113 research outputs found
Structural transformations and disordering in zirconolite (CaZrTi2O7) at high pressure
No full textThere 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/
Stability of Cs-Ionsiv in Portland cement blends for radioactive waste disposal
No full textThe suitability of Portland cement blends for encapsulation of Cs-Ionsiv in a monolithic wasteform was investigated. No evidence of reaction or dissolution of the Cs-Ionsiv in the cementitious environment was found by scanning electron microscopy and X-ray diffraction. However, a small fraction (≤1.6 wt%) of the Cs inventory was released from the encapsulated Ionsiv during leaching experiments carried out on hydrated samples. Cs release was enhanced by exchange of K and Na present in the cementitious pore water. Cement systems lower in K and Na, such as slag based blends, showed lower Cs release than the fly ash based analogues. © 2010 Materials Research Society
The effect of specimen thickness on the shock propagation along the in-fibre direction of an aerospace-grade CFRP laminate
No full textIn-fibre measurements of the Hugoniot have been carried out on a carbon fibre-reinforced polymer composite. For this material, we have shown at high shock stresses, a two component wave was formed consisting of a fast moving ramped portion and a slower moving shock wave. Changing the thickness of test specimen for a given shock stress resulted in a change in the magnitude and duration of the ramped portion of the wave front. As the shock stress imparted to the target was reduced, or the thickness of the target was increased, the steep shock wave in the rear surface gauge was no longer apparent. Instead a relatively slow rising wave was measured. Consequently, to establish a Hugoniot at lower shock stress levels, relatively thin specimens of target material are required
Solution composition effects on the dissolution of a CeO<sub>2</sub> analogue for UO<sub>2</sub> and ThO<sub>2</sub> nuclear fuels
No full textThis study investigates the dissolution of CeO2, an isostructural analogue for U02 and ThO2, which was synthesized to closely approximate the microstructure of a spent nuclear fuel matrix. Dissolution of CeO2 particles was performed in simplified solutions representative of saline, near-neutral and alkaline ground waters that may be encountered in geological disposal scenarios, and in acidic medium for comparison. The normalized mass loss of cerium was found to be significantly influenced by the formation of colloidal particles, especially in the near- neutral and alkaline solutions investigated. The normalized dissolution rate, RL(Ce), k (g m-2 d-1, in these two solutions was found to be similar, but significantly lower than in a nitric acid medium. The activation energies based on the normalized release rate of cerium, at 40°C, 70°C and 90°C in each solution, were in the range of 24 ± 3 kJ mol-1 to 27 ± 7 kJ mol', indicative of a surface-mediated dissolution mechanism. The mechanism of dissolution was postulated to be similar in each of the solutions investigated, and further work is proposed to investigate the role of carbonate on the CeO2 dissolution mechanism.</p
Sintering of CaF2 pellets as nuclear fuel analog for surface stability experiments
No full textTo enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel, it is necessary to produce analogs that closely resemble nuclear fuel in terms of crystallography and microstructure. One such analog can be obtained by sintering CaF2 powder. This paper reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240 °C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900 °C and 1000 °C. When CaF2 was sintered above 1100 °C the formation of CaO at the grain boundaries caused the disintegration of the pellet due to hydration occurring after sintering. First results from a novel set-up of dissolution experiments show that changes in roughness, dissolution rate and etch pit shape of fluorite surfaces are strongly dependent on the crystallographic orientation of the expose surface. Consequently, the differences observed for each orientation will affect the overall dissolution rate and will lead to uncertainties in the estimation of dissolution rates of spent nuclear fuel.</p
Molten salt synthesis of Ce doped zirconolite for the immobilisation of pyroprocessing wastes and separated plutonium
No full textMolten 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
Reactive spark plasma synthesis of CaZrTi2O7 zirconolite ceramics for plutonium disposition
No full textNear single phase zirconolite ceramics, prototypically CaZrTi 2 O 7 , were fabricated by reactive spark plasma sintering (RSPS), from commercially available CaTiO 3 , ZrO 2 and TiO 2 reagents, after processing at 1200 °C for only 1 h. Ceramics were of theoretical density and formed with a controlled mean grain size of 1.9 ± 0.6 μm. The reducing conditions of RSPS afforded the presence of paramagnetic Ti 3+ , as demonstrated by EPR spectroscopy. Overall, this study demonstrates the potential for RSPS to be a disruptive technology for disposition of surplus separated plutonium stockpiles in ceramic wasteforms, given its inherent advantage of near net shape products and rapid throughput
Preparation, characterisation and dissolution of a CeO<sub>2</sub> analogue for UO<sub>2</sub> nuclear fuel
No full textThe behaviour of spent nuclear fuel under geological conditions is a major issue underpinning the safety case for final disposal. This work describes the preparation and characterisation of a non-radioactive UO2 fuel analogue, CeO2, to be used to investigate nuclear fuel dissolution under realistic repository conditions as part of a developing EU research programme. The densification behaviour of several cerium dioxide powders, derived from cerium oxalate, were investigated to aid the selection of a suitable powder for fabrication of fuel analogues for powder dissolution tests. CeO2 powders prepared by calcination of cerium oxalate at 800 °C and sintering at 1700 °C gave samples with similar microstructure to UO 2 fuel and SIMFUEL. The suitability of the optimised synthesis route for dissolution was tested in a dissolution experiment conducted at 90 °C in 0.01 M HNO3.</p
Ultra-facile mechanochemical synthesis of sodium and silver metaperiodate at room temperature
No full textUltra-facile mechanochemical synthesis of single phase metaperiodates, AgIO4 and NaIO4, was achieved in a matter of minutes by gentle grinding of AgNO3 or NaNO3 with orthoperiodic acid, H5IO6, at room temperature. The reaction mechanism is hypothesised to involve the equilibrium between H5IO6 and HIO4, effected under the tribological conditions of grinding, leading to the release of H2O, production of metaperiodate species, and formation of the products by dissolution – precipitation, as evidenced by powder X-ray diffraction and Scanning Electron Microscopy data. Time resolved X-ray diffraction demonstrated that initial grinding of reagents, over 60 s, is sufficient to initiate the reaction. The mechanochemical method presented here meets the need for a simple, safe, scalable and sustainable approach to the synthesis of sodium and silver metaperiodates.</p
Reducing the uncertainty of nuclear fuel dissolution:An investigation of UO<sub>2</sub> analogue CeO<sub>2</sub>
No full textIn this investigation, CeO2 analogues, which approximate as closely as possible the characteristics of fuel-grade UO2, were characterised after dissolution under a wide range of conditions. Powdered samples were subject to a range of aggressive and environmentally relevant alteration media with different solubility controls, and reacted at 70°C and 90°C. Dissolution kinetics were monitored through analysis of the coexisting aqueous solution. Monolith samples were monitored for development of surface defects such as pores and dissolution pits, in addition to morphological changes at grain boundaries and surface pores upon dissolution under aggressive conditions. The surfaces were analysed using confocal profilometry, vertical scanning interferometry and scanning electron microscopy. Dissolution rates were found to be greatest in low pH solutions and at higher temperatures. Preferential dissolution appears to occur at grain boundaries and on particular grains, suggesting a crystallographic control on dissolution.</p
- …
