Effects of Th doping on mechanical properties of U<sub>1–</sub><i><sub>x</sub></i>Th<i><sub>x</sub></i>O<sub>2</sub>: An atomistic simulation

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Since thorium (Th) owns high conversion ratio in thermal neutron spectrum, high melting temperature, high thermal conductivity and good corrosion resistance in high-temperature water, it can be doped into UO<sub>2</sub> based fuel to initiate the fission reaction, and improve the physical properties of UO<sub>2</sub>. Owing to the challenging experimental conditions and technologies, molecular dynamics (MD) simulations are conducted to investigate the influences of Th doping on the mechanical properties of U<sub>1–</sub><i><sub>x</sub></i>Th<i><sub>x</sub></i>O<sub>2</sub>. The phase transition from initial fluorite structure to the metastable scrutinyite phase when loading along the [001] direction is observed, which accords well with the previous density functional theory calculations. However, if U<sub>1–</sub><i><sub>x</sub></i>Th<i><sub>x</sub></i>O<sub>2</sub> is loaded along the [111] direction, only brittle fracture is observed. It is found that both the elastic modulus and fracture stress decrease linearly with elevating temperature but the fracture strain increases. As the Th concentration increases from 0 to 0.55, the elastic modulus first decreases and then increases; if the Th concentration is larger than 0.1, the fracture stress increases and the fracture strain decreases monotonically. The cracks are nucleated with an angle of 45º to the loading direction, propagate rapidly, and are characteristic of brittle fracture, which accords well with the classical failure criteria and experimental results for brittle materials. By comparison, the uniaxial tensile loading is also performed for polycrystalline U<sub>1–</sub><i><sub>x</sub></i>Th<i><sub>x</sub></i>O<sub>2</sub>. It is found that the elastic modulus and fracture stress decrease as the temperature increases. However, the elastic modulus is not sensitive to the Th concentration and the fracture increases as the Th concentration increases. The brittle intergranular fracture is observed in each of all polycrystalline samples. The obtained physical parameters are useful for designing the fuels in nuclear reactors.