Microstructural Characterization of U Coprecipitated Phases Formed in Bentonic-Granitic Groundwater and under Anoxic Conditions

2006 ◽  
Vol 985 ◽  
Author(s):  
Javier Quinones ◽  
Eduardo Iglesias ◽  
Jose M. Cobo ◽  
Aurora Martinez Esparza ◽  
Jose Maria Gomez de Salazar
Keyword(s):  
Dissolution Rate ◽  
Iron Powder ◽  
Microstructural Characterization ◽  
Secondary Phase ◽  
Radiation Shielding ◽  
Localized Corrosion ◽  
Spent Fuel ◽  
Secondary Phases ◽  
Anoxic Conditions

AbstractFor improving the accuracy of the performance assessment studies related to the spent fuel safety under storage conditions it is necessary to develop a new matrix alteration model. These models must be based on laboratory experiences and they should be capable to extrapolate to storing environmental conditions. Most of recently models developed included the oxidation and dissolution process of the spent fuel matrix, but the influence of a possible process of secondary phase formation over the spent fuel surface is not taken into account yet. This is a key process that could produce a reduction of the matrix dissolution rate, radiation shielding behaviour; however, the surface precipitation of the secondary phase could induce a localized corrosion process, which in this case dissolution rate of the spent fuel would be increased. This paper is focussed on microstructural characterization of secondary phases formed in coprecipitation experiments performed under anoxic conditions in granitic-bentonitic simulated groundwater. In order to simulate the influence of the container material, the coprecipitation experiments were performed in absence and presence of iron powder. The solid phases formed were characterized using the following techniques: XRD; SEM-EDX and TEM-EDX. The XRD diffraction pattern showed that under anoxic conditions a mixture of phases were obtained (sodium and potassium uranate and schoepite), whereas uranate phases were detected when only iron was present. The characterization study indicates that the U secondary phase formed (under reducing conditions and presence of iron powder) growth from iron surface. The crystal size of the secondary phase is independent of the presence of iron powder (and it is always less than 3 μm). Furthermore, the microstructural study showed the growing of U phases over iron powder.

Identification of Secondary Phases Formed During Unsaturated Reaction of UO2 with EJ-13 Water

1989 ◽  
Vol 176 ◽  
Author(s):  
J. K. Bates ◽  
B. S. Tani ◽  
E. Veleckis ◽  
O. J. Wronklewicz
Keyword(s):  
Phase Formation ◽  
Secondary Phase ◽  
Yucca Mountain ◽  
Solution Chemistry ◽  
Spent Fuel ◽  
Secondary Phases ◽  
Secondary Phase Formation

ABSTRACTA set of experiments, wherein UO2 has been contacted by dripping water, has been conducted over a period of 182.5 weeks. The experiments are being conducted to develop procedures to study spent fuel reaction under unsaturated conditions that are expected to exist over the lifetime of the proposed Yucca Mountain repository site. One half of the experiments have been terminated, while one half are ongoing. Analyses of solutions that have dripped from the reacted UO2 have been performed for all experiments, while reacted UO2 surfaces have been examined for the terminated experiments. A pulse of uranium release from the UO2 solid, combined with the formation of schoepite on the surface of the UO2, was observed between 39 and 96 weeks of reaction. Thereafter, the uranium release decreased and a second set of secondary phases was observed. The latter phases incorporated cations from the EJ-13 water and include boltwoodite, uranophane, sklodowskite, compreignacite, and schoepite. The experiments are continuing to monitor whether additional changes in solution chemistry or secondary phase formation occurs.

Synthesis and characterization of β-Sialon powders from Si, halloysite clay and AlN powders

2017 ◽  
Vol 31 (16-19) ◽  
pp. 1744080
Author(s):  
Li Yin ◽  
Mark Ian Jones
Keyword(s):  
Crystal Growth ◽  
Phase Transformations ◽  
Firing Temperature ◽  
Secondary Phase ◽  
Nitrogen Pressure ◽  
Main Phase ◽  
Synthesis And Characterization ◽  
Secondary Phases

Two [Formula: see text]-Sialons, with [Formula: see text]-values of 1 and 4, respectively, were successfully synthesized by silicothermal reduction and nitridation method under 0.4 MPa nitrogen pressure. The effect of firing temperatures on the phase transformations and morphologies of [Formula: see text]-Sialons were analyzed by XRD and SEM. For [Formula: see text]-Sialons ([Formula: see text]), the product was finally composed of targeted [Formula: see text]-Sialon ([Formula: see text]) and secondary phase [Formula: see text]-Si3N4; for [Formula: see text], [Formula: see text]-Sialon ([Formula: see text]) was the main phase, and 15R-Sialon and [Formula: see text]-Al2O3 co-existed as secondary phases. A higher firing temperature is more beneficial for the phase transformations and crystal growth of [Formula: see text]-Sialons, however, the most suitable firing temperature was 1400[Formula: see text]C.

Calcium Vanadinite: An Alternative Apatite Host for Cl-rich Wastes

2014 ◽  
Vol 1665 ◽  
pp. 319-324 ◽  
Author(s):  
M. R. Gilbert
Keyword(s):  
Elevated Temperatures ◽  
Secondary Phase ◽  
Phase System ◽  
Secondary Phases ◽  
Waste Forms ◽  
Good Potential ◽  
Multi Phase

ABSTRACTApatites are often seen as good potential candidates for the immobilization of halide-rich wastes and, in particular, chlorapatite (Ca5(PO4)3Cl) has received much attention in recent years. However, synthesis of chlorapatite waste-forms can produce a complicated multi-phase system, with a number of secondary phases forming, including β-TCP (Ca3(PO4)2), spodiosite (Ca2(PO4)Cl) and pyrophosphate (Ca2P2O7), many of which require elevated temperatures and extended calcinations times to reduce. Calcium vanadinite (Ca5(VO4)3Cl) demonstrates a much simpler phase system, with calcination at 750 °C yielding Ca5(VO4)3Cl together a small quantity of a Ca2V2O7secondary phase, the formation of which can be retarded by the addition of excess CaCl2. Characterization of compositions doped with SmCl3as an inactive analogue for AnCl3show the Cl to be immobilized in the vanadinite whilst the Sm forms a wakefieldite (SmVO4) phase.

scholarly journals Prediction of Radioactive Waste Glass Durability by the Hydration Thermodynamic Model: Application to Saturated Repository Environments

1989 ◽  
Vol 176 ◽  
Author(s):  
Carol M. Jantzen ◽  
W. Gene Ramsey
Keyword(s):  
Free Energy ◽  
Dissolution Rate ◽  
Phase Formation ◽  
Thermodynamic Model ◽  
Functional Dependence ◽  
Secondary Phase ◽  
Deionized Water ◽  
Secondary Phases ◽  
Durability Test ◽  
Secondary Phase Formation

ABSTRACTThe effects of groundwater chemistry on glass durability were examined using the hydration thermodynamic model. The relative durabilities of SiO2, obsidian, basalt, nuclear waste glasses, medieval window glasses, and a frit glass were determined in tuffaceous (J–13) groundwater, basaltic (GR–4) groundwater, WIPP–A brine, and Permian Basin brine (PBB–3) using the monolithic MCC–I durability test. In the groundwater–dominated MCC–l experiments, the interaction of the glasses and the initial groundwater (leachant) caused the formation of unique assemblages of secondary phases. The secondary phase formation, in turn, controlled the final groundwater (leachate) pH and ionic strength, I[t].Correlations of the final leachate pH and I[t] with the Si release from the glass indicated that it is the influence of the secondary phase formation on the leachate pH and I[t] that controls the final dissolution rate of the glass. Since I[t] and the pH of the leachates are functions of the precipitation reactions, inclusion of the experimentally determined solution pH in the free energy of hydration model provides for the functional dependence of the dissolution rate on the secondary precipitation. Therefore, superposition of the linear equation for the groundwater and deionized water experiments occurs and the hydration free energy model can be used to compare glass durability in deionized water and in repository groundwaters.

Effect of Secondary Phases Formation in the Leaching of UO2 Under Simulated Radiolytic Products

1994 ◽  
Vol 353 ◽  
Author(s):  
P. Díaz-Arocas ◽  
J. Quinoñes ◽  
C. Maffiotte ◽  
J. Serrano ◽  
J. Garcia ◽  
...  
Keyword(s):  
Secondary Phase ◽  
Point Of View ◽  
Spent Fuel ◽  
Secondary Phases ◽  
X Ray Diffraction ◽  
X Ray ◽  
Ph Values ◽  
Leaching Process ◽  
Leaching Experiments ◽  
Solid Phases

AbstractThe leaching of the spent fuel matrix (UO2) is function of the radiolytic products formation. The effect of each radioiytic product on the leaching process is not totally understood. In the literature, the influence of H2O2 on the dissolution process is described from the qualitative point of view, and most of the studies were performed for pH values from 8 to 12. In this paper we report on the effect of the H2O2 in the leaching process of UO2 by dissolution experiments at various H2O2 concentrations. Also, it was tested the influence of S/V ratio (surface area exposed to the leaching media) on the UO2 leaching and secondary phases formation. It was identified the formation of secondary phases on the UO2 surface. Solid phases characterization was carried out by x-ray Photoelectron Spectrometry (XPS), x-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. By XPS studies the secondary phase formed corresponded to a U(VI) phase. By XRD analyses the solid was identified as studtite, UO4 - 4H2O, (card n0 16–206, [I]). A comparison of the U(VI) phases formed in spent fuel and UO, leaching experiments in various media has been carried out.

Modelling of the (U1−y238Puy)O2+x Leaching Behaviour in Deionised Water under Anoxic Conditions

2003 ◽  
Vol 807 ◽  
Author(s):  
J. Quiñones ◽  
J. Cobos ◽  
P. Diaz-Arocas ◽  
V. V. Rondinella
Keyword(s):  
Radiation Field ◽  
Heterogeneous Systems ◽  
Surface Oxidation ◽  
Secondary Phase ◽  
Dissolution Process ◽  
Published Data ◽  
Spent Fuel ◽  
Anoxic Conditions ◽  
Pellet Surface ◽  
The Matrix

ABSTRACTThe aim of this work is to provide credibility to the radiolytic models developed for performance assessment studies. Nowadays, there is a tendency to consider them useful tools for predicting the behaviour of spent fuel matrix under repository conditions, although they still suffer of limitations due, among others, to the following reasons: limited availability of kinetic data, difficulty to handle heterogeneous systems, lack of model validation.A kinetics-based model to predict the dissolution of UO2 α-doped pellets under initial anoxic conditions is presented and compared with experimental results previously obtained. The uranium and plutonium concentrations in solution are predicted by considering the presence of an α-radiation field and its influence due to radiolysis of water on the pellet surface oxidation and subsequent dissolution.The initial parameters required by the model in order to reproduce the pellet alteration process are: system geometry, chemical composition of the leachant, physico-chemical characteristics of the leachate and oxidation conditions of the pellet surface (expressed in terms of U(VI)/U(IV) ratio). The last one is the key parameter in the model for simulating the initial quick dissolution process. The results obtained are compared with experimental data. The agreement between the predictions obtained and the experimental published data is good.The influence on the matrix oxidation – dissolution process due to the α-radiation field as well as the release of Pu are reproducible by the model. The Pu concentration trends as a function of time are explained in connection with the matrix dissolution process and are controlled by the formation of the secondary phase “Pu(OH)4(s)”.

Microstructural Characterization of Ferroelectric Bismuth Ferrite (BiFeO3) Ceramic by Electron Backscattered Diffraction

2011 ◽  
Vol 702-703 ◽  
pp. 1011-1014
Author(s):  
Jayant Kolte ◽  
Devidas Gulwade ◽  
Aatish Daryapurkar ◽  
Prakash Gopalan
Keyword(s):  
Direct Evidence ◽  
Bismuth Ferrite ◽  
Microstructural Characterization ◽  
Secondary Phase ◽  
Future Generation ◽  
Potential Candidate ◽  
Electron Backscattered Diffraction ◽  
Secondary Phase Formation ◽  
Backscattered Electron

Ferroelectric BiFeO3 (BFO) is potential candidate for future generation of FeRAM due to its large polarization. However, BFO is very sensitive to secondary phase formation during synthesis because of volatility issues related to Bismuth. Investigation of the microstructure for phase purity is the key as impurities can destroy the desired properties. We have used backscattered electron diffraction to study the microstructure of BFO ceramic. The EBSD results provide a direct evidence of the appearance of secondary phase that XRD could not be detected in XRD.

Microstructural characterization of CoPtCr/Cr thin film disks by cross-section TEM and elongated probe micro-diffraction

1991 ◽  
Vol 49 ◽  
pp. 762-763
Author(s):  
M.A. Parker ◽  
K.E. Johnson ◽  
C. Hwang ◽  
A. Bermea
Keyword(s):  
Magnetic Recording ◽  
Electron Probe ◽  
Microstructural Characterization ◽  
Crystallographic Structure ◽  
Recording Media ◽  
Layer By Layer ◽  
Cross Sectional ◽  
New Techniques ◽  
Polished Side

We have reported the dependence of the magnetic and recording properties of CoPtCr recording media on the thickness of the Cr underlayer. It was inferred from XRD data that grain-to-grain epitaxy of the Cr with the CoPtCr was responsible for the interaction observed between these layers. However, no cross-sectional TEM (XTEM) work was performed to confirm this inference. In this paper, we report the application of new techniques for preparing XTEM specimens from actual magnetic recording disks, and for layer-by-layer micro-diffraction with an electron probe elongated parallel to the surface of the deposited structure which elucidate the effect of the crystallographic structure of the Cr on that of the CoPtCr.XTEM specimens were prepared from magnetic recording disks by modifying a technique used to prepare semiconductor specimens. After 3mm disks were prepared per the standard XTEM procedure, these disks were then lapped using a tripod polishing device. A grid with a single 1mmx2mm hole was then glued with M-bond 610 to the polished side of the disk.

Microstructural Characterization of Au-Ge-Ni based ohmic contacts to GaAs/AlGaAs modfet device

1987 ◽  
Vol 45 ◽  
pp. 326-327
Author(s):  
A.K. Rai ◽  
A.K. Petford-Long ◽  
A. Ezis ◽  
D.W. Langer
Keyword(s):  
Contact Resistance ◽  
Ohmic Contact ◽  
Ohmic Contacts ◽  
Microstructural Characterization ◽  
Almost Everywhere ◽  
Spacer Layer ◽  
Good Contact ◽  
The Relationship ◽  
Lower Contact

Considerable amount of work has been done in studying the relationship between the contact resistance and the microstructure of the Au-Ge-Ni based ohmic contacts to n-GaAs. It has been found that the lower contact resistivity is due to the presence of Ge rich and Au free regions (good contact area) in contact with GaAs. Thus in order to obtain an ohmic contact with lower contact resistance one should obtain a uniformly alloyed region of good contact areas almost everywhere. This can possibly be accomplished by utilizing various alloying schemes. In this work microstructural characterization, employing TEM techniques, of the sequentially deposited Au-Ge-Ni based ohmic contact to the MODFET device is presented.The substrate used in the present work consists of 1 μm thick buffer layer of GaAs grown on a semi-insulating GaAs substrate followed by a 25 Å spacer layer of undoped AlGaAs.

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