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We refine the crystal structures of a systematic series of compounds with the general composition Ho (x) Yb (2-x) TiO 5 (x = 2, 1.6, 1.2, 1, 0.8, 0.4, 0) and Er 2 TiO 5 and find a transition from defect-pyrochlore to defect-fluorite structure with increasing ytterbium content, decreasing lanthanide radius. Short-range structure modulations consisting of pyrochlore-like nano-domains are systematically characterised using transmission electron microscopy. We test the Kr 2+ 1 MeV ion-irradiation response of Ho 2 TiO 5 , HoYbTiO 5 , Yb 2 TiO 5 , and Er 2 TiO 5 , via the crystalline to amorphous transition observed by using the in-situ TEM approach. The critical dose of amorphisation, D c , was measured at various temperatures and used to calculate the critical temperature for maintaining crystallinity, T c . A trend of lower T c values with decreasing lanthanide radius is found. We describe a new approach for determining T c values using cross-sectional TEM analysis of ex-situ bulk irradiated, 1 MeV Se + , samples; Ho 2 TiO 5 , HoYbTiO 5 and Yb 2 TiO 5 . The results of D c and T c values using the two approaches vary; however the trends across the sample system remain the same. [ABSTRACT FROM AUTHOR]

The response of Ln 2 TiO 5 (where Ln is a lanthanide) compounds exposed to high-energy ions was used to test their suitability for nuclear-based applications, under two different but complementary conditions. Eight samples with nominal stoichiometry Ln 2 TiO 5 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy), of orthorhombic ( Pnma ) structure were irradiated, at various temperatures, with 1 MeV Kr 2+ ions in-situ within a transmission electron microscope. In each case, the fluence was increased until a phase transition from crystalline to amorphous was observed, termed critical dose D c . At certain elevated temperatures, the crystallinity was maintained irrespective of fluence. The critical temperature for maintaining crystallinity, T c , varied non-uniformly across the series. The T c was consistently high for La, Pr, Nd and Sm 2 TiO 5 before sequential improvement from Eu to Dy 2 TiO 5 with T c 's dropping from 974 K to 712 K. In addition, bulk Dy 2 TiO 5 was irradiated with 12 MeV Au + ions at 300 K, 723 K and 823 K and monitored via grazing-incidence X-ray diffraction (GIXRD). At 300 K, only amorphisation is observed, with no transition to other structures, whilst at higher temperatures, specimens retained their original structure. The improved radiation tolerance of compounds containing smaller lanthanides has previously been attributed to their ability to form radiation-induced phase transitions. No such transitions were observed here. [ABSTRACT FROM AUTHOR]

In situ neutron powder diffraction experiments are applied to physical, kinetic, and microstructural characterization of uranium mononitride as a promising light water reactor fuel material. The temperature-variable coefficient of thermal expansion and isotropic Debye Waller factors are obtained by sequential Rietveld refinement over 499–1873 K. Oxidation of a UN pellet (95.2% density) under flow of 11 mg/min D 2 O is observed to initiate above 623 K and the rate increases by a factor of approximately 10 from 673 to 773 K, with activation energy 50.6 ± 1.3 kJ/mol; uranium oxide is the only solid corrosion product. [ABSTRACT FROM AUTHOR]

3C-β SiC was implanted with He and H ions using plasma immersion ion implantation (PIII). Regions of damage were created at various depths by applying a sample stage bias of 5 kV, 10 kV, 20 kV or 30 kV. Raman spectroscopy results indicate that He irradiation leads to more damage compared to H irradiation, as observed via increased disordered C and Si signals, as well as broadening of the SiC peaks. X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) results indicate significant change to the SiC structure and that surface oxidation has occurred following irradiation, with the degree of change varying dependent on impinging He fluence. The distributions of implanted species were measured using elastic recoil detection analysis. Despite the varying degree and depth of damage created in the SiC by the He ion irradiations, the retained H distribution was observed to not be affected by preceding He implantation. [ABSTRACT FROM AUTHOR]

Garnets have been considered as host phases for the safe immobilisation of high-level nuclear waste, as they have been shown to accommodate a wide range of elements across three different cation sites, such as Ca, Y, Mn on the a-site, Fe, Al, U, Zr, and Ti on the b-site, and Si, Fe, Al on the c-site. Garnets, due to their ability to have variable composition, make ideal model materials for the examination of radiation damage and recovery in nuclear materials, including as potential waste forms. Kimzeyite, Ca 3 Zr 2 FeAlSiO 12 , has been shown naturally to contain up to 30 wt% Zr, and has previously been examined to elucidate both the structure and ordering within the lattice. This study examines the effects of radiation damage and recovery using in-situ ion beam irradiation with 1 MeV Kr ions at the IVEM-TANDEM facility, Argonne National Laboratory. The complementary Hf containing system Ca 3 Hf 2 FeAlSiO 12 was also examined, and found to have a different response to irradiation damage. A sample of irradiated Ca 3 Zr 2 FeAlSiO 12 , at 1000 K, was characterised using aberration corrected (S)TEM and found to contain discreet, nano-sized, crystalline Fe rich particles, indicating a competing process during recovery is occurring. [ABSTRACT FROM AUTHOR]

The incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. [ABSTRACT FROM AUTHOR]

Abstract: Ceramics with the sodium zirconium phosphate or NZP type structure have potential as nuclear waste form and inert matrix materials. For both applications the material will be subjected to self-radiation damage from α-decay of the incorporated actinides. In this study, ion-beam irradiation using Au- and He-ions has been used to simulate the consequences of α-decay and the effects of irradiation on the structural and macroscopic properties (density and hardness) have been investigated. Irradiation by Au-ions resulted in a significant volume contraction of ∼7%, a reduction in hardness of ∼30% and a loss in long-range order at fluences above 1014 Au-ions/cm2. In contrast, little effect on the material properties was noted for samples irradiated with He-ions up to a fluence of 1017 ions/cm2. Thermal annealing was investigated for the highest fluence Au-ion irradiated sample and significant decomposition was observed. [Copyright &y& Elsevier]

Abstract: Raman spectra of synthetic non-substituted and U/Np/Pu- substituted thorutite (ThTi2O6) samples were collected. Factor group analysis was used to analyse the possible vibration modes. Theoretical simulations based on density functional theory (DFT) were performed on both CeTi2O6 and ThTi2O6. All possible vibrational modes for thorutite have been identified and assigned to the measured spectra. The presence and the effect of higher valence of U in thorutite has been discussed and the U–O bond lengths have been calculated by using the measured wavenumbers of ν 1 (UO2)2+ symmetric stretching vibrations. The presence of U6+ in uranyl form can be inferred from the wavenumbers of U–O vibrations. Corresponding Np–O and Pu–O vibrations were not observed in the Raman spectra. Thus the presence of Np6+ and Pu6+ in the studied samples was not established. [Copyright &y& Elsevier]

Abstract: The incorporation of plutonium (Pu) within lanthanum zirconate pyrochlore was investigated using air, argon, and N2–3.5%H2 sintering atmospheres together with Ca2+ and Sr2+ incorporation for charge compensation. The samples have been characterised in the first instance by X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The results show Pu can be exchanged for La3+ on the A-site with and without charge compensation and for Zr4+ on the B-site. DRS measurements were made over the wavenumber range of 4000–19,000cm−1 and the Pu in all air- and argon-sintered samples was found to be present as Pu4+ while that in samples sintered in N2–3.5%H2 was present as Pu3+. The Pu valence was confirmed for three of the samples using X-ray near-edge absorption spectroscopy (XANES). Pu valences >4+ were not observed in any of the samples. [Copyright &y& Elsevier]

The thermophysical and structural properties of calcium and barium zirconium phosphate ceramics (CZP and BZP) have been investigated for their potential candidacy as actinide hosts for inert matrix fuels (IMF) in nuclear reactors. These phosphate ceramics, which can accommodate minor actinides as well as the resulting fission products, are found to be thermally stable to 1600°C in air, however they begin to decompose in an inert atmosphere above approximately 1400°C. The heat capacity, thermal conductivity and bulk thermal-expansion were measured from room temperature up to 1200°C. Structural changes in this temperature region as well as the anisotropic thermal-expansion behaviour were studied using high-temperature X-ray diffraction. A phase change from R-3 to R-3c was identified for Ba0.5Zr2(PO4)3 near 880°C. The thermal conductivity for these ceramics at 1000°C was found to be 1.0Wm−1 K−1, a relatively low thermal conductivity that was increased to 5.0Wm−1 K−1 at 1000°C for BZP:Ni (25:75 mass ratio) cermet composites. [ABSTRACT FROM AUTHOR]

Abstract: A series of uranium-containing gadolinium zirconate samples have been fabricated at 1450°C in oxidizing, inert and reducing atmospheres. X-ray diffraction, Raman spectroscopy and transmission electron microscopy have been utilized to confirm adoption of pyrochlore or defect fluorite structures. X-ray diffraction allowed determination of the bulk averaged structure while Raman spectroscopy and transmission electron microscopy were used to determine ordering at the microdomain scale. Diffuse reflectance, X-ray absorption near edge structure and X-ray photoelectron spectroscopies indicated a predominantly U6+ oxidation state for all the air-sintered samples, even when Ca2+ or A-site vacancies were incorporated to charge balance for U4+, a mixed U5+/U6+ oxidation state was found for samples sintered in argon, while a mixed U4+/U5+ oxidation state occurred for sintering under N2–3.5%H2. This demonstrates a degree of uranium oxidation state control through sintering conditions, and the potential of using gadolinium zirconates as host materials for uranium in nuclear waste applications. [Copyright &y& Elsevier]

Abstract: Raman spectra of a well-characterized natural brannerite and some synthetic brannerite samples have been collected and analysed. For the first time, all 12 theoretical Raman vibrational modes of CeTi2O6 with brannerite structure have been positively identified by using quantum chemical calculations based on density functional theory (DFT) and assigned to the spectra. For synthetic samples with Y or Ca substitution on the U site and Fe substitution on the Ti site, Y–O, Fe–O and U–O vibrational modes are tentatively assigned. The U–O bond lengths for the uranyl (UO2)2+ groups, have been calculated by using the measured wavenumbers of ν 1 (UO2)2+ symmetric stretching vibrations and compared with the published U–O bond lengths of a natural brannerite. [Copyright &y& Elsevier]

The mechanical properties of zirconolite glass-ceramics designed for immobilising actinide-rich nuclear wastes, have been investigated through micro-compression tests pre and post 12 MeV gold ion irradiation at room temperature to a peak damage level of 100 displacements per atom (dpa). Whilst zirconolite crystals have become amorphized because of irradiation, transmission electron microscope analysis found no voids within the irradiated zirconolite. Nevertheless, voids of ∼25 nm in diameter were observed within the glass phase and appear to be homogeneously distributed. Surprisingly, micro-compression testing found the zirconolite glass-ceramic to exhibit a lower modulus, lower compression yield and greater toughness post irradiation. [ABSTRACT FROM AUTHOR]