Microchemical and crystallographic characterisation of fluorite-based ceramic wasteforms

ABSTRACTA number of possible options have been proposed for the encapsulation and immobilisation of long lived actinide (Act) fractions in nuclear waste. Ceramics offer superior durability against chemical migration and the ability to be tailored to accommodate a variety ofdifferent waste streams. Research on the fabrication of dense, durable crystalline matrices for the safe disposal of fissile plutonium is ongoing and this study reports quantitative chemical, structural and spectroscopic analysis on fluorite based host phases.Ceramics based on the fluorite structure are known to be able to incorporate a variety of actinides and in this work two candidate ceramic matrices were investigated: a pyrochlore, Gd2Zr1.60Ce0.20Hf0.20O7; and a zirconolite, (Ca0.90Gd0.10)(Zr0.50Ce0.20Hf0.20Gd0.10)Ti2O7. The chemical compositions of the two major phases observed in the ‘zirconolite’ sample were consistent with the 2M and 4M zirconolite polytypes and the presence of the 4M structure was confirmed by Electron Diffraction (ED). The major phase in the ‘pyrochlore’ ceramic was confirmed by ED to have the pyrochlore structure. Electron Energy Loss Spectroscopy (EELS) data indicated the presence of both Ce3+ and Ce4+ in all the samples.

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Ion Irradiation Effects for Two Pyrochlore Compositions: Gd2Ti2O7 and Gd2Zr207

MRS Proceedings ◽

10.1557/proc-540-355 ◽

1998 ◽

Vol 540 ◽

Cited By ~ 20

Author(s):

S. X. Wang ◽

L. M. Wang ◽

R. C. Ewing ◽

K. V. Govindan Kutty

Keyword(s):

Nuclear Waste ◽

Ion Irradiation ◽

Pyrochlore Structure ◽

Fluorite Structure ◽

Irradiation Effects ◽

Critical Temperatures ◽

Short Range Ordering ◽

Radiation Induced ◽

Diffraction Patterns ◽

Ion Species

AbstractPyrochlore is an important nuclear waste form phase for actinide immobilization. Two synthetic pyrochlores, Gd2Ti2O7 and Gd2Zr2O7 were irradiated at various temperatures (25 K to 1073 K) by different ion species (1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+). The titanate pyrochlore amorphized at relatively low doses (0.5 ∼ 0.6 dpa). Temperature dependence of the amorphization dose for titanate pyrochlore was measured, and the critical temperatures for amorphization were 1300 K, 1100 K and 950 K by 1.5 MeV Xe+, 1.0 MeV Kr+ and 0.6 MeV Ar+, respectively. The higher critical temperature for the heavier ion irradiation is consistent with an amorphization mechanism by which the heavier ion produces a larger cascade. The zirconate pyrochlore, Gd2Zr2O7, showed a strong amorphization “resistance”. Gd2Zr2O7did not become amorphous under 1.0 MeV Kr+ and 1.5 MeV Xe+ irradiation. After prolonged irradiation (up to 7 dpa) even at a temperature of 25 K, no amorphization was observed. The irradiated zirconate pyrochlore showed abundant dislocations as observed by TEM. The pyrochlore structure of Gd2Zr2O7 transformed to the fluorite structure after irradiation. The diffraction patterns of irradiated Gd2Zr2O7 showed the existence of short-range ordering of cations. The large difference between these two pyrochlores emphasizes the strong effect of chemical composition on radiation-induced amorphization.

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Oxygen K near edge structures studied with multiple scattering calculations

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100104583 ◽

1988 ◽

Vol 46 ◽

pp. 504-505

Author(s):

Xudong Weng ◽

Peter Rez

Keyword(s):

Cross Section ◽

Cross Sections ◽

Partial Cross Section ◽

Energy Window ◽

Edge Structure ◽

Fine Structures ◽

Hydrogenic Model ◽

Electron Energy Loss ◽

Quantitative Chemical

In electron energy loss spectroscopy, quantitative chemical microanalysis is performed by comparison of the intensity under a specific inner shell edge with the corresponding partial cross section. There are two commonly used models for calculations of atomic partial cross sections, the hydrogenic model and the Hartree-Slater model. Partial cross sections could also be measured from standards of known compositions. These partial cross sections are complicated by variations in the edge shapes, such as the near edge structure (ELNES) and extended fine structures (ELEXFS). The role of these solid state effects in the partial cross sections, and the transferability of the partial cross sections from material to material, has yet to be fully explored. In this work, we consider the oxygen K edge in several oxides as oxygen is present in many materials. Since the energy window of interest is in the range of 20-100 eV, we limit ourselves to the near edge structures.

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The Kinetics of Ordering in Gadolinium Zirconate: an Unusual Oxygen Ion Conductor

MRS Proceedings ◽

10.1557/proc-398-599 ◽

1995 ◽

Vol 398 ◽

Cited By ~ 3

Author(s):

Sossina M. Haile ◽

Scott Meilicke

Keyword(s):

Impedance Spectroscopy ◽

Pyrochlore Structure ◽

Fluorite Structure ◽

Transformation Kinetics ◽

X Ray Diffraction ◽

Ion Conductor ◽

Gadolinium Zirconate ◽

X Ray ◽

Oxygen Ion ◽

Kinetics Of

ABSTRACTGadolinium zirconate, Gd2Zr2O7, undergoes an order-disorder transition at ∼1550°C, transforming from a defect fluorite structure (Fm3m) to a pyrochlore structure (Fd3m). Both cations and anions are ordered in the low-temperature, pyrochlore structure. In order to understand the interplay between anion and cation order parameters and ordering rates, the transformation kinetics of Gd2Zr2O7 have been examined via X-ray diffraction. Gadolinium zirconate is of particular interest because the oxygen ion conductivity of the ordered phase is significantly greatly than that of the disordered phase, in contrast to virtually every other known solid electrolyte. This difference in conductivity has provided a second technique for characterizing the transformation kinetics: in situ A.C. impedance spectroscopy. Results of the X-ray diffraction showed the growth of superstructure peak intensity to follow an apparent (time)½ dependence, rather than that expected from a nucleation and growth model. The impedance spectroscopy measurements, on the other hand, showed the conductivity to increase linearly with time. These results suggest the transition is second order in nature.

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SREX: A NEWPROCESS FOR THE EXTRACTION AND RECOVERY OF STRONTIUM FROM ACIDIC NUCLEAR WASTE STREAMS

Solvent Extraction and Ion Exchange ◽

10.1080/07366299108918039 ◽

1991 ◽

Vol 9 (1) ◽

pp. 1-25 ◽

Cited By ~ 177

Author(s):

E. Philip Horwitz ◽

Mark L. Dietz ◽

Dan E. Fisher

Keyword(s):

Nuclear Waste ◽

Waste Streams

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Second Difference Electron Energy-Loss Spectroscopy with the Gatan Imaging Filter

Microscopy and Microanalysis ◽

10.1017/s1431927600017360 ◽

1999 ◽

Vol 5 (S2) ◽

pp. 808-809

Author(s):

E.C. Buck

Keyword(s):

Nuclear Waste ◽

High Frequency ◽

Waste Materials ◽

White Line ◽

Minor Elements ◽

Gain Variation ◽

Line Absorption ◽

Electron Energy Loss ◽

Frequency Features ◽

Array Gain

The large number of minor elements present in geological specimens and nuclear waste materials, can make TEM/EDS analysis of such samples troublesome. With a parallel detector such as the Gatan PEELS 666, the second difference technique has been shown to be effective at removing the channel-to-channel gain variation [1]. As spectroscopy performed with the Gatan Imaging Filter (GIF200) averages over a 2D array, gain variations are minimal; however, the second-difference technique selectively enhances the high frequency features such as the “white line” absorption edges, particularly of rare earth elements (REE) and transuranics (TRU). The second difference method may thus still have merit with the GIF200. A script was created within the controlling software program (DigitalMicrograph ™) which permitted second difference acquisition [2]. The Spectroscopy Package was also modified with ResEdit and the required values were added to the Global Tags to enable easy application of the second difference routine.

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Removal of TcO4– from Representative Nuclear Waste Streams with Layered Potassium Metal Sulfide Materials

Chemistry of Materials ◽

10.1021/acs.chemmater.6b01296 ◽

2016 ◽

Vol 28 (11) ◽

pp. 3976-3983 ◽

Cited By ~ 33

Author(s):

James J. Neeway ◽

R. Matthew Asmussen ◽

Amanda R. Lawter ◽

Mark E. Bowden ◽

Wayne W. Lukens ◽

...

Keyword(s):

Nuclear Waste ◽

Metal Sulfide ◽

Waste Streams

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Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore: Bond-type effect

Journal of Materials Research ◽

10.1557/jmr.2004.0178 ◽

2004 ◽

Vol 19 (5) ◽

pp. 1575-1580 ◽

Cited By ~ 65

Author(s):

Jie Lian ◽

Rodney C. Ewing ◽

L.M. Wang ◽

K.B. Helean

Keyword(s):

Room Temperature ◽

Ion Beam ◽

Pyrochlore Structure ◽

Fluorite Structure ◽

Structure Type ◽

Radiation Response ◽

Beam Irradiation ◽

Ion Beam Irradiation ◽

Bond Type ◽

Wide Range

Ceramics with III-IV pyrochlore compositions, A3+2B4+2O7 (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability ofthe pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by1 MeV Kr+. The results are discussed in terms of the ionic size and type ofbonding of Sn4+ and Hf4+ and compared to previous results for titanate andzirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam–induced amorphizationwith a critical amorphization dose of approximately 3.4 displacements per atom(dpa) (2.62 × 1015 ions/cm2) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at adose of approximately 4.54 displacement per [lattice] atom (3.13 × 1015 ions/cm2) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the 〈Sn–O〉 bond in determining the radiation response.

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A Recovery Process of Strontium from Acidic Nuclear Waste Streams

Separation Science and Technology ◽

10.1080/01496399708000731 ◽

1997 ◽

Vol 32 (10) ◽

pp. 1725-1737 ◽

Cited By ~ 6

Author(s):

M. Draye ◽

G. Le Buzit ◽

J. Foos ◽

A. Guy ◽

B. Leclere ◽

...

Keyword(s):

Nuclear Waste ◽

Recovery Process ◽

Waste Streams

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Apatite- and Monazite-Bearing Glass-Crystal Composites for the Immobilization of Low-Level Nuclear and Hazardous Wastes

MRS Proceedings ◽

10.1557/proc-412-345 ◽

1995 ◽

Vol 412 ◽

Cited By ~ 6

Author(s):

D. J. Wronkiewicz ◽

S. F. Wolf ◽

T. S. DiSanto

Keyword(s):

Transition Metals ◽

Nuclear Waste ◽

Toxic Metals ◽

Hazardous Wastes ◽

Waste Streams ◽

Waste Forms ◽

Glass Crystal ◽

High Level Nuclear Waste ◽

High Level ◽

Crystal Composite

AbstractThis study demonstrates that glass-crystal composite waste forms can be produced from waste streams containing high proportions of phosphorus, transition metals, and/or halides. The crystalline phases produced in crucible-scale melts include apatite, monazite, spinels, and a Zr-Si-Fe-Ti phase. These phases readily incorporated radionuclide and toxic metals into their crystal structures, while corrosion tests have demonstrated that glass-crystal composites can be up to 300-fold more durable than simulated high-level nuclear waste glasses, such as SRL 202U.

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