[en] Highlights: • Mechanical and thermal properties of UO₂ in the presence of lanthanide fission products are calculated using Density functional Theory. • Lanthanide substituted UO₂ has lower bulk and Young's moduli than undoped UO₂, which reduces the mechanical stability of the fuel pellet. • Phonon frequencies are obtained from Density Functional Perturbation Theory. • Thermal Properties such as specific heat capacity and thermal expansion coefficient are evaluated using Quasi-Harmonic Approximation. • Mechanical and thermal properties of pure and doped UO₂ obtained from our simulations match well with available experimental observations. - Abstract: The structural, mechanical and thermal properties of fuel materials going through fission process are prime concern in the context of safe nuclear reactor operations. The interaction of fission products with the nuclear fuel significantly changes the fuel performance. Understanding their impact on fuel behavior is very important for efficient reactor operations. For this purpose, the mechanical and thermal properties of Uranium dioxide (UO₂) fuel in the presence of lanthanide (Ln) fission products are calculated using first principles based electronic structure calculations. The density functional theory (DFT) calculations employed here is improved for strongly correlated 5f electrons of uranium using Hubbard-U corrections. The elastic properties such as bulk modulus, Young's modulus, Poisson's ratio etc. are evaluated from optimized structures of UO₂ and Ln-doped UO₂. The results obtained match very well with available experimental observations. Further, the thermal properties like heat capacity and coefficient of thermal expansion are calculated using quasi-harmonic approximation from phonon frequencies obtained from density functional perturbation theory. The calculated thermal properties gave excellent agreement with trends reported in available experimental studies. Atomic scale simulations performed here provide insight to most relevant fuel properties which will be helpful in designing advanced fuel materials.