Bulk and Young’s modulus of doped UO2 by synchrotron diffraction under high pressure and Knoop indentation

The elastic constants of doped UO2 simulating a burn-up in the reactor of up to 200 GWd/t M (≃20% U-atoms replaced by fission products) were measured by synchrotron diffraction under high pressure and by Knoop indentation. As a complement, also the corresponding thermal expansion was determined by X-ray diffraction at high temperature. It is shown that the elastic constants of the simulated fuels are increased with burn-up, while the thermal expansion is decreased, satisfying the Grüneisen equation. An increasing Grüneisen constant γ with burn-up is also suggested. The results are, however, in contradiction with empirical bulk modulus estimates and previous data from ultrasonic measurements indicating either slightly decreasing or unchanged to strongly decreasing stiffness with burn-up. The need to perform detailed cohesive energy calculations of the doped fuel (ab initio or empirical potential methods), as well as to review the role of microstructure heterogeneities in the ultrasonic E-modulus determinations are therefore suggested.