Eric O'Quinn1 Jacob Shamblin1 Brandon Perlov1 Rodney Ewing2 Joerg Neuefeind3 Igor Gussev1 Maik Lang1

1, University of Tennessee, Knoxville, Tennessee, United States
2, Stanford University, Stanford, California, United States
3, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

A major obstacle to the disposal of high-level nuclear waste is the fabrication of durable materials that can safely immobilize radionuclides. Ceramic structures proposed as waste forms for underground repositories must demonstrate stability against self-irradiation of the incorporated actinides as well as chemical durability such that radioisotopes are not leached into the environment. One proposed ceramic waste form, the isometric spinel structure (AB2O4), is adopted by many chemical compositions and exhibits excellent radiation tolerance due to its ability to accommodate atomic-scale disordering. However, it is currently not yet fully understood how this disordering mechanism proceeds over a range of length scales and how it influences particle transport and phase stability under operational conditions, which entail high temperatures and self-irradiation. We have shown with neutron total scattering experiments that the short-range structure of disordered Mg1-xNixAl2O4 spinel is much more complex than previously thought with highly local cation-ordered distortions affecting the long-range lattice. Pair distribution function analysis suggests that this short-range ordering influences the response of spinel to ion irradiation. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for the immobilization of nuclear wastes.