Spark plasma sintering (SPS) densified U3Si2 pellets: Microstructure control and enhanced mechanical and oxidation properties
Introduction
As a promising nuclear fuel form, U3Si2 [[1], [2], [3], [4]] has been considered as an alternative for UO2 for a long time among many other candidates, including uranium nitride (UN) [5,6], uranium molybdenum (U–Mo) [[7], [8], [9]], and several other doped fuel forms [[10], [11], [12]]. UN has been tested in sodium-cooled reactors for years [13] and is currently being considered as a potential candidate for the GEN IV powder system [14], while U–Mo was being developed mainly for research and test reactors [15]. The development and study of U3Si2 receive particular attention since the 1960s [16] due to its higher uranium loading density [17] and higher thermal conductivity [[18], [19], [20]] at elevated temperatures. Higher uranium density leads to improved fissionable content [21] and extended cycle length [22]. Higher thermal conductivity allows a better heat release rate and lower working temperature, thus leading to improved safety margin and enhanced tolerance [23] under accidental conditions.
The manufacture of U3Si2 powder is complicated, which involves the conversion from UF6 to UF4, the reduction from UF4 to metallic uranium, and induction melting to produce the intermetallic U3Si2 [24,25]. Another approach developed by Idaho National Laboratory for a scalable product of uranium silicide [22] is associated with mixing the powders of uranium and silicon with a ratio close to stoichiometric and using arc melting technique to alloy the ingredients. The phase purity can reach 97% using this method. U3Si2 powder was then produced through ball milling, and dense pellets were consolidated using conventional vacuum sintering at temperatures higher than 1200 °C for several hours. Recently, spark plasma sintering (SPS) has become popular in sintering nuclear fuels. Yao [26] successfully synthesized dense UO2+x pellets with nanocrystalline grains under high pressure and low temperature. Yeo [27] sintered UO2 with 10 vol% SiC and obtained a 62% enhancement in thermal conductivity. Johnson [28] and Muta [29] sintered UN-U3Si2 and UN respectively with SPS and confirmed that SPS is capable of sintering high-density nuclear pellets with shorter sintering time. One of our recent work [30] also indicates that SPS is able to sinter commercial-size pellets with uniform densification and stoichiometry. In terms of U3Si2, Mohamad [19] and Lopes [31] manufactured U3Si2 at 850 °C, 75 MPa, 10 min, and 1200 °C, 50 MPa, 6 min respectively and both authors obtained more than 95% physical density. Eugene [32] systematically studied the potential industrial application of SPS in order to understand the process scalability of this advanced technique. They conducted a series of experiments with different setups and found that specimens with consistent properties can be achieved with uniform density and grain size, which demonstrated the excellent scalability of SPS. These authors proved the possibility and feasibility of using SPS to fabricate nuclear fuels that traditionally require high temperature and long time to sinter and revealed the great potential of SPS as a new means for rapid fabrication.
Thermal conductivity is a critical thermophysical property of the nuclear fuel materials since it determines temperature profile in the fuel during operation. When designing nuclear fuels, one of the major concerns is heat removal efficiency. One of the limitations with UO2 is its low thermal conductivity, especially at elevated temperatures. Low thermal conductivity leads to higher centerline temperature and higher thermal gradient between the centerline of the fuel and the cladding, thus increasing the likelihood of the thermal stress induced cracks and fuel failures [18]. A common way to measure thermal diffusivity of the material is through laser flash analysis, which utilizes a high-energy pulse to heat one side of the specimen and measure the time required for the temperature rise on the other side [33]. Microindentation testing was commonly applied as a means to determine mechanical properties of nuclear materials. Speidel [34] and Moore [35] determined the microhardness of UN to be 5.7 GPa and 6.3 GPa at 100 g load. Cappia [36] studied the microhardness of high burn-up UO2 and derived the relationship between microhardness and porosity. Recently, Mohamad [19] and Metzger [37] derived the hardness and fracture toughness of U3Si2, and hardness was found to be highly related to porosity.
The oxidation and corrosion resistance of U3Si2 is critical for fuel performance and accident tolerance as silicide fuels are prone to rapid oxidation and severe corrosion when exposed to water vapor and/or ambient conditions. At scenarios such as loss of coolant, steam oxidation and air oxidation occur and lead to the degradation of structural integrity, rapid pulverization and washout of the fuel materials to the primary loop [38]. The dynamic oxidation behavior of U3Si2 is generally tested with thermogravimetric analysis, where the mass of the specimen can be continuously measured, and the onset temperature can be obtained. Accompanying with X-ray analysis, the detailed oxidation process and the oxidation production during each step can be analyzed. Wood [39] studied the oxidation behavior of U–Si compounds and determined the onset temperature of U3Si2, U3Si, and U3Si5 to be 384 °C, 390 °C, and 185 °C respectively. Johnson [40] performed thermogravimetric analysis (TGA) on U3Si2 and a series of UN specimens and their results indicate that SPS sintered U3Si2 has an onset temperature of 470 °C, which is improved comparing to that of UN’s (320–440 °C) [40] and the reference sample UO2 (the onset temperature: 405 °C [40] or 455 °C [39]). The development of oxidation and corrosion resistance of U3Si2 is critical for the realization of the U3Si2 as the leading concept of accident tolerant fuels. The key questions include: (1) how to further increase the onset temperature of oxidation and (2) how to reduce the kinetics of oxidation and corrosion kinetics and increase the coping time of fuels under accident conditions.
In the current work, we report the synthesis of dense U3Si2 pellets using spark plasma sintering with controlled microstructure and length scales from micron to nano-sized regimes and drastically improved oxidation behavior. The SPS densified pellets with a theoretical density of 95% were synthesized at 1000 °C and 40 MPa for 5 min, and the grain sizes were determined to be 5.65 μm (microcrystalline) and 280 nm (nanocrystalline). Thermal conductivity was determined based on the measured thermal diffusivity and compared with literature data. Microhardness and fracture toughness measurement indicated that the SPS densified U3Si2 pellets are both mechanically strong and tough as compared with the counterparts sintered by conventional sintering. Of particular importance, our SPS densified U3Si2 pellets show enhanced oxidation resistance with an enhanced onset temperature of oxidation above 500 °C as measured by TGA analysis. Dense nanocrystalline (referred as nc-hereafter) silicide pellets shows significantly improved oxidation resistance with reduced oxidation kinetics than its microcrystalline (referred as mc-hereafter) counterpart, in which the full oxidation and weight gain of the dense nanocrystalline pellets are completed at a temperature beyond 900 °C during dynamic oxidation testing. These results show immerse potentials of microstructure control in improving the key characteristics of U3Si2 as the potential ATF fuel.
Section snippets
Powder preparation and spark plasma sintering
The original powder was produced through the powder metallurgy in INL, which includes the mixing of uranium and silicon powders with near stoichiometric quantities, arc melting at various current, thermal annealing at 800 °C, and a quick heating up to 1450 °C. The detailed manufacturing procedure can be found in previous literature [22]. The as-received powders were subsequently refined into smaller particle size using high energy ball milling (HEBM), which was a mechanochemical process
Microstructure and phase composition of the SPS sintered U3Si2
Fig. 1 shows SEM images and EDS scan results of the SPS-sintered mc- and nc-specimens. It can be seen from the polished surface that both specimens are dense and uniform. Fig. 1A was chemically etched, which reflected the grain size of the mc-specimen being ∼5.65 μm. Fig. 1B is the fracture surface of the nanocrystalline sample, and the grain size is determined to be ∼280 nm. Fig. 1D shows the EDS mapping results scanned on a randomly selected region of mc-U3Si2 indicated in Fig. 1C, which
Conclusions
In summary, dense U3Si2 pellets were fabricated by SPS with controlled microstructure from micron-to nano-metered scales, improved thermal-mechanical properties, and oxidation resistance. The microstructure of the SPS sintered specimens is characterized, and thermal-mechanical properties and oxidation behavior are characterized on the SPS densified pellets by SEM, XRD, diffusivity measurements, TGA analysis, and micro-hardness testing. The correlation between microstructure and sintering
CRediT authorship contribution statement
Bowen Gong: Conceptualization, Investigation, Visualization, Writing - original draft, Writing - review & editing. Tiankai Yao: Investigation, Visualization, Writing - review & editing. Penghui Lei: Investigation, Visualization, Writing - review & editing. Jason Harp: Resources, Writing - review & editing. Andrew T. Nelson: Resources, Writing - review & editing. Jie Lian: Conceptualization, Funding acquisition, Resources, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the U.S. Department of Energy, Office of Nuclear Energy, under a Nuclear Engineer University Program [award number: DE-NE0008532].
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