Analytical Chemistry Seminar – Xiaodong Zhao

About the event

Title: Zircon: Host Form for Actinide Radioisotopes Disposal

Abstract: Zircon (MSiO4) has been proposed for permanent disposal of actinides (e.g., Pu) produced from the U.S. defense program and other actinide radioisotopes from civil nuclear sectors (spent fuel from nuclear power plant and high-level nuclear waste from reprocessing plant). Due to the high radiation resistance, low leachability, and good long-term stability, zircon is considered as a promising host structure to accommodate actinide cations in its large polyhedral sites, especially for plutonium immobilization. Previously, structures and thermodynamics of ThSiO4 and USiO4 have been determined by diffraction, calorimetry investigation and solubility studies, explaining their natural occurrence and potential alter phase for spent fuel disposal. However, there are still limited structural and thermodynamic understanding for PuSiO4, due to the synthetic difficulties in acquiring a pure phase. Here, I present a preliminary DFT calculation on PuSiO4 and a literature review of experimental work using CeSiO4 as a surrogate from our group, together showing a non-negligible formation energy barrier of pure PuSiO4. I also propose a forward strategy that may stabilize Pu in zircon is adding other elements to the matrix. This has been partially validated by recent published work on solid solutions of Th and U in zircon, where it was demonstrated their non-ideal mixing proves an extra stabilization enthalpy to U so it can be immobilized under the conditions where the endmember USiO4 cannot.