The Effect of Radiation Damage on Zirconolite Dissolution
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II6.12.1
The Effect of Radiation Damage on Zirconolite Dissolution Katherine L. Smith1, Zhaoming Zhang1, Peter McGlinn1, Darren Attard1, Huijing Li1, Gregory R. Lumpkin1, Michael Colella1, Terry McLeod1, Zaynab Aly1, Elaine Loi1, Sammy Leung1, Kaye P. Hart1, Mark Ridgway2, William J. Weber3 and Suntharampillai Thevuthasan3 1
Materials Division, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia. 2 Research School of Physical Sciences, The Australian National University, Canberra, ACT 0200, Australia. 3 Pacific Northwest National Laboratory, Richland, WA 99352, USA.
ABSTRACT Polished tiles (7x7x2 mm3) of Nd-bearing zirconolite were fabricated and then some were irradiated on both large faces with 3 MeV or 2 MeV Au2+ ions (total fluence of ≥ 1 x 1015 ions/cm2) in order to render the zirconolite amorphous and so simulate displacement damage caused by alpha decay. Both the irradiated and non-irradiated tiles were then subjected to static dissolution tests in 0.01M nitric solution (pH2) at 90 C, for periods of 0-1, 1-7, 7-14 and 14-28 days. It was found that radiation damage did not affect the dissolution rate of zirconolite as indicated by the elemental leach rates of Nd, Ti, Ca and Al. The results of solution analyses are consistent with those obtained from X-ray Photoelectron Spectroscopy (XPS) in that the Ca, Nd, Ti and Al concentrations in the top surface layer (< 5 nm) all decreased with respect to that of Zr after dissolution testing, and the leached surface composition of the non-irradiated zirconolite is very similar to that of the two irradiated specimens. The implications of these results are discussed in the context of previous work.
INTRODUCTION Zirconolite and pyrochlore have both been proposed as major constituent phases of titanate waste forms for actinide-rich wastes [1] such as impure Pu or waste comprised of minor actinides. Recently a pyrochlore-rich ceramic was advanced for Pu immobilisation [2]. After fabrication it is envisioned that titanate waste forms will be loaded into canisters and stored in a geological repository. During storage, the waste forms (and their incorporated phases) will suffer alpha decay damage from incorporated actinides. Zirconolite and pyrochlore have closely related structures and can be considered to be anion deficient derivatives of the fluorite structure [3]. However there is some evidence from natural zirconolites and pyrochlores, that zirconolite may be more retentive of actinides than pyrochlore in natural environments [4]. Specifically actinide loss from natural zirconolites has only been reported in rocks that have been subjected to high temperature (>500°C) hydrothermal fluids, whereas actinide loss from pyrochlores can result from hydration at low temperatures ( Ca > Nd ~ Ti> Zr.
CONCLUDING COMMENTS In conjunction, data from this and previous studies [5, 8, 11] suggest a) that amorphisation affects zirconolite leach rates less than pyrochlore leach rates and b) dissolution rates are phase dependent even when
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