Evaluation of mechanical properties of (Ce,Gd)O2-x as a surrogate for (U,Gd)O2-x solid solutions

The mechanical properties of CeO2-Gd2O3 solid solutions as a surrogate for UO2-Gd2O3 burnable poison fuel were evaluated by both experimental and molecular dynamics (MD) analyses. Surface observation and X-ray diffraction measurement showed that Gd2O3 doping into CeO2 depressed the grain size of CeO2-Gd2O3 solid solution and caused the expansion of the lattice parameter up to 15 wt% of Gd2O3 content. The mechanical properties were estimated from the sound velocities measured by the ultrasonic pulse-echo method. The sound velocities in CeO2-Gd2O3 were found to decrease with increasing Gd2O3 content, which caused the reduction in the elastic moduli and Debye temperature. On the other hand, the results obtained from MD analyses showed the lattice expansion of CeO2-Gd2O3, the decrease of the average nearest distance between Ce (or Gd) and O ions, and the decrease of oxygen coordination number around each cation while the Gd2O3 content was increasing. It was, therefore, found that the bond valence sum considered to be the effective cation charge slightly decreased with increasing Gd2O3 content. Thus, the crystal binding energy decreased and consequently, the elastic moduli decreased. In addition, the lower value of experimental elastic moduli may be partly caused by crystal imperfection, e.g. grain boundaries, pores, and cracks.