A high throughput computational investigation of the solid solution mechanisms of actinides and lanthanides in zirconolite

Abstract

In this work, we perform a theoretical investigation of the actinide and lanthanide solid solution mechanisms of zirconolite-2M, prototypically CaZrTi₂O₇, as a candidate immobilisation matrix for plutonium. Solid solution energies were calculated using static atomistic simulations by means of the General Utility Lattice Program, for formulations of relevance to ceramic wasteform deployment, with substitution on the Ca²⁺ and Zr⁴⁺ sites by Ce⁴⁺, Pu⁴⁺, Th⁴⁺, and U⁴⁺, and appropriate charge balance by substitution of Al³⁺ or Fe³⁺ on Ti⁴⁺ sites. In simple solid solutions involving substitution on the Zr⁴⁺ site, we found that whereas substitution of Ce⁴⁺, U⁴⁺ and Pu⁴⁺ were energetically favoured, substitution of Th⁴⁺ was not energetically favoured. For more complex solid solutions involving Ce⁴⁺, Pu⁴⁺, Th⁴⁺, and U⁴⁺ substitution on the Ca²⁺ site, we found the most energetically favoured scheme involved co-substitution of Al³⁺ or Fe³⁺ on the five-fold co-ordinate Ti⁴⁺ site in the zirconolite-2M structure. Comparison of these computational data with experimental evidence, where available, demonstrated broad agreement. Consequently, this study provides useful insight into formulation design and the efficacy of Ce⁴⁺, U⁴⁺ and Th⁴⁺ as Pu⁴⁺ surrogates in zirconolite-2M ceramic wasteforms for plutonium disposition.