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Lattice parameter functions of (AmyU1−y)O2−x based on XRD and XANES measurements
Journal of Solid State Chemistry 256 (2017) 252–255
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Lattice parameter functions of (AmyU1−y)O2−x based on XRD and XANES measurements
MARK
⁎
Tsuyoshi Nishia, , Masami Nakadab, Masaru Hiratac a b c
Graduate school of Science and Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan Nuclear Science and Engineering Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan Advanced Fast Reactor Cycle System R & D Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1112, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: AmyU1−y)O2−x X-ray diffraction Lattice parameter functions Am content X-ray absorption near-edge structure spectrum
The lattice parameters of (Am0.50U0.50)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2−x were determined by powder X-ray diffraction with Cu Kα radiation. In addition, the lattice parameter functions of (AmyU1−y)O2−x (0.00 < x < 0.25, 0.00 < y < 0.50) were evaluated using models of (Am3+yU4+1−2yU5+y)O2 and (Am3+yU4+1−y) O2−y/2 based on the results of X-ray diffraction and the ionic radii of Am3+, U4+, and U5+. In order to confirm the valence state of Am and U in (AmyU1−y)O2−x, the X-ray absorption near-edge structure measurements were performed in the transmission mode at the Am-LIII and U-LIII absorption edges of (Am0.50U0.50)O2.0, (Am0.50U0.50)O2−x, and UO2.0.
1. Introduction The operation of commercial light-water reactors (LWRs) generates plutonium and minor actinides (MAs: neptunium, americium, curium) in the irradiated nuclear fuels. Among the MAs, Am has especially high and long-lasting radiotoxicity. To reduce the long-term radiotoxicity of high-level radioactive waste and to efficiently use repositories, one of the options in future nuclear fuel cycles is heterogeneous recycling of MAs using fast reactors (FRs). Among the many types of advanced fuels that contain MAs, fuels containing a mixed oxide (MOX) of uranium and americium, (AmyU1−y)O2−x, are one of the targets for the transmutation of Am in FRs [1,2]. Moreover, Zakari-Issoufou et al. investigated the feasibility of Am transmutation in MOX fuels of commercial pressurized-water reactors (PWRs) [3]. Because excess oxygen can oxidize the cladding tubes during irradiation, one of the major factors of effective transmutation of Am is the change in oxygen potential with regard to changes in the oxidation states of both uranium and americium. Therefore, the Am redox behavior in (AmyU1−y)O2−x can provide valuable design information for fuel development. In particular, the lattice parameter of (AmyU1−y)O2−x provides very useful information for the design of MOX fuels for commercial PWRs. Nakamura et al. suggested a lattice parameter model of fluorite oxide defects as a function of the ionic radius [4]. However, the lattice
parameter of (AmyU1−y)O2−x was not available owing to the lack of the experimental data and information on the valence state of Am and U in (AmyU1−y)O2−x. Recently, Nishi et al. reported X-ray absorption near-edge structure (XANES) measurements at the Am-LIII absorption edge of (Am0.05U0.95)O2.0 [5]. In addition, Prieur et al. studied the local structure and charge distribution in (AmyU1−y)O2−x [6]. Using the XANES spectra at the Am-LIII and U-LIII absorption edge of (AmyU1−y) O2−x, they suggested a value for the lattice parameter of (AmyU1−y)O2−x (−0.04 < x < 0.04, 0.00 < y < 0.09). They clarified that the estimation of this lattice parameter was not based on Vegard's law; however, no value was suggested for the lattice parameter of (AmyU1−y)O2−x with high Am content because the XANES data was limited to an Am atomic ratio of y < 0.20. In this work, the lattice parameters of (Am0.50U0.50)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2−x were determined by powder X-ray diffraction (XRD). In addition, the lattice parameter functions of (AmyU1−y)O2−x (0.00 < x < 0.25, 0.00 < y < 0.50) were evaluated using models of (Am3+yU4+1−2yU5+y)O2 and (Am3+yU4+1−y)O2−y/2 based on the results of X-ray diffraction and ionic radii of Am3+, U4+, and U5+. In order to confirm the valence state of Am and U in (AmyU1−y)O2−x, the XANES measurements were performed in the transmission mode at the Am-LIII and U-LIII absorption edges of (Am0.50U0.50)O2.0, (Am0.50U0.50)O2−x, and UO2.0.
Abbreviations: EXAFS, expanded X-ray absorption fine structure; KEK, High Energy Accelerator Research Organization; MOX, mixed oxide; XAFS, X-ray absorption fine structure; XANES, X-ray absorption near-edge structure; XRD, X-ray diffraction ⁎ Correspondence to: Department of Materials Science and Engineering, College of Engineering, Ibaraki University, Nakanarusawa 4-12-1, Hitachi, Ibaraki 316-8511, Japan. E-mail address: [email protected] (T. Nishi). http://dx.doi.org/10.1016/j.jssc.2017.09.011 Received 28 June 2017; Received in revised form 27 August 2017; Accepted 11 September 2017 Available online 19 September 2017 0022-4596/ © 2017 Elsevier Inc. All rights reserved.
Journal of Solid State Chemistry 256 (2017) 252–255
T. Nishi et al.
2. Experimental 2.1. Sample preparation The (AmyU1−y)O2−x powders for the XRD and X-ray absorption fine structure (XAFS) measurements were prepared as follows. The UO2 powder was first mixed with AmO2 powder (purity: 99.7%; nonradioactive impurities were virtually absent [7]) in various proportions, and then the different batches of mixed powders were pressed into green disks. The disks of (Am0.50U0.50)O2.0 and (Am0.37U0.63)O2.0 were heated at 1723 K for 4 h in a N2 gas flow and then at the same temperature for 4 h in a gas flow of N2 with 4% H2. In order to obtain the actinide dioxide powder, the disks were heated at 1173 K for 1 h in a gas flow of N2 with 10 ppm of O2. The preparation conditions were determined based on data on the oxygen potential [8]. It was revealed that the O/(U+Am) ratio of the (U0.95Am0.05)O2.0 powder was 2.0 ± 0.02. On the other hand, the (Am0.50U0.50)O2−x disks were heated at 1723 K for 4 h in a N2 gas flow. The (AmyU1−y)O2−x and graphite powder were stirred together and pressed into a disk measuring 6 mm in diameter. The graphite powder was used to dilute the pressed sample for the transmission XAFS measurements [9]. The disk was doubly sealed in two polyethylene terephthalate (PET) containers using epoxy resin (Stycast 1266, Henkel Loctite, Germany) for the XAFS measurements. 2.2. XRD and XANES measurements The (AmyU1−y)O2−x powder was examined by powder XRD analysis with Cu Kα radiation to confirm the presence of a single phase and to determine the lattice parameter. Since the powder consisted of a single face-centered cubic (fcc) phase, the lattice parameter was determined by the Nelson–Riley extrapolation function. XANES measurements at the U-LIII and Am-LIII absorption edges of (AmyU1−y)O2−x took place in the transmission mode at the hard X-ray station BL-27 in the Photon Factory of High Energy Accelerator Research Organization (KEK), Japan, at an energy level of 2.5 GeV. The intensities of the incident and transmitted beams were monitored in an ionization chamber under an Ar–N2 gas flow and an Ar gas flow, respectively. The radiation was monochromatized using a double-crystal Si(111) monochromator. The XANES spectra were recorded at 293 K. The energy shown in the XANES spectra was calibrated against the expanded XAFS (EXAFS) spectrum of (AmyU1−y)O2−x and the Zr K absorption edge of Zr foil (Kedge: 17.998 keV [10]). Based on the XAFS spectra for energy calibration, the uncertainty of the energy value of the white-line peak was ± 0.5 eV.
Fig. 1. XANES spectrum of the Am-LIII absorption edge of (Am0.50U0.50)O2.0, together with the XANES spectra of AmO2 and AmO1.5 [9].
absorption coefficient μ is represented by μt. Two peaks (A, B) and a tail peak (A′) can be seen in all three XANES spectra. Moreover, the XANES spectrum of the Am-LIII edge of (Am0.50U0.50)O2.0 is in good agreement with that of AmO1.5, demonstrating that Am in (Am0.50U0.50)O2.0 was in the trivalent state. The same results were also obtained for (Am0.50U0.50)O2−x and (Am0.05U0.95)O2.0 [5]. Fig. 2 shows the XANES spectra of the U-LIII absorption edge of UO2.0, (Am0.50U0.50)O2−x, BaUO3, and NaUO3 [11]. According to the electroneutrality rule, U in BaU4+O3 was in the tetravalent state, and U in NaU5+O3 was in the pentavalent state. The XANES spectrum of the U-LIII edge of UO2.0 is in good agreement with that of BaUO3, and the XANES spectrum of the U-LIII edge of (Am0.50U0.50)O2−x is in good agreement with that of NaUO3. In particular, the energy shift of the white-line peak between (Am0.50U0.50)O2−x and UO2.0 was observed at about 2 eV, which is consistent with the chemical shift between U4+ and U5+. Thus, it can be concluded that U in (Am0.50U0.50)O2−x was in the pentavalent state. The same results were obtained for (Am0.50U0.50) O2.0 and (Am0.05U0.95)O2.0 [5]. As shown in Figs. 1 and 2, Am in (AmyU1−y)O2−x was in the trivalent state, while U in (Am0.50U0.50)O2.0 and (Am0.50U0.50)O2−x was in the pentavalent state. Based on the electroneutrality rule, (AmyU1−y) O2−x was expected to consist of Am3+, U4+, and U5+. Nakamura et al. suggested the following lattice parameter model of fluorite oxide defects as a function of the ionic radius [4]. Assuming a random distribution of cations in the cation sub-lattice and anions in the anion sub-lattice, the coordination number (CN) of Am3+, U4+, and U5+ is 8 and that of O2− ion is 4. The ionic radius of Am3+, U4+, and U5+ at CN = 8 are summarized in Table 2. The ionic radii were obtained from values reported in the literature [12,13] and the lattice parameters of AmO1.5 [14] and UO2 [15]. The lattice parameter of (AmyU1−y)O2.0 was estimated from the (Am3+yU4+1−2yU5+y)O2 model suggested by Nakamura et al. (0 < y < 1) [4]. The lattice parameter functions of (AmyU1−y)O2.0 are represented by the following equations:
3. Results and discussion The measured lattice parameters of (AmyU1−y)O2−x and the UO2.0 powder are summarized in Table 1. Since the lattice parameter of (Am0.50U0.50)O2−x was larger than that of (Am0.50U0.50)O2.0, the (Am0.50U0.50)O2−x powder was determined to have hypo-stoichiometric composition. Fig. 1 shows the XANES spectra of the Am-LIII absorption edge of (Am0.50U0.50)O2.0, Am4+O2, and Am3+O1.5 [9]. The normalized intensity of an X-ray beam passing through a material of thickness t with the Table 1 Measured lattice parameters of (AmyU1−y)O2−x and UO2.0. Lattice parameter (nm) (Am0.50U0.50)O2.0 (Am0.37U0.63)O2.0 (Am0.50U0.50)O2−x UO2.0
0.5446 0.5452 0.5465 0.5470
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Fig. 3. Lattice parameter of (AmyU1−y)O2.0 as a function of the Am content, obtained using the (Am3+yU4+1−2yU5+y)O2 model together with the lattice parameter of (Am0.50U0.50)O2.0 reported by Mayer et al. [8].
fC = 0.40693 + 0.03741rc + 14.7973rc 2(nm),
(5)
rC(ss) = (1 − y )rU 4+ + yrAm 3+,
(6)
rU 4+ = 0.0996 + 0.00520[(8 − 2 y ) − 8] − 3.397 × 10−12[(8 − 2 y ) − 8]2 , (7)
rAm
Table 2 Ionic radii of U4+, U5+, and Am3+.
U U5+ Am3+
Ionic radius (nm)
Ref.
0.0996 0.0880 0.1091
[12] [13] [12]
a 0 = 0.3571 + 1.5016rc + 4.076rc 2(nm),
(1)
rc = 0.0996(1 − 2y ) + 0.0880 y + 0.1091 y(nm),
(2)
where a0, rc, and y are the lattice parameter of (AmyU1−y)O2.0, mean ionic radius of the (Am3+yU4+1−2yU5+y)O2 model, and Am content in (AmyU1−y)O2.0, respectively. Fig. 3 shows the lattice parameter of (AmyU1−y)O2.0 as a function of the Am content in the (Am3+yU4+1−2yU5+y)O2 model. It can be seen that the lattice parameters of (Am0.05U0.95)O2.0 (Am0.05U0.95)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2.0 agree with values obtained from Eqs. (1) and (2). According to these two equations, the lattice parameter of (Am0.50U0.50)O2.0 was 0.5447 nm. For comparison, Mayer et al. it reported a value of 0.5452 nm for the lattice parameter of (Am0.50U0.50)O2.0 [8]. Thus, the results of the lattice parameters also suggest that the (Am0.50U0.50)O2.0 solid solution sample had a stoichiometric composition. Next, based on the model proposed by Nakamura et al. [4], the lattice parameter functions of the AmO1.5–UO2 mixed-oxide solid solution (ss) can be represented by the following equations:
a 0(ss) = (1 − y )fF + yfC , fF = 0.3571 + 1.5016rc + 4.076rc 2(nm),
2
= 0.1091 + 0.00307[(8 − 2 y ) − 8] − 0.00134[(8 − 2y ) − 8] ,
(8)
where a0(ss) is the lattice parameter of the AmO1.5–UO2 mixed-oxide solid solution, fF is the lattice parameter of UO2.0, fC is the lattice parameter of AmO1.5, rC(ss) is the mean ionic radius of the AmO1.5– UO2 mixed-oxide solid solution, rU4+ is the mean ionic radius of U4+ from the fitting equation, and rAm3+ is the mean ionic radius of Am3+ from the fitting equation. Using Eqs. (3)–(8), the lattice parameter of (Am0.50U0.50)O1.75 was calculated to be 0.5521 nm. Fig. 4 shows the lattice parameter of (AmyU1−y)O2−x as a function of the Am content, obtained using the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models together with those of (Am0.10U0.90)O2.00, (Am0.15U0.85)O2.00, and (Am0.20U0.80)O1.99 [5]. It can be seen that the lattice parameters of (Am0.10U0.90)O2.00, (Am0.15U0.85)O2.00, and (Am0.20U0.80)O1.99 were almost the same as the values of the lattice parameter functions of (AmyU1−y)O2−x based on the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models. Using the lattice parameter functions, the lattice parameter of (Am0.50U0.50)O2.0 was calculated to be 0.5447 nm. Using the lattice parameters of (Am0.50U0.50)O2.0 and (Am0.50U0.50)O1.75, the O/(Am+U) ratio of (Am0.50U0.50)O2−x was presumed to be 1.94.
Fig. 2. XANES spectra of the U-LIII absorption edge of UO2.0 and (Am0.50U0.50)O2−x, together with the XANES spectra of BaUO3 and NaUO3 [11].
4+
3+
(3) Fig. 4. Lattice parameter of (AmyU1−y)O2−x as a function of the Am content, obtained using the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models together with the lattice parameters of (Am0.10U0.90)O2.00, (Am0.15U0.85)O2.00, and (Am0.20U0.80)O1.99 [6].
(4) 254
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4. Conclusions
References
The lattice parameter functions of (AmyU1−y)O2−x (0.00 < x < 0.25, 0.00 < y < 0.50) were evaluated using the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models based on the analysis results of the X-ray diffraction and XANES measurements. The conclusions of this study are as follows:
[1] F. Varaine, L. Buiron, L. Boucher, D. Verrier, S. Massara, Study of minor actinide transmutation in sodium fast reactor depleted uranium radial blanket, in: Proceedings of the 10th OECD/NEA Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation, Mito, Japan, 2008 Oct 6–10, OECD/NEA, Issy-les-Moulineaux, France, 2010, pp. 421–430. [2] M.A. Denecke, Actinide speciation using X-ray absorption fine structure spectroscopy, Coord. Chem. Rev. 250 (2006) 730–754. [3] A.A. Zakari-Issoufou, X. Doligez, A. Somaini, Q. Hoarau, S. David, S. Bouneau, F. Courtin, B. Leniau, N. Thiollière, B. Mouginot, A. Bidaud, N. Capellan, O. Meplan, A. Nuttin, R. Sogbadji, Americium mono-recycling in PWR: a step towards transmutation, Ann. Nucl. Energy 102 (2017) 220–230. [4] A. Nakamura, N. Masaki, H. Otobe, Y. Hinatsu, J. Wang, M. Takeda, Defect-fluorite oxides M1-yLnyO2-y/2(Ln = lanthanide; M = Hf, Zr, Ce, U, Th): structure, property, and applications, Pure Appl. Chem. 79 (2007) 1691–1729. [5] T. Nishi, M. Nakada, C. Suzuki, H. Shibata, Y. Okamoto, M. Akabori, M. Hirata, Valence state of Am in (U0.95Am0.05)O2.0, J. Nucl. Mater. 418 (2011) 311–312. [6] D. Prieur, P.M. Martin, A. Jankowiak, E. Gavilan, A.C. Scheinost, N. Herlet, P. Dehaudt, P. Blanchart, Local structure and charge distribution in mixed uranium–americium oxides: effects of oxygen potential and Am content, Inorg. Chem. 50 (2011) 12437–12445. [7] T. Nishi, M. Takano, A. Itoh, M. Akabori, K. Minato, M. Kizaki, Thermal diffusivity of Americium mononitride from 373 to 1473 K, J. Nucl. Mater. 355 (2006) 114–118. [8] K. Mayer, B. Kanellakopoulos, J. Naegele, L. Koch, On the valency state of americium in (U0.5Am0.5)O2−x, J. Alloy. Compd. 213–214 (1994) 456–459. [9] T. Nishi, M. Nakada, C. Suzuki, H. Shibata, A. Itoh, M. Akabori, M. Hirata, Local and electronic structure of Am2O3 and AmO2 with XAFS spectroscopy, J. Nucl. Mater. 401 (2010) 138–142. [10] J.A. Bearden, A.F. Burr, Reevaluation of X-ray atomic energy levels, Rev. Mod. Phys. 39 (1967) 125–142. [11] A.V. Soldatov, D. Lamoen, M.J. Konstantinovic´, S. Van den Berghe, A.C. Scheinost, M. Verwerft, Local structure and oxidation state of uranium in some ternary oxides: x-ray absorption analysis, J. Solid State Chem. 180 (2007) 54–61. [12] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A32 (1976) 751–767. [13] T. Ohmichi, S. Fukushima, A. Madea, H. Watanabe, On the relation between lattice parameter and O/M ratio for uranium dioxide-trivalent rare earth oxide solid solution, J. Nucl. Mater. 102 (1981) 40–46. [14] C. Huntgen, J. Fuger, Self-irradiation effects in americium oxides, Inorg. Nucl. Chem. Lett. 13 (1977) 179–188. [15] T. Yamashita, N. Nitani, T. Tsuji, H. Inagaki, Thermal expansions of NpO2 and some other actinide dioxides, J. Nucl. Mater. 245 (1997) 72–78.
(1) The XANES spectra of the Am-LIII edge of (Am0.50U0.50)O2.0 and (Am0.50U0.50)O1.94 are in good agreement with that of AmO1.5, indicating that Am in (Am0.50U0.50)O2−x was in the trivalent state. On the other hand, the energy shift of a white-line peak between (Am0.50U0.50)O1.94 and UO2.0 was observed at about 2 eV, which is consistent with the chemical shift between U4+ and U5+. Thus, it can be concluded that U in (Am0.50U0.50)O2−x was in the pentavalent state. (2) The lattice parameter of (AmyU1−y)O2.0 was estimated from the (Am3+yU4+1−2yU5+y)O2 model (0 < y < 1). The lattice parameter functions of (AmyU1−y)O2.0 are represented by a0 = 0.3571 + 1.5016rc + 4.076rc2 (nm) and rc = 0.0996(1−2y) + 0.0880y + 0.1091y (nm), where, a0, rc, and y are the lattice parameter of (AmyU1−y)O2.0, mean ionic radius of the (Am3+yU4+1-2yU5+y)O2 model, and the Am content in (AmyU1−y)O2.0, respectively. (3) By using the Nakamura's model, the lattice parameter functions of AmO1.5–UO2 mixed oxide solid solution were obtained. Acknowledgments The authors would like to express their gratitude to Prof. K. Kobayashi, the Institute of Materials Structure Science of High Energy Accelerator Research Organization (KEK), and Dr. Y. Okamoto, Dr. M. Akabori, Dr. C. Suzuki, Dr. H. Shibata, and Dr. A. Nakamura of JAEA for their helpful advice. They also thank Mr. A. Itoh of Nuclear Engineering Co., Ltd. and Mr. M. Kamoshida of Chiyoda Maintenance Corporation for their kind support.
255
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Lattice parameter functions of (AmyU1−y)O2−x based on XRD and XANES measurements
MARK
⁎
Tsuyoshi Nishia, , Masami Nakadab, Masaru Hiratac a b c
Graduate school of Science and Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan Nuclear Science and Engineering Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan Advanced Fast Reactor Cycle System R & D Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1112, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: AmyU1−y)O2−x X-ray diffraction Lattice parameter functions Am content X-ray absorption near-edge structure spectrum
The lattice parameters of (Am0.50U0.50)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2−x were determined by powder X-ray diffraction with Cu Kα radiation. In addition, the lattice parameter functions of (AmyU1−y)O2−x (0.00 < x < 0.25, 0.00 < y < 0.50) were evaluated using models of (Am3+yU4+1−2yU5+y)O2 and (Am3+yU4+1−y) O2−y/2 based on the results of X-ray diffraction and the ionic radii of Am3+, U4+, and U5+. In order to confirm the valence state of Am and U in (AmyU1−y)O2−x, the X-ray absorption near-edge structure measurements were performed in the transmission mode at the Am-LIII and U-LIII absorption edges of (Am0.50U0.50)O2.0, (Am0.50U0.50)O2−x, and UO2.0.
1. Introduction The operation of commercial light-water reactors (LWRs) generates plutonium and minor actinides (MAs: neptunium, americium, curium) in the irradiated nuclear fuels. Among the MAs, Am has especially high and long-lasting radiotoxicity. To reduce the long-term radiotoxicity of high-level radioactive waste and to efficiently use repositories, one of the options in future nuclear fuel cycles is heterogeneous recycling of MAs using fast reactors (FRs). Among the many types of advanced fuels that contain MAs, fuels containing a mixed oxide (MOX) of uranium and americium, (AmyU1−y)O2−x, are one of the targets for the transmutation of Am in FRs [1,2]. Moreover, Zakari-Issoufou et al. investigated the feasibility of Am transmutation in MOX fuels of commercial pressurized-water reactors (PWRs) [3]. Because excess oxygen can oxidize the cladding tubes during irradiation, one of the major factors of effective transmutation of Am is the change in oxygen potential with regard to changes in the oxidation states of both uranium and americium. Therefore, the Am redox behavior in (AmyU1−y)O2−x can provide valuable design information for fuel development. In particular, the lattice parameter of (AmyU1−y)O2−x provides very useful information for the design of MOX fuels for commercial PWRs. Nakamura et al. suggested a lattice parameter model of fluorite oxide defects as a function of the ionic radius [4]. However, the lattice
parameter of (AmyU1−y)O2−x was not available owing to the lack of the experimental data and information on the valence state of Am and U in (AmyU1−y)O2−x. Recently, Nishi et al. reported X-ray absorption near-edge structure (XANES) measurements at the Am-LIII absorption edge of (Am0.05U0.95)O2.0 [5]. In addition, Prieur et al. studied the local structure and charge distribution in (AmyU1−y)O2−x [6]. Using the XANES spectra at the Am-LIII and U-LIII absorption edge of (AmyU1−y) O2−x, they suggested a value for the lattice parameter of (AmyU1−y)O2−x (−0.04 < x < 0.04, 0.00 < y < 0.09). They clarified that the estimation of this lattice parameter was not based on Vegard's law; however, no value was suggested for the lattice parameter of (AmyU1−y)O2−x with high Am content because the XANES data was limited to an Am atomic ratio of y < 0.20. In this work, the lattice parameters of (Am0.50U0.50)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2−x were determined by powder X-ray diffraction (XRD). In addition, the lattice parameter functions of (AmyU1−y)O2−x (0.00 < x < 0.25, 0.00 < y < 0.50) were evaluated using models of (Am3+yU4+1−2yU5+y)O2 and (Am3+yU4+1−y)O2−y/2 based on the results of X-ray diffraction and ionic radii of Am3+, U4+, and U5+. In order to confirm the valence state of Am and U in (AmyU1−y)O2−x, the XANES measurements were performed in the transmission mode at the Am-LIII and U-LIII absorption edges of (Am0.50U0.50)O2.0, (Am0.50U0.50)O2−x, and UO2.0.
Abbreviations: EXAFS, expanded X-ray absorption fine structure; KEK, High Energy Accelerator Research Organization; MOX, mixed oxide; XAFS, X-ray absorption fine structure; XANES, X-ray absorption near-edge structure; XRD, X-ray diffraction ⁎ Correspondence to: Department of Materials Science and Engineering, College of Engineering, Ibaraki University, Nakanarusawa 4-12-1, Hitachi, Ibaraki 316-8511, Japan. E-mail address: [email protected] (T. Nishi). http://dx.doi.org/10.1016/j.jssc.2017.09.011 Received 28 June 2017; Received in revised form 27 August 2017; Accepted 11 September 2017 Available online 19 September 2017 0022-4596/ © 2017 Elsevier Inc. All rights reserved.
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2. Experimental 2.1. Sample preparation The (AmyU1−y)O2−x powders for the XRD and X-ray absorption fine structure (XAFS) measurements were prepared as follows. The UO2 powder was first mixed with AmO2 powder (purity: 99.7%; nonradioactive impurities were virtually absent [7]) in various proportions, and then the different batches of mixed powders were pressed into green disks. The disks of (Am0.50U0.50)O2.0 and (Am0.37U0.63)O2.0 were heated at 1723 K for 4 h in a N2 gas flow and then at the same temperature for 4 h in a gas flow of N2 with 4% H2. In order to obtain the actinide dioxide powder, the disks were heated at 1173 K for 1 h in a gas flow of N2 with 10 ppm of O2. The preparation conditions were determined based on data on the oxygen potential [8]. It was revealed that the O/(U+Am) ratio of the (U0.95Am0.05)O2.0 powder was 2.0 ± 0.02. On the other hand, the (Am0.50U0.50)O2−x disks were heated at 1723 K for 4 h in a N2 gas flow. The (AmyU1−y)O2−x and graphite powder were stirred together and pressed into a disk measuring 6 mm in diameter. The graphite powder was used to dilute the pressed sample for the transmission XAFS measurements [9]. The disk was doubly sealed in two polyethylene terephthalate (PET) containers using epoxy resin (Stycast 1266, Henkel Loctite, Germany) for the XAFS measurements. 2.2. XRD and XANES measurements The (AmyU1−y)O2−x powder was examined by powder XRD analysis with Cu Kα radiation to confirm the presence of a single phase and to determine the lattice parameter. Since the powder consisted of a single face-centered cubic (fcc) phase, the lattice parameter was determined by the Nelson–Riley extrapolation function. XANES measurements at the U-LIII and Am-LIII absorption edges of (AmyU1−y)O2−x took place in the transmission mode at the hard X-ray station BL-27 in the Photon Factory of High Energy Accelerator Research Organization (KEK), Japan, at an energy level of 2.5 GeV. The intensities of the incident and transmitted beams were monitored in an ionization chamber under an Ar–N2 gas flow and an Ar gas flow, respectively. The radiation was monochromatized using a double-crystal Si(111) monochromator. The XANES spectra were recorded at 293 K. The energy shown in the XANES spectra was calibrated against the expanded XAFS (EXAFS) spectrum of (AmyU1−y)O2−x and the Zr K absorption edge of Zr foil (Kedge: 17.998 keV [10]). Based on the XAFS spectra for energy calibration, the uncertainty of the energy value of the white-line peak was ± 0.5 eV.
Fig. 1. XANES spectrum of the Am-LIII absorption edge of (Am0.50U0.50)O2.0, together with the XANES spectra of AmO2 and AmO1.5 [9].
absorption coefficient μ is represented by μt. Two peaks (A, B) and a tail peak (A′) can be seen in all three XANES spectra. Moreover, the XANES spectrum of the Am-LIII edge of (Am0.50U0.50)O2.0 is in good agreement with that of AmO1.5, demonstrating that Am in (Am0.50U0.50)O2.0 was in the trivalent state. The same results were also obtained for (Am0.50U0.50)O2−x and (Am0.05U0.95)O2.0 [5]. Fig. 2 shows the XANES spectra of the U-LIII absorption edge of UO2.0, (Am0.50U0.50)O2−x, BaUO3, and NaUO3 [11]. According to the electroneutrality rule, U in BaU4+O3 was in the tetravalent state, and U in NaU5+O3 was in the pentavalent state. The XANES spectrum of the U-LIII edge of UO2.0 is in good agreement with that of BaUO3, and the XANES spectrum of the U-LIII edge of (Am0.50U0.50)O2−x is in good agreement with that of NaUO3. In particular, the energy shift of the white-line peak between (Am0.50U0.50)O2−x and UO2.0 was observed at about 2 eV, which is consistent with the chemical shift between U4+ and U5+. Thus, it can be concluded that U in (Am0.50U0.50)O2−x was in the pentavalent state. The same results were obtained for (Am0.50U0.50) O2.0 and (Am0.05U0.95)O2.0 [5]. As shown in Figs. 1 and 2, Am in (AmyU1−y)O2−x was in the trivalent state, while U in (Am0.50U0.50)O2.0 and (Am0.50U0.50)O2−x was in the pentavalent state. Based on the electroneutrality rule, (AmyU1−y) O2−x was expected to consist of Am3+, U4+, and U5+. Nakamura et al. suggested the following lattice parameter model of fluorite oxide defects as a function of the ionic radius [4]. Assuming a random distribution of cations in the cation sub-lattice and anions in the anion sub-lattice, the coordination number (CN) of Am3+, U4+, and U5+ is 8 and that of O2− ion is 4. The ionic radius of Am3+, U4+, and U5+ at CN = 8 are summarized in Table 2. The ionic radii were obtained from values reported in the literature [12,13] and the lattice parameters of AmO1.5 [14] and UO2 [15]. The lattice parameter of (AmyU1−y)O2.0 was estimated from the (Am3+yU4+1−2yU5+y)O2 model suggested by Nakamura et al. (0 < y < 1) [4]. The lattice parameter functions of (AmyU1−y)O2.0 are represented by the following equations:
3. Results and discussion The measured lattice parameters of (AmyU1−y)O2−x and the UO2.0 powder are summarized in Table 1. Since the lattice parameter of (Am0.50U0.50)O2−x was larger than that of (Am0.50U0.50)O2.0, the (Am0.50U0.50)O2−x powder was determined to have hypo-stoichiometric composition. Fig. 1 shows the XANES spectra of the Am-LIII absorption edge of (Am0.50U0.50)O2.0, Am4+O2, and Am3+O1.5 [9]. The normalized intensity of an X-ray beam passing through a material of thickness t with the Table 1 Measured lattice parameters of (AmyU1−y)O2−x and UO2.0. Lattice parameter (nm) (Am0.50U0.50)O2.0 (Am0.37U0.63)O2.0 (Am0.50U0.50)O2−x UO2.0
0.5446 0.5452 0.5465 0.5470
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Fig. 3. Lattice parameter of (AmyU1−y)O2.0 as a function of the Am content, obtained using the (Am3+yU4+1−2yU5+y)O2 model together with the lattice parameter of (Am0.50U0.50)O2.0 reported by Mayer et al. [8].
fC = 0.40693 + 0.03741rc + 14.7973rc 2(nm),
(5)
rC(ss) = (1 − y )rU 4+ + yrAm 3+,
(6)
rU 4+ = 0.0996 + 0.00520[(8 − 2 y ) − 8] − 3.397 × 10−12[(8 − 2 y ) − 8]2 , (7)
rAm
Table 2 Ionic radii of U4+, U5+, and Am3+.
U U5+ Am3+
Ionic radius (nm)
Ref.
0.0996 0.0880 0.1091
[12] [13] [12]
a 0 = 0.3571 + 1.5016rc + 4.076rc 2(nm),
(1)
rc = 0.0996(1 − 2y ) + 0.0880 y + 0.1091 y(nm),
(2)
where a0, rc, and y are the lattice parameter of (AmyU1−y)O2.0, mean ionic radius of the (Am3+yU4+1−2yU5+y)O2 model, and Am content in (AmyU1−y)O2.0, respectively. Fig. 3 shows the lattice parameter of (AmyU1−y)O2.0 as a function of the Am content in the (Am3+yU4+1−2yU5+y)O2 model. It can be seen that the lattice parameters of (Am0.05U0.95)O2.0 (Am0.05U0.95)O2.0, (Am0.37U0.63)O2.0, and (Am0.50U0.50)O2.0 agree with values obtained from Eqs. (1) and (2). According to these two equations, the lattice parameter of (Am0.50U0.50)O2.0 was 0.5447 nm. For comparison, Mayer et al. it reported a value of 0.5452 nm for the lattice parameter of (Am0.50U0.50)O2.0 [8]. Thus, the results of the lattice parameters also suggest that the (Am0.50U0.50)O2.0 solid solution sample had a stoichiometric composition. Next, based on the model proposed by Nakamura et al. [4], the lattice parameter functions of the AmO1.5–UO2 mixed-oxide solid solution (ss) can be represented by the following equations:
a 0(ss) = (1 − y )fF + yfC , fF = 0.3571 + 1.5016rc + 4.076rc 2(nm),
2
= 0.1091 + 0.00307[(8 − 2 y ) − 8] − 0.00134[(8 − 2y ) − 8] ,
(8)
where a0(ss) is the lattice parameter of the AmO1.5–UO2 mixed-oxide solid solution, fF is the lattice parameter of UO2.0, fC is the lattice parameter of AmO1.5, rC(ss) is the mean ionic radius of the AmO1.5– UO2 mixed-oxide solid solution, rU4+ is the mean ionic radius of U4+ from the fitting equation, and rAm3+ is the mean ionic radius of Am3+ from the fitting equation. Using Eqs. (3)–(8), the lattice parameter of (Am0.50U0.50)O1.75 was calculated to be 0.5521 nm. Fig. 4 shows the lattice parameter of (AmyU1−y)O2−x as a function of the Am content, obtained using the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models together with those of (Am0.10U0.90)O2.00, (Am0.15U0.85)O2.00, and (Am0.20U0.80)O1.99 [5]. It can be seen that the lattice parameters of (Am0.10U0.90)O2.00, (Am0.15U0.85)O2.00, and (Am0.20U0.80)O1.99 were almost the same as the values of the lattice parameter functions of (AmyU1−y)O2−x based on the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models. Using the lattice parameter functions, the lattice parameter of (Am0.50U0.50)O2.0 was calculated to be 0.5447 nm. Using the lattice parameters of (Am0.50U0.50)O2.0 and (Am0.50U0.50)O1.75, the O/(Am+U) ratio of (Am0.50U0.50)O2−x was presumed to be 1.94.
Fig. 2. XANES spectra of the U-LIII absorption edge of UO2.0 and (Am0.50U0.50)O2−x, together with the XANES spectra of BaUO3 and NaUO3 [11].
4+
3+
(3) Fig. 4. Lattice parameter of (AmyU1−y)O2−x as a function of the Am content, obtained using the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models together with the lattice parameters of (Am0.10U0.90)O2.00, (Am0.15U0.85)O2.00, and (Am0.20U0.80)O1.99 [6].
(4) 254
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4. Conclusions
References
The lattice parameter functions of (AmyU1−y)O2−x (0.00 < x < 0.25, 0.00 < y < 0.50) were evaluated using the (Am3+yU4+1−2yU5+y)O2 and (Am3+yU5+1−y)O2−y/2 models based on the analysis results of the X-ray diffraction and XANES measurements. The conclusions of this study are as follows:
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(1) The XANES spectra of the Am-LIII edge of (Am0.50U0.50)O2.0 and (Am0.50U0.50)O1.94 are in good agreement with that of AmO1.5, indicating that Am in (Am0.50U0.50)O2−x was in the trivalent state. On the other hand, the energy shift of a white-line peak between (Am0.50U0.50)O1.94 and UO2.0 was observed at about 2 eV, which is consistent with the chemical shift between U4+ and U5+. Thus, it can be concluded that U in (Am0.50U0.50)O2−x was in the pentavalent state. (2) The lattice parameter of (AmyU1−y)O2.0 was estimated from the (Am3+yU4+1−2yU5+y)O2 model (0 < y < 1). The lattice parameter functions of (AmyU1−y)O2.0 are represented by a0 = 0.3571 + 1.5016rc + 4.076rc2 (nm) and rc = 0.0996(1−2y) + 0.0880y + 0.1091y (nm), where, a0, rc, and y are the lattice parameter of (AmyU1−y)O2.0, mean ionic radius of the (Am3+yU4+1-2yU5+y)O2 model, and the Am content in (AmyU1−y)O2.0, respectively. (3) By using the Nakamura's model, the lattice parameter functions of AmO1.5–UO2 mixed oxide solid solution were obtained. Acknowledgments The authors would like to express their gratitude to Prof. K. Kobayashi, the Institute of Materials Structure Science of High Energy Accelerator Research Organization (KEK), and Dr. Y. Okamoto, Dr. M. Akabori, Dr. C. Suzuki, Dr. H. Shibata, and Dr. A. Nakamura of JAEA for their helpful advice. They also thank Mr. A. Itoh of Nuclear Engineering Co., Ltd. and Mr. M. Kamoshida of Chiyoda Maintenance Corporation for their kind support.
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