characterize ThO2–UO2 mixtures, the proposed fuel for the AHWR, in India. Although all the peak positions due to UO2 substitution show a linear relationship with the composition, the intensity of defectinduced modes due to UO2 increases with the addition of UO2. The presence of an intense broad peak at around 620 cm1 in SIMFUEL samples indicates oxygen sublattice damage, whereas in ThO2–UO2 mixtures the oxygen sublattice remains intact. Although the defects in the ThO2 –UO 2 mixtures are point defects, the SIMFUELs were found to have extended defects. Thus, Raman spectroscopy is an effective alternate tool in this important class of nuclear fuels to examine composition and defects that are otherwise difficult to detect. ACKNOWLEDGMENT R.R. and N.P.S. thank Dr. S.L. Chaplot for support and encouragement. 1. International Atomic Energy Agency. 2005. ‘‘Thorium Fuel Cycle– Potential Benefits and Challenges’’. IAEA-TECDOC-1450, International Atomic Energy Agency. www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf [accessed Sept 4 2013]. 2. R.K. Sinha, A. Kakodkar. ‘‘Design and Development of the AHWR— The Indian Thorium Fuelled Innovative Nuclear Reactor’’. Nucl. Eng. Des./Fusion. 2006. 236(7-8): 683-700. 3. N.P. Pushpam, A. Kumar, U. Kannan, A. Kumar, P.D. Krishnani. ‘‘Study for Use of LEU Along with Thorium in Advanced Heavy Water Reactor (AHWR) to Enhance Proliferation Resistance Characteristics of Fuel’’. Paper presented at: International Conference on Peaceful Uses of Atomic Energy. New Delhi: 2009. Paper Number: IAEA-CN-184/207. 4. R.D. Robinson, J. Tang, M.L. Steigerwald, L.E. Brus, I.P. Herman. ‘‘Raman Scattering in HfxZr1xO2 Nanoparticles’’. Phys. Rev. 2005. 71(11): 115408 (1-8). doi: 10.1103/PhysRevB.71.115408. 5. D. Horlait, N. Clavier, N. Dacheux, R. Cavalier, R. Podor. ‘‘Synthesis and Characterization of Th1-xLnxO2-x/2 Mixed-Oxides’’. Mater. Res. Bull. 2012. 47(12): 4017-4025. 6. D. Manara, B. Renker. ‘‘Raman Spectra of Stoichiometric and Hyperstoichiometric Uranium Dioxide’’. J. Nucl. Mater. 2003. 321(2-3): 233-237. 7. F. Pointurier, O. Marie. ‘‘Identification of the Chemical Forms of Uranium Compounds in Micrometer-Size Particles by Means of Micro-Raman Spectrometry and Scanning Electron Microscope’’. Spectrochim. Acta, Part B. 2010. 65(9-10): 797-804. 8. C. Je´gou, R. Caraballo, J. De Bonfils, V. Broudic, S. Peuget, T. Vercouter, D. Roudil. ‘‘Oxidizing Dissolution of Spent MOX47 Fuel Subjected to Water Radiolysis: Solution Chemistry and Surface Characterization by Raman Spectroscopy’’. J. Nucl. Mater. 2010. 399(1): 68-80. 9. H. He, D. Shoesmith. ‘‘Raman Spectroscopic Studies of Defect Structures and Phase Transition in Hyper-Stoichiometric UO2þx’’. Phys. Chem. Chem. Phys. 2010. 12(28): 8108-8117.
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10. T. Livneh, E. Sterer. ‘‘Effect of Pressure on the Resonant Multiphonon Raman Scattering in UO2’’. Phys. Rev. B. 2006. 73(8): 085118(1-9). doi: 10.1103/PhysRevB.73.085118. 11. M. Amme, B. Renker, B. Schmid, M.P. Feth, H. Bertagnolli, W. Dobelin. ‘‘Raman Microspectrometric identification of Corrosion Products Formed on UO2 Nuclear Fuel During Leaching Experiments’’. J. Nucl. Mater. 2002. 306(2-3): 202-212. 12. G. Guimbretie´re, L. Desgranges, A. Canizare´s, G. Carlot, R. Caraballo, C. Je´gou, F. Duval, N. Raimboux, M.R. Ammar, P. Simon. ‘‘Determination of In-depth Damaged Profile by Raman Line Scan in a Pre-Cut He2þ Irradiated UO2’’. Appl. Phys. Lett. 2012. 100(25): 251914(1-4). 13. M.J. Sarsfield, R.J. Taylor, C. Puxley, H.M. Steele. ‘‘Raman Spectroscopy of Plutonium Dioxide and Related Materials’’. J. Nucl. Mater. 2012. 427(1-3): 333-342. 14. C. Je´gou, R. Caraballo, S. Peuget, D. Roudil, L. Desgranges, M. Magnin. ‘‘Raman Spectroscopy Characterization of Actinide Oxides (U1-yPuy)O2: Resistance to Oxidation by the Laser Beam and Examination of Defects’’. J. Nucl. Mater. 2010. 405(3): 235-243. 15. R.K. Chang, B. Lacina, P.S. Persan. ‘‘Raman Scattering from Mixed Crystals (CaxSr1-x)F2 and (SrxBa1-x)F2’’. Phys. Rev. Lett. 1966. 17(14): 755-758. 16. R.G. Alonso, Y.R. Lee, E. Oh, A.K. Ramdas, H. Luo, N. Samarth, J.K. Furdyna, H. Pascher. ‘‘Raman and Reflectivity Spectra of Cubic Cd1xMnxSe Epilayers Grown by Molecular-Beam Epitaxy’’. Phys. Rev. B. 1991. 43(12): 9610-9620. 17. J. Belle, R.M. Berman. Thorium Dioxide: Properties and Nuclear Application. Washington, DC: 1984. DOE/NE-0060. http://www.osti. gov/bridge/servlets/purl/5986642-LhiWnw/5986642.pdf [accessed Sept 4 2013]. 18. W.A. Lambertson, M.H. Mueller, F.H. Gunzel. ‘‘Uranium Oxide Phase Equilibrium Systems: IV, UO2-ThO2’’. J. Am. Ceram. Soc. 1953. 36(12): 397-399. 19. R.K. Bhagat, K. Krishnan, T.R.G. Kutty, A. Kumar, H.S. Kamath, S. Banerjee. ‘‘Thermal Expansion of Simulated Thoria–Urania Fuel by High Temperature XRD’’. J. Nucl. Mater. 2012. 422(1-3): 152-157. 20. A.P. Roy, S.K. Deb, M.A. Rekha, A.K. Sinha. ‘‘Multichannel Raman Spectroscopy’’. Indian J. Pure Appl. Phys. 1992. 30(12): 724. 21. W.H. Weber, K.C. Hass, J.R. McBride. ‘‘Raman Study of CeO2: Second-Order Scattering, Lattice Dynamics, and Particle Size Effects’’. Phys. Rev. B. 1993. 48(1): 178-185. 22. A. Nakajima, A. Yoshihara, M. Ishigame. ‘‘Defect Induced Raman Spectra in Doped CeO2’’. Phys. Rev. B. 1994. 50(18): 13297-13307. 23. S.D. Senanayake, R. Rousseau, D. Colegrave, H. Idriss. ‘‘The Reaction of Water on Polycrystalline UO2: Pathways to Surface and Bulk Oxidation’’. J. Nucl. Mater. 2005. 342(1-3): 179-187. 24. E.A. Stefaniak, A. Alsecz, I.E. Sajo´, A. Worobiec, Z. Ma´the´, S. To¨ro¨k, R. Grieken. ‘‘Recognition of Uranium Oxides in Soil Particulate Matter by Means of l-Raman Spectrometry’’. J. Nucl. Mater. 2008. 381(3): 278-283. 25. L. Desgranges, G. Baldinozzi, P. Simon, G. Guimbretie`re, A. Canizares. ‘‘Raman Spectrum of U4O9: A New Interpretation of Damage Lines in UO2’’. J. Raman Spectrosc. 2012. 43(3): 455-458. 26. H. He, P.G. Keech, M.E. Broczkowski, J.J. Noel, D.W. Shoesmith. ‘‘Characterization of the Influence of Fission Product Doping on the Anodic Reactivity of Uranium Oxide’’. Can. J. Chem. 2007. 85(10): 702-713.
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400 085, India Radiometallurgy Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
Raman spectroscopic investigations were carried out on proposed nuclear fuel thorium dioxide–uranium dioxide (ThO2–UO2) solid solutions and simulated fuels based on ThO2–UO2. Raman spectra of ThO2–UO2 solid solutions exhibited two-mode behavior in the entire composition range. Variations in mode frequencies and relative intensities of Raman modes enabled estimation of composition, defects, and oxygen stoichiometry in these compounds that are essential for their application. The present study shows that Raman spectroscopy is a simple, promising analytical tool for nondestructive characterization of this important class of nuclear fuel materials. Index Headings: Raman spectroscopy; Thorium dioxide (ThO2); Uranium dioxide (UO2); Solid solution.
INTRODUCTION There has been continued interest in thorium fuels and fuel cycles in India because of large deposits of thorium (nearly one third of the world resources) compared with very modest reserves of low-grade uranium.1 India has planned a nuclear power program based on its domestic reserves of uranium and thorium.2 Considering the large thorium reserves in India, the future nuclear power program will be based on Th–233U fuel cycle with the objective of providing long-term energy security to the country. The Advanced Heavy Water Reactor (AHWR) has been designed in the Bhabha Atomic Research Centre in Mumbai, India, for development of thoriumbased technologies.2 Thorium dioxide (ThO2) containing 18–22.5 wt% low-enriched uranium dioxide (UO2, with 19.75% enriched uranium) is being considered as a fuel for AHWR.3 Hence, a database relating to the ThO2–UO2 system is very important for India’s nuclear power program. Although ThO2 and UO2 form a homogeneous solid solution, the quantitative composition analysis of the ThO2–UO2 solid solution by wet chemical analysis involves dissolution of the solid solution, a process that is difficult due to the highly stable nature of ThO2. Whereas other methods of analysis require extensive sample preparation, Raman spectroscopy is an easy, quick, nondestructive technique that can help in quantitative analysis. Raman spectroscopy has been used for characterization and quantitative composition analyses in many mixed oxides, including HfxZr1-xO24 and Th 1-x Ln x O 2-x/2 systems. 5 Because the fluorite Received 5 June 2013; accepted 5 September 2013. * Author to whom correspondence should be sent. E-mail: rekhar@ barc.gov.in. DOI: 10.1366/13-07172
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structure of UO2 can accommodate large amounts of interstitial oxygen, it can form hyper-stoichiometric UO2þx or other higher oxides of uranium such as triuranium octoxide or uranium(IV,V) oxide (U4O9) that have different structures, with accompanying disorder only in the anion sublattice; the cation sublattice remains mostly unaffected. Identifying the phase of the oxide as well as oxygen stoichiometry is a challenging field where Raman spectroscopy is now contributing significantly. Recently, Raman spectroscopy has been used extensively to characterize nuclear fuel materials, especially to identify different phases of uranium oxides; to characterize defects and oxygen stoichiometry in unirradiated uranium oxides; 6 to provide supportive structural information about the surface;7,8 to study phase transitions due to oxidation;9 and to investigate phonon behavior at high pressures.10 Raman spectroscopy also has been used for identification of corrosion products formed on UO211 and to probe ion-beam, irradiation-induced damage in UO2.12 There are reports of the use of Raman spectroscopy to characterize plutonium dioxide (PuO2) and (U1-yPuy)O2 fuels, as well as to identify the nature of defects.13,14 The present studies are aimed at providing a sensitive, alternate characterization tool for ThO2–UO2 compounds. In general, mixed systems can show two different types of phonon behavior: (i) one-mode behavior in which the zone center optical phonon frequency varies continuously with composition from one end member to that of the other, for example, as in Ca1-xSrxF2 and Sr1-xBaxF215; and (ii) two-mode behavior in which zone center optical phonon frequencies of both the end members appear in all the intermediate compositions, for example, as in Cd1-xMnxSe.16 It is of interest to investigate the Raman spectra of a ThO2–UO2 mixed system to understand its phonon behavior. Two sets of nuclear fuels were investigated by Raman spectroscopy in the present study. The first set consists of ThO2–UO2 solid solutions with a wide range of UO2 contents. The second set of samples consists of simulated fuel (SIMFUEL), an unirradiated analog of spent nuclear fuel, made by doping ThO2–UO2 fuel with nonradioactive oxides in appropriate proportions to mimic the chemical effects of a nuclear reactor. SIMFUEL replicates the chemical composition of spent fuel so that detailed experiments to estimate the extent of degradation in the properties of the fuel due to the presence of fission products can be undertaken in the absence of radiation. Here, we report the results of Raman spectroscopic investigations carried out on ThO2–UO2 fuels and SIMFUELs based on ThO2–UO2 for
0003-7028/14/6801-0044/0 Q 2014 Society for Applied Spectroscopy
APPLIED SPECTROSCOPY
FIG. 2. Raman spectra of ThO2–UO2 solid solution for different UO2 contents. (a) Region of F2g mode. (b) Region of F1u (LO) and higher order modes. Arrow in panel a indicates asymmetry in F2g due to UO2. The curves in panel (b) have been multiplied by a factor of five for clarity. FIG. 1. Variation of the lattice parameter with composition for the ThO2–UO2 system, indicating a linear relationship obeying the Vegard’s law.
composition analyses, estimation of extent of damage, and oxygen stoichiometry. Experimental Details. Thorium dioxide and UO2 form an ideal, homogeneous solid solution in the entire composition range17 and crystallize in the fluorite structure, space group Fm3m.18 ThO2–UO2 pellets of various compositions (0–100 wt% of UO2) were synthesized by a conventional powder metallurgical method, starting from UO2 and ThO2 powders, followed by comilling, compaction, and sintering at around 1700 8C, in a reducing atmosphere (92% argon and 8% hydrogen) to avoid formation of other oxides of uranium. The solid solutions of different compositions were characterized by X-ray diffraction. The diffraction patterns indicated that the samples were single-phase solid solutions and that the overall structure was a fluorite structure. Figure 1 shows the lattice parameter for each composition, obtained by Rietveld refinement of the powder diffraction data that varied linearly with composition, following Vegard’s law. The SIMFUELs corresponding to an average burn-up of 28 000 and 43 000 MWd/t (megawatt day per tonne) also were synthesized by the powder metallurgical route by adding high-purity compounds such as strontium oxide (SrO), yttrium oxide, zirconium dioxide, molybdenum trioxide, barium carbonate, lanthanum oxide,
cerium(IV) oxide (CeO2), neodymium(III) oxide, ruthenium(IV) oxide, rhodium(III) oxide, cesium carbonate, praseodymium(III, IV) oxide, samarium(III) oxide, and palladium to ThO2–3.45 wt% UO2 powders. Powder X-ray diffraction showed that all three compounds: ThO2-3.45 wt% UO2, SIMFUEL corresponding to 28 000 MWd/t, and SIMFUEL 43 000 MWd/t have fluorite structure.19 Raman spectroscopic measurements were carried out from polycrystalline pellets using an ;15 mW 532 nm laser. The focused spot size on the sample was about 20 lm, and the power density was low enough to avoid surface oxidation of the samples due to local heating by the laser beam, an effect commonly observed in this class of oxides.14 Raman spectra recorded from different regions of the pellet were reproducible, confirming homogeneity of the sample. Scattered light was analyzed using a home-built 0.9 m single monochromator,20 coupled with an edge filter, and was detected by a Peltier cooled, charge-coupled device. The spectrograph was calibrated using the Raman spectra of indene. Entrance slit was kept at 50 lm, thereby giving a spectral band pass of 3 cm1. Raman spectra could be reproduced with an accuracy of 60.2 cm1.
RESULTS AND DISCUSSION Raman Spectroscopy of ThO2–UO2 Solid Solutions. As mentioned, both ThO2 and UO2 crystallize in fluorite structure. Group theory predicts six optic phonon
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FIG. 3. Fitting of the observed Raman spectra of ThO2–UO2 solid solution for 20 wt% UO2 (a) in the region of first order modes of ThO2, and (b) in the region of higher order modes of ThO2. Continuous lines represent the experimental data, and the dotted and the dashed lines denote the individual Lorentzian and fitted curves respectively. The broad bands at around 1335 and 1575 cm1 are due to carbon.
branches, a triply degenerate Raman active mode F2g, and an infrared (IR) active mode F1u for fluorite structure. Figures 2a and 2b show Raman spectra of different compositions of the ThO2–UO2 solid solution. Pure ThO2 exhibits first-order Raman mode at 467 cm1 due to the symmetric breathing mode of the oxygen atoms around each thorium ion. Raman spectrum of stoichiometric UO2 has the corresponding F2g vibrational mode at around 448 cm1. The presence of defects distorts the translational symmetry and relaxes the selection rules, making several normally dipole-forbidden optical transitions observable. A weak feature observed in the Raman spectra of UO2 around 575 cm1, assigned to be the IR active F1u longitudinal optic (1LO) mode, appears to be due to the breakdown of selection rules as a result of presence of defects.21,22 A broad band observed in UO2 around 1150 cm1, a band that was initially assigned to an electronic crystal field transition, is now attributed to the second-order longitudinal optic (2LO) phonon based on the similarity of 1150 cm1 mode with 575 cm1 in terms of resonance profile, pressure dependence of intensity, and frequency.10 Its intensity is highly sensitive to minor changes in oxygen stoichiometry; it is more intense in stoichiometric UO2 samples,9 hence it is interesting to monitor this band as an indicator of the extent of UO2 oxidation. Although pure ThO2 has only one intense peak in the Raman spectrum, several new features are observed with the addition of UO2. Apart from the appearance of 1LO and 2LO modes, a clear asymmetry is observed in the F2g peak of ThO2, the position and intensity of which change with UO2 content (Figs. 2a and 2b). The relative intensity of 1LO and 2LO modes with regard to the F2g mode increases exponentially with addition of UO2. All the new features seen in the Raman spectra of a typical solid solution ThO2–20 wt% UO2 are shown more clearly in Figs. 3a and 3b. After appropriate background
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FIG. 4. Variation of Raman mode frequency with composition of ThO2– UO2.
subtraction, all the Raman bands are fitted to Lorentzians. The region of F2g peak with asymmetry toward low frequency is fitted to two Lorentzians. Although the peak due to ThO 2 remains at the same position, the asymmetry due to UO2 shifts toward lower frequency with and increase in UO2 content. The intensity of the asymmetry due to UO2 increases with addition of UO2, at the cost of intensity of the ThO2 peak. The region of the F2g mode shows a clear two-mode behavior in the entire composition. There is a weak broad mode at around 270 cm1 that is assigned to be F1u transverse optic (TO).9 In addition to overtones of 1LO, a Raman band is observed at around 1335 cm1, the position of which is found to be highly sensitive to the composition. To ascertain that it is not a luminescence band, we also have recorded Raman spectra of these compounds with a wavelength of 473 nm, and we found the presence of this band. This band was observed in UO2 by previous research when recorded with 785 nm laser23,24 and was attributed to the presence of carbon in the samples. However, carbon content in our samples was below the detection limit of the combustion analysis carried out to estimate carbon. Variation of peak positions and intensity in Raman spectra of ThO2–UO2 solid solutions can form the basis of quantitative analysis of the solid solution. Figure 4 shows the peak positions of all the features in the Raman spectra for different compositions of the solid solution.
FIG. 5. Raman spectra of ThO2–3.45 wt% UO2 (a), and SIMFUELs for 28 000 MWd/t (b) and 43 000 MWd/t (c).
All the peak positions vary smoothly and linearly with composition, and there is a considerable change in the position of the Raman modes with composition that may be useful in quantitative analysis of ThO2–UO2 solid solutions. The position of the asymmetry in the F2g mode shows a variation of 0.17 cm1 per percentage of UO2. From the present results, composition of any solid solution of ThO2–UO2 can be estimated. The composition of a ThO2–UO2 solid solution estimated using the position of asymmetry in F2g mode agrees within 3 wt% with that measured by powder X-ray diffraction (XRD) in the composition of interest for nuclear applications (18– 22.5 wt% UO2). Two-mode behavior of the mixed system in the Raman spectra enables us to qualitatively detect any higher oxide of uranium and hence the oxidation state of uranium in the mixed system. Raman Spectroscopy of SIMFUELs. Figure 5 shows the Raman spectrum of ThO2–3.45 wt% UO2 (Fig. 5a) and that of the SIMFUELs corresponding to burn-up 28 000 MWd/t (Fig. 5b) and 43 000 MWd/t (Fig. 5c) under ambient conditions. The strong narrow mode at 467 cm1 corresponds to the F2g symmetry in ThO2–3.45 wt% UO2 and has a full width half-maximum (FWHM) of about 6.5 cm1. In the SIMFUELs samples, this mode is slightly shifted by about 2 cm1 toward the higher frequency and is broader (FWHM of ;9 cm1). No significant decrease in the intensity of the F2g mode was observed. Higher frequency and broadening of the F2g mode indicates higher oxidation of the compound.6,9 In the region 520– 700 cm1, bands are seen in the spectra of SIMFUELs. The band at around 575 cm1 is assigned to F1u(LO), as already mentioned. The second band at around 620 cm1 also has been observed in UO2 samples due to oxidation by laser heating, when a higher laser power was used.14
As mentioned, we recorded all the Raman spectra at power levels below the threshold required to induce oxidation of the surface. A comparison with the report on irradiated fuels indicated the band at around 620 cm1 is attributed to oxygen sublattice damage either by the formation of Frenkel pairs (vacancy and interstitial oxygen atom) or hyperstoichiometry12,14 due to simulated radiation damage. This band appears to be due to oxygen ions placed in the interstitial sites of fluorite structure, leading to breakdown of translational symmetry and resulting in new disorder-induced bands assigned as an LO(X) band.10 The relative intensity of this band with respect to that of the F1u band is reported to be a measure of hyperstoichiometry x in UO2þx samples.9 Desgranges et al. 25 carried out polarized Raman measurements on a single crystallite of U4O 9, a superstructure of UO2, and they identified a Raman mode at 630 cm1 to be an Ag mode, along with another mode at 160 cm1 due to zone folding. Because we do not have polarized Raman data, it is not possible to comment on whether there is formation of an U4O9 type of phase in SIMFUELs. However, in the absence of the peak around 160 cm1, we assign the band at 620 cm1 to be due to oxygen sublattice damage. The presence of both 575 and 620 cm1 bands in the Raman spectra of SIMFUEL indicates presence of point defects as well as oxygen sublattice damage due to doping in the fluorite matrix. It may be noted that the oxygen sublattice damage is observed only in SIMFUEL samples and not in the first set of ThO2–UO2 samples where the defects are point defects. A very weak band around 1150 cm1 due to 2LO is observed in all the three samples (not shown in Fig. 5). Both of the SIMFUEL samples showed similar relative intensity of the defect peaks, indicating no significant difference in the extent of damage for the two different burn-ups. The present Raman results show that the major matrix in SIMFUEL is a fluorite-type, face-centered cubic. The presence of peak around 620 cm1 could be due to (i) the formation of U4O9 or (ii) the replacement of Th4þ sites by dopant cations. When Th4þ ions are substituted by dopants ions of different radii, the oxygen sublattice undergoes distortion to accommodate the dopant. The second possibility is based on the appearance of a defect peak in doped (with Zr4þ/Y3þ/La3þ) fluorite-structured systems, such as CeO2,22 and is the more probable reason for the appearance of the peak around 620 cm1; because sintering of SIMFUEL pellets was carried out in a reducing atmosphere, the possibility of formation of the U4O9 phase is less. It may be noted that no such clues about any structural changes were obtained from the corresponding XRD data.19 The present study of SIMFUELs indicates that the damage due to doping is minimal, unlike the loss of cubic symmetry observed in previous studies on SIMFUELs, due to the higher level of doping corresponding to higher burn-up and resulting in ordering of vacancies and structural changes.26
SUMMARY AND CONCLUSIONS A detailed Raman spectroscopic investigation was carried out on ThO2–UO2 solid solutions and SIMFUELs based on ThO2–UO2. The results suggest that Raman
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spectroscopy is feasible as a simple, nondestructive technique to characterize ThO2–UO2 mixtures, the proposed fuel for the AHWR, in India. Although all the peak positions due to UO2 substitution show a linear relationship with the composition, the intensity of defectinduced modes due to UO2 increases with the addition of UO2. The presence of an intense broad peak at around 620 cm1 in SIMFUEL samples indicates oxygen sublattice damage, whereas in ThO2–UO2 mixtures the oxygen sublattice remains intact. Although the defects in the ThO2 –UO 2 mixtures are point defects, the SIMFUELs were found to have extended defects. Thus, Raman spectroscopy is an effective alternate tool in this important class of nuclear fuels to examine composition and defects that are otherwise difficult to detect. ACKNOWLEDGMENT R.R. and N.P.S. thank Dr. S.L. Chaplot for support and encouragement. 1. International Atomic Energy Agency. 2005. ‘‘Thorium Fuel Cycle– Potential Benefits and Challenges’’. IAEA-TECDOC-1450, International Atomic Energy Agency. www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf [accessed Sept 4 2013]. 2. R.K. Sinha, A. Kakodkar. ‘‘Design and Development of the AHWR— The Indian Thorium Fuelled Innovative Nuclear Reactor’’. Nucl. Eng. Des./Fusion. 2006. 236(7-8): 683-700. 3. N.P. Pushpam, A. Kumar, U. Kannan, A. Kumar, P.D. Krishnani. ‘‘Study for Use of LEU Along with Thorium in Advanced Heavy Water Reactor (AHWR) to Enhance Proliferation Resistance Characteristics of Fuel’’. Paper presented at: International Conference on Peaceful Uses of Atomic Energy. New Delhi: 2009. Paper Number: IAEA-CN-184/207. 4. R.D. Robinson, J. Tang, M.L. Steigerwald, L.E. Brus, I.P. Herman. ‘‘Raman Scattering in HfxZr1xO2 Nanoparticles’’. Phys. Rev. 2005. 71(11): 115408 (1-8). doi: 10.1103/PhysRevB.71.115408. 5. D. Horlait, N. Clavier, N. Dacheux, R. Cavalier, R. Podor. ‘‘Synthesis and Characterization of Th1-xLnxO2-x/2 Mixed-Oxides’’. Mater. Res. Bull. 2012. 47(12): 4017-4025. 6. D. Manara, B. Renker. ‘‘Raman Spectra of Stoichiometric and Hyperstoichiometric Uranium Dioxide’’. J. Nucl. Mater. 2003. 321(2-3): 233-237. 7. F. Pointurier, O. Marie. ‘‘Identification of the Chemical Forms of Uranium Compounds in Micrometer-Size Particles by Means of Micro-Raman Spectrometry and Scanning Electron Microscope’’. Spectrochim. Acta, Part B. 2010. 65(9-10): 797-804. 8. C. Je´gou, R. Caraballo, J. De Bonfils, V. Broudic, S. Peuget, T. Vercouter, D. Roudil. ‘‘Oxidizing Dissolution of Spent MOX47 Fuel Subjected to Water Radiolysis: Solution Chemistry and Surface Characterization by Raman Spectroscopy’’. J. Nucl. Mater. 2010. 399(1): 68-80. 9. H. He, D. Shoesmith. ‘‘Raman Spectroscopic Studies of Defect Structures and Phase Transition in Hyper-Stoichiometric UO2þx’’. Phys. Chem. Chem. Phys. 2010. 12(28): 8108-8117.
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10. T. Livneh, E. Sterer. ‘‘Effect of Pressure on the Resonant Multiphonon Raman Scattering in UO2’’. Phys. Rev. B. 2006. 73(8): 085118(1-9). doi: 10.1103/PhysRevB.73.085118. 11. M. Amme, B. Renker, B. Schmid, M.P. Feth, H. Bertagnolli, W. Dobelin. ‘‘Raman Microspectrometric identification of Corrosion Products Formed on UO2 Nuclear Fuel During Leaching Experiments’’. J. Nucl. Mater. 2002. 306(2-3): 202-212. 12. G. Guimbretie´re, L. Desgranges, A. Canizare´s, G. Carlot, R. Caraballo, C. Je´gou, F. Duval, N. Raimboux, M.R. Ammar, P. Simon. ‘‘Determination of In-depth Damaged Profile by Raman Line Scan in a Pre-Cut He2þ Irradiated UO2’’. Appl. Phys. Lett. 2012. 100(25): 251914(1-4). 13. M.J. Sarsfield, R.J. Taylor, C. Puxley, H.M. Steele. ‘‘Raman Spectroscopy of Plutonium Dioxide and Related Materials’’. J. Nucl. Mater. 2012. 427(1-3): 333-342. 14. C. Je´gou, R. Caraballo, S. Peuget, D. Roudil, L. Desgranges, M. Magnin. ‘‘Raman Spectroscopy Characterization of Actinide Oxides (U1-yPuy)O2: Resistance to Oxidation by the Laser Beam and Examination of Defects’’. J. Nucl. Mater. 2010. 405(3): 235-243. 15. R.K. Chang, B. Lacina, P.S. Persan. ‘‘Raman Scattering from Mixed Crystals (CaxSr1-x)F2 and (SrxBa1-x)F2’’. Phys. Rev. Lett. 1966. 17(14): 755-758. 16. R.G. Alonso, Y.R. Lee, E. Oh, A.K. Ramdas, H. Luo, N. Samarth, J.K. Furdyna, H. Pascher. ‘‘Raman and Reflectivity Spectra of Cubic Cd1xMnxSe Epilayers Grown by Molecular-Beam Epitaxy’’. Phys. Rev. B. 1991. 43(12): 9610-9620. 17. J. Belle, R.M. Berman. Thorium Dioxide: Properties and Nuclear Application. Washington, DC: 1984. DOE/NE-0060. http://www.osti. gov/bridge/servlets/purl/5986642-LhiWnw/5986642.pdf [accessed Sept 4 2013]. 18. W.A. Lambertson, M.H. Mueller, F.H. Gunzel. ‘‘Uranium Oxide Phase Equilibrium Systems: IV, UO2-ThO2’’. J. Am. Ceram. Soc. 1953. 36(12): 397-399. 19. R.K. Bhagat, K. Krishnan, T.R.G. Kutty, A. Kumar, H.S. Kamath, S. Banerjee. ‘‘Thermal Expansion of Simulated Thoria–Urania Fuel by High Temperature XRD’’. J. Nucl. Mater. 2012. 422(1-3): 152-157. 20. A.P. Roy, S.K. Deb, M.A. Rekha, A.K. Sinha. ‘‘Multichannel Raman Spectroscopy’’. Indian J. Pure Appl. Phys. 1992. 30(12): 724. 21. W.H. Weber, K.C. Hass, J.R. McBride. ‘‘Raman Study of CeO2: Second-Order Scattering, Lattice Dynamics, and Particle Size Effects’’. Phys. Rev. B. 1993. 48(1): 178-185. 22. A. Nakajima, A. Yoshihara, M. Ishigame. ‘‘Defect Induced Raman Spectra in Doped CeO2’’. Phys. Rev. B. 1994. 50(18): 13297-13307. 23. S.D. Senanayake, R. Rousseau, D. Colegrave, H. Idriss. ‘‘The Reaction of Water on Polycrystalline UO2: Pathways to Surface and Bulk Oxidation’’. J. Nucl. Mater. 2005. 342(1-3): 179-187. 24. E.A. Stefaniak, A. Alsecz, I.E. Sajo´, A. Worobiec, Z. Ma´the´, S. To¨ro¨k, R. Grieken. ‘‘Recognition of Uranium Oxides in Soil Particulate Matter by Means of l-Raman Spectrometry’’. J. Nucl. Mater. 2008. 381(3): 278-283. 25. L. Desgranges, G. Baldinozzi, P. Simon, G. Guimbretie`re, A. Canizares. ‘‘Raman Spectrum of U4O9: A New Interpretation of Damage Lines in UO2’’. J. Raman Spectrosc. 2012. 43(3): 455-458. 26. H. He, P.G. Keech, M.E. Broczkowski, J.J. Noel, D.W. Shoesmith. ‘‘Characterization of the Influence of Fission Product Doping on the Anodic Reactivity of Uranium Oxide’’. Can. J. Chem. 2007. 85(10): 702-713.
