Article pubs.acs.org/IC
Insight into the Am−O Phase Equilibria: A Thermodynamic Study Coupling High-Temperature XRD and CALPHAD Modeling Enrica Epifano,*,† Christine Guéneau,‡ Renaud C. Belin,§ Romain Vauchy,† Florent Lebreton,† Jean-Christophe Richaud,∥ Alexis Joly,† Christophe Valot,† and Philippe M. Martin† †
CEA, Nuclear Energy Division, Research Department on Mining and Fuel Recycling Processes, SFMA, LCC, F-30207 Bagnols-sur-Cèze, France ‡ DENService de la Corrosion et du Comportement des Matériaux dans leur Environnement (SCCME), CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France § CEA, DEN, DEC, SESC, LLCC, Cadarache, 13108 Saint-Paul-Lez-Durance, France ∥ CEA, DEN, DEC, SA3C, LAMIR, Cadarache, 13108 Saint-Paul-Lez-Durance, France ABSTRACT: In the frame of minor actinide transmutation, americium can be diluted in UO2 and (U, Pu)O2 fuels burned in fast neutron reactors. The first mandatory step to foresee the influence of Am on the in-reactor behavior of transmutation targets or fuel is to have fundamental knowledge of the Am−O binary system and, in particular, of the AmO2−x phase. In this study, we coupled HT-XRD (high-temperature X-ray diffraction) experiments with CALPHAD thermodynamic modeling to provide new insights into the structural properties and phase equilibria in the AmO2−x−AmO1.61+x− Am2O3 domain. Because of this approach, we were able for the first time to assess the relationships between temperature, lattice parameter, and hypostoichiometry for fcc AmO2−x. We showed the presence of a hyperstoichiometric existence domain for the bcc AmO1.61+x phase and the absence of a miscibility gap in the fcc AmO2−x phase, contrary to previous representations of the phase diagram. Finally, with the new experimental data, a new CALPHAD thermodynamic model of the Am−O system was developed, and an improved version of the phase diagram is presented.
1. INTRODUCTION Minor actinides (MAs) like Am, Np, and Cm significantly contribute to the long-term radiotoxicity of spent nuclear fuel. One of the options considered for reducing nuclear waste inventory is the transmutation of these elements in fast neutron reactors (FNRs). The development of advanced fuels containing MAs implies the need to determine their structural and thermodynamic properties. In the transmutation research community, attention is mainly focused on mixed actinide dioxides, e.g., (U, Am)O2 or (U, Pu, Am)O2.1−3 The assessment of the thermodynamic behavior of ternary and multielement systems such as U−Am−O and U−Am−Pu−O is not possible without a thorough knowledge of each binary. Among the respective binaries, very few experimental data concerning the Am−O system are available in the literature. This is due to the scarcity of pure Am and to its high radioactivity, both of which have limited experimental investigations. For this reason, even if various versions were proposed in the literature, the Am−O phase diagram is not completely determined yet. In this study, the data available for americium oxides are critically revised, with the most important uncertainties underlined. More specifically, the starting point of our work is the latest thermodynamic model of the Am−O system, proposed by Gotcu-Freis et al.3 using the CALPHAD (calculation of phase diagrams) method4 and integrated in the © 2017 American Chemical Society
Thermodynamics of Advanced Fuel International Database (TAF-ID).5 The model predictions were used to determine the most appropriate temperature and atmospheres to partially fill the gap in structural and phase-diagram data using in situ hightemperature X-ray diffraction (HT-XRD) to answer some open questions concerning the binary phase diagram. The agreement between experimental results and computations is discussed. In particular, some important discrepancies are highlighted. Finally, because of the new experimental data acquired in this work, a new, improved CALPHAD assessment was performed, and the resulting phase diagram is proposed here.
2. Am−O SYSTEM: STATE OF THE ART First attempts to define the phase diagram of the Am−O system date back to the 1960s.6,7 Various representations were then proposed in the literature, without being completely satisfying. Uncertainties are due to the lack of, and in some cases discrepancies between, experimental data. In this study, we focus on the domain with the oxygen/americium ratio (O/ Am) ranging from 1.5 to 2, which is the most interesting for nuclear fuel applications. Crystallographic, thermodynamic, and Received: March 9, 2017 Published: June 16, 2017 7416
DOI: 10.1021/acs.inorgchem.7b00572 Inorg. Chem. 2017, 56, 7416−7432
Article
Inorganic Chemistry Table 1. Structures, O/Am Ratios, and Lattice Parameters at Room Temperature for Americium Oxidesa structure
O/Am
AmO2−x
phase
Fm3̅m (225)
C-AmO1.61+x
Ia3̅ (206)
2 2 2 1.96 2 2 ∼2 1.646 1.633 1.616
C-AmO1.5+x
Ia3̅ (206) 1.513
a
A-Am2O3
P3̅m1 (164)
B-Am2O3
C2/m (12)
1.5 1.5 1.5
lattice parameters [Å] a a a a a a a a a a a a a a a a a a a
= = = = = = = = = = = = = = = = = = =
5.377(3) 5.3772(4) 5.3724(4) 5.3897(2) 5.373(2) 5.375(1) 5.376(1) 10.966(5) 10.92(2) 10.97(2) 11.03(1) 11.013 11.023 11.02 b = 3.805 b = 3.815 b = 3.810 b = 3.821 14.38, b = 3.52
c c c c c
= = = = =
5.96 5.975 5.957 5.984 8.92
comments
ref
purity unknown purity unknown 1.7 domain. Because of the sluggishness of DTA measurements for O/ Am < 1.6, only ceramographic analyses were performed on the more hypostoichiometric samples, allowing an estimate for the homogeneity of the microstructure. The oxides annealed below 573 K were single-phased for O/Am ratios between 1.5 and 1.56, whereas two phases were found for an O/Am ratio equal 7418
DOI: 10.1021/acs.inorgchem.7b00572 Inorg. Chem. 2017, 56, 7416−7432
Article
Inorganic Chemistry
domain of the phase diagram established in the 1.5 < O/Am < 2.0 range. Furthermore, the lack of thermodynamic data for the C-type phases made a complete assessment of all the phases reported in the previous phase diagrams impossible, and some simplifications were necessary. In the 1.5 < O/Am < 2 range, three phases (instead of four) were included: the fluorite-type americium dioxide (AmO2−x), the intermediate bcc phase (CAmO1.61+x, named AmO1.62 by Gotcu-Freis et al.3), and the hexagonal sesquioxide (A-Am2O3). Moreover, the intermediate AmO1.61+x was modeled as a stoichiometric compound. The miscibility gap in the AmO2−x region suggested by Sari and Zamorani23 was included in the assessment. In the AmO2−x region at high temperature, a choice between the oxygen potential measurements reported in the literature was necessary. The data of Chikalla and Eyring7 were selected, but only for compositions and temperatures outside the miscibility gap. 2.4. Open Questions. From this review, it is clear that several important data are missing for Am oxides, and the phase equilibria remain challenging to assess. For instance, the phase boundaries between the two sesquioxides A-Am2O3 and CAmO1.5+x are not defined because very few data are available for the cubic form. The limits of the existence domain (T and O/ Am ratio ranges) of the intermediate C-AmO1.61+x phase are not identified. A lot of uncertainties also exist for the AmO2−x phase. In fact, while it is known that a hypostoichiometric region exists at high temperature, the relative phase boundaries are not clearly identified. Moreover, no crystallographic data are available for hypostoichiometric AmO2−x at high temperature. A miscibility gap for O/Am > 1.7 and 1100 < T < 1300 K was introduced by Sari and Zamorani23 on the basis of DTA measurements, but no further confirmation was reported. Finally, none of the oxygen potential measurements are fully consistent with the data of Sari and Zamorani,23 and no XRD observation of the two fluorite structures was reported in the literature. Therefore, the existence of a miscibility gap is controversial. The experimental investigation in the present work aims at answering these open questions. HT-XRD has proven to be very useful for this purpose in several studies.10,27−33
to 1.63. Finally, the samples quenched from temperatures above 573 K exhibited two phases for O/Am < 1.62 and only one phase for O/Am = 1.638. Nevertheless, at these low temperatures, whether the equilibrium was reached or not is questionable. On the basis of these data, the first Am−O phase diagram was proposed by Sari and Zamorani.23 In this representation, the americium dioxide is stoichiometric at room temperature, and it has a large hypostoichiometric domain for T > 1300 K. The lower O/Am ratio limit of this region is not well-defined, and it was estimated to be around 1.62. AmO2−x exhibits a miscibility gap for 1.7 ≤ O/Am ≤ 1.94, with a critical temperature around 1300 K. For T > 570 K, the intermediate C-AmO1.61+x phase has a narrow existence domain for 1.62 < O/Am < 1.67; the upper temperature limit of this phase is not defined. The C-AmO1.5+x is considered as the stable sesquioxide form at room temperature. An existence domain with maximum O/Am = 1.59 is attributed to this phase. Finally, the stoichiometric phase A-Am2O3 appears at T > 500 K. The Am−O phase diagram proposed by Sari and Zamorani23 was later updated by Thiriet and Konings.26 Two main modifications were introduced. First, the two-phase region between the intermediate C-AmO1.61+x phase and the two sesquioxides was redefined, with respect to the phase rule. Second, the representation was extended to higher temperatures, up to the liquid region, taking into account the melting temperature measurements performed on A-Am2O3 and AmO2−x.25 The values are 2481 ± 15 K for A-Am2O3 and 2386 K for AmO2−x (the O/Am ratio reached during the melting process is unknown). Recently, Gotcu-Freis et al. presented an Am−O phase diagram based on a CALPHAD assessment,3 which is reported in Figure 1, together with the phase-diagram data previously
3. HT-XRD EXPERIMENTS 3.1. Materials. Two different batches of AmO2−x powder were used for the HT-XRD investigations, here named AmO2A and AmO2-B. The latter was only used for the isothermal XRD measurements shown in Section 3.5.3. The isotopic and chemical characteristics of the two powders are reported in Table 4. The major americium isotope in both batches is 241Am, which is characterized by a high α activity (1.3 × 1011 Bq g−1). For this reason, only a few dozen milligrams of material could have been used for the experiments, and the handling was always performed in shielded gloveboxes. Table 4 shows that several percents of impurities are present in both batches: Np from α-decay, mostly Ce for AmO2-A, and Pu for AmO2-B. As already mentioned, the influence of impurities is hard to predict. 3.2. Experimental XRD Setup and Sample Preparation. XRD measurements were performed at the LEFCA facility of CEA/Cadarache with a Bragg−Brentano θ−θ Bruker D8 Advance X-ray diffractometer using copper radiation from a conventional tube source (Kα1 + Kα2 radiation: λ = 1.5406 and 1.5444 Å). The device, implemented in a glovebox dedicated to handling nuclear materials, was described in detail in a previous
Figure 1. Calculated phase diagram of Gotcu-Freis et al. and experimental data. (Reprinted in part with permission from ref 3. Copyright 2011 Elsevier.)
discussed. The CALPHAD method involves the Gibbs energy modeling of each phase, by evaluating all the available experimental data (structure, phase-diagram data, heats of formation, heat increments, oxygen potentials, etc.). Therefore, all the phases (solid, liquid, and gas) were modeled, and all the data discussed above were taken into account by Gotcu-Freis et al.3 in the CALPHAD assessment. However, since almost no data were available for the high-temperature domains of the liquid and gas phases, the model mostly focused on the solid 7419
DOI: 10.1021/acs.inorgchem.7b00572 Inorg. Chem. 2017, 56, 7416−7432
Article
Inorganic Chemistry
of fit. When more than one phase was detected, a Rietveld refinement38 was performed to retrieve the fraction of each phase. 3.4. Choice of Experimental Conditions. Thermodynamic computations were performed with Thermo-Calc software, using the Am−O CALPHAD model of Gotcu-Freis et al.3 Calculations were set according to two different strategies. The first was selecting a point in the phase diagram, which fixes the temperature T and composition O/Am, and then computing the corresponding oxygen potential of the solid phase [ΔG̅ O2(s)]; at equilibrium, this latter value must coincide with the oxygen potential of the surrounding atmosphere [ΔG̅ O2(g)], and therefore one can derive the oxygen partial pressure pO2 using the relation
Table 4. Isotopy and Chemical Impurities of the Two AmO2 Powders Used in This Work AmO2-A technique 241
Am Am 243 Am 242
Pu Np Na Fe Ce Nd
237
a
AmO2-B value
technique
Isotopic Composition (atom %) TIMS 98.84(1) TIMS TIMS
