Thermochemical investigations of zirconolite, pyrochlore and brannerite: Candidate materials for the immobilization of plutonium

The thermochemical investigations of the end-member phases, pyrochlore and zirconolite and brannerite, a common impurity in titanium waste form assemblages were carried out, using the high-temperature oxide melt solution calorimetry. Cerium and U-bearing compositions were used to infer the energetics of plutonium-containing materials. A significant compositional region of thermodynamic instability as measured by the enthalpy change was defined for the waste form owing to the positive Gibbs formation energies. The results show that the resistance of a phase to radiation damage was related to its thermodynamic stability and the stability of radiation-induced isolated defects and damaged volumes.