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Abstract Ti 3 SiC 2 with a continuous network structure was introduced to UO 2 to improve the thermal conductivity of UO 2. A dense microstructure with a clean Ti 3 SiC 2 -UO 2 interface was obtained using the spark plasma sintering (SPS) method. Ti 3 SiC 2 exhibited high thermal conductivity at elevated temperature, and a coefficient of thermal expansion (CTE) close to UO 2. Therefore, the thermal conductivity of Ti 3 SiC 2 /UO 2 was greatly improved compared to that of UO 2 , particularly at high temperature. The enhancing effect of Ti 3 SiC 2 even exceeded those of SiC and BeO above the working temperature of fuel pellets. This improvement in thermal conductivity is of great importance for enhancing the fuel safety under accident conditions. [ABSTRACT FROM AUTHOR]

The high uranium density and good thermal conductivity at high temperature render U 3 Si 2 an excellent candidate for accident tolerant fuels (ATF). Spark plasma sintering (SPS) is a convenient method to produce U 3 Si 2 fuel and U 3 Si 2 -X fuels. However, the densification mechanism of U 3 Si 2 by SPS has been rarely investigated. In this work, U 3 Si 2 pellets were fabricated by SPS with 2 sets of different sintering parameters (the soak samples and the ramp samples). The grain size of U 3 Si 2 pellets sintered by SPS persisted ∼25 μm when soaked at 900–1300 °C and increased to ∼35 μm at 1400 °C. The morphology of the 900 °C soak sample was supposed to be due to spark plasma and plastic deformation. Utilizing the Helle-Granger model, the apparent activation energy (Q) and the stress exponent factor (n) can be determined. The soak samples exported n as 2.88, 3.16, and 3.55 (average 3.20) at 900, 1000, and 1100 °C, respectively. The ramp samples exported Q as 355.57, 325.48, 343.04, and 377.04 kJ/mol (average 350.28 kJ/mol) and n as 4.67, 4.22, 4.04, and 3.52 (average 4.11) at 65, 70, 75, and 80%TD, respectively. The n values of the two set of samples were both consistent with the suspected deformation hints in SEM. These results suggest that densification of SPS-sintered U 3 Si 2 occurred via plastic deformation controlled by dislocation motion. [ABSTRACT FROM AUTHOR]

The current distribution structure of the thermal conductivity (TC) enhancements in UO 2 matrix can be divided into three categories: dispersed structure, continuous structure and oriented structure. This paper develops a new semi-continuous structure to improve the TC of UO 2. Ti 3 SiC 2 is selected as the enhancement in this paper due to its excellent irradiation resistance and high TC at elevated temperature. The semi-continuous structure of Ti 3 SiC 2 is formed through a simple centrifugal spheroidization of UO 2 and a followed powder mixing process. Both the theoretical prediction and experimental results indicate the semi-continuous structure has a close TC enhancing efficiency to the continuous structure. Meanwhile, the semi-continuous structure has the advantages of simplifying the fabrication process and can be applied to more kinds of enhancements compared to the continuous structure. The TC of Ti 3 SiC 2 /UO 2 is remarkably improved at elevated temperature compared with pure UO 2 , which is beneficial to the fuel safety. [ABSTRACT FROM AUTHOR]

U 3 Si 2 -UO 2 composite fuel combines the advantages of high uranium density and thermal conductivity of U 3 Si 2 and excellent stability of UO 2 under light water reactor (LWR) condition. Since commercialized pressureless sintering of UO 2 is usually performed above the melting point of U 3 Si 2 , spark plasma sintering (SPS) is considered as a convenient method to produce U 3 Si 2 -UO 2 composite fuel. However, the potential interaction between U 3 Si 2 and UO 2 , an important aspect for composite materials, has not been sufficiently investigated. In this report, U 3 Si 2 -UO 2 pellets with 1:1 wt ratio were fabricated by SPS. Next, the interaction between U 3 Si 2 and UO 2 during the sintering process was examined with X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that U 3 Si 2 and UO 2 reacted to yield USi as the main product. The reaction occurred at a relatively low temperature of 1100 °C, at which the composite fuel was less than 90% theoretical density (TD). [ABSTRACT FROM AUTHOR]

The development of UO 2 /Mo composite pellets with enhanced thermal conductivity (TC) has been considered to be a promising route for improving the safety of nuclear reactors. The interfacial microstructure and especially the interfacial thermal resistance (ITR) of UO 2 /Mo composites could dominate the effective thermal conductivity (ETC) of the composites. In this work, the interfacial microstructure of UO 2 /Mo composites fabricated by spark plasma sintering (SPS) was investigated by high-resolution transmission electron microscopy (HRTEM) with energy dispersive spectroscopy (EDS) and auger electron spectroscopy (AES). The results indicated that UO 2 /Mo interface was a semi-coherent interfacial microstructure with an orientation of (220) UO 2 / (110) Mo and a lattice misfit degree of 0.117. Furthermore, there were no distinct interfacial products at the UO 2 /Mo interface. Based on the structure characterized above, the calculated ITR of the UO 2 /Mo composite was on the order of approximately 10-9 m2K/W. [ABSTRACT FROM AUTHOR]

SiC matrix microencapsulated composites with particle-free buffer (PB) layer were produced by spark plasma sintering for the first time. Microstructure and thermal physical performance of the composites from room temperature to 1900 °C were investigated. Dense composites with rational layout of tristructural isotropic (TRISO) particles and PB layer were obtained. The TRISO particles in the composites kept intact spherical shell structure. Thermal physical properties of the composites approached those of the SiC materials. The composites with high thermal conductivity and low coefficient of thermal expansion were suitable for nuclear applications. The effect of oxide additives on comprehensive performance of the composites was reappraised. Dissolution and recrystallization of SiC in the eutectic liquid phase revealed a new high temperature failure mechanism for the composites. [ABSTRACT FROM AUTHOR]

UO 2 –Mo composites are expected to be candidate materials for use as enhanced thermal fuel in the next generation of high-level safety reactors. Nevertheless, the UO 2 matrix tends to create micro-cracks caused by thermal stress generated during the sintering process. A novel type of UO 2 –Mo-SiC w composite is fabricated by spark plasma sintering (SPS) to enhance the thermal-mechanical properties of UO 2. In the composite, SiC w is dispersed uniformly in the UO 2 matrix to form microspheres, and Mo is distributed in a three-dimensional network to form heat-conduction channels. Micro-cracks in the UO 2 matrix are significantly suppressed in the UO 2 –Mo composite, since only a small amount of SiC w is incorporated into the UO 2. Indentation testing indicates a great capability of the SiC w to sharply enhance the crack propagation resistance of the UO 2 matrix. The estimated indentation fracture toughness reaches 0.91 MPa m1/2 with the incorporation of 0.5 vol% SiC w , representing a 68.5% improvement over the value of 0.54 MPa m1/2 obtained for the UO 2 –Mo composite. The thermal conductivity of the UO 2 –4Mo composite at 1000 °C was enhanced by approximately 54.3%, 60.1% and 67.2% for a SiC w content of 0.5, 2, and 4 vol%, respectively, compared with pure UO 2. These results provide new insight for further improving the safety and lifespan of UO 2 fuel in reactors. [ABSTRACT FROM AUTHOR]