Effect of the substitution of f-electron elements on the structure and elastic properties of UO2

The chemistry of nuclear reactor fuel initially is complex, and continuous loss of uranium and plutonium, and formation of a broad range of new species due to fission introduce a challenging time-dependence to this chemistry. Lanthanides and/or Actinides substitution on the uranium sublattice occurs (a) during fission, (b) when mixed oxide fuel is used, and (c) when minor Actinides are reprocessed in UO2 matrix fuel as part of a closed nuclear fuel cycle. These fission products and minor Actinides influence a variety of thermo-physical properties, which depend on structure and elastic properties. How these structural and elastic properties vary with Lanthanide and Actinide substitution is not well studied. In this study we use atomic level simulations to investigate the effect of 4+ and 3+ ion substitutions on the structural and elastic properties of urania matrix. Our results show that most of the 4+ ions reduce the overall lattice parameter, while all the 3+ ions considered here increased the lattice parameter of the urania matrix. This effect is guided by the interplay between the elastic and electrostatic effect of the substituted ions. We calculate the chemical expansion and chemical expansion coefficient with the change in concentration based on the ionic radii of the substituted 3+ and 4+ ions. In general, elastic properties are enhanced for 4+ ions substitution and reduced for 3+ ion substitution.