2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 65-12
Presentation Time: 4:50 PM


GUO, Xiaofeng, Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, NAVROTSKY, Alexandra, Peter A. Rock Thermochemistry Laboratory and NEAT ORU, UC Davis, 4415 Chemistry Annex, One Shields Ave, Davis, CA 95616, KUKKADAPU, Ravi K., Environmental Molecular Sciences Laboratory, 3335 Innovation Boulevard, Richland, WA 99354, ENGELHARD, Mark, Pacific Northwest National Laboratory, EMSL, Richland, WA 99352, LANZIROTTI, Antonio, National Synchrotron Light Source, Brookhaven National Laboratory, The University of Chicago - CARS, Upton, NY 11973, NEWVILLE, Matt, Consortium for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, ILTON, Eugene, Fundamental and Computational Science, Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99354, SUTTON, Stephen R., CARS, University of Chicago, Buldg 434A, APS, 9700 S.Cass Ave, Argonne, IL 60439 and XU, Hongwu, Los Alamos National Laboratory, Los Alamos, NM 87545, xguo@lanl.gov

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, Ca3UxZr2-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

Ca3UxZr2-xFe3O12 phases as viable waste forms for U and other actinides.

  • LANL_Guo_GSA_Mtg_Nov_2015.ppt (8.4 MB)