STRUCTURE AND STABILITY OF URANIUM CONTAINING IRON GARNETS

Safe disposal of highly radioactive wastes from civilian nuclear reactors is a daunting challenge for sustained nuclear energy growth and associated environmental remediation. Use of crystalline garnets as waste forms to immobilize actinide radionuclides from nuclear wastes appears to be advantageous. Previous studies show that large loadings of uranium (U), and its analogues such as cerium (Ce) and thorium (Th) can be incorporated in garnet host phases, which have good aqueous durability at natural conditions. In this study, we synthesized a series of U-bearing garnet phases, Ca₃U_xZr_2-xFe₃O₁₂ (x = 0.5 - 0.7), as well as the Ca₃(Zr₂)SiFe³⁺₂O₁₂ end-member for comparison. The oxidation states of U in these phases were examined by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. U was found in mixed pentavalent and hexavalent oxidation states in the phases with x = 0.6 and 0.7. The oxidation state and coordination environment of Fe were investigated by ⁵⁷Fe-Mössbauer spectroscopy. Our results show that all the Fe cations in the U-substituted garnets are tetrahedrally coordinated Fe³⁺but in two different local environments. The enthalpies of formation of garnets from constituent oxides and elements were determined by high temperature oxide melt solution calorimetry. The thermodynamic analyses demonstrate that these substituted garnets can be stabilized under reducing conditions. Structural and thermodynamic analysis further provides explanations for the formation and stabilization of the natural uranium garnet elbrusite-(Zr), and supports the potential use of

Ca₃U_xZr_2-xFe₃O₁₂ phases as viable waste forms for U and other actinides.