Tetragonal tungsten bronzes (TTBs), an important class of oxides known to exhibit ferroelectricity, undergo complex distortions, including rotations of oxygen octahedra, which give rise to either ...incommensurately or commensurately modulated superstructures. Many TTBs display broad, frequency-dependent relaxor dielectric behavior rather than sharper frequency-independent normal ferroelectric anomalies, but the exact reasons that favor a particular type of dielectric response for a given composition remain unclear. In this contribution the influence of incommensurate/commensurate displacive modulations on the onset of relaxor/ferroelectric behavior in TTBs is assessed in the context of basic crystal-chemical factors, such as positional disorder, ionic radii and polarizabilities, and point defects. We present a predictive crystal-chemical model that rationalizes composition–structure–properties relations for a broad range of TTB systems.
Research and innovation is key to delivering UK Government's civil nuclear energy policy, in particular to accelerate reduction in the hazard, timescale and cost of legacy decommissioning and ...geological disposal of radioactive wastes. To address this challenge, a national centre of excellence, the HADES Facility, has been established to support research and innovation in High Activity Decommissioning Engineering & Science, as part of the wider network of UK National Nuclear User Facilities. Herein, we describe the development of this user facility, the current status of its capability, and functional equipment specifications. The unique capabilities of the HADES Facility, in the UK academic landscape, are emphasised, including: handling of weighable quantities of 99Tc and transuranics; quantitative electron probe microanalysis of radioactive materials; hot isostatic pressing of radioactive materials; and laboratory-based X-ray absorption and emission spectroscopy. An example case study of the application of the HADES capability is described, involving thermal treatment of a real radioactive ion exchange resin waste to produce a conceptual vitrified waste form.
Rare earth zirconates, such as Nd
2
Zr
2
O
7
, crystallise with the pyrochlore structure and are a group of materials which have been suggested as potential nuclear waste forms for actinide ...immobilisation. In this work, a new hydroxide co-precipitation route is presented to investigate the incorporation of Pu into Nd
2
Zr
2
O
7
. The plutonium content was varied between 5 and 10 mol% and the structural uptake and Pu oxidation state were probed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS). The experimental findings were complemented by DFT
ab initio
calculations. For all the incorporation mechanisms studied PuO
2
was used as the reference reactant state to allow for a direct comparison between the possible Pu uptake scenarios. Analysis of the experimental data suggests that Pu(
iv
) cations substitute for Nd(
iii
) cations leading to structural distortion of the pyrochlore A-sites. The computed solution energies and bond-distances corroborate the experimental findings and indicate that the excess charge is balanced
via
the introduction of oxygen at formerly vacant sites.
Experimental & theoretical insights into the structural response of complex oxides to dopant elements - the example of plutonium within pyrochlore.
The basic principles of incorporating high level radioactive waste into glasses, ceramics (Synroc type) and glass composites including glass ceramics are described. Current UK technology uses glass ...wasteforms for the products of reprocessing, although many countries are temporarily storing the ceramic spent fuel for eventual disposal. Some waste streams may be incorporated into ceramics, but difficult or legacy wastes will require the development of other wasteforms comprising composite systems of crystals and glass. The importance of processing-property-structure (especially durability) relations in such systems over size scales from the atomic to the geological and on timescales to hundreds of thousands of years is highlighted.
A suite of uranium brannerites for the disposal of MOX residues, formulated (U
0.9
Ce
0.1
)
1−
x
M
x
Ti
2
O
6
(M = Ca
2+
and/or Gd
3+
), were prepared using a mixed oxide route under oxidising, inert ...and reducing atmospheres (air, argon and H
2
/N
2
). Gd
3+
was added to act as a neutron absorber in the final Pu bearing wasteform and Ce added to function as a structural analogue for Pu. X-ray powder diffraction of the synthesised specimens found that phase distribution was strongly affected by the processing atmosphere and Gd content. In all cases prototypical brannerite was formed, accompanied by different secondary phases dependent on processing atmosphere. Microstructural analysis (SEM) of the sintered samples confirmed the results of the X-ray powder diffraction. Bulk XANES found that Ti remained in the Ti
4+
oxidation state whereas Ce was uniformly reduced to the Ce
3+
oxidation state regardless of processing conditions or stoichiometry. Micro-focus XANES was used to determine U oxidation in the brannerite phase and showed that U oxidised to higher U oxidation states to charge compensate. It was concluded that the charge balance mechanism was a combination of U oxidation and A-site vacancies.
This work presents the synthesis and characterisation of novel brannerite ceramics designed for the disposal of mixed oxide fuel residues.
SEM–EDX and Raman spectroscopy analysis of radioactive compounds is often restricted to dedicated instrumentation, within radiological working areas, to manage the hazard and risk of contamination. ...Here, we demonstrate application of WetSEM® capsules for containment of technetium powder materials, enabling routine multimodal characterisation with general user instrumentation, outside of a controlled radiological working area. The electron transparent membrane of WetSEM® capsules enables SEM imaging of submicron non-conducting technetium powders and acquisition of Tc Lα X-ray emission, using a low cost desktop SEM–EDX system, as well as acquisition of good quality μ-Raman spectra using a 532 nm laser.
To study radiation stability of iron phosphate glasses, cerium is used as surrogate of actinides. Cerium doped iron phosphate glasses have been synthesised. Heavy gold ion irradiations have been ...performed on pure and cerium doped iron phosphate glasses to mimic ballistic damage due to cascade of recoil atoms. Pure and cerium doped glasses are irradiated with gold ions of energy 750 keV at fluence of 2 × 10
15
ions/cm
2
. In this paper, ion irradiation effects on glass network modification and change in speciation of network former have been discussed. Significant changes in glass network structure and speciation of network former is observed.
Iron K-edge EXAFS and XANES have been used to study the local structural environment of dilute quantities (0.2mol% Fe2O3) of Fe3+ in seven representative SiO2–R2O–R’O glasses where R=(Li, Na, K), ...R’=(Mg, Ca, Sr, Ba). Sample preparation was carried out by adding excess CeO2 as an oxidising agent and it was confirmed by optical absorption spectroscopy that iron was fully oxidised to Fe3+. This methodology has made possible, for the first time, an unambiguous EXAFS study of the coordination and local structural environment of dilute levels of Fe3+ in alkali–alkaline earth–silica glasses without the complications presented by the presence of Fe in multiple oxidation states. Coordination of Fe3+ in these glasses is strongly affected by the nature of the alkali and alkaline earth cations, with Fe3+–O CN increasing from 4.0±0.4 to 6.2±0.9 with decreasing alkali/alkaline earth ionic radius ratio (IRR). Respective FeO bond lengths of 1.87±0.01Å to 1.92±0.01Å were also obtained. Dilute Fe3+ in the studied glasses displays selective behaviour, as demonstrated by the opposing effects of alkali and alkaline earth ions on measured Fe3+ CN and Fe3+O bond length.
•EXAFS of dilute, redox-pure Fe3+ in ternary silicate glasses•Fe3+ coordination strongly affected by nature of alkali and alkaline earth ions•Fe3+–O CN increases from 4 to 6 with decreasing alkali alkaline earth radius ratio.•Corresponding FeO bond lengths of 1.87Å to ca. 1.92Å obtained•Supports selective behaviour of Fe3+ with wider implications for glass structure
Technetium and transuranic elements (TRUs) are long-lived radionuclides, produced as a result of nuclear power generation. Co-immobilisation of these radionuclides in a ceramic wasteform is ...attractive as they are problematic for vitrification and would reduce the demand on a future geological disposal facility. A range of zirconolite ceramics have been produced via an oxide route using the surrogates Mo and Ce with a view to the co-immobilisation of Tc and TRUs. The resultant materials were characterised by XRD, SEM-EDX, TEM and XAS. Final phase assemblage was found to be affected by target stoichiometry, the Ca precursor used, processing temperature and processing atmosphere. Through appropriate optimisation of processing conditions and target stoichiometry, the results of this study show co-immobilisation of Tc and TRUs is a promising approach.
The effect of uranium on glass structure is of interest in the context both of geological and nuclear waste glasses. In this study, solid state Magic Angle Spinning Nuclear Magnetic Resonance ...(MAS-NMR) spectroscopy was used to establish the effect of UO3 on the bulk structure of mixed alkali borosilicate glass. The effects of increasing UO3 content on boron co-ordination, alkali co-ordination and network polymerisation, determined using MAS-NMR, show that UO3 acts essentially as an intermediate within the glass, occupying sites in the interstices of the glass network related to the alkali channels. This requires little structural rearrangement of the glass and so allows for the high solubility of UO3. It is shown that each mole of UO3 requires 2moles of alkali for charge balance. 7Li MAS-NMR spectra indicate that with the addition of UO3 the structural role of Li changes.
•UO22+ species acts essentially as an intermediate in borosilicate glasses.•XAS and NMR data show that UO22+ species occupy sites in the alkali channels.•1mol of UO3 requires charge compensation by 2mol of A2O.•UO3 addition results in increased silicate network polymerisation.
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