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Fabrication and microstructural analysis of UN-U3Si2 composites for accident tolerant fuel applications
Journal of Nuclear Materials 477 (2016) 18e23
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Fabrication and microstructural analysis of UN-U3Si2 composites for accident tolerant fuel applications Kyle D. Johnson*, Alicia M. Raftery, Denise Adorno Lopes, Janne Wallenius €gskolan, Stockholm, Sweden Albanova Universitets Centrum, Kungliga Tekniska Ho
h i g h l i g h t s U3Si2 fabricated from elemental uranium and silicon through arc melting. Homogeneity of the silicides assessed through densitometry, XRD, SEM and EDS, chemical etching and optical microscopy. UN powder fabricated using hydriding-nitriding method. No phase transformations detected when sintering using silicide particle sizes less than UN particle size. High density composite (98%TD) fabricated with silicide grain coating using spark plasma sintering at 1450 C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2016 Received in revised form 28 April 2016 Accepted 2 May 2016 Available online 4 May 2016
In this study, U3Si2 was synthesized via the use of arc-melting and mixed with UN powders, which together were sintered using the SPS method. The study revealed a number of interesting conclusions regarding the stability of the system e namely the formation of a probable but as yet unidentified ternary phase coupled with the reduction of the stoichiometry in the nitride phase e as well as some insights into the mechanics of the sintering process itself. By milling the silicide powders and reducing its particle size ratio compared to UN, it was possible to form a high density UN-U3Si2 composite, with desirable microstructural characteristics for accident tolerant fuel applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nuclear fuel Accident tolerant fuel Uranium nitride Uranium silicide SPS U-N-Si system
1. Introduction Following the Fukushima-Daiichi nuclear disaster in 2011, the United States Department of Energy began funding intense and coordinated research into improving the safety, reliability, and economy of nuclear fuel systems through the consideration of alternative fuel and cladding concepts, namely the so-called Accident Tolerant Fuels (ATF). Candidate fuel-clad systems should exhibit a reduced hydrogen generation rate e specifically feature a compatibility with non-zircaloy cladding material e improved fission product retention, reduced cladding interaction with steam, and reduced fuel-cladding interactions. Among the candidates being considered are uranium silicide e principally the U3Si2 phase
* Corresponding author. E-mail address: [email protected] (K.D. Johnson). http://dx.doi.org/10.1016/j.jnucmat.2016.05.004 0022-3115/© 2016 Elsevier B.V. All rights reserved.
but also, to a lesser extent, U3Si5 e as well as uranium mononitride (UN) [1e3]. A comparison of selected material properties of interest is presented below in Table 1. As seen from Table 1, all of the ATF candidates exhibit greatly improved thermal conductivities when compared to UO2. More importantly for accident tolerance considerations, their respective conductivities increase with increasing temperature e in stark contrast to UO2. These improvements in thermal conductivity allow for peak fuel and cladding temperatures to be substantially reduced during normal operation and accident scenarios, and enhance the overall ability of the system to tolerate severe accident conditions [4,5]. The silicides both feature reduced melting temperatures and multiple phase transformations, behaviours which are not desired, while on the other hand they have been shown to provide improved resistance to oxidation by steam when compared to UN [6]. Of the silicides, only U3Si2 provides an increased metal density
K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
19
Table 1 Properties of ATF Fuel Candidates compared to UO2 Property
UO2
U3Si2
U3Si5
UN
Theoretical density (g/cm3) Uranium density (g/cm3) Thermal Conductivity (BoL) (W/m*K from 600 to 1400 K) Melting Point (K) Water Tolerance
10.96 9.66 6.0e2.9 [15] 3130 [15] Excellent [15]
12.2 [13] 11.31 15e27.5 [16] 1938 [13] Good [13]
9.06 [14] 7.57 8.0e16 [14] 2043 [13] Good [13]
14.32 13.52 19e25 [17] 2953 [15] Poor [15]
as compared to the oxide, which is why this phase was chosen for mixing and co-sintering with UN. UN, meanwhile, shows the highest uranium density of all of the candidates, which greatly improves economy of operation. UN has received considerable attention over the years, with substantial efforts aimed at improving the sinterability of the material using conventional sintering techniques. Temperatures in excess of 2300 C [7e9] have been required to sinter UN compacts up to 95% relative density, with additional undesired consequences in terms of the microstructure of the material, i.e. grain size, stoichiometry, etc. Aside from these undesired effects, sintering at these temperatures itself is known to be both costly and difficult [10]. Various proposals have been put forth to decrease the sintering temperatures e ergo costs e associated with such a process, perhaps most notably the use of a second phase with a lower melting point and so-called ”liquid phase sintering,” [11] which, using U3Si2, has succeeded in reaching 90% density at 1700 C, which otherwise required over 1900 C for UN alone [7,12]. Nevertheless, the use of these temperatures was observed to produce geometric deformation in the final sintered compact, as well as interactions between the UN and U3Si2 phases, which can perhaps result in undesirable material properties for the fuel. The objective of this study, therefore, was to explore the applicability of the SPS technique to produce UN-U3Si2 composites featuring both high density and a desirable microstructure, i.e. a ”coating” of U3Si2 able to provide improved resistance to oxidation, while at the same time suppressing the interaction of UN and U3Si2. 2. Experimental 2.1. Synthesis and characterisation of nitrides and silicides High purity uranium nitride powder was fabricated at KTH using a hydriding-nitriding method described previously [18,19] These powders were found to contain 5.4% nitrogen, 800 ppm oxygen, and 400 ppm carbon by elemental analysis. In parallel, U3Si2 ingots were fabricated using arc melting of metallic uranium and silicon in an argon atmosphere as described by Raftery [20]. Elemental silicon was obtained from AlfaAesar and metallic uranium from Materials Science Corporation. Samples were taken for impurity analysis using ICP-OES, while a LECO TC436DR was used for determination of oxygen and nitrogen, and a LECO series CS440 for carbon measurement. Analyses of the initial metal showed concentrations of less than 100 ppm oxygen, and 400 ppm carbon, as well as various minor impurities such as nickel, calcium, zinc, and aluminum, all of which were found to be less than 25 ppm using ICP-OES. Arc-melted samples were prepared using a slight hyperstoichiometric content of silicon to suppress the formation of U3Si in the event of silicon volatility, as noted by White [16]. The microstructure of the manufactured silicide was evaluated using SEM and LOM with 8 M nitric acid as an etchant, while crystallography was assessed using XRD, which was performed at room temperature under Cu K-a radiation using an Ni filter. The following conditions were adopted: voltage 40 kV; 30 mA; angle (2q) ranging from 20 to 80 , angular step of 0.02 and counting time
of 1 s. The phases present were indexed based on crystallographic data contained in records JCPDS [21]. This alloy was then ground into particles with widely varying particle sizes (up to 300 mm) and manually mixed with UN powders featuring an average particle size of 4 mm. These powders were characterized using XRD and XRF, and elemental analysis was performed.
2.2. Co-sintering Following characterisation, the powders were sintered using a modified Dr. Sinter SPS machine, contained within a glovebox under an inert, argon atmosphere. For sintering, a 30 mm tall, 30 mm wide, and 2 mm inner-diameter graphite die was used, with thin graphite paper used to protect the sample and die from interaction during sintering. These were provided by the National SPS Facility at Department of Materials and Environmental Chemistry at Stockholm University. After sintering, this bonded graphite paper was removed from the samples via grinding. The first batch of pellets produced in this study were sintered at 1450 C and 135 MPa, which was predicted to produce pellets at around 96% TD but at a fairly low temperature so as to avoid high temperature reactions [22]. Samples were prepared with 5%, 10%, 20%, and 25% weight fraction U3Si2. The sintered pellets were then measured for density as described previously and examined using LOM, SEM, EDS; and EBSD [22]. In Fig. 1, the relevant sintering parameters, i.e. voltage, current, applied pressure, temperature, and Z-axis displacement for a given pellet, can be seen. A second route was attempted whereby U3Si2 was milled in a planetary ball mill contained within an argon glove box using stainless a steel cup and balls for a period of 30 min. Following this, the balls were removed and as-synthesized UN powder was added to the cup to form 10% weight fraction U3Si2, and the powders were then mixed by rotation for an additional 30 min. This mixture was then sintered using the previously described parameters.
Fig. 1. Sintering curves used for each pellet showing voltage, current, applied pressure, temperature, and typical Z-axis displacement.
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K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
3. Results 3.1. Silicide fabrication Multiple samples of U3Si2 were synthesized with densities ranging between 12.06 and 12.14 g/cm3, which compares favorably to the theoretical value of 12.2 g/cm3. An investigation was also made to determine the homogeneity of the samples, through the use of EDS and LOM. When analyzed by EDS, samples appeared homogenous with small inclusions of UO2 and U-rich phases, as seen in Fig. 2. Elemental analysis showed 600 ppm oxygen present within the material, while XRF showed 7.1 wt% Si and 92.9 wt% U, corresponding to 39.4 at% SI and 60.6 at% U. Etching and analysis using LOM revealed, however, the presence of a wide inhomogeneity within the material, demonstrating the need for heat treatment and annealing. Fig. 3 presents a view of the obtained alloy in the as-cast condition, using optical microscopy and metallographic etching. It can be observed that the sample has
a microstructure consisting of a primary precipitate with different sizes and geometries, and dendritic structures, which are an indication of the occurrence of a very inhomogeneous liquid during the solidification process. The etching produces a contrast in microsegration samples due to differing concentrations of alloying elements, which leads to different corrosion behaviors. In order to eliminate this variability within the silicide, subsequent samples were subjected to several e between 5 and 7 e additional melts, in lieu of prolonged heat treatment. In Fig. 4 it can be observed that the re-melting procedure produced a microstructure composed of a primary precipitate and low inter phase space. This morphology is in accordance with the invariant reaction proposal for U3Si2 in the phase diagram indicating that during the solidification process the liquid was in a homogenous condition. No increase in oxygen content was observed. X-ray diffraction performed on the sample revealed a predominant crystal structure matching that of the U3Si2 phase and a smaller UO2 phase, as seen in Fig. 5. 3.2. Co-sintering The mixture of nitride and silicide powders was sintered using the sintering and mixture parameters listed in Table 2 and resulted in the listed absolute densities, as measured by densitometry. Elemental analysis of the sintered compacts showed oxygen between 600 and 1000 ppm, and carbon ranging from 500 to 600 ppm, consistent with the initial materials. Nitrogen content varied in proportion to the admixed silicide content.
Fig. 2. EDS mapping of arc-melted U3Si2 sample showing UO2 and U-rich phases.
3.2.1. Varying silicide fraction In the pellets fabricated using manual grinding and mixing, the resultant pellets displayed a highly heterogeneous structure, characterized by large inclusions of U3Si2 and U3Si5 e as evaluated by EDS e surrounded by regions of UN of exceptionally high density, and smaller silicide inclusions which had segregated into the grain boundary. One such region can be seen in Fig. 6. Elsewhere were poorly sintered regions of very high porosity and small
Fig. 3. LOM of etched silicides, showing wide inhomogeneity within the fabricated sample.
K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
21
Fig. 4. LOM of etched samples following multiple additional melts, revealing homogenous composition.
Fig. 5. XRD pattern of homogenized U3Si2, UN, co-milled powders, and the sintered composite. Peak broadening and displacement in the sintered composite are due to diffraction being performed on solid samples.
Table 2 Parameters of sintered pellets. Pellet
Mixing method
Silicide content
Sintering temperature, pressure
UNUSi1 UNUSi2 UNUSi3 UNUSi4 UNUSi5
Manual Manual Manual Manual Milled
5% 10% 20% 25% 10%
1450 1450 1450 1450 1450
C, C, C, C, C,
135 135 135 135 135
MPa MPa MPa MPa MPa
Density (g/ cm3) 13.92 13.72 13.72 13.82 13.66
inclusions of silicides of varying stoichiometry. The silicide phases present in the grain boundary and sintered under these conditions also contained trace amounts of nitrogen as
Fig. 6. A) EDS image of large U3Si2 inclusion, surrounded by dense UN structure, in turn surrounded by poorly sintered regions of high porosity and B) an overview of the pellet structure showing the repetition of the microstructure seen in A. Each sample is composed of 90% UN and 10% U3Si2, while the dominant phases in each region are noted in the micrograph.
measured by EDS, but due to the known issues regarding quantification of elements of dissimilar energy lines e such as uranium
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K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
and nitrogen e a precise quantification could not be made. However, the observation was taken as evidence of the formation of a ternary compound. Further attempts to analyze this phase were attempted by the use of EBSD, a technique used to identify the crystal structure of materials by comparing the electron diffraction of a material with a crystallographic database. These examinations indicated the observed phase was non-amorphous, possessing a crystal structure distinct from either UN or any known USi compound. However, the technique was unable to match the phase to any known structure, which suggests the need for more detailed studies of this ternary phase and its formation. 3.2.2. Milling of the silicide Given the nature of the large silicide inclusions present in the manually mixed samples, it was decided to first mill the silicide, and then mix using a planetary ball mill as described previously. The results of this yielded a very fine silicide powder e less than 1 mm e compared to the relatively larger, unmilled particles of UN at roughly 4 mm e Identical sintering parameters were used, and the resulting composite material displayed high homogeneity as well as a desirable dispersal and confinement of the silicide into the grain boundary. Moreover, the resulting silicides were evaluated to be of a uniform phase, i.e. no fraction of U3Si5 was reported when evaluated using EDS, as shown in Fig. 7. The use of chemical etching and LOM served to highlight the presence of the silicide phase within the material, as well as highlighting the microstructure of the UN matrix. The results of this analysis can be seen in Fig. 8. Image analysis of the pellet surface revealed a porosity of 1.4%, which is somewhat smaller than the measured value of 2.3% based upon densitometry using the Archimedean method with chloroform as the immersion medium and the calculated theoretical density. The UN grain size was evaluated using ASTM E112 and found to be 9.1 mm [23]. 4. Discussion 4.1. Silicide fabrication The samples obtained from arc-melting contained a wide range of heterogeneity within the material, which necessitated the implementation of a series of re-melts of the silicide in lieu of heat treatment. Similar studies have shown this to result in the volatilization and loss of some Si during the procedure, but this was not
Fig. 8. LOM image of composite fabricated using milling. When compared to the manual mixing procedure, silicide inclusions have been removed and confined to grain interfaces.
found to be problematic in this study [16]. Increased oxidation due to re-melting also did not present a concern. 4.2. Co-sintering 4.2.1. Varying silicide fraction The presence of large silicide inclusions surrounded by regions of very high density, as noted in the manually mixed samples, suggests the important role of the electrical resistivity and structure of the two materials. As the electrical resistivity of U3Si2 UN are very similar e 2 mU/cm and 1.7 mU/cm, respectively e and since the inclusions themselves were several times larger than the average UN particle size, the large, conductive silicide phases likely served to act as conduction nodes within the bulk material, i.e. the electrical current flowed preferentially through these phases rather than distributing homogeneously [24,25]. This served to create local temperatures well above the measured bulk values and caused both complete densification in the vicinity as well as phase transformations leading to the formation of U3Si5 and other possible interactions near the melting temperature. 4.2.2. Milling of the silicide By milling the silicide, the average particle size was reduced well below that obtained through manual mixing. This reduction of the particle size to below that of UN assured that the electrical current would preferentially flow through the bulk UN instead, and resulted in greatly enhanced homogenization of the composite. When comparing to the microstructure observed in the sintering of pure UN, the grain geometry can also be observed to follow a far more random and meandering path due to the presence of the silicide at the boundary [22]. Of greater consequence, interdiffusion of nitrogen into the USi matrix has been successfully suppressed, which has resulted in the preservation of the intended U3Si2 phase at the UN grain boundary. Complete saturation of the UN grain boundaries has not yet been achieved, which therefore suggests the need for greater optimization between silicide concentration and UN grain size. 5. Conclusions
Fig. 7. EDS image of composite fabricated using milling. When compared to the manual mixing procedure, silicide inclusions have been removed and confined to grain interfaces.
In this work, samples of U3Si2 were fabricated and composites of UN-U3Si2 were sintered to high density using the SPS method. Sintering of the composites using unmilled fractions of U3Si2 resulted in a highly heterogeneous material, featuring widely
K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
varying porosity, silicide inclusion size, and resultant silicide phases, all of which seems to support the conclusion that local temperatures around these inclusions were not adequately reflected by the bulk temperature measured using pyrometry. These conditions also precipitated nitrogen interdiffusion into the USI matrix, forming a probably ternary phase with unknown consequences for fuel performance. While resulting in potentially undesirable microstructural characteristics the phenomenon itself is of interest with respect to understanding the physical processes at work in the SPS technique. However, by reducing the particle size of the U3Si2, it has been possible to create a high density UN-U3Si2 composite fuel, with the silicide phase driven into the grain boundary of the bulk UN. Moreover, the particle size reduction has permitted a greater homogenization of the electrical current distribution, and therefore temperature, such that the high temperature reactions between U3Si2 and UN have been successfully suppressed. Acknowledgements
[6] [7] [8] [9]
[10] [11] [12]
[13] [14]
[15] [16]
The preceding work has been conducted within the framework of the ASGARD project under the European Atomic Energy Community’s 7th Framework Programme. The author also wishes to acknowledge the cooperation of Dr. Mohsin Saleemi and the Materials and Environmental Chemistry Department at Stockholm University as well as Oskar Karlsson and KIMAB for providing the use of the EBSD technique. References [1] J. Carmack, F. Goldner, S. Bragg-Sitton, L. Snead, LWR Fuel Performance Meeting, TopFuel, Overview of the U.S. DoE Accident Tolerant Fuel Development Program, vol. 2, 2013, pp. 734e739. [2] A.T. Nelson, J.T. White, D.D. Byler, J.T. Dunwoody, J.A. Valdez, K.J. McClellan, Overview of Properties and Performance of Uranium-Silicide Compounts for Light Water Applications, vol. 110, Transactions of the American Nuclear Society, Reno, Nevada, June 15e19e2014. [3] J.M. Harp, P.A. Lessing, R.E. Hoggan, Uranium silicide pellet fabrication by powder metallurgy for accident tolerant fuel evaluation and irradiation, J. Nucl. Mater. 466 (November 2015) 728e738. [4] S.J. Zinkle, K.A. Terrani, J.C. Gehin, L.J. Ott, L.L. Snead, Accident tolerant fuels for LWRs: a perspective, J. Nucl. Mater. 448 (1e3) (May 2014) 374e379. [5] K.A. Terrani, D. Wang, L.J. Ott, R.O. Montgomery, The effect of fuel thermal
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conductivity on the behavior of LWR cores during loss-of-coolant accidents, J. Nucl. Mater. 448 (2014) 512e519. S. Sunder, N.H. Miller, Xps and XRD studies of corrosion of uranium nitride by water, J. Alloys Compd. 271 (1998) 568e572. R.R. Metroka, Fabrication of Uranium Mononitride Compacts, NASA, July 1970. NASA Technical Note TN-5876. Y. Arai, S. Fukushima, K. Shiozawa, M. . Handa, Fabrication of (U,Pu)N fuel pellets, J. Nucl. Mater. 168 (1989) 280e289. R.B. Matthews, K.M. Chidester, C.W. Hoth, R.E. Mason, R.L. Petty, Fabrication and testing of uranium nitridce fuel for space power reactors, J. Nucl. Mater. 151 (1988) 334e344. R.M. German, Sintering: from Empirical Observations to Scientific Principles, Butterworth-Heinemann, Oxford, UK, 2014 (Print). P.A. Lessing, Oxidation Protection of Uranium Nitride Fuel Using Liquid Phase Sintering, March 2012. INL/EXT-12e24974. L.H. Ortega, B.J. Blamer, J.A. Evans, S.M. McDeavitt, Development of an accident-tolerant fuel composite from uranium mononitride (UN) and uranium sesquisilicide (U3Si2) with increased uranium loading, J. Nucl. Mater. 471 (2016) 116e121. Y.S. Kim, Uranium intermetallic fuels, Compr. Nucl. Mater. 3 (2012) 391e422. J.T. White, A.T. Nelson, D.D. Byler, D.J. Safarik, J.T. Dunwoody, K.J. McClellan, Thermophysical properties of U3Si5 to 1773 K, J. Nucl. Mater. 456 (2015) 442e448. J. Wallenius, Transmutation of Nuclear Waste, LeadCold Reactors, Stockholm, Sweden, 2011 (Print). J.T. White, A.T. Nelson, D.D. Byler, D.J. Safarik, J.T. Dunwoody, K.J. McClellan, Thermophysical properties of U3Si2 to 1773 K, J. Nucl. Mater. 464 (2015) 275e280. S.L. Hayes, J.K. Thomas, K.L. Peddicord, Material property correlations for uranium mononitride III. Transport properties, J. Nucl. Mater. 171 (1990) 289e299. P. Malkki, M. Jolkkonen, T. Hollmer, J. Wallenius, Manufacture of fully dense uranium nitride pellets using hydride derived powders with spare plasma sintering, J. Nucl. Mater. 452 (1e3) (September 2014) 548e551. K. Johnson, J. Wallenius, M. Jolkkonen The Fabrication of Low Impurity Uranium Nitrides in an Unprotected Environment. Journal of Nuclear Materials. Submitted, awaiting publication. A.M. Raftery, Xps and XRD studies of corrosion of uranium nitride by water, J. Alloys Compd. 271 (1998) 568e572. Powder Diffraction File Inorganic Phases: Alphabetical Index, Inorganic Phases, JCPDS/Internation Center for Diffraction Data, Swarthmore, Pennsylvania, 1988. K. Johnson, J. Wallenius, M. Jolkkonen, A. Claisse Spark Plasma Sintering and Porosity Studies of Uranium Nitride. Journal of Nuclear Materials. Submitted, awaiting publication. ASTM E112-96, Standard Test Methods for Determining Average Grain Size, ASM International, West Conshohocken, PA, 2004. T. Miyadai, H. Mori, T. Oguchi, Y. Tazuke, H. Amitsuka, T. Kuwai, Y. Miyako, Magnetic and electrical properties of the U-Si system (part II), J. Magnetism Magnetic Mater. 104e107 (Feb. 1992) 47e48. N.A. Curry, An investigation of the magnetic structure of uranium nitride by neutron diffraction, Proc. Phys. Soc. 86 (1965).
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Fabrication and microstructural analysis of UN-U3Si2 composites for accident tolerant fuel applications Kyle D. Johnson*, Alicia M. Raftery, Denise Adorno Lopes, Janne Wallenius €gskolan, Stockholm, Sweden Albanova Universitets Centrum, Kungliga Tekniska Ho
h i g h l i g h t s U3Si2 fabricated from elemental uranium and silicon through arc melting. Homogeneity of the silicides assessed through densitometry, XRD, SEM and EDS, chemical etching and optical microscopy. UN powder fabricated using hydriding-nitriding method. No phase transformations detected when sintering using silicide particle sizes less than UN particle size. High density composite (98%TD) fabricated with silicide grain coating using spark plasma sintering at 1450 C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2016 Received in revised form 28 April 2016 Accepted 2 May 2016 Available online 4 May 2016
In this study, U3Si2 was synthesized via the use of arc-melting and mixed with UN powders, which together were sintered using the SPS method. The study revealed a number of interesting conclusions regarding the stability of the system e namely the formation of a probable but as yet unidentified ternary phase coupled with the reduction of the stoichiometry in the nitride phase e as well as some insights into the mechanics of the sintering process itself. By milling the silicide powders and reducing its particle size ratio compared to UN, it was possible to form a high density UN-U3Si2 composite, with desirable microstructural characteristics for accident tolerant fuel applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nuclear fuel Accident tolerant fuel Uranium nitride Uranium silicide SPS U-N-Si system
1. Introduction Following the Fukushima-Daiichi nuclear disaster in 2011, the United States Department of Energy began funding intense and coordinated research into improving the safety, reliability, and economy of nuclear fuel systems through the consideration of alternative fuel and cladding concepts, namely the so-called Accident Tolerant Fuels (ATF). Candidate fuel-clad systems should exhibit a reduced hydrogen generation rate e specifically feature a compatibility with non-zircaloy cladding material e improved fission product retention, reduced cladding interaction with steam, and reduced fuel-cladding interactions. Among the candidates being considered are uranium silicide e principally the U3Si2 phase
* Corresponding author. E-mail address: [email protected] (K.D. Johnson). http://dx.doi.org/10.1016/j.jnucmat.2016.05.004 0022-3115/© 2016 Elsevier B.V. All rights reserved.
but also, to a lesser extent, U3Si5 e as well as uranium mononitride (UN) [1e3]. A comparison of selected material properties of interest is presented below in Table 1. As seen from Table 1, all of the ATF candidates exhibit greatly improved thermal conductivities when compared to UO2. More importantly for accident tolerance considerations, their respective conductivities increase with increasing temperature e in stark contrast to UO2. These improvements in thermal conductivity allow for peak fuel and cladding temperatures to be substantially reduced during normal operation and accident scenarios, and enhance the overall ability of the system to tolerate severe accident conditions [4,5]. The silicides both feature reduced melting temperatures and multiple phase transformations, behaviours which are not desired, while on the other hand they have been shown to provide improved resistance to oxidation by steam when compared to UN [6]. Of the silicides, only U3Si2 provides an increased metal density
K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
19
Table 1 Properties of ATF Fuel Candidates compared to UO2 Property
UO2
U3Si2
U3Si5
UN
Theoretical density (g/cm3) Uranium density (g/cm3) Thermal Conductivity (BoL) (W/m*K from 600 to 1400 K) Melting Point (K) Water Tolerance
10.96 9.66 6.0e2.9 [15] 3130 [15] Excellent [15]
12.2 [13] 11.31 15e27.5 [16] 1938 [13] Good [13]
9.06 [14] 7.57 8.0e16 [14] 2043 [13] Good [13]
14.32 13.52 19e25 [17] 2953 [15] Poor [15]
as compared to the oxide, which is why this phase was chosen for mixing and co-sintering with UN. UN, meanwhile, shows the highest uranium density of all of the candidates, which greatly improves economy of operation. UN has received considerable attention over the years, with substantial efforts aimed at improving the sinterability of the material using conventional sintering techniques. Temperatures in excess of 2300 C [7e9] have been required to sinter UN compacts up to 95% relative density, with additional undesired consequences in terms of the microstructure of the material, i.e. grain size, stoichiometry, etc. Aside from these undesired effects, sintering at these temperatures itself is known to be both costly and difficult [10]. Various proposals have been put forth to decrease the sintering temperatures e ergo costs e associated with such a process, perhaps most notably the use of a second phase with a lower melting point and so-called ”liquid phase sintering,” [11] which, using U3Si2, has succeeded in reaching 90% density at 1700 C, which otherwise required over 1900 C for UN alone [7,12]. Nevertheless, the use of these temperatures was observed to produce geometric deformation in the final sintered compact, as well as interactions between the UN and U3Si2 phases, which can perhaps result in undesirable material properties for the fuel. The objective of this study, therefore, was to explore the applicability of the SPS technique to produce UN-U3Si2 composites featuring both high density and a desirable microstructure, i.e. a ”coating” of U3Si2 able to provide improved resistance to oxidation, while at the same time suppressing the interaction of UN and U3Si2. 2. Experimental 2.1. Synthesis and characterisation of nitrides and silicides High purity uranium nitride powder was fabricated at KTH using a hydriding-nitriding method described previously [18,19] These powders were found to contain 5.4% nitrogen, 800 ppm oxygen, and 400 ppm carbon by elemental analysis. In parallel, U3Si2 ingots were fabricated using arc melting of metallic uranium and silicon in an argon atmosphere as described by Raftery [20]. Elemental silicon was obtained from AlfaAesar and metallic uranium from Materials Science Corporation. Samples were taken for impurity analysis using ICP-OES, while a LECO TC436DR was used for determination of oxygen and nitrogen, and a LECO series CS440 for carbon measurement. Analyses of the initial metal showed concentrations of less than 100 ppm oxygen, and 400 ppm carbon, as well as various minor impurities such as nickel, calcium, zinc, and aluminum, all of which were found to be less than 25 ppm using ICP-OES. Arc-melted samples were prepared using a slight hyperstoichiometric content of silicon to suppress the formation of U3Si in the event of silicon volatility, as noted by White [16]. The microstructure of the manufactured silicide was evaluated using SEM and LOM with 8 M nitric acid as an etchant, while crystallography was assessed using XRD, which was performed at room temperature under Cu K-a radiation using an Ni filter. The following conditions were adopted: voltage 40 kV; 30 mA; angle (2q) ranging from 20 to 80 , angular step of 0.02 and counting time
of 1 s. The phases present were indexed based on crystallographic data contained in records JCPDS [21]. This alloy was then ground into particles with widely varying particle sizes (up to 300 mm) and manually mixed with UN powders featuring an average particle size of 4 mm. These powders were characterized using XRD and XRF, and elemental analysis was performed.
2.2. Co-sintering Following characterisation, the powders were sintered using a modified Dr. Sinter SPS machine, contained within a glovebox under an inert, argon atmosphere. For sintering, a 30 mm tall, 30 mm wide, and 2 mm inner-diameter graphite die was used, with thin graphite paper used to protect the sample and die from interaction during sintering. These were provided by the National SPS Facility at Department of Materials and Environmental Chemistry at Stockholm University. After sintering, this bonded graphite paper was removed from the samples via grinding. The first batch of pellets produced in this study were sintered at 1450 C and 135 MPa, which was predicted to produce pellets at around 96% TD but at a fairly low temperature so as to avoid high temperature reactions [22]. Samples were prepared with 5%, 10%, 20%, and 25% weight fraction U3Si2. The sintered pellets were then measured for density as described previously and examined using LOM, SEM, EDS; and EBSD [22]. In Fig. 1, the relevant sintering parameters, i.e. voltage, current, applied pressure, temperature, and Z-axis displacement for a given pellet, can be seen. A second route was attempted whereby U3Si2 was milled in a planetary ball mill contained within an argon glove box using stainless a steel cup and balls for a period of 30 min. Following this, the balls were removed and as-synthesized UN powder was added to the cup to form 10% weight fraction U3Si2, and the powders were then mixed by rotation for an additional 30 min. This mixture was then sintered using the previously described parameters.
Fig. 1. Sintering curves used for each pellet showing voltage, current, applied pressure, temperature, and typical Z-axis displacement.
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K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
3. Results 3.1. Silicide fabrication Multiple samples of U3Si2 were synthesized with densities ranging between 12.06 and 12.14 g/cm3, which compares favorably to the theoretical value of 12.2 g/cm3. An investigation was also made to determine the homogeneity of the samples, through the use of EDS and LOM. When analyzed by EDS, samples appeared homogenous with small inclusions of UO2 and U-rich phases, as seen in Fig. 2. Elemental analysis showed 600 ppm oxygen present within the material, while XRF showed 7.1 wt% Si and 92.9 wt% U, corresponding to 39.4 at% SI and 60.6 at% U. Etching and analysis using LOM revealed, however, the presence of a wide inhomogeneity within the material, demonstrating the need for heat treatment and annealing. Fig. 3 presents a view of the obtained alloy in the as-cast condition, using optical microscopy and metallographic etching. It can be observed that the sample has
a microstructure consisting of a primary precipitate with different sizes and geometries, and dendritic structures, which are an indication of the occurrence of a very inhomogeneous liquid during the solidification process. The etching produces a contrast in microsegration samples due to differing concentrations of alloying elements, which leads to different corrosion behaviors. In order to eliminate this variability within the silicide, subsequent samples were subjected to several e between 5 and 7 e additional melts, in lieu of prolonged heat treatment. In Fig. 4 it can be observed that the re-melting procedure produced a microstructure composed of a primary precipitate and low inter phase space. This morphology is in accordance with the invariant reaction proposal for U3Si2 in the phase diagram indicating that during the solidification process the liquid was in a homogenous condition. No increase in oxygen content was observed. X-ray diffraction performed on the sample revealed a predominant crystal structure matching that of the U3Si2 phase and a smaller UO2 phase, as seen in Fig. 5. 3.2. Co-sintering The mixture of nitride and silicide powders was sintered using the sintering and mixture parameters listed in Table 2 and resulted in the listed absolute densities, as measured by densitometry. Elemental analysis of the sintered compacts showed oxygen between 600 and 1000 ppm, and carbon ranging from 500 to 600 ppm, consistent with the initial materials. Nitrogen content varied in proportion to the admixed silicide content.
Fig. 2. EDS mapping of arc-melted U3Si2 sample showing UO2 and U-rich phases.
3.2.1. Varying silicide fraction In the pellets fabricated using manual grinding and mixing, the resultant pellets displayed a highly heterogeneous structure, characterized by large inclusions of U3Si2 and U3Si5 e as evaluated by EDS e surrounded by regions of UN of exceptionally high density, and smaller silicide inclusions which had segregated into the grain boundary. One such region can be seen in Fig. 6. Elsewhere were poorly sintered regions of very high porosity and small
Fig. 3. LOM of etched silicides, showing wide inhomogeneity within the fabricated sample.
K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
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Fig. 4. LOM of etched samples following multiple additional melts, revealing homogenous composition.
Fig. 5. XRD pattern of homogenized U3Si2, UN, co-milled powders, and the sintered composite. Peak broadening and displacement in the sintered composite are due to diffraction being performed on solid samples.
Table 2 Parameters of sintered pellets. Pellet
Mixing method
Silicide content
Sintering temperature, pressure
UNUSi1 UNUSi2 UNUSi3 UNUSi4 UNUSi5
Manual Manual Manual Manual Milled
5% 10% 20% 25% 10%
1450 1450 1450 1450 1450
C, C, C, C, C,
135 135 135 135 135
MPa MPa MPa MPa MPa
Density (g/ cm3) 13.92 13.72 13.72 13.82 13.66
inclusions of silicides of varying stoichiometry. The silicide phases present in the grain boundary and sintered under these conditions also contained trace amounts of nitrogen as
Fig. 6. A) EDS image of large U3Si2 inclusion, surrounded by dense UN structure, in turn surrounded by poorly sintered regions of high porosity and B) an overview of the pellet structure showing the repetition of the microstructure seen in A. Each sample is composed of 90% UN and 10% U3Si2, while the dominant phases in each region are noted in the micrograph.
measured by EDS, but due to the known issues regarding quantification of elements of dissimilar energy lines e such as uranium
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K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
and nitrogen e a precise quantification could not be made. However, the observation was taken as evidence of the formation of a ternary compound. Further attempts to analyze this phase were attempted by the use of EBSD, a technique used to identify the crystal structure of materials by comparing the electron diffraction of a material with a crystallographic database. These examinations indicated the observed phase was non-amorphous, possessing a crystal structure distinct from either UN or any known USi compound. However, the technique was unable to match the phase to any known structure, which suggests the need for more detailed studies of this ternary phase and its formation. 3.2.2. Milling of the silicide Given the nature of the large silicide inclusions present in the manually mixed samples, it was decided to first mill the silicide, and then mix using a planetary ball mill as described previously. The results of this yielded a very fine silicide powder e less than 1 mm e compared to the relatively larger, unmilled particles of UN at roughly 4 mm e Identical sintering parameters were used, and the resulting composite material displayed high homogeneity as well as a desirable dispersal and confinement of the silicide into the grain boundary. Moreover, the resulting silicides were evaluated to be of a uniform phase, i.e. no fraction of U3Si5 was reported when evaluated using EDS, as shown in Fig. 7. The use of chemical etching and LOM served to highlight the presence of the silicide phase within the material, as well as highlighting the microstructure of the UN matrix. The results of this analysis can be seen in Fig. 8. Image analysis of the pellet surface revealed a porosity of 1.4%, which is somewhat smaller than the measured value of 2.3% based upon densitometry using the Archimedean method with chloroform as the immersion medium and the calculated theoretical density. The UN grain size was evaluated using ASTM E112 and found to be 9.1 mm [23]. 4. Discussion 4.1. Silicide fabrication The samples obtained from arc-melting contained a wide range of heterogeneity within the material, which necessitated the implementation of a series of re-melts of the silicide in lieu of heat treatment. Similar studies have shown this to result in the volatilization and loss of some Si during the procedure, but this was not
Fig. 8. LOM image of composite fabricated using milling. When compared to the manual mixing procedure, silicide inclusions have been removed and confined to grain interfaces.
found to be problematic in this study [16]. Increased oxidation due to re-melting also did not present a concern. 4.2. Co-sintering 4.2.1. Varying silicide fraction The presence of large silicide inclusions surrounded by regions of very high density, as noted in the manually mixed samples, suggests the important role of the electrical resistivity and structure of the two materials. As the electrical resistivity of U3Si2 UN are very similar e 2 mU/cm and 1.7 mU/cm, respectively e and since the inclusions themselves were several times larger than the average UN particle size, the large, conductive silicide phases likely served to act as conduction nodes within the bulk material, i.e. the electrical current flowed preferentially through these phases rather than distributing homogeneously [24,25]. This served to create local temperatures well above the measured bulk values and caused both complete densification in the vicinity as well as phase transformations leading to the formation of U3Si5 and other possible interactions near the melting temperature. 4.2.2. Milling of the silicide By milling the silicide, the average particle size was reduced well below that obtained through manual mixing. This reduction of the particle size to below that of UN assured that the electrical current would preferentially flow through the bulk UN instead, and resulted in greatly enhanced homogenization of the composite. When comparing to the microstructure observed in the sintering of pure UN, the grain geometry can also be observed to follow a far more random and meandering path due to the presence of the silicide at the boundary [22]. Of greater consequence, interdiffusion of nitrogen into the USi matrix has been successfully suppressed, which has resulted in the preservation of the intended U3Si2 phase at the UN grain boundary. Complete saturation of the UN grain boundaries has not yet been achieved, which therefore suggests the need for greater optimization between silicide concentration and UN grain size. 5. Conclusions
Fig. 7. EDS image of composite fabricated using milling. When compared to the manual mixing procedure, silicide inclusions have been removed and confined to grain interfaces.
In this work, samples of U3Si2 were fabricated and composites of UN-U3Si2 were sintered to high density using the SPS method. Sintering of the composites using unmilled fractions of U3Si2 resulted in a highly heterogeneous material, featuring widely
K.D. Johnson et al. / Journal of Nuclear Materials 477 (2016) 18e23
varying porosity, silicide inclusion size, and resultant silicide phases, all of which seems to support the conclusion that local temperatures around these inclusions were not adequately reflected by the bulk temperature measured using pyrometry. These conditions also precipitated nitrogen interdiffusion into the USI matrix, forming a probably ternary phase with unknown consequences for fuel performance. While resulting in potentially undesirable microstructural characteristics the phenomenon itself is of interest with respect to understanding the physical processes at work in the SPS technique. However, by reducing the particle size of the U3Si2, it has been possible to create a high density UN-U3Si2 composite fuel, with the silicide phase driven into the grain boundary of the bulk UN. Moreover, the particle size reduction has permitted a greater homogenization of the electrical current distribution, and therefore temperature, such that the high temperature reactions between U3Si2 and UN have been successfully suppressed. Acknowledgements
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The preceding work has been conducted within the framework of the ASGARD project under the European Atomic Energy Community’s 7th Framework Programme. The author also wishes to acknowledge the cooperation of Dr. Mohsin Saleemi and the Materials and Environmental Chemistry Department at Stockholm University as well as Oskar Karlsson and KIMAB for providing the use of the EBSD technique. References [1] J. Carmack, F. Goldner, S. Bragg-Sitton, L. Snead, LWR Fuel Performance Meeting, TopFuel, Overview of the U.S. DoE Accident Tolerant Fuel Development Program, vol. 2, 2013, pp. 734e739. [2] A.T. Nelson, J.T. White, D.D. Byler, J.T. Dunwoody, J.A. Valdez, K.J. McClellan, Overview of Properties and Performance of Uranium-Silicide Compounts for Light Water Applications, vol. 110, Transactions of the American Nuclear Society, Reno, Nevada, June 15e19e2014. [3] J.M. Harp, P.A. Lessing, R.E. Hoggan, Uranium silicide pellet fabrication by powder metallurgy for accident tolerant fuel evaluation and irradiation, J. Nucl. Mater. 466 (November 2015) 728e738. [4] S.J. Zinkle, K.A. Terrani, J.C. Gehin, L.J. Ott, L.L. Snead, Accident tolerant fuels for LWRs: a perspective, J. Nucl. Mater. 448 (1e3) (May 2014) 374e379. [5] K.A. Terrani, D. Wang, L.J. Ott, R.O. Montgomery, The effect of fuel thermal
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