Crystal chemistry of uranyl tungstates and actinide-mineral interactions

Nuclear power is a carbon-free source of electricity generation and represents one solution to increasing energy demands and the potential for reduced CO2 emissions. The actinide elements are the fuel of the nuclear cycle, but despite their importance, questions remain unanswered regarding the fundamental chemistry and geochemistry of this group of elements. Experimental work with actinide elements provides the basis for filling this gap in knowledge. The information obtained through experimental work will be essential for safe disposal of radioactive materials, the design of a cleaner fuel cycle, and control of the fate of these elements in the environment. In this work two major topics related to actinide chemistry are explored. The first is the crystal chemistry of uranyl tungstates, and how they relate to known uranyl compounds. Synthetic materials were characterized by single crystal X-ray diffraction. A series of isotypical uranyl tungstates with cation-cation interactions, an uncommon structural feature is described. The second topic addresses the interaction between actinide elements and minerals, and is divided into two themes. The first analyzes factors that affect the structural incorporation of neptunium(V) and uranium(VI) in synthetic carbonate and sulfate mineral. Synthetic materials were characterized by inductively coupled plasma mass spectrometry and luminescence spectroscopy. Structural incorporation of actinide elements in minerals may affect their migration in the subsurface. This study highlights the structural and chemical constraints that affect incorporation of the neptunyl and uranyl units into seven distinct minerals, and demonstrates different incorporation behaviors for Np(V) and U(VI). The second theme probes how the formation of uranyl peroxide nanocage clusters affects the aqueous solubility of U(IV) and U(VI) minerals. This work was conducted by reacting mineral powder with relative dilute peroxide bearing aqueous solutions with neutral to basic pH. A combination of chemical, spectrometric, and X-ray techniques was used to characterized solutions and reacted powders. Results indicate that the formation of uranyl peroxide nanoclusters can enhance the solubility of U(IV) and U(VI) solids by as much as orders of magnitude. This process may enhance the release of uranium and other radionuclides in the environment from spent fuel or secondary U(VI) minerals.