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Pyrochlore-Type Phases for Actinides and Rare Earth Elements Immobilization

Published online by Cambridge University Press:  10 February 2011

S. V. Stefanovsky ,
S. V. Yudintsev ,
B. S. Nikonov ,
B. I. Omelianenko ,
A. I. Gorshkov ,
A. V. Sivtsov ,
M. I. Lapina  and
R. C. Ewing
S. V. Stefanovsky
Affiliation:
SIA "Radon", 7-th Rostovskii per., 2/14, Moscow 119121, Russia
S. V. Yudintsev
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 1091017, Russia
B. S. Nikonov
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 1091017, Russia
B. I. Omelianenko
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 1091017, Russia
A. I. Gorshkov
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 1091017, Russia
A. V. Sivtsov
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 1091017, Russia
M. I. Lapina
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 1091017, Russia
R. C. Ewing
Affiliation:
The Unniversity of Michigan, Ann Arbor, Michigan, 87109
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Abstract

Pyrochlore is a complex oxide with the nominal formula A2B2X6Y, where A and B are cations in VIII and VI-fold co-ordination, X and Y are anions. Its structure is derived from the cubic fluorite structure. In natural pyrochlores A = Na, Mg, K, Ca, Mn, Fe, Sr, Sb, Cs, Ba, REEs, Pb, Bi, Th, and U; B = Nb, Ta, Ti, Zr, Sn, W, Fe, and Al; X = O; Y = O, OH, or F. Synthetic pyrochlores have been repeatedly described as matrices designed for actinide-bearing waste immobilization. In synthetic pyrochlores site "A" is mainly occupied by Ca, U, An, and REEs; B = Ti and Zr; X and Y = O. In this work we have studied pyrochlores in crystalline titanate-based waste forms. The ceramics were fabricated in the system: Ca-Mn-U-REE-Zr-Ti-Al-O by cold pressing and sintering, melting in a high-temperature furnace, and inductive melting in a cold crucible. All specimens were studied by XRD, SEM/EDS and TEM methods. The amount of pyrochlore in the samples varied from 10 to 70%. Other phases in these ceramics were brannerite, perovskite, zirconolite, murataite, hibonite, loverengite, pseudobrookite, and rutile. Compositions of the pyrochlores correspond to stoichiometry: A2B2O7-x, 0.1<x<0.4, where A = Ca, Mn, REEs, U, Zr; B = Ti, Zr, Al, Mn. The positions and intensities of the peaks of pyrochlores from various ceramics were: d222=2.89-2.93A, 1=100; d400=2.51, 1=10-25; d440=1.779-1.809, 1=20-60; d622=1.512-1.540, 1=20-35; d444=1.451-1.477, I=10-15; d662=1.158-1.173, I=10-15. These data allowed the determination of the unit-cell dimensions of the pyrochlores as 1.00-1.02 nm. Results obtained from TEM research agree well with these values. Distribution of U and REEs among all phases of the ceramics was characterized. The main substitutions which have influenced the pyrochlore compositions are discussed.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 556: Symposium QQ – Scientific Basis for Nuclear Waste Management XXII , 1999 , 27
Copyright
Copyright © Materials Research Society 1999

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References

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