UO 2 /Mo composite pellets with enhanced thermal conductivity have been considered for the novel construction of high-safety fuel systems. UO 2 /Mo core-shell composite pellets with a reasonable porosity of 4–5% were fabricated by spark plasma sintering (SPS). In the SPS UO 2 /Mo composite, the majority of pores remained homogeneous in the UO 2 matrix, while it had a dense morphology in the continuous Mo channel for the heat conduction . The sintering behaviour of the UO 2 /Mo composite indicated that the incorporating Mo impeded densification of UO 2 , and the primary densification temperature ranged from 903 K to 1270 K. To introduce 2 vol% Mo to UO 2 , 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 UO 2 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 UO 2 pellet. The reduction effect on the CTE was superior to that of other UO 2 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 UO 2 /Mo core-shell pellet as an accident tolerant fuel.
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 m 1/2 with the incorporation of 0.5 vol% SiC w , representing a 68.5% improvement over the value of 0.54 MPa m 1/2 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.
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