Energetics of Ce and Pu incorporation into zirconolite waste-forms

Abstract

The General Utility Lattice Program (GULP) has been used to model the zirconolite lattice, calculate the energies of substituting Ce³⁺, Ce⁴⁺, Pu³⁺, Pu⁴⁺ and Fe³⁺ into the lattice both as single and multi-defect systems and model the formation of Ce³⁺, Ce⁴⁺, Pu³⁺ and Pu⁴⁺ doped zirconolite lattices. These results have been compared against experimental observations, with particular emphasis on those Ce containing solid solutions that exhibit Ce³⁺/Ce⁴⁺ mixed valence characteristics. It is found that the Ce³⁺/Ce⁴⁺ mixed valence is as a result of reduction within the lattice, with the Ce³⁺ being stabilised on the Ca site, and that this behaviour would not be expected for the corresponding Pu solid solutions.