Solubility, speciation and local environment of chlorine in zirconolite glass–ceramics for the immobilisation of plutonium residues

The immobilisation and disposal of fissile materials from civil and defence nuclear programmes requires compatible, passively safe and proliferation resistant wasteforms. In this study, we demonstrate the application of an albite glass–zirconolite ceramic material for immobilisation of chloride contaminated plutonium oxide residues in the United Kingdom. The chlorine solubility limit in the albite glass phase was determined to be 1.0 ± 0.1 wt%, above the maximum envisaged chorine inventory of 0.5 wt%, attainable at a 20 wt% PuO2 incorporation rate within the ceramic. Cl K-edge of X-ray Absorption Near Edge Spectroscopy (XANES) was exploited to confirm partitioning of Cl to the glass phase, speciated as the chloride anion, with exsolution of crystalline NaCl above the chlorine solubility limit. Combinatorial fitting of Cl XANES data, utilising a library of chemically plausible reference spectra, demonstrated the association of Cl with Na and Ca modifier cations, with environments characteristic of the aluminosilicate chloride minerals eudialyte, sodalite, chlorellestadite and afghanite. Adventitious incorporation of Ca, Zr and Ti within the albite glass phase apparently assists chlorine solubility, by templating a local chemical environment characteristic of the mineral reference compounds. The partitioning of Ce, as a Pu analogue, within the glass–ceramic was not adversely impacted by incorporation of Cl. The significance of this research is in demonstrating the compatibility of the glass–ceramic wasteform toward Cl solubility at the expected incorporation rate, below the determined solubility limit. Thus, an upstream heat treatment facility to remove chloride contamination, as specified in the current conceptual flowsheet, would not be required from the perspective of wasteform compatibility, thus providing scope to de-risk the technology roadmap and reduce the projected capital and operational plant costs

The response function of a sphere in a viscoelastic two-fluid medium

In order to address basic questions of importance to microrheology, we study the dynamics of a rigid sphere embedded in a model viscoelastic medium consisting of an elastic network permeated by a viscous fluid. We calculate the complete response of a single bead in this medium to an external force and compare the result to the commonly-accepted, generalized Stokes-Einstein relation (GSER). We find that our response function is well approximated by the GSER only within a particular frequency range determined by the material parameters of both the bead and the network. We then discuss the relevance of this result to recent experiments. Finally we discuss the approximations made in our solution of the response function by comparing our results to the exact solution for the response function of a bead in a viscous (Newtonian) fluid.Comment: 12 pages, 2 figure

Synthesis and characterisation of the hollandite solid solution Ba1.2-xCsxFe2.4-xTi5.6+xO16 for partitioning and conditioning of radiocaesium

The geological disposal of high level radioactive waste requires careful budgeting of the heat load produced by radiogenic decay. Removal of high-heat generating radionuclides, such as 137 Cs, reduces the heat load in the repository allowing the remaining high level waste to be packed closer together therefore reducing demand for repository space and the cost of the disposal of the remaining wastes. Hollandites have been proposed as a possible host matrix for the long-term disposal of Cs separated from HLW raffinate. The incorporation of Cs into the hollandite phase is aided by substitution of cations on the B-site of the hollandite structure, including iron. A range of Cs containing iron hollandites were synthesised via an alkoxide-nitrate route and the structural environment of Fe in the resultant material characterised by Mössbauer and X-ray Absorption Near Edge Spectroscopy. The results of spectroscopic analysis found that Fe was present as octahedrally co-ordinated Fe (III) in all cases and acts as an effective charge compensator over a wide solid solution range

The role of a disulfide bridge in the stability and folding kinetics of Arabidopsis thaliana cytochrome c6A

Cytochrome c 6A is a eukaryotic member of the Class I cytochrome c family possessing a high structural homology with photosynthetic cytochrome c 6 from cyanobacteria, but structurally and functionally distinct through the presence of a disulfide bond and a heme mid-point redox potential of + 71 mV (vs normal hydrogen electrode). The disulfide bond is part of a loop insertion peptide that forms a cap-like structure on top of the core α-helical fold. We have investigated the contribution of the disulfide bond to thermodynamic stability and (un)folding kinetics in cytochrome c 6A from Arabidopsis thaliana by making comparison with a photosynthetic cytochrome c 6 from Phormidium laminosum and through a mutant in which the Cys residues have been replaced with Ser residues (C67/73S). We find that the disulfide bond makes a significant contribution to overall stability in both the ferric and ferrous heme states. Both cytochromes c 6A and c 6 fold rapidly at neutral pH through an on-pathway intermediate. The unfolding rate for the C67/73S variant is significantly increased indicating that the formation of this region occurs late in the folding pathway. We conclude that the disulfide bridge in cytochrome c 6A acts as a conformational restraint in both the folding intermediate and native state of the protein and that it likely serves a structural rather than a previously proposed catalytic role. © 2011 Elsevier B.V. All rights reserved

Idling Magnetic White Dwarf in the Synchronizing Polar BY Cam. The Noah-2 Project

Results of a multi-color study of the variability of the magnetic cataclysmic variable BY Cam are presented. The observations were obtained at the Korean 1.8m and Ukrainian 2.6m, 1.2m and 38-cm telescopes in 2003-2005, 56 observational runs cover 189 hours. The variations of the mean brightness in different colors are correlated with a slope dR/dV=1.29(4), where the number in brackets denotes the error estimates in the last digits. For individual runs, this slope is much smaller ranging from 0.98(3) to 1.24(3), with a mean value of 1.11(1). Near the maximum, the slope becomes smaller for some nights, indicating more blue spectral energy distribution, whereas the night-to-night variability has an infrared character. For the simultaneous UBVRI photometry, the slopes increase with wavelength from dU/dR=0.23(1) to dI/dR=1.18(1). Such wavelength dependence is opposite to that observed in non-magnetic cataclysmic variables, in an agreement to the model of cyclotron emission. The principal component analysis shows two (with a third at the limit of detection) components of variablitity with different spectral energy distribution, which possibly correspond to different regions of emission. The scalegram analysis shows a highest peak corresponding to the 200-min spin variability, its quarter and to the 30-min and 8-min QPOs. The amplitudes of all these components are dependent on wavelength and luminosity state. The light curves were fitted by a statistically optimal trigonometrical polynomial (up to 4-th order) to take into account a 4-hump structure. The dependences of these parameters on the phase of the beat period and on mean brightness are discussed. The amplitude of spin variations increases with an increasing wavelength and with decreasing brightnessComment: 30pages, 11figures, accepted in Cent.Eur.J.Phy

Relationship between incommensurability and superconductivity in Peierls distorted charge-density-wave systems

We study the pairing potential induced by fluctuations around a charge-density wave (CDW) with scattering vector Q by means of the Froehlich transformation. For general commensurability M, defined as |k+M*Q>=|k>, we find that the intraband pair scattering within the M subbands scales with M whereas the interband pair scattering becomes suppressed with increasing CDW order parameter. As a consequence superconductivity is suppressed when the Fermi energy is located between the subbands as it is usually the case for nesting induced CDW's, but due to the vertex renormalization it can be substantially enhanced when the chemical potential is shifted sufficiently inside one of the subbands. The model can help to understand the experimentally observed dependence of the superconducting transition temperature from the stripe phase incommensurability in the lanthanum cuprates.Comment: 6 pages, 3 figure

Genomic DNA transposition induced by human PGBD5

Transposons are mobile genetic elements that are found in nearly all organisms, including humans. Mobilization of DNA transposons by transposase enzymes can cause genomic rearrangements, but our knowledge of human genes derived from transposases is limited. In this study, we find that the protein encoded by human PGBD5, the most evolutionarily conserved transposable element-derived gene in vertebrates, can induce stereotypical cut-and-paste DNA transposition in human cells. Genomic integration activity of PGBD5 requires distinct aspartic acid residues in its transposase domain, and specific DNA sequences containing inverted terminal repeats with similarity to piggyBac transposons. DNA transposition catalyzed by PGBD5 in human cells occurs genome-wide, with precise transposon excision and preference for insertion at TTAA sites. The apparent conservation of DNA transposition activity by PGBD5 suggests that genomic remodeling contributes to its biological function

The Magnetic Sun: Reversals and Long-Term Variations

A didactic introduction to current thinking on some aspects of the solar dynamo is given for geophysicists and planetary scientists.Comment: 17 pages, 9 figures; Space Science Rev., in pres

Revisiting the Local Scaling Hypothesis in Stably Stratified Atmospheric Boundary Layer Turbulence: an Integration of Field and Laboratory Measurements with Large-eddy Simulations

The `local scaling' hypothesis, first introduced by Nieuwstadt two decades ago, describes the turbulence structure of stable boundary layers in a very succinct way and is an integral part of numerous local closure-based numerical weather prediction models. However, the validity of this hypothesis under very stable conditions is a subject of on-going debate. In this work, we attempt to address this controversial issue by performing extensive analyses of turbulence data from several field campaigns, wind-tunnel experiments and large-eddy simulations. Wide range of stabilities, diverse field conditions and a comprehensive set of turbulence statistics make this study distinct

Partially filled stripes in the two dimensional Hubbard model: statics and dynamics

The internal structure of stripes in the two dimensional Hubbard model is studied by going beyond the Hartree-Fock approximation. Partially filled stripes, consistent with experimental observations, are stabilized by quantum fluctuations, included through the Configuration Interaction method. Hopping of short regions of the stripes in the transverse direction is comparable to the bare hopping element. The integrated value of $n_{\bf \vec{k}}$ compares well with experimental results.Comment: 4 page