Abstract
Low thermal transport behavior along the radial direction of nuclear fuel pellets and pellet-cladding mechanical interaction significantly impact fuel performance and the safety of current nuclear energy systems. Here we report a new strategy of advanced fuel design in which highly thermally-conductive and mechanically-robust graphene nanoplatelets are incorporated into UO2 fuel matrix to improve fuel thermal-mechanical properties. The 2D geometry of the graphene nanoplatelets enables a unique lamellar structure upon fuel consolidation by spark plasma sintering. The thermal conductivity along the radial direction of the sintered fuel pellets at room temperature reaches 12.7 and 19.1 wm−1K−1 at 1 wt.% and 5 wt.% loadings of the graphene nanoplatelets, respectively, representing at least 74% and 162% enhancements as compared to pure UO2 fuel pellets. Indentation testing suggests great capability of the 2D graphene nanoplatelets to deflect and pin crack propagation, drastically improving the crack propagation resistance of fuel matrix. The estimated indentation fracture toughness reaches 3.5 MPa·m1/2 by 1 wt.% loading of graphene nano-platelets, representing a 150% improvement over 1.4 MPa·m1/2 for pure UO2 fuel pellets. Isothermal annealing of the composite fuel indicates that the graphene nano-platelet is able to retain its structure and properties against reaction with UO2 matrix up to 1150 °C.
Highlights
Advanced fuels with enhanced safety margin and accident tolerance are of vital importance for the safe operation of current light water reactor fleets and the development of future advanced nuclear energy systems
The average size of the equiaxial UO2 grains is 12 ± 1 μm, which falls into the typical grain size range of 10–15 μm for commercial LWRs UO2 fuels fabricated by conventional sintering[10]
The UO2-GNP composite fuel form prepared in this study shows the greatest promise with a simultaneously improved thermal conductivity and fracture toughness at a minimum loading of GNP (Note, the 1 wt.% GNP is 4.6 vol.% if density of graphite, 2.26 g/cm[3] is used for GNP)
Summary
Advanced fuels with enhanced safety margin and accident tolerance are of vital importance for the safe operation of current light water reactor fleets and the development of future advanced nuclear energy systems. Thermally-conductive secondary phases, such as SiC and carbon nanotubes, were incorporated into UO2 fuel pellets to increase thermal conductivity. A UO2–10 vol.% silicon carbide (SiC) composite pellet shows a 62% improvement of thermal conductivity as compared with pure UO26, and UO2-5 vol.% carbon nanotube shows 29.7% increase in thermal conductivity[5]. A 2D single sheet of carbon atoms bonding in a hexagonal arrangement, has extremely high in-plane thermal conductivity (3000 W m−1 K−1)[7] and mechanical stiffness (1060 GPa)[8], and is a potential filler as the reinforced component of composite fuels to improve their thermal-mechanical properties. A theoretical study[9] shows UO2-graphene composite fuels have a better performance that monolithic UO2 fuels due to the potential benefits of incorporating highly thermally-conductive graphene nano-platelet with low neutron absorption cross section. A shift of peaks from high 2θ to lower ones indicates the concurrent reduction and sintering of UO2 + x powder by SPS
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