Fracture surface energy of UO2 and ThO2 containing additives.

The Hertzian indentation technique is used to study the cracking behavior and to measure the fracture surface energy of UO2 and ThO2of high density containing simulated fission products (Nb, Zr). UO2 + ZrO2 of 98% density showed permanent “plastic” deformation in addition to Hertzian cone crack formation thus making a quantitative analysis impossible. The Nb2O5 containing oxides, in particular ThO2 + Nb2O5, had a more suitable microstructure and showed less or no permanent deformation. The fracture surface energy of ThO2 + 0.1 mYo Nb2O5 was γ= (2.6±0.4) Jm-2, and thus comparable to that of ThO2 without additives, (2.5±0.3)Jm-2, indicating that the effect of Nb impurities and of excess oxygen introduced together with the Nb5+ cancel each other.