Structural transformations and disordering in zirconolite (CaZrTi2O7) at high pressure

There is interest in identifying novel materials for use in radioactive waste applications and studying their behavior under high pressure conditions. The mineral zirconolite (CaZrTi2O7) exists naturally in trace amounts in diamond-bearing deep-seated metamorphic/igneous environments, and it is also identified as a potential ceramic phase for radionuclide sequestration. However, it has been shown to undergo radiation-induced metamictization resulting in amorphous forms. In this study we probed the high pressure structural properties of this pyrochlore-like structure to study its phase transformations and possible amorphization behavior. Combined synchrotron X-ray diffraction and Raman spectroscopy studies reveal a series of high pressure phase transformations. Starting from the ambient pressure monoclinic structure, an intermediate phase with P21/m symmetry is produced above 15.6 GPa via a first order transformation resulting in a wide coexistence range. Upon compression to above 56 GPa a disordered metastable phase III with a cotunnite-related structure appears that is recoverable to ambient conditions. We examine the similarity between the zirconolite behavior and the structural evolution of analogous pyrochlore systems under pressure.<br/

Designing a mobile augmented memory system for people with traumatic brain injuries

Augmented memory systems help people remember events in their lives. Individuals with Traumatic Brain Injury (TBI) often have memory impairments. We conducted a user study to learn about strategies individuals with TBI use to remember events in their lives. We explored what characteristics individuals with TBI expect of an augmented memory system. We then investigated these aspects in an initial mobile app design, and propose here a concept for a rehearsal application that addresses the issues found in our studies

Plutonium management policy in the United Kingdom: The need for a dual track strategy

The United Kingdom holds the largest stockpile of separated civil plutonium in the world, projected to reach 140 t, at the end of this decade, when reprocessing operations are complete. UK Government policy is that this material should be reused as MOX fuel in Light Water Reactors. This policy is re-examined in the light of recent experience of the US plutonium disposition programme, in which the MOX Fuel Fabrication Facility is now considered to be potentially unaffordable. Problematic aspects of US programme, relevant to the UK scenario, are reviewed, to understand the possible impact on UK policy. Based on the US experience and inherent uncertainty regarding the capital and operational costs of MOX fuel fabrication and plutonium immobilisation facilities, and the associated technical risks, it is concluded that the UK policy should explicitly adopt a dual track strategy to plutonium management, with commitment that: any remaining plutonium which is not converted into MOX fuel, or otherwise reused, will be immobilised and treated as waste for disposal. This will also ensure that the UK is positioned and prepared to take forward an immobilisation and disposal programme for the plutonium stockpile, should reuse as MOX fuel not prove an economic or viable option

Nonresonant valence-to-core x-ray emission spectroscopy of niobium

The valence-to-core (V2C) portion of x-ray emission spectroscopy (XES) measures the electron states close to the Fermi level. These states are involved in bonding, thus providing a measure of the chemistry of the material. In this article, we show the V2C XES spectra for several niobium compounds. The K β ′′ peak in the V2C XES results from the transition of a ligand 2 s electron into the 1 s core-hole of the niobium, a transition allowed by hybridization with the niobium 4 p . This location in energy of this weak peak shows a strong ligand dependence, thus providing a sensitive probe of the ligand environment about the niobium

Synthesis and characterisation of Ca1-xCexZrTi2-2xCr2xO7: Analogue zirconolite wasteform for the immobilisation of stockpiled UK plutonium

A series of Ca1-xCexZrTi2-2xCr2xO7 zirconolite ceramics (0 ≤ x ≤ 0.35) were reactively sintered in air at 1350 °C for 20 h. Single phase zirconolite-2M was formed for x ≤ 0.15, with Cr2O3 and an undesirable Ce-bearing perovskite phase present above x = 0.20. Electron diffraction analysis confirmed that the zirconolite-2M polytype was maintained over the solid solution. X-ray absorption near edge structure (XANES) data determined that between 10–20% Ce was speciated as Ce3+, and Cr was present uniformly as Cr3+ with near edge features consistent with occupation of octahedral sites within the zirconolite-2M structure. A sample corresponding to x = 0.20 was processed by reactive spark plasma sintering (RSPS), with a rapid processing time of less than 1 h. XANES data confirmed complete reduction to Ce3+ during RSPS, promoting the formation of a Ce-bearing perovskite, comprising 19.3 ± 0.4 wt. % of the phase assemblage

Combined Quantitative X-ray Diffraction, Scanning Electron Microscopy, and Transmission Electron Microscopy Investigations of Crystal Evolution in CaO–Al2O3–SiO2–TiO2–ZrO2–Nd2O3–Na2O System

Glass-ceramics, with a specific crystalline phase assembly, can combine the advantages of glass and ceramic and avoid their disadvantages. In this study, both cubic-zirconia and zirconolite-based glass-ceramics were obtained by the crystallization of SiO2-CaO-Al2O3-TiO2-ZrO2-Nd2O3-Na2O glass. Results show that all samples underwent a phase transformation from cubic-zirconia to zirconolite when crystallized at 900, 950, and 1000 °C. The size of the cubic-zirconia crystal could be controlled by temperature and dwelling time. Both cubic-zirconia and zirconolite crystals/particles show dendrite shapes, but with different dendrite branching. The dendrite cubic-zirconia showed highly oriented growth. Scanning electron microscopy images show that the branches of the cubic-zirconia crystal had a snowflake-like appearance, while those in zirconolite were composed of many individual crystals. Rietveld quantitative analysis revealed that the maximum amount of zirconolite was ∼19 wt %. A two-stage crystallization method was used to obtain different microstructures of zirconolite-based glass-ceramic. The amount of zirconolite remained approximately 19 wt %, but the individual crystals were smaller and more homogeneously dispersed in the dendrite structure than those obtained from one-stage crystallization. This process-control feature can result in different sizes and morphologies of cubic-zirconia and zirconolite crystals to facilitate the design of glass-ceramic waste forms for nuclear wastes

Iron phosphate glasses: Bulk properties and atomic scale structure

© 2017 The Authors Bulk properties such as glass transition temperature, density and thermal expansion of iron phosphate glass compositions, with replacement of Cs by Ba, are investigated as a surrogate for the transmutation of 137 Cs to 137 Ba, relevant to the immobilisation of Cs in glass. These studies are required to establish the appropriate incorporation rate of 137 Cs in iron phosphate glass. Density and glass transition temperature increases with the addition of BaO indicating the shrinkage and reticulation of the iron phosphate glass network. The average thermal expansion coefficient reduces from 19.8 × 10 −6 K −1 to 13.4 × 10 −6 K −1 , when 25 wt. % of Cs 2 O was replaced by 25 wt. % of BaO in caesium loaded iron phosphate glass. In addition to the above bulk properties, the role of Ba as a network modifier in the structure of iron phosphate glass is examined using various spectroscopic techniques. The Fe II content and average coordination number of iron in the glass network was estimated using Mössbauer spectroscopy. The Fe II content in the un-doped iron phosphate glass and barium doped iron phosphate glasses was 20, 21 and 22 ± 1% respectively and the average Fe coordination varied from 5.3 ± 0.2 to 5.7 ± 0.2 with increasing Ba content. The atomic scale structure was further probed by Fe K-edge X-ray absorption spectroscopy. The average coordination number provided by extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure was in good agreement with that given by the Mössbauer data

Role of microstructure and surface defects on the dissolution kinetics of CeO2, a UO2 fuel analogue.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesised CeO2 analogue for UO2 fuel. Dissolution was performed on: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects; and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90°C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce3+ influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long-term performance of spent fuel in geological disposal

Molten salt synthesis of Ce doped zirconolite for the immobilisation of pyroprocessing wastes and separated plutonium

Molten salt mediated synthesis of zirconolite Ca0.9Zr0.9Ce0.2Ti2O7 was investigated, as a target ceramic matrix for the clean-up of waste molten salts from pyroprocessing of spent nuclear fuels and the immobilisation of separated plutonium. A systematic study of reaction variables, including, reaction temperature, time, atmosphere, reagents and composition, was made to optimise the yield of the target zirconolite phase. Zirconolite 2 M and 3T polytypes were formed as the major phase (with minor perovskite) between 1000 – 1400 °C, in air, with the relative proportion of 2 M polytype increasing with temperature. Synthesis under 5% H2/N2 or Ar increased the proportion of minor perovskite phase and reduced the yield of the zirconolite phase. The yield of zirconolite polytypes was maximised with the addition of 10 wt% TiO2 and 5 wt% TiO2, yielding 91.7 ± 2.0 wt% zirconolite, primarily as the 2 M polytype, after reaction at 1200 °C for 2 h, in air. The particle size and morphology of the zirconolite product bears a close resemblance to that of the TiO2 precursor, demonstrating a dominant template growth mechanism. Although the molten salt mediated synthesis of zirconolite is effective at lower reaction temperature and time, compared to reactive sintering, this investigation has demonstrated that the approach does not offer any clear advantage with over conventional reactive sintering for the envisaged application

Transformation of Cs-IONSIV® into a ceramic wasteform by hot isostatic pressing

A simple method to directly convert Cs-exchanged IONSIV® IE-911 into a ceramic wasteform by hot isostatic pressing (1100 °C/190 MPa/2 hr) is presented. Two major Cs-containing phases, Cs2TiNb6O18 and Cs2ZrSi6O15, and a series of mixed oxides form. The microstructure and phase assemblage of the samples as a function of Cs content were examined using XRD, XRF, SEM and TEM/EDX. The chemical aqueous durability of the materials was investigated using the MCC-1 and PCT-B standard test methods. For HIPed Cs-IONSIV® samples, the MCC-1 normalised release rates of Cs were <1.57 × 10−1 g m−2 d−1 at 0–28 days, and <3.78 × 10−2 g m−2 d−1 for PCT-B at 7 days. The low rates are indicative of a safe long-term immobilisation matrix for Cs formed directly from spent IONSIV®. It was also demonstrated that the phase formation can be altered by adding Ti metal due to a controlled redox environment