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Assessment of Te as a U-Zr fuel additive to mitigate fuel-cladding chemical interactions
Accepted Manuscript Assessment of Te as a U Zr fuel additive to mitigate fuel-cladding chemical interactions Yi Xie, Jinsuo Zhang, Michael T. Benson, James A. King, Robert D. Mariani PII:
S0022-3115(18)30636-6
DOI:
https://doi.org/10.1016/j.jnucmat.2018.10.050
Reference:
NUMA 51289
To appear in:
Journal of Nuclear Materials
Received Date: 5 May 2018 Revised Date:
31 October 2018
Accepted Date: 31 October 2018
Please cite this article as: Y. Xie, J. Zhang, M.T. Benson, J.A. King, R.D. Mariani, Assessment of Te as a U Zr fuel additive to mitigate fuel-cladding chemical interactions, Journal of Nuclear Materials (2018), doi: https://doi.org/10.1016/j.jnucmat.2018.10.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Assessment of Te as a U-Zr Fuel Additive to Mitigate Fuel-Cladding Chemical Interactions Yi Xiea,b, Jinsuo Zhanga*, Michael T. Bensonb, James A. Kingb, Robert D. Marianib
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a. Nuclear Engineering Program, Mechanical Engineering Department, Virginia Tech, Blacksburg, VA 24060, USA b. Idaho National Laboratory, P.O. Box 1625, MS 6188, Idaho Falls, ID 83415, USA
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*Corresponding author, Email: [email protected], Phone: (540) 231-1198, Postal address: 635 Prices Fork Rd, Blacksburg, VA 24060, USA.
Abstract
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Tellurium was investigated as a potential additive to metallic fuel (e.g. U-10Zr) to mitigate fuel-cladding chemical interaction (FCCI). A primary cause of FCCI is the lanthanide fission products migrating to the fuel periphery and interacting with cladding, Te therefore was used to bind the lanthanides into stable compounds in the U-10Zr. The present work investigates the microstructures of as-cast and annealed U-10Zr-4Te and U-10Zr-4Te-4Ce alloys (weight percent) with scanning electron microscope and transmission electron microscopy. Tellurium was found to bind Ce and form CeTe compound, which will mitigate lanthanides migration to the cladding.
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Keywords: Fuel additive; Te; Metallic fuel; FCCI
1. Introduction
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Lanthanide fission products are recognized as one of the key causes of fuel-cladding chemical interaction (FCCI) in metallic fuels during operation [1]. They can migrate toward fuel surface and react with the cladding to form interaction layers, the phenomenon is called lanthanide-induced FCCI. To mitigate FCCIs, barriers on the inner surface of cladding have been broadly investigated [2] [3] [4]. One of the other methods is to use fuel additives in the fuel alloy to immobilize lanthanide fission products [5].
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The potential fuel additives, Pd [5], In [6], Sn [7], Sb [8] and the mixture of Sn and Sb [9], have been investigated through casting with the U-Zr fuel. In the fuel-additive systems, additiveZr-bearing compounds were found, such as PdZr2 and U(Pd,Zr) in U-15Zr-4Pd alloy [5], Sn3Zr5 and Sn4Zr5 in U-10Zr-4Sn alloy [7], SbZr2 in U-10Zr-4Sb alloy [8], and Zr5(Sn,Sb)3 in U-10Zr2Sn-2Sb alloy [9]. Upon addition of lanthanides, the additives are leased from the additive-Zrbearing compounds to form stable additive-lanthanide compounds, such as PdLn and Ln-rich PdLn in U-15Zr-2Pd-2Ln and U-15Zr-4Pd-4Ln alloys [5], Sn3Ln5 in U-10Zr-4Sn-4Ln alloy [7], SbCe2 and Sb3Ce4 in U-10Zr-4Sb-4Ce alloy [8], and Ln5(Sn,Sb)4 in U-10Zr-2Sn-2Sb-4Ln alloy [9] (where Ln = 53Nd-25Ce-16Pr-6La wt. %). Other than binding lanthanides, the redundant fuel additives are present in the fuel matrix in the form of small (< 1 µm) precipitates. Most of these small precipitates are in the fuel matrix region with a higher Zr content.
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The enthalpies of formation of In-lanthanide compounds, such as In3Ce, are more positive than the other additive-lanthanide compounds [8], which indicates that In-lanthanide compounds have lower thermodynamic stability than the other additive-lanthanide compounds. The thermodynamics of Sn-lanthanide and Sb-lanthanide compounds are similar; however, in the UZr alloys with Sn and Sb, the additive-lanthanide compounds contain a much higher percentage of Sb than Sn, indicating a higher thermodynamic stability of Sb-lanthanide than Sn-lanthanide [9]. The study shows that though the known thermodynamic stability between Sn- and Sblanthanides should be roughly equivalent, experimentally that is not the case.
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In the U-Zr alloys with Pd, In, Sn, Sb or (Sn+Sb) and lanthanide, a small amount of Zr has been found in the Pd/In/Sn/(Sn+Sb)-Ln compounds, but not found in the SbCe2 or Sb3Ce4 compounds. Zr-precipitates have been found in the U-Zr alloys with Pd, In, or Sb [5] [6] [8], but not found in those with Sn or (Sn+Sb) [7] [9]. The formation of a more stable additive-Zr compound is not advantageous to the fuel performance. Additives may consume a non-negligible amount of Zr from the fuel matrix, resulting in the decrease of solidus temperature. The proportions of lanthanides bound to Sb are in the order of Ce/La > Nd >> Pr [9]; while the proportions of lanthanides bound to Pd are in the order of Pr/La > Nd >> Ce [5]. Therefore, multiple additives may be needed to immobilize the most prevalent lanthanide fission products. Multiple additives will lead to a combined effect of immobilizing lanthanides, which is expected to be more effective than a single additive.
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Tellurium has also been known as a promising fuel additive [5]. Enthalpies of formation of Te-lanthanide compounds [10] are much more negative than the other additive-lanthanide compounds. Although a more negative entropy of formation can produce a more negative free energy of formation (∆G = ∆H-T∆S), to our knowledge, most of the entropy and free energy of formation data have not been reported. Based on the limited data, such as in Refs. [11] [12] [13], the absolute value of entropy of formation is very small, and the change in free energy of formation follows the same trend in enthalpy of formation, so the enthalpy data can still be used to estimate the thermodynamic stability. Enthalpies of formation of lanthanide-additive compounds are listed in Table 1. Tellurium and Sb have more negative enthalpies of formation than Pd and Sn. Tellurium also shows more negative enthalpies of formation than Sb, except for the case of La. Te-Ln compounds also have high melting temperatures, for example, NdTe at 2296 K [14], CeTe at 2093 K [15], LaTe at 1993 K [16], and PrTe at 2223 K [17].
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Table 1 Enthalpies of formation (kJ/g-atom) of lanthanide-additive intermetallic compounds. Pd (~ Sn Sb Te 873 K) (298 K) (298 K) (298 K) N -83 -106 -111 -67 [18] d [13] [19] [10] C -86 -127 -138 -64 [18] e [13] [20] [10] L -90 -131 -124 -56 [18] a [13] [21] [10] P -86 -109 -123 -63 [18] r [13] [19] [10]
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2. Experiment
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The present study is the microstructural characterization of as-cast and annealed alloys U10Zr-4Te and U-10Zr-4Te-4Ce. Microstructure of these alloys was investigated, with a focus on the Te-Zr and Te-Ce compounds. These are the initial data needed in determining the efficacy of applying Te as a fuel additive to mitigate FCCI by binding the lanthanide fission products.
Two alloys were made for the microstructure analysis. The cast ingredients are listed as below. U-10Zr-4Te alloy: 71.5U-21.8Zr-6.7Te (at. %) U-10Zr-4Te-4Ce alloy: 66.6U-21.3Zr-6.5Te-5.5Ce (at. %)
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All materials, except U, were obtained from Alfa Aesar and used as received. Cerium was obtained as rods, packaged in mylar under argon. Uranium was cleaned by submersion in nitric acid, followed by a water wash, and then an ethanol wash. All casting operations were carried out in an arc-melter within an argon atmosphere glovebox with high purity argon as a cover gas. After each addition step, the resulting button was flipped and re-melted three times to ensure homogeneity. To prepare the U-10Zr-4Te alloy, appropriate amount of Te, Zr and U were arcmelted together in two steps. A button of U-Zr was prepared, followed by the addition of Te. To prepare the U-10Zr-4Te-4Ce alloy, the appropriate amount of the Ce was added to a pre-alloy button of U-Zr-Te, prepared as described for the U-10Zr-4Te. The buttons were cast into 5 mm diameter pins. Due to the low boiling point of Te, boiling was observed when adding Te and when dropping the pins. The amount of Te loss because of boiling was not recorded.
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Approximately 4 mm from each pin was cut for annealing. The samples were wrapped with Ta foil and placed in a furnace in the glovebox at 923 K for 500 hours. After the heat treatment, the samples were cooled in flowing argon gas, and then cut to expose a fresh surface for analysis.
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Scanning electron microscope (SEM) analysis was performed on a section of pin from each alloy. The samples were mounted in a 31.8 mm diameter phenolic metallographic (met) mount filled with epoxy. They were polished by grinding the surfaces flat with SiC grinding paper followed by polishing with polycrystalline diamond suspensions, starting with 9 µm, then 3 µm, and finally 1 µm. The surface was polished with 3 µm and 1 µm suspensions again to remove the oxide film immediately before the microstructure analysis. The environmental SEM (model: FEI Quanta 600 FEG) was used, equipped with a Bruker energy dispersive X-ray spectrometer (EDS). The EDS was controlled by Esprit 1.9 software for image acquisition and data quantification. The measurement uncertainty of EDS is within a few percent of the measured value. A number of points for each investigated phase were measured to demonstrate the consistency of EDS measurement. The SEM was operated at an accelerating voltage of 20 kV and spot size 5. Under this voltage, the EDS spatial resolution was 1-1.5 µm. Spectra were collected over the energy range 0-10 keV. Transmission electron microscopy (TEM) analysis was carried out to identify the compound of local position. The TEM sample was prepared using the ion milling method. First, a 3 mm diameter by 1 mm thick disc was cut from the alloy bulk with a slow speed saw, and then the disc was ground and polished, both top and bottom surfaces, to 50 µm thick with SiC grinding papers followed by polishing with polycrystalline diamond suspensions. The damage caused by 3
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mechanical polishing was carefully avoided by removing the appropriate depth of material with each sandpaper. The disc was finally thinned by ion milling and polishing system (model: Fischione 1010 Ion Mill) with ion milling top and bottom surfaces till the disc center was transparent to transmitted (chamber) light. At this point, the periphery around the disc center/hole was considered thin enough for TEM analysis.
3. Results and discussion
3.1 U-10Zr-4Te
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When casting these alloy compositions, there was obvious loss of Te, primarily when casting the 5 mm pin, due to the low melting temperature of Te (723 K). In order to have the alloy drop into the mold in one charge to form a continuous pin, superheat is necessary. Boiling occurred in the button surface during this process, and therefore the exact composition is not known. The loss is not expected to be enough to change the microstructure present, though there will be less precipitates for analysis. The low melting temperature is also a consideration during irradiation, since the melting temperature is lower than the operating fuel temperature (typically around 923 K) in a reactor. For this reason, keeping Te bound with a higher melting intermetallic is necessary. Ideally, in a fresh fuel, Te will bind Zr, and after fission products burn-in, Te will bind lanthanides. Some Te-Zr compounds also have low melting temperatures, for example, the melting temperature of ZrTe5, on the Te-rich side is 822 K. Since Te will be a minor additive, with a much higher abundance of Zr present, the compounds will be on the Zr-rich side of Zr-Te phase diagram, such as Zr3Te and Zr5Te4, the solidus temperatures are 1616 K and 1627 K, respectively. Therefore, the low melting temperature of Te-rich Zr-Te compounds should not be an issue.
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Figure 1 is the overview of as-cast U-10Zr-4Te. The precipitates are round or sub-round, while in the previous study on U-Zr-Sn/Sb/(Sn+Sb) alloys, the precipitates were dendritic, hexagonal or triangular [7] [8] [9]. The precipitates are generally less than 50 µm in diameter (the largest distance between any two ends of a precipitate), with a small number of larger precipitates in hundreds of microns. The large number of precipitates indicates a reasonable amount of Te remaining in the alloy after casting.
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The precipitates with diameter smaller than 10 µm, shown in Figure 2, are homogeneous. The precipitates with diameter larger than 10 µm are speckled, with U- and Zr-precipitates trapped. Uranium and Zr are white and black in the SEM-BSE image, respectively. The Zrprecipitates are in the microstructures of line, bulk and ring, while U, which may include some Zr, is present as small round inclusions. The compositions of these inclusions were not quantified due to the spatial resolution of SEM-EDS. The magnification of the box in Figure 2a, shown in Figure 4, is the microstructure of line Zr-precipitates. There is an abundance of nano-scale U-rich white inclusions dispersed in the precipitate. The non-spherical precipitates examined in the previous studies on the U-Zr-Sn/Sb/(Sn+Sb) systems did not contain these inclusions. The spherical precipitates tend to trap matrix material, as has been observed in the spherical additivelanthanide precipitates [5] [7] [8] [9]. The small homogeneous precipitates shown in Figure 2a (points 4-6), with EDS data listed in Table 2, contain a lower content of Zr and U than the larger precipitate (points 1-3). The inclusions (U, Zr, and U-Zr) dispersed in the precipitates raise the 4
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measured concentration of Zr and U. The representative EDS spectra of elements U, Zr and Te for points 1-7 are shown in Figure 3. The main energy peaks of U, Zr and Te used for the quantification are Mα (~ 3.2 keV), Lα (~ 2.0 keV), and Lα (~ 3.8 keV), respectively, well separated and resolved by EDS.
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Figure 1 Low magnification SEM backscattered electron (BSE) image of the as-cast U-10Zr-4Te.
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Figure 2 SEM-BSE images of precipitates in the as-cast U-10Zr-4Te. Corresponding EDS data are listed in Table 2, with EDS spectra shown in Figure 3.
Table 2 EDS data1 for the points shown in Figure 2a (at. %). Z T U r e 6 2 1 5.2 3.1 1.7 6 2 1 1
The measurement uncertainty of EDS is within a few percent of the measured value, applicable to Table 2-10. 5
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Figure 3 Representative EDS spectra of elements U, Zr and Te for points 1-7 shown in Figure 2a. Main energy peaks of each element are separated.
Figure 4 High magnification SEM-BSE image of the white box in Figure 2a, showing the line Zr-precipitates with U-rich inclusions.
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In the annealed U-10Zr-4Te, the content of U in the small homogeneous precipitates, shown in Figure 5a and Table 3, is relatively reduced compared with the as-cast sample. The reduction is attributed to U migrating out of the precipitates during annealing. The remaining U measured in the precipitates could be from surrounding U being included in the EDS measurement, or could be U substitution of atoms in the precipitates. In addition, the contents of Zr and U in the larger speckled precipitates do not reduce after annealing. The Zr-precipitates and U-rich inclusions, shown in Figure 5b, are still trapped, so they contribute to the measured concentration of Zr and U.
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Figure 5 (a) Low magnification SEM-BSE image of the annealed U-10Zr-4Te. (b) Magnification of precipitates. Corresponding EDS data are listed in Table 3.
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Table 3 EDS data for the points shown in Figure 5a (at. %). Z T U r e 6 2 1 4.8 4.8 0.4 6 2 1 4.0 5.9 0.1 6 2 1 4.5 5.3 0.2 6 2 8 5.7 6.0 .3 6 3 5 3.6 1.4 .0 6 3 3 4.2 2.7 .1 6 2 8 3.2 8.8 .0
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The precipitates in the as-cast U-10Zr-4Te were further analyzed with TEM to identify the phase, shown in Figure 6, with EDS data listed in Table 4. The two round precipitates are homogeneous, without U- or Zr-inclusions. The EDS spectrum, shown in Figure 6f, indicates the energy peaks of U, Zr and Te are well separated. A number of points were randomly selected and the compositions were measured, consistent to be ~ 52 Zr, 26 Te, and 22 U (at. %). Three selected area electron diffraction (SAED) patterns for the precipitate shown in Figure 6a were carried out at different tilt conditions, with the resulting patterns shown in Figure 7. The diffraction patterns cannot be indexed by Te-Zr, Te-U, or U-Zr binary phase. The precipitate is likely to be a Te-Zr-U ternary phase; however, a Te-Zr-U ternary phase and its crystal structure are not available in literature. It is difficult to identify the Te-Zr-U phase based on the current information. Identifying the crystal structure of the phase is not the scope of this paper, but this issue is pointed out that needs further investigation.
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Figure 6 (a-b) STEM bright field images of precipitates in the as-cast U-10Zr-4Te. (c-e) Elemental mappings of Te, Zr and U, respectively, corresponding to the box in (a). (f) Representative EDS spectrum for an EDS data. Corresponding EDS data are listed in Table 4.
Table 4 EDS data for the points in Figure 6 (at. %). Zr
Te
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20.3 22.1 21.8 19.8 23.1 21.3 21.1 22.0
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Point 5 Point 6 Point 7 Point 8
Figure 6a 53.1 26.6 51.4 26.6 52.4 25.7 52.2 27.9 Figure 6b 50.5 26.3 52.3 26.4 51.2 27.7 51.3 26.7
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Point 1 Point 2 Point 3 Point 4
Figure 7 SAED patterns collected at different tilt conditions for the precipitate in Figure 6a. The patterns cannot be indexed by Te-Zr, Te-U, or U-Zr binary phase reported in literature.
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The fuel matrix in the as-cast U-10Zr-4Te, shown in Figure 8, has dark and light grey phases, which consist of ~ 90 Zr and 10 U, and ~ 80 Zr and 20 U (at. %), respectively. These light and dark regions have been observed in other U-10Zr based fuels, both with additives [5] [7] [8] and without [22]. A number of nano-scale black precipitates are dispersed in the dark grey phase. The EDS data listed in Table 5 indicate that the nano-scale black phases consist of Zr, a small amount of Te, and possibly some U. Due to the spatial resolution of SEM-EDS, the precipitates could not be quantified.
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The annealed microstructure, shown in Figure 9, with EDS data listed in Table 6, shows the fuel matrix separates to α-U (points 7-9) and δ-UZr2 (points 1-6), which have been commonly found in U-Zr alloys [7] [9]. δ-UZr2 is an intermediate phase and formed at temperatures between 773 and 883 K with compositions around UZr2 [23]. The nano-scale black precipitates present in the as-cast fuel matrix were also found in the annealed, and are dispersed primarily in the δ-UZr2 phase. The precipitates are likely high-Zr precipitates [24] [25], with a small amount of Te present, either dissolved or in the form of Te-Zr or Te-Zr-U phase. In the annealed U-12Zr4Pd alloy [26], β-Zr was identified in the TEM using EDS and diffraction analysis.
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Figure 8 (a) High magnification SEM-BSE image of fuel matrix in the as-cast U-10Zr-4Te. (b-c) Elemental mappings of U and Zr. Corresponding EDS data are listed in Table 5.
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Table 5 EDS data for the points and boxes shown in Figure 8 (at. %). Zr Te U box 1 2 3 point 4 5 6 7 8
31.1 28.2 27.4
0.8 0.4 0.8
68.0 71.4 71.8
19.4 16.0 11.6 8.8 8.1
0.0 0.0 0.0 0.0 0.0
80.7 84.0 88.4 91.2 92.0
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Figure 9 High magnification SEM-BSE image of fuel matrix in the annealed U-10Zr-4Te. Corresponding EDS data are listed in Table 6.
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Table 6 EDS data for the points shown in Figure 9 (at. %). T Ph Z U e asea r 6 0 3 δ1.4 .0 8.6 UZr2 6 0 3 δ0.2 .0 9.8 UZr2 5 0 4 δ8.9 .0 1.1 UZr2 4 δ5 0 9.8 .0 0.2 UZr2 5 0 4 δ7.5 .0 2.4 UZr2 5 0 4 δ6.2 .0 3.8 UZr2 0 0 9 α.8 .0 9.2 U 1 0 9 α.0 .0 9.0 U 0 0 9 α.1 .0 9.9 U a. Suggested phase based on EDS analysis.
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3.2 U-10Zr-4Te-4Ce
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The as-cast microstructure of U-10Zr-4Te-4Ce is shown in Figure 10. The sub-round and angular grey phases are Ce-Te precipitates. Most of them are less than 50 µm in diameter. A few large precipitates, which are hundreds of microns, were also found in the alloy. Black precipitates that are generally smaller than the Ce-Te precipitates and randomly dispersed in the fuel matrix are Zr-rich.
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The Ce-Te precipitates, shown in Figure 11, with EDS data listed in Table 7, are composed of ~ 50 at. % Ce and 50 at. % Te (points 1-4). Based on the EDS data, the precipitates are identified as CeTe. The small white inclusions, with diameter less than 2 µm, i.e. points 5-7 in Figure 11a, were commonly found in the larger CeTe precipitates with a diameter larger than 10 µm. The EDS data indicate that they are rich with U and Zr. Whether these U-Zr inclusions are the same as the nano-scale U-rich inclusions found in the precipitates in U-10Zr-4Te, is unclear. Nevertheless, they have been reported previously in the lanthanide-Sb/Sn compounds in the UZr-Sb/Sn-lanthanide alloys [7] [8] [9].
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We noted that most of the Zr-precipitates are in the fuel matrix but not the CeTe phase, which is different from the Zr-precipitates in U-10Zr-4Te. The Zr-precipitates, shown in Figure 12, reveal that the largest distance between any two ends is less than 5 µm. The EDS data listed in Table 8 indicate that these precipitates contain more than 92 at. % Zr. Uranium from the surrounding matrix is likely contributing to the EDS data, artificially raising the measured concentration of U. Accordingly, the EDS data are suspect for α-Zr in the U-Zr system [23].
Figure 10 Low magnification SEM-BSE image of the as-cast U-10Zr-4Te-4Ce.
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Figure 11 SEM-BSE images of Ce-Te precipitates in the as-cast U-10Zr-4Te-4Ce. Corresponding EDS data are listed in Table 7.
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Table 7 EDS data for the points shown in Figure 11 (at. %). Z T C Ph U r e e asea 0 4 4 0 Ce .2 9.8 9.4 .6 Te 0 4 4 0 Ce .4 9.7 9.2 .7 Te 0 5 4 0 Ce .1 0.3 9.2 .4 Te 0 5 4 0 Ce .2 0.0 9.5 .3 Te 3 0 0 6 6.0 .0 .4 3.6 3 1 1 6 6.0 .8 .8 0.5 4 0 0 5 0.4 .1 .7 8.8 a. Suggested phase based on EDS analysis.
Figure 12 SEM-BSE images of Zr-precipitates (black) in the as-cast U-10Zr-4Te-4Ce. Corresponding EDS data are listed in Table 8.
Table 8 EDS data for the points indicated in Figure 12a (at. %). Z T C U 14
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The annealed microstructure of U-10Zr-4Te-4Ce is shown in Figure 13. Similar to the as-cast microstructure, the Ce-Te precipitates are sub-round, and the Zr-precipitates are dispersed in the fuel matrix, which is now resolved into α-U and δ-UZr2 phases. Due to the high melting point of CeTe (2093 K), the precipitates were not altered by the heat treatment. The Ce-Te precipitates with diameter larger than 10 µm are speckled, with U and Zr trapped. The trapped U and Zr are different with the line Zr-precipitates present in the precipitates in U-10Zr-4Te.
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The grey phases, i.e. points 1-3 in Figure 13b, with EDS data in Table 9, are composed of ~ 50 at. % Ce and 50 at. % Te and identified as CeTe. The dark grey phases, i.e. points 4-6, are composed of ~ 66 Ce and 33 Te (at. %) and suspected for Ce2Te, which has not been reported previously. CeTe is in all of the Ce-Te precipitates, therefore, it has the highest thermodynamic stability between the Ce-Te compounds. On the contrary, Ce2Te was only found in some of the large Ce-Te precipitates. As mentioned, there was loss of Te when casting the alloys, which accounts for a slight excess of Ce, causing the formation of the Ce-rich Ce2Te phase. Representative EDS spectra used for the quantification are shown in Figure 14. Except for U, Zr and Te mentioned in Section 3.1, Ce shows strong Lα peak (~ 4.8 keV). The energy peaks are well separated and resolved by EDS.
Figure 13 (a) Low magnification SEM-BSE image of the annealed U-10Zr-4Te-4Ce. (b) Magnification of the Ce-Te precipitate. Corresponding EDS data are listed in Table 9, with representative EDS spectra shown in Figure 14.
Table 9 EDS data for the points shown in Figure 13b (at. %). 15
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50. 49. 1 3 49. 50. 2 0.2 0 6 48. 51. 3 0.5 3 0 33. 66. 4 0.2 3 4 33. 65. 5 0.3 9 7 35. 64. 6 0.0 4 1 a. Suggested phase based on EDS analysis. 0.2
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Figure 14 Representative EDS spectra of elements U, Zr, Te and Ce for the points 1-6 shown in Figure 13b. Main energy peaks of each element are separated.
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The fuel matrix of as-cast U-10Zr-4Te-4Ce, shown in Figure 15, with EDS data listed in Table 10, has light and dark grey phases consisting of ~ 87 U and 13 Zr (points 1-3), and ~ 57 U and 43 Zr (at. %) (points 4-6), respectively. The nano-scale black precipitates such as points 7-12 and area 13 are rich with Ce, and some other points such as points 11-12 have Te. Given the spatial resolution of SEM-EDS, the composition could not be quantified, and whether the precipitates included U or Zr is unclear. The phase diagrams of Ce-U [27] and Ce-Zr [28] both indicate that there are no binary phases between Ce and U and Zr, so most of the nano-scale black precipitates are likely to be pure Ce, and the others are Ce-Te-U-Zr precipitates with unknown composition. As the loss of Te when casting the alloys, the excess of Ce precipitated upon solidification. In the light grey matrix phase, the Ce- and Ce-Te-U-Zr precipitates are larger and sparser than those in the dark grey, which might be influenced by the solidus temperatures of the two matrix phases and the precipitation temperature of Ce. It is well known that the precipitate morphology is mainly 16
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controlled by the temperature and alloy concentration during solidification, so the microstructures of the nano-scale precipitates shown in Figure 15 are different with different matrix phases.
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The annealed microstructure, shown in Figure 16, is separated into α-U and δ-UZr2, which is the same as the annealed U-10Zr-4Te in Figure 9. The elemental mapping images indicate that the black precipitates are rich with Ce, and some have Te. Most of Ce- and Ce-Te-U-Zr precipitates in the annealed alloy are present on the grain boundary between α-U and δ-UZr2, which is different with those in the as-cast alloy (see Figure 15). These nano-scale precipitates possibly migrated during annealing and formed larger precipitates on the grain boundary, displaying an equilibrium status between the Ce- and Ce-Te-U-Zr phases and α-U and δ-UZr2. These nano-scale precipitates indicate that a higher concentration of Te is needed.
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Figure 15 High magnification SEM-BSE image of fuel matrix in the as-cast U-10Zr-4Te-4Ce. Corresponding EDS data are listed in Table 10.
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Table 10 EDS data for points/area indicated in Figure 15 (at. %). Points 7-12 are not correctly quantitative as limited by the EDS spatial resolution. Z T C U r e e 1 0 0 8 4.0 .0 .0 6.0 1 0 0 8 2.5 .0 .2 7.3 1 0 0 8 2.5 .0 .0 7.5 4 0 0 5 3.5 .0 .5 6.0 17
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Figure 16 (a) High magnification SEM-BSE image of fuel matrix in the annealed U-10Zr-4Te4Ce. (b-e) Elemental mappings of Ce, Te, Zr and U, respectively.
4. Conclusion In the present study, the as-cast and annealed microstructures of U-10Zr-4Te and U-10Zr4Te-4Ce were analyzed with SEM and TEM. In the U-10Zr-4Te alloy, both the homogeneous Te-Zr-bearing precipitates and the speckled precipitates have Zr-line precipitates and U inclusions trapped within, and the Zr-line precipitates and U inclusions are not mitigated after 18
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annealing. This feature was found in the U-10Zr-4Te sample but not reported in the U-10Zr alloy with Pd/Sn/Sb additive. The Te-Zr-bearing precipitates are likely to be Te-Zr-U ternary phase, however, available literature about the Te-Zr-U ternary phase is insufficient to make a solid conclusion.
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Upon addition of Ce, the Te present in the Te-Zr-bearing precipitate released Zr and reacted with Ce, forming the stable CeTe compound. The CeTe compound is a known phase based on phase diagram between Ce and Te. Some U-Zr inclusions were found in the relatively large CeTe compounds, but do not change the microstructure present; they have been found previously in the U-Zr alloys with Sn/Sb/Pd present with lanthanides.
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Free Te was not found, which is outcome requirement if using Te as a fuel additive, since its melting temperature is below a reactor operating temperature. The excess Te is in the nano-scale precipitates, which are composed of Te and Zr and possibly U, in the fuel matrix. Why these small precipitates form, as opposed to the larger agglomerations or precipitates, is not understood in the present study.
Acknowledgement
References
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The authors gratefully acknowledge U.S. Department of Energy for the Nuclear Energy University Program grant DE-NE0008574, and DOE-NE Idaho Operations Office Contract DEAC07-05ID14517. We thank Weiqian Zhuo for supervising the glovebox system. The help rendered by the operation staff of Nanoscale Characterization and Fabrication Laboratory at Virginia Tech is also thankfully acknowledged.
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DOI:
https://doi.org/10.1016/j.jnucmat.2018.10.050
Reference:
NUMA 51289
To appear in:
Journal of Nuclear Materials
Received Date: 5 May 2018 Revised Date:
31 October 2018
Accepted Date: 31 October 2018
Please cite this article as: Y. Xie, J. Zhang, M.T. Benson, J.A. King, R.D. Mariani, Assessment of Te as a U Zr fuel additive to mitigate fuel-cladding chemical interactions, Journal of Nuclear Materials (2018), doi: https://doi.org/10.1016/j.jnucmat.2018.10.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Assessment of Te as a U-Zr Fuel Additive to Mitigate Fuel-Cladding Chemical Interactions Yi Xiea,b, Jinsuo Zhanga*, Michael T. Bensonb, James A. Kingb, Robert D. Marianib
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a. Nuclear Engineering Program, Mechanical Engineering Department, Virginia Tech, Blacksburg, VA 24060, USA b. Idaho National Laboratory, P.O. Box 1625, MS 6188, Idaho Falls, ID 83415, USA
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*Corresponding author, Email: [email protected], Phone: (540) 231-1198, Postal address: 635 Prices Fork Rd, Blacksburg, VA 24060, USA.
Abstract
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Tellurium was investigated as a potential additive to metallic fuel (e.g. U-10Zr) to mitigate fuel-cladding chemical interaction (FCCI). A primary cause of FCCI is the lanthanide fission products migrating to the fuel periphery and interacting with cladding, Te therefore was used to bind the lanthanides into stable compounds in the U-10Zr. The present work investigates the microstructures of as-cast and annealed U-10Zr-4Te and U-10Zr-4Te-4Ce alloys (weight percent) with scanning electron microscope and transmission electron microscopy. Tellurium was found to bind Ce and form CeTe compound, which will mitigate lanthanides migration to the cladding.
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Keywords: Fuel additive; Te; Metallic fuel; FCCI
1. Introduction
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Lanthanide fission products are recognized as one of the key causes of fuel-cladding chemical interaction (FCCI) in metallic fuels during operation [1]. They can migrate toward fuel surface and react with the cladding to form interaction layers, the phenomenon is called lanthanide-induced FCCI. To mitigate FCCIs, barriers on the inner surface of cladding have been broadly investigated [2] [3] [4]. One of the other methods is to use fuel additives in the fuel alloy to immobilize lanthanide fission products [5].
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The potential fuel additives, Pd [5], In [6], Sn [7], Sb [8] and the mixture of Sn and Sb [9], have been investigated through casting with the U-Zr fuel. In the fuel-additive systems, additiveZr-bearing compounds were found, such as PdZr2 and U(Pd,Zr) in U-15Zr-4Pd alloy [5], Sn3Zr5 and Sn4Zr5 in U-10Zr-4Sn alloy [7], SbZr2 in U-10Zr-4Sb alloy [8], and Zr5(Sn,Sb)3 in U-10Zr2Sn-2Sb alloy [9]. Upon addition of lanthanides, the additives are leased from the additive-Zrbearing compounds to form stable additive-lanthanide compounds, such as PdLn and Ln-rich PdLn in U-15Zr-2Pd-2Ln and U-15Zr-4Pd-4Ln alloys [5], Sn3Ln5 in U-10Zr-4Sn-4Ln alloy [7], SbCe2 and Sb3Ce4 in U-10Zr-4Sb-4Ce alloy [8], and Ln5(Sn,Sb)4 in U-10Zr-2Sn-2Sb-4Ln alloy [9] (where Ln = 53Nd-25Ce-16Pr-6La wt. %). Other than binding lanthanides, the redundant fuel additives are present in the fuel matrix in the form of small (< 1 µm) precipitates. Most of these small precipitates are in the fuel matrix region with a higher Zr content.
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The enthalpies of formation of In-lanthanide compounds, such as In3Ce, are more positive than the other additive-lanthanide compounds [8], which indicates that In-lanthanide compounds have lower thermodynamic stability than the other additive-lanthanide compounds. The thermodynamics of Sn-lanthanide and Sb-lanthanide compounds are similar; however, in the UZr alloys with Sn and Sb, the additive-lanthanide compounds contain a much higher percentage of Sb than Sn, indicating a higher thermodynamic stability of Sb-lanthanide than Sn-lanthanide [9]. The study shows that though the known thermodynamic stability between Sn- and Sblanthanides should be roughly equivalent, experimentally that is not the case.
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In the U-Zr alloys with Pd, In, Sn, Sb or (Sn+Sb) and lanthanide, a small amount of Zr has been found in the Pd/In/Sn/(Sn+Sb)-Ln compounds, but not found in the SbCe2 or Sb3Ce4 compounds. Zr-precipitates have been found in the U-Zr alloys with Pd, In, or Sb [5] [6] [8], but not found in those with Sn or (Sn+Sb) [7] [9]. The formation of a more stable additive-Zr compound is not advantageous to the fuel performance. Additives may consume a non-negligible amount of Zr from the fuel matrix, resulting in the decrease of solidus temperature. The proportions of lanthanides bound to Sb are in the order of Ce/La > Nd >> Pr [9]; while the proportions of lanthanides bound to Pd are in the order of Pr/La > Nd >> Ce [5]. Therefore, multiple additives may be needed to immobilize the most prevalent lanthanide fission products. Multiple additives will lead to a combined effect of immobilizing lanthanides, which is expected to be more effective than a single additive.
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Tellurium has also been known as a promising fuel additive [5]. Enthalpies of formation of Te-lanthanide compounds [10] are much more negative than the other additive-lanthanide compounds. Although a more negative entropy of formation can produce a more negative free energy of formation (∆G = ∆H-T∆S), to our knowledge, most of the entropy and free energy of formation data have not been reported. Based on the limited data, such as in Refs. [11] [12] [13], the absolute value of entropy of formation is very small, and the change in free energy of formation follows the same trend in enthalpy of formation, so the enthalpy data can still be used to estimate the thermodynamic stability. Enthalpies of formation of lanthanide-additive compounds are listed in Table 1. Tellurium and Sb have more negative enthalpies of formation than Pd and Sn. Tellurium also shows more negative enthalpies of formation than Sb, except for the case of La. Te-Ln compounds also have high melting temperatures, for example, NdTe at 2296 K [14], CeTe at 2093 K [15], LaTe at 1993 K [16], and PrTe at 2223 K [17].
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Table 1 Enthalpies of formation (kJ/g-atom) of lanthanide-additive intermetallic compounds. Pd (~ Sn Sb Te 873 K) (298 K) (298 K) (298 K) N -83 -106 -111 -67 [18] d [13] [19] [10] C -86 -127 -138 -64 [18] e [13] [20] [10] L -90 -131 -124 -56 [18] a [13] [21] [10] P -86 -109 -123 -63 [18] r [13] [19] [10]
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2. Experiment
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The present study is the microstructural characterization of as-cast and annealed alloys U10Zr-4Te and U-10Zr-4Te-4Ce. Microstructure of these alloys was investigated, with a focus on the Te-Zr and Te-Ce compounds. These are the initial data needed in determining the efficacy of applying Te as a fuel additive to mitigate FCCI by binding the lanthanide fission products.
Two alloys were made for the microstructure analysis. The cast ingredients are listed as below. U-10Zr-4Te alloy: 71.5U-21.8Zr-6.7Te (at. %) U-10Zr-4Te-4Ce alloy: 66.6U-21.3Zr-6.5Te-5.5Ce (at. %)
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All materials, except U, were obtained from Alfa Aesar and used as received. Cerium was obtained as rods, packaged in mylar under argon. Uranium was cleaned by submersion in nitric acid, followed by a water wash, and then an ethanol wash. All casting operations were carried out in an arc-melter within an argon atmosphere glovebox with high purity argon as a cover gas. After each addition step, the resulting button was flipped and re-melted three times to ensure homogeneity. To prepare the U-10Zr-4Te alloy, appropriate amount of Te, Zr and U were arcmelted together in two steps. A button of U-Zr was prepared, followed by the addition of Te. To prepare the U-10Zr-4Te-4Ce alloy, the appropriate amount of the Ce was added to a pre-alloy button of U-Zr-Te, prepared as described for the U-10Zr-4Te. The buttons were cast into 5 mm diameter pins. Due to the low boiling point of Te, boiling was observed when adding Te and when dropping the pins. The amount of Te loss because of boiling was not recorded.
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Approximately 4 mm from each pin was cut for annealing. The samples were wrapped with Ta foil and placed in a furnace in the glovebox at 923 K for 500 hours. After the heat treatment, the samples were cooled in flowing argon gas, and then cut to expose a fresh surface for analysis.
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Scanning electron microscope (SEM) analysis was performed on a section of pin from each alloy. The samples were mounted in a 31.8 mm diameter phenolic metallographic (met) mount filled with epoxy. They were polished by grinding the surfaces flat with SiC grinding paper followed by polishing with polycrystalline diamond suspensions, starting with 9 µm, then 3 µm, and finally 1 µm. The surface was polished with 3 µm and 1 µm suspensions again to remove the oxide film immediately before the microstructure analysis. The environmental SEM (model: FEI Quanta 600 FEG) was used, equipped with a Bruker energy dispersive X-ray spectrometer (EDS). The EDS was controlled by Esprit 1.9 software for image acquisition and data quantification. The measurement uncertainty of EDS is within a few percent of the measured value. A number of points for each investigated phase were measured to demonstrate the consistency of EDS measurement. The SEM was operated at an accelerating voltage of 20 kV and spot size 5. Under this voltage, the EDS spatial resolution was 1-1.5 µm. Spectra were collected over the energy range 0-10 keV. Transmission electron microscopy (TEM) analysis was carried out to identify the compound of local position. The TEM sample was prepared using the ion milling method. First, a 3 mm diameter by 1 mm thick disc was cut from the alloy bulk with a slow speed saw, and then the disc was ground and polished, both top and bottom surfaces, to 50 µm thick with SiC grinding papers followed by polishing with polycrystalline diamond suspensions. The damage caused by 3
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mechanical polishing was carefully avoided by removing the appropriate depth of material with each sandpaper. The disc was finally thinned by ion milling and polishing system (model: Fischione 1010 Ion Mill) with ion milling top and bottom surfaces till the disc center was transparent to transmitted (chamber) light. At this point, the periphery around the disc center/hole was considered thin enough for TEM analysis.
3. Results and discussion
3.1 U-10Zr-4Te
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When casting these alloy compositions, there was obvious loss of Te, primarily when casting the 5 mm pin, due to the low melting temperature of Te (723 K). In order to have the alloy drop into the mold in one charge to form a continuous pin, superheat is necessary. Boiling occurred in the button surface during this process, and therefore the exact composition is not known. The loss is not expected to be enough to change the microstructure present, though there will be less precipitates for analysis. The low melting temperature is also a consideration during irradiation, since the melting temperature is lower than the operating fuel temperature (typically around 923 K) in a reactor. For this reason, keeping Te bound with a higher melting intermetallic is necessary. Ideally, in a fresh fuel, Te will bind Zr, and after fission products burn-in, Te will bind lanthanides. Some Te-Zr compounds also have low melting temperatures, for example, the melting temperature of ZrTe5, on the Te-rich side is 822 K. Since Te will be a minor additive, with a much higher abundance of Zr present, the compounds will be on the Zr-rich side of Zr-Te phase diagram, such as Zr3Te and Zr5Te4, the solidus temperatures are 1616 K and 1627 K, respectively. Therefore, the low melting temperature of Te-rich Zr-Te compounds should not be an issue.
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Figure 1 is the overview of as-cast U-10Zr-4Te. The precipitates are round or sub-round, while in the previous study on U-Zr-Sn/Sb/(Sn+Sb) alloys, the precipitates were dendritic, hexagonal or triangular [7] [8] [9]. The precipitates are generally less than 50 µm in diameter (the largest distance between any two ends of a precipitate), with a small number of larger precipitates in hundreds of microns. The large number of precipitates indicates a reasonable amount of Te remaining in the alloy after casting.
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The precipitates with diameter smaller than 10 µm, shown in Figure 2, are homogeneous. The precipitates with diameter larger than 10 µm are speckled, with U- and Zr-precipitates trapped. Uranium and Zr are white and black in the SEM-BSE image, respectively. The Zrprecipitates are in the microstructures of line, bulk and ring, while U, which may include some Zr, is present as small round inclusions. The compositions of these inclusions were not quantified due to the spatial resolution of SEM-EDS. The magnification of the box in Figure 2a, shown in Figure 4, is the microstructure of line Zr-precipitates. There is an abundance of nano-scale U-rich white inclusions dispersed in the precipitate. The non-spherical precipitates examined in the previous studies on the U-Zr-Sn/Sb/(Sn+Sb) systems did not contain these inclusions. The spherical precipitates tend to trap matrix material, as has been observed in the spherical additivelanthanide precipitates [5] [7] [8] [9]. The small homogeneous precipitates shown in Figure 2a (points 4-6), with EDS data listed in Table 2, contain a lower content of Zr and U than the larger precipitate (points 1-3). The inclusions (U, Zr, and U-Zr) dispersed in the precipitates raise the 4
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measured concentration of Zr and U. The representative EDS spectra of elements U, Zr and Te for points 1-7 are shown in Figure 3. The main energy peaks of U, Zr and Te used for the quantification are Mα (~ 3.2 keV), Lα (~ 2.0 keV), and Lα (~ 3.8 keV), respectively, well separated and resolved by EDS.
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Figure 1 Low magnification SEM backscattered electron (BSE) image of the as-cast U-10Zr-4Te.
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Figure 2 SEM-BSE images of precipitates in the as-cast U-10Zr-4Te. Corresponding EDS data are listed in Table 2, with EDS spectra shown in Figure 3.
Table 2 EDS data1 for the points shown in Figure 2a (at. %). Z T U r e 6 2 1 5.2 3.1 1.7 6 2 1 1
The measurement uncertainty of EDS is within a few percent of the measured value, applicable to Table 2-10. 5
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Figure 3 Representative EDS spectra of elements U, Zr and Te for points 1-7 shown in Figure 2a. Main energy peaks of each element are separated.
Figure 4 High magnification SEM-BSE image of the white box in Figure 2a, showing the line Zr-precipitates with U-rich inclusions.
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In the annealed U-10Zr-4Te, the content of U in the small homogeneous precipitates, shown in Figure 5a and Table 3, is relatively reduced compared with the as-cast sample. The reduction is attributed to U migrating out of the precipitates during annealing. The remaining U measured in the precipitates could be from surrounding U being included in the EDS measurement, or could be U substitution of atoms in the precipitates. In addition, the contents of Zr and U in the larger speckled precipitates do not reduce after annealing. The Zr-precipitates and U-rich inclusions, shown in Figure 5b, are still trapped, so they contribute to the measured concentration of Zr and U.
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Figure 5 (a) Low magnification SEM-BSE image of the annealed U-10Zr-4Te. (b) Magnification of precipitates. Corresponding EDS data are listed in Table 3.
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Table 3 EDS data for the points shown in Figure 5a (at. %). Z T U r e 6 2 1 4.8 4.8 0.4 6 2 1 4.0 5.9 0.1 6 2 1 4.5 5.3 0.2 6 2 8 5.7 6.0 .3 6 3 5 3.6 1.4 .0 6 3 3 4.2 2.7 .1 6 2 8 3.2 8.8 .0
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The precipitates in the as-cast U-10Zr-4Te were further analyzed with TEM to identify the phase, shown in Figure 6, with EDS data listed in Table 4. The two round precipitates are homogeneous, without U- or Zr-inclusions. The EDS spectrum, shown in Figure 6f, indicates the energy peaks of U, Zr and Te are well separated. A number of points were randomly selected and the compositions were measured, consistent to be ~ 52 Zr, 26 Te, and 22 U (at. %). Three selected area electron diffraction (SAED) patterns for the precipitate shown in Figure 6a were carried out at different tilt conditions, with the resulting patterns shown in Figure 7. The diffraction patterns cannot be indexed by Te-Zr, Te-U, or U-Zr binary phase. The precipitate is likely to be a Te-Zr-U ternary phase; however, a Te-Zr-U ternary phase and its crystal structure are not available in literature. It is difficult to identify the Te-Zr-U phase based on the current information. Identifying the crystal structure of the phase is not the scope of this paper, but this issue is pointed out that needs further investigation.
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Figure 6 (a-b) STEM bright field images of precipitates in the as-cast U-10Zr-4Te. (c-e) Elemental mappings of Te, Zr and U, respectively, corresponding to the box in (a). (f) Representative EDS spectrum for an EDS data. Corresponding EDS data are listed in Table 4.
Table 4 EDS data for the points in Figure 6 (at. %). Zr
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20.3 22.1 21.8 19.8 23.1 21.3 21.1 22.0
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Point 5 Point 6 Point 7 Point 8
Figure 6a 53.1 26.6 51.4 26.6 52.4 25.7 52.2 27.9 Figure 6b 50.5 26.3 52.3 26.4 51.2 27.7 51.3 26.7
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Figure 7 SAED patterns collected at different tilt conditions for the precipitate in Figure 6a. The patterns cannot be indexed by Te-Zr, Te-U, or U-Zr binary phase reported in literature.
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The fuel matrix in the as-cast U-10Zr-4Te, shown in Figure 8, has dark and light grey phases, which consist of ~ 90 Zr and 10 U, and ~ 80 Zr and 20 U (at. %), respectively. These light and dark regions have been observed in other U-10Zr based fuels, both with additives [5] [7] [8] and without [22]. A number of nano-scale black precipitates are dispersed in the dark grey phase. The EDS data listed in Table 5 indicate that the nano-scale black phases consist of Zr, a small amount of Te, and possibly some U. Due to the spatial resolution of SEM-EDS, the precipitates could not be quantified.
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The annealed microstructure, shown in Figure 9, with EDS data listed in Table 6, shows the fuel matrix separates to α-U (points 7-9) and δ-UZr2 (points 1-6), which have been commonly found in U-Zr alloys [7] [9]. δ-UZr2 is an intermediate phase and formed at temperatures between 773 and 883 K with compositions around UZr2 [23]. The nano-scale black precipitates present in the as-cast fuel matrix were also found in the annealed, and are dispersed primarily in the δ-UZr2 phase. The precipitates are likely high-Zr precipitates [24] [25], with a small amount of Te present, either dissolved or in the form of Te-Zr or Te-Zr-U phase. In the annealed U-12Zr4Pd alloy [26], β-Zr was identified in the TEM using EDS and diffraction analysis.
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Figure 8 (a) High magnification SEM-BSE image of fuel matrix in the as-cast U-10Zr-4Te. (b-c) Elemental mappings of U and Zr. Corresponding EDS data are listed in Table 5.
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Table 5 EDS data for the points and boxes shown in Figure 8 (at. %). Zr Te U box 1 2 3 point 4 5 6 7 8
31.1 28.2 27.4
0.8 0.4 0.8
68.0 71.4 71.8
19.4 16.0 11.6 8.8 8.1
0.0 0.0 0.0 0.0 0.0
80.7 84.0 88.4 91.2 92.0
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Figure 9 High magnification SEM-BSE image of fuel matrix in the annealed U-10Zr-4Te. Corresponding EDS data are listed in Table 6.
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Table 6 EDS data for the points shown in Figure 9 (at. %). T Ph Z U e asea r 6 0 3 δ1.4 .0 8.6 UZr2 6 0 3 δ0.2 .0 9.8 UZr2 5 0 4 δ8.9 .0 1.1 UZr2 4 δ5 0 9.8 .0 0.2 UZr2 5 0 4 δ7.5 .0 2.4 UZr2 5 0 4 δ6.2 .0 3.8 UZr2 0 0 9 α.8 .0 9.2 U 1 0 9 α.0 .0 9.0 U 0 0 9 α.1 .0 9.9 U a. Suggested phase based on EDS analysis.
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3.2 U-10Zr-4Te-4Ce
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The as-cast microstructure of U-10Zr-4Te-4Ce is shown in Figure 10. The sub-round and angular grey phases are Ce-Te precipitates. Most of them are less than 50 µm in diameter. A few large precipitates, which are hundreds of microns, were also found in the alloy. Black precipitates that are generally smaller than the Ce-Te precipitates and randomly dispersed in the fuel matrix are Zr-rich.
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The Ce-Te precipitates, shown in Figure 11, with EDS data listed in Table 7, are composed of ~ 50 at. % Ce and 50 at. % Te (points 1-4). Based on the EDS data, the precipitates are identified as CeTe. The small white inclusions, with diameter less than 2 µm, i.e. points 5-7 in Figure 11a, were commonly found in the larger CeTe precipitates with a diameter larger than 10 µm. The EDS data indicate that they are rich with U and Zr. Whether these U-Zr inclusions are the same as the nano-scale U-rich inclusions found in the precipitates in U-10Zr-4Te, is unclear. Nevertheless, they have been reported previously in the lanthanide-Sb/Sn compounds in the UZr-Sb/Sn-lanthanide alloys [7] [8] [9].
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We noted that most of the Zr-precipitates are in the fuel matrix but not the CeTe phase, which is different from the Zr-precipitates in U-10Zr-4Te. The Zr-precipitates, shown in Figure 12, reveal that the largest distance between any two ends is less than 5 µm. The EDS data listed in Table 8 indicate that these precipitates contain more than 92 at. % Zr. Uranium from the surrounding matrix is likely contributing to the EDS data, artificially raising the measured concentration of U. Accordingly, the EDS data are suspect for α-Zr in the U-Zr system [23].
Figure 10 Low magnification SEM-BSE image of the as-cast U-10Zr-4Te-4Ce.
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Figure 11 SEM-BSE images of Ce-Te precipitates in the as-cast U-10Zr-4Te-4Ce. Corresponding EDS data are listed in Table 7.
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Table 7 EDS data for the points shown in Figure 11 (at. %). Z T C Ph U r e e asea 0 4 4 0 Ce .2 9.8 9.4 .6 Te 0 4 4 0 Ce .4 9.7 9.2 .7 Te 0 5 4 0 Ce .1 0.3 9.2 .4 Te 0 5 4 0 Ce .2 0.0 9.5 .3 Te 3 0 0 6 6.0 .0 .4 3.6 3 1 1 6 6.0 .8 .8 0.5 4 0 0 5 0.4 .1 .7 8.8 a. Suggested phase based on EDS analysis.
Figure 12 SEM-BSE images of Zr-precipitates (black) in the as-cast U-10Zr-4Te-4Ce. Corresponding EDS data are listed in Table 8.
Table 8 EDS data for the points indicated in Figure 12a (at. %). Z T C U 14
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The annealed microstructure of U-10Zr-4Te-4Ce is shown in Figure 13. Similar to the as-cast microstructure, the Ce-Te precipitates are sub-round, and the Zr-precipitates are dispersed in the fuel matrix, which is now resolved into α-U and δ-UZr2 phases. Due to the high melting point of CeTe (2093 K), the precipitates were not altered by the heat treatment. The Ce-Te precipitates with diameter larger than 10 µm are speckled, with U and Zr trapped. The trapped U and Zr are different with the line Zr-precipitates present in the precipitates in U-10Zr-4Te.
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The grey phases, i.e. points 1-3 in Figure 13b, with EDS data in Table 9, are composed of ~ 50 at. % Ce and 50 at. % Te and identified as CeTe. The dark grey phases, i.e. points 4-6, are composed of ~ 66 Ce and 33 Te (at. %) and suspected for Ce2Te, which has not been reported previously. CeTe is in all of the Ce-Te precipitates, therefore, it has the highest thermodynamic stability between the Ce-Te compounds. On the contrary, Ce2Te was only found in some of the large Ce-Te precipitates. As mentioned, there was loss of Te when casting the alloys, which accounts for a slight excess of Ce, causing the formation of the Ce-rich Ce2Te phase. Representative EDS spectra used for the quantification are shown in Figure 14. Except for U, Zr and Te mentioned in Section 3.1, Ce shows strong Lα peak (~ 4.8 keV). The energy peaks are well separated and resolved by EDS.
Figure 13 (a) Low magnification SEM-BSE image of the annealed U-10Zr-4Te-4Ce. (b) Magnification of the Ce-Te precipitate. Corresponding EDS data are listed in Table 9, with representative EDS spectra shown in Figure 14.
Table 9 EDS data for the points shown in Figure 13b (at. %). 15
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50. 49. 1 3 49. 50. 2 0.2 0 6 48. 51. 3 0.5 3 0 33. 66. 4 0.2 3 4 33. 65. 5 0.3 9 7 35. 64. 6 0.0 4 1 a. Suggested phase based on EDS analysis. 0.2
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Figure 14 Representative EDS spectra of elements U, Zr, Te and Ce for the points 1-6 shown in Figure 13b. Main energy peaks of each element are separated.
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The fuel matrix of as-cast U-10Zr-4Te-4Ce, shown in Figure 15, with EDS data listed in Table 10, has light and dark grey phases consisting of ~ 87 U and 13 Zr (points 1-3), and ~ 57 U and 43 Zr (at. %) (points 4-6), respectively. The nano-scale black precipitates such as points 7-12 and area 13 are rich with Ce, and some other points such as points 11-12 have Te. Given the spatial resolution of SEM-EDS, the composition could not be quantified, and whether the precipitates included U or Zr is unclear. The phase diagrams of Ce-U [27] and Ce-Zr [28] both indicate that there are no binary phases between Ce and U and Zr, so most of the nano-scale black precipitates are likely to be pure Ce, and the others are Ce-Te-U-Zr precipitates with unknown composition. As the loss of Te when casting the alloys, the excess of Ce precipitated upon solidification. In the light grey matrix phase, the Ce- and Ce-Te-U-Zr precipitates are larger and sparser than those in the dark grey, which might be influenced by the solidus temperatures of the two matrix phases and the precipitation temperature of Ce. It is well known that the precipitate morphology is mainly 16
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controlled by the temperature and alloy concentration during solidification, so the microstructures of the nano-scale precipitates shown in Figure 15 are different with different matrix phases.
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The annealed microstructure, shown in Figure 16, is separated into α-U and δ-UZr2, which is the same as the annealed U-10Zr-4Te in Figure 9. The elemental mapping images indicate that the black precipitates are rich with Ce, and some have Te. Most of Ce- and Ce-Te-U-Zr precipitates in the annealed alloy are present on the grain boundary between α-U and δ-UZr2, which is different with those in the as-cast alloy (see Figure 15). These nano-scale precipitates possibly migrated during annealing and formed larger precipitates on the grain boundary, displaying an equilibrium status between the Ce- and Ce-Te-U-Zr phases and α-U and δ-UZr2. These nano-scale precipitates indicate that a higher concentration of Te is needed.
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Figure 15 High magnification SEM-BSE image of fuel matrix in the as-cast U-10Zr-4Te-4Ce. Corresponding EDS data are listed in Table 10.
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Table 10 EDS data for points/area indicated in Figure 15 (at. %). Points 7-12 are not correctly quantitative as limited by the EDS spatial resolution. Z T C U r e e 1 0 0 8 4.0 .0 .0 6.0 1 0 0 8 2.5 .0 .2 7.3 1 0 0 8 2.5 .0 .0 7.5 4 0 0 5 3.5 .0 .5 6.0 17
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Figure 16 (a) High magnification SEM-BSE image of fuel matrix in the annealed U-10Zr-4Te4Ce. (b-e) Elemental mappings of Ce, Te, Zr and U, respectively.
4. Conclusion In the present study, the as-cast and annealed microstructures of U-10Zr-4Te and U-10Zr4Te-4Ce were analyzed with SEM and TEM. In the U-10Zr-4Te alloy, both the homogeneous Te-Zr-bearing precipitates and the speckled precipitates have Zr-line precipitates and U inclusions trapped within, and the Zr-line precipitates and U inclusions are not mitigated after 18
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annealing. This feature was found in the U-10Zr-4Te sample but not reported in the U-10Zr alloy with Pd/Sn/Sb additive. The Te-Zr-bearing precipitates are likely to be Te-Zr-U ternary phase, however, available literature about the Te-Zr-U ternary phase is insufficient to make a solid conclusion.
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Upon addition of Ce, the Te present in the Te-Zr-bearing precipitate released Zr and reacted with Ce, forming the stable CeTe compound. The CeTe compound is a known phase based on phase diagram between Ce and Te. Some U-Zr inclusions were found in the relatively large CeTe compounds, but do not change the microstructure present; they have been found previously in the U-Zr alloys with Sn/Sb/Pd present with lanthanides.
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Free Te was not found, which is outcome requirement if using Te as a fuel additive, since its melting temperature is below a reactor operating temperature. The excess Te is in the nano-scale precipitates, which are composed of Te and Zr and possibly U, in the fuel matrix. Why these small precipitates form, as opposed to the larger agglomerations or precipitates, is not understood in the present study.
Acknowledgement
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The authors gratefully acknowledge U.S. Department of Energy for the Nuclear Energy University Program grant DE-NE0008574, and DOE-NE Idaho Operations Office Contract DEAC07-05ID14517. We thank Weiqian Zhuo for supervising the glovebox system. The help rendered by the operation staff of Nanoscale Characterization and Fabrication Laboratory at Virginia Tech is also thankfully acknowledged.
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