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The development of UO2/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 UO2/Mo composites could dominate the effective thermal conductivity (ETC) of the composites. In this work, the interfacial microstructure of UO2/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 UO2/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 UO2/Mo interface. Based on the structure characterized above, the calculated ITR of the UO2/Mo composite was on the order of approximately 10-9 m2K/W.

UO2-Mo composites with a core-shell structure have been considered candidates for the thermal conductivity (TC)-enhanced UO2 pellets and have demonstrated commercial potential for use in novel high-level safety reactors. Nevertheless, UO2-Mo composites tend to form micro-cracks that are caused by the presence of residual stress (RS) during manufacturing. In this work, neutron diffraction measurements were employed to analyse the RS in UO2-Mo core-shell structured composites fabricated by spark plasma sintering (SPS) for the first time. It was found that in the UO2-Mo composites, the RS state present in the UO2 matrix was tensile in nature. The RS in the UO2 matrix increased with increaseing Mo content. There was a maximum value of 148 ± 15 MPa in the UO2-10 vol% Mo composite. The micro-cracks produced in the high-Mo content composites were explained by the results of the neutron diffraction measurements. These results could provide significant guidance for the manufacturing and improvement of the operational performance of UO2-Mo composites as next-generation fuels.

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.

UO2/Mo composite pellets with enhanced thermal conductivity have been considered for the novel construction of high-safety fuel systems. UO2/Mo core-shell composite pellets with a reasonable porosity of 4–5% were fabricated by spark plasma sintering (SPS). In the SPS UO2/Mo composite, the majority of pores remained homogeneous in the UO2 matrix, while it had a dense morphology in the continuous Mo channel for the heat conduction. The sintering behaviour of the UO2/Mo composite indicated that the incorporating Mo impeded densification of UO2, and the primary densification temperature ranged from 903 K to 1270 K. To introduce 2 vol% Mo to UO2, the thermal conductivity (TC) was enhanced to 4.02 W/m·K at 1073 K. The above result represented a 23.31% improvement over the value of 3.26 W/m·K (Fink) for a pure UO2 pellet, which was approximated by the revised Hasselman-Johnson model. In particular, the coefficient of thermal expansion (CTE) was reduced to 10.0 × 10−6/K from 298 K to 1673 K, representing an 8.83% reduction from the value of 10.98 × 10−6/K (Martin) for a pure UO2 pellet. The reduction effect on the CTE was superior to that of other UO2 matrix composites fuel systems and, hence, offers an external safety aspect for reactors at the elevated temperatures close to accident conditions. These results provide a feasible method for the fabrication of a UO2/Mo core-shell pellet as an accident tolerant fuel.

UO2–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 UO2 matrix tends to create micro-cracks caused by thermal stress generated during the sintering process. A novel type of UO2–Mo-SiCw composite is fabricated by spark plasma sintering (SPS) to enhance the thermal-mechanical properties of UO2. In the composite, SiCw is dispersed uniformly in the UO2 matrix to form microspheres, and Mo is distributed in a three-dimensional network to form heat-conduction channels. Micro-cracks in the UO2 matrix are significantly suppressed in the UO2–Mo composite, since only a small amount of SiCw is incorporated into the UO2. Indentation testing indicates a great capability of the SiCw to sharply enhance the crack propagation resistance of the UO2 matrix. The estimated indentation fracture toughness reaches 0.91 MPa m1/2 with the incorporation of 0.5 vol% SiCw, representing a 68.5% improvement over the value of 0.54 MPa m1/2 obtained for the UO2–Mo composite. The thermal conductivity of the UO2–4Mo composite at 1000 °C was enhanced by approximately 54.3%, 60.1% and 67.2% for a SiCw content of 0.5, 2, and 4 vol%, respectively, compared with pure UO2. These results provide new insight for further improving the safety and lifespan of UO2 fuel in reactors.

Large grain UO2 is considered as an accident tolerant fuel with great application potential due to its competitive advantage of good fission gas retention. In this paper, the influence of preparation parameters such as sintering atmosphere, mixing process, powder pretreatment and grain growth additives on the grain size of UO2 is systematically studied. The result shows that the factors mentioned above have different effects on the grain size of UO2. The grain growth of UO2 pellet sintered in oxidizing atmosphere is better than those in reducing atmosphere. The wet mixing process has a significant advantage over the dry mixing process. In addition, the powder pretreatment has little effect on grain growth while the influence of additives plays the main role. Large grain UO2 pellets with uniform grain size up to 150 μm are successfully prepared. Finally, the thermo-physical properties of the pellets are investigated.

Ti3SiC2 with a continuous network structure was introduced to UO2 to improve the thermal conductivity of UO2. A dense microstructure with a clean Ti3SiC2-UO2 interface was obtained using the spark plasma sintering (SPS) method. Ti3SiC2 exhibited high thermal conductivity at elevated temperature, and a coefficient of thermal expansion (CTE) close to UO2. Therefore, the thermal conductivity of Ti3SiC2/UO2 was greatly improved compared to that of UO2, particularly at high temperature. The enhancing effect of Ti3SiC2 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.

In this paper, the fabrication process of UO2-BeO composite pellets was optimized for improving the thermal conductivity based on multi-parameter theoretical analyses and experimental investigations. It was found that the density of BeO and UO2/BeO interfacial thermal resistance (ITR) are crucial parameters that affect the thermal conductivity of UO2-BeO. To effectively increase BeO density and decrease UO2/BeO ITR, the fabrication method of pressureless sintering with a spheroidizing process was proposed. Through this method, UO2-BeO composite with high thermal conductivity was obtained. 89.2% and 71.4% enhancements of the thermal conductivity over UO2 were achieved at room temperature and 673 K, respectively. This enhancement is higher than all the reported results in the previous literatures that fabricated UO2-BeO using normal sintering temperatures (