Melody L. Carter

Enthalpies of formation of Ce-pyrochlore, Ca0.93Ce1.00Ti2.035O7.00, U-pyrochlore, Ca1.46U4+0.23U6+0.46Ti1.85O7.00 and Gd-pyrochlore, Gd2Ti2O7: three materials relevant to the proposed waste form for excess weapons plutonium

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔH0f (kJ/mol), for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (−3656.0±5.6), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−3610.6±4.1) and Gd2Ti2O7 (−3822.5±4.9). Enthalpy of drop solution data, ΔHds, were used to calculate enthalpies of formation with respect to an oxide phase assemblage, ΔH0f−ox: CaO+MO2+2TiO2=CaMTi2O7 or Gd2O3+2TiO2=Gd2Ti2O7, and an oxide/perovskite phase assemblage, ΔH0f−pv+ox: CaTiO3+MO2+TiO2=CaMTi2O7, where M=Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔH0f−ox (kJ/mol) (Gd2Ti2O7)=−113.4±2.8; ΔH0f−ox(Ca1.46U4+0.23U6+0.46Ti1.85O7.00)=−123.1±3.4; ΔH0f−ox(Ca0.93Ce1.00Ti2.035O7.00)=−54.1±5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔH0f−pv+ox=−5.1±4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔH0f−pv+ox=+21.0±5.5 kJ/mol). A significant metastability field was defined with respect to an oxide/perovskite phase assemblage. However, the proposed waste form baseline composition lies in the stable regions of the phase diagrams.

Examination of U valence states in the brannerite structure by near-infrared diffuse reflectance and X-ray photoelectron spectroscopies

Abstract The valence state of uranium doped into a f 0 thorium analog of brannerite (i.e., thorutite) has been examined using near-infrared (NIR) diffuse reflectance (DRS) and X-ray photoelectron (XPS) spectroscopies. NIR transitions of U 4+ , which are not observed in spectra of brannerite, have been detected in the samples of U x Th 1− x Ti 2 O 6 , and we propose that strong specular reflectance is responsible for the lack of U 4+ features in UTi 2 O 6 . Characteristic U 5+ bands have been identified in samples in which sufficient Ca 2+ has been added to nominally effect complete oxidation to U 5+ . XPS results support the assignments of U 4+ and U 5+ by DRS. The presence of residual U 4+ bands in the spectra of the Ca-doped samples is consistent with segregation of Ca 2+ to the grain boundaries during high temperature sintering.

Structural phase transitions and magnetic order in SrTcO3

The structure of the perovskite SrTcO(3) has been investigated using both synchrotron X-ray and neutron powder diffraction. At room temperature SrTcO(3) is orthorhombic as a consequence of cooperative tilting of the corner sharing TcO(6) octahedra. The tilts are sequentially removed as the sample is heated with the oxide displaying the sequence of structres Pnma→Imma→I4/mcm→Pm ̅3m. Neutron powder diffraction data collected in the temperature range 4-1023 K indicate that SrTcO(3) has G-type antiferromagnetic structure, in which each Tc moment is antiparallel to its six nearest neighbours, below ∼1000 K. The magnetic structure is collinear antiferromagnetic with the technetium moments parallel to c-axis and can be described by the propagation vector k = [0,0,0] and the basis vector (0,0,A(z)). The same magnetic structure is observed in each of the four crystal structures.

The synthesis, crystal chemistry and structures of Y-doped brannerite (U1-xYxTi2O6) and thorutite (Th1-xYxTi2O6-δ) phases

Abstract Yttrium-doped uranium brannerite (U 1− x Y x Ti 2 O 6 ) and thorutite (Th 1− x Y x Ti 2 O 6− δ ) phases were synthesized in air at 1400°C. Powder X-ray diffraction revealed that these phases crystallized to form monoclinic ( C 2/ m ) structures. Crystal structures of U 0.54 Y 0.46 Ti 2 O 6 ( 1 ) ( a =9.8008(2); b =3.7276(1); c =6.8745(1); β =118.38(1); V =220.97(1); Z =2; R P =7.3%; R B =4.6%) and Th 0.91 Y 0.09 Ti 2 O 6− δ ( 2 ) ( a =9.8002(7); b =3.7510(3); c =6.9990(5); β =118.37(4); V =226.40(3); Z =2; R P =4.5%; R B =2.9%) were refined from powder neutron diffraction data. These two phases were isostructural, revealing planes of corner and edge-sharing TiO 6 octahedra separated by irregular eight-fold coordinate U/Y or Th/Y atoms. The oxygen sites within the structure of 1 were found to be fully occupied, confirming that the doping of lower valence Y atoms occurs in conjunction with the oxidation of U(IV) to U(V). Y doping of the thorutite phase 2 does not lead to oxidation but rather the formation of oxygen vacancies within the structure.

The Zn-MnO2 Battery : The Influence of Aqueous LiOH and KOH Electrolytes on the Intercalation Mechanism

Intercalation chemistry of the zinc–manganese dioxide (Zn–MnO2) electrochemical cell aiming at the development of aqueous rechargeable batteries is presented. This study includes electrochemical characterization of MnO2 in saturated aqueous lithium hydroxide (LiOH) and potassium hydroxide (KOH) electrolytes. The lithium insertion into MnO2 results in the formation of LixMnO2. The reversible deintercalation process prevails in the presence of LiOH electrolyte. Rather than the usual protonation (H+) which is apparent in the literature while using KOH electrolyte, in this work, K+ ion insertion into MnO2 is observed. However, the K+ ion insertion is found to be irreversible. The intercalation mechanism is confirmed using various techniques to characterize the discharged MnO2 cathode in LiOH and KOH electrolytes. The influence of small amounts of Bi2O3 (bismuth oxide) additive on the discharge behavior of MnO2 is also discussed.

Antiferromagnetism in a Technetium Oxide. Structure of CaTcO3

The technetium perovskite CaTcO(3) has been synthesized. Combining synchrotron X-ray and neutron diffraction, we found that CaTcO(3) is an antiferromagnetic with a surprisingly high Neel temperature of ∼800 K. The transition to the magnetic state does not involve a structural change, but there is obvious magnetostriction. Electronic structure calculations confirm the experimental results.

Leaching Behavior of Zirconolite-Rich Synroc Used to Immobilize High-Fired Plutonium Oxide

This study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ Pu0 2 . The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≃ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low ( −4 g m −2 d −1 ) and comparable rates from the waste form.

An electron and X-ray diffraction study of the compositely modulated barium nickel hollandite Bax(NixTi8—x)O16, 1.16 < x < 1.32, solid solution

Abstract Hollandite samples, Bax(NixTi8—x)O16 (1.1 < x < 1.40) made by ceramic techniques, have been studied by electron and X-ray diffraction. As a result, the barium nickel hollandite system is described as a composite modulated single phase solid solution made up of mutually incommensurable (along b) framework and Ba ion sub-structures. The magnitude of the incommensurate primary modulation wave-vector when defined with respect to the framework sub-structure q = bBa* (measured directly from electron diffraction patterns (EDPs)) acts as a chemical ruler in that it is directly related to composition and given by x/2. The superspace group symmetry was found to be I′2/m(0, x/2, 0)1. Both the framework and the Ba sub-structures have the same I2/m average structure space group symmetry. The solid solution limits for Bax(NixTi8—x)O16 were found to correspond to 1.16 < x < 1.32 . These limits were found by calculating the lattice parameters of both the frame work and sub-lattice from the powder X-ray diffraction patterns.

Further Studies of Synroc Immobilization of HLW Sludges and Tc for Hanford Tank Waste Remediation

Synroc/glass composites were designed for simulated Hanford HLW sludges containing U (the current “All-Blend” formulation). The composite contained ∼ 50 wt% of simulated HLW (oxide equivalent), to which ∼ 6, 10, 10, and 24 wt% of CaO, Al 2 O 3 , TiO 2 and SiO 2 were added, and melted under an argon atmosphere at 1350°C. The phase assemblage consisted of zirconolite, perovskite, spinel, nepheline, whitlockite and glass as major phases. Seven-day PCT tests yielded values of 2 for all elements studied. The PCT results were tolerant to changes of ∼ 20% of the inventories of the additives, and to variations in redox conditions. Technetium separated out during decontamination of liquid Hanford wastes can be incorporated as metal in hot-pressed Synroc prepared under reducing conditions, and its leach resistance is good (∼ 10 ∼3 g/m 2 /day at 70°C), and can be improved by alloying with iron group metals. With a choice of “neutral” (P(O 2 ) ∼ 10 −4 atm., near the Ni/NiO buffer) hot-pressing conditions, Tc can also be incorporated as Tc 4+ , substituting for Ti 4+ in the ceramic phases, and in this form, it should be highly leach resistant also.

HIPed Tailored Pyrochlore-Rich Glass-Ceramic Waste Forms for the Immobilization of Nuclear Waste

Hot isostatically pressed (HIPed) glass-ceramics for the immobilization of uranium-rich intermediate-level wastes and Hanford K-basin sludges were designed. These were based on pyrochlore-structured Ca (1-x) U (1+y) Ti 2 O 7 in glass, together with minor crystalline phases. Detailed microstructural, diffraction and spectroscopic characterization of selected glass-ceramic samples has been performed, and chemical durability is adequate, as measured by both MCC-1 and PCT-B leach tests.