A MECHANISTIC UNDERSTANDING CATION INCORPORATION INTO SOLID PHASES

Cation incorporation into a host phase is a critical process for sequestering radionuclides in an engineered nuclear waste form, limiting the mobility of radionuclides in the environment, and suggesting processing signatures in actinide oxides. Understanding the atomistic-scale mechanism of incorporation enables the prediction of phase stability based on calculated thermodynamic data compared with experimental measurements. For example, the long term stability of a nuclear waste form depends on the stability of the structure with the ingrowth of radionuclide progeny. Thus, the energetics of mixing of parent and daughter radionuclides in a crystalline matrix provides insight into the long term stability of the waste form.

Quantum-mechanical calculations offer a tool for a detailed comparison of atomic-scale incorporation mechanisms. The thermodynamic stability of a phase can be quantified based on the enthalpy of formation, calculated with respect to the simple oxide components, and compared with calorimetric measurements. The advantage of using quantum-mechanical calculations to evaluate incorporation is that charge-compensation mechanisms can be quantitatively compared, where the incorporation energy is calculated as the reaction energy for a reaction describing the mechanism of incorporation based on realistic sources and sinks for the atoms involved in the incorporation process. The methodology for quantum-mechanical incorporation calculations is described using a series of examples relevant to nuclear waste management, including Cs-incorporation into hollandite (e.g., Ba₂Ga₄Ti₄O₁₆) and U-incorporation into hematite (Fe₂O₃).