Structural and Spectroscopic Investigation of Nuclear Fuel Cycle Related Materials

This thesis focuses on investigating structural and spectroscopic properties of phases relevant to the nuclear fuel cycle, with a focus on actinide oxalates. One such series of actinide oxalates relevant to the fuel cycle, actinide(IV) oxalate hexahydrates (An(C2O4)2·6H2O; An = Th, U, Np, and Pu), were synthesized and characterized to solve their single crystal structures. It was found that there were errors in previous work in the location of two of the water molecules. We present the first crystal structures for Th(C2O4)2·6H2O and Pu(C2O4)2·6H2O along with improved solutions of U(C2O4)2·6H2O and Np(C2O4)2·6H2O, which include two water molecules coordinated to the actinide metal centers, resulting in variable oxalate coordination orientation, which is not reported in the literature. To further probe the role of water molecules and the structural environment of water molecules in these structures, we performed inelastic neutron scattering (INS) and neutron diffraction studies on the hexahydrate and dihydrate phases of thorium and uranium(IV) oxalate. Through INS data, and supported by infrared spectroscopy on deuterated samples, we reveal two unreported phonon modes related to hydrogen vibrations. The assignment of the unreported mode is also supported by past computational work. The locations of the hydrogen/deuterium atoms could not be solved with Rietveld refinement, but the neutron diffraction patterns were otherwise in good agreement with the calculated patterns. Next, we investigate present the single crystal structures of a series of seven unreported uranyl oxalate phases, each with monovalent counter cations. These structures contribute unique solid-state topologies and expand our understanding of uranyl oxalate phases. The structures examined include a series of three structural isomers with different monovalent counter cations, M2(UO2)(C2O4)2(H2O)·H2O (M = NH4+, Rb+, Cs+), consisting of a monomer that composes is the fundamental building unit of the chains present in the next structures, Rb2(UO2)(C2O4)2 and Rb2(UO2)(C2O4)2·2H2O. The latter of these two phases contains an unreported chain topology in uranyl oxalate compounds. The final two compounds, (NH4)(UO2)OH(C2O4) and Li(UO2)2(C2O4)2(OH)(H2O)·4.5H2O, also present two unprecedented topologies of uranyl oxalate materials. Moving away from actinide oxalates, we present the Raman spectrum of uranium tetrachloride, which is a phase relevant for molten salt reactors (MSRs). Because of the air-sensitivity of this phase, UCl4 remains understudied. We present the first Raman spectra of UCl4 from 45 to 3200 cm-1 collected with 532 and 785 nm excitation sources as well as the assignment of 10 identified peaks to their respective vibrational modes. We compare the spectrum to those for isostructural ThCl4, the compositionally related UCl3, and the computed Raman bands for UCl4. The observed spectrum of UCl4 agrees with that of ThCl4 and the computed spectra of UCl4. It is possible that the observed differences between the spectra of UCl4 and UCl3 may be useful in differentiating these species for applications such as in situ monitoring of MSR operations. Finally, we examine the optical vibrational spectra of uranyl oxy-hydroxy-hydrate mineral phases, which are common alteration products of uraninite (UO2+x), a phase chemically and structurally analogous to uranium dioxide nuclear fuel. This work contributes vibrational spectroscopic data for uranyl minerals to the Compendium of Uranium Raman and Infrared Experimental Spectra (CURIES). Originally, only 37% of known uranyl oxy-hydroxy-hydrate minerals had spectra present in CURIES, and no infrared spectra for this mineral group was included in CURIES. We collected, and now include in CURIES, Raman and infrared spectra for an additional twelve uranyl hydroxide phases. We compare Raman spectra of different anion sheet topological groups and of analogous phases hosting different counter cations, and we identify spectroscopic variations related to differences in equatorial bonding and structural changes as a result of cation substitution. We also prepared a uranyl hydroxide phase via hydrolysis of uranyl fluoride (UO2F2) as an analog of hydrolysis reactions that occur in nuclear fuel cycle materials, and find that the alteration product of UO2F2, despite chemical and structural similarities to uranyl oxy-hydroxy-hydrate minerals, is readily distinguishable from related mineral phases using Raman spectroscopy.<p></p>