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Open AccessArticle

Chemical Stability of Zirconolite for Proliferation Resistance under Conditions Typically Required for the Leaching of Highly Refractory Uranium Minerals

1
College of Science, Health, Engineering and Education, Murdoch University, Perth 6150, Australia
2
Sellafield Ltd., Seascale, Cumbria CA20 1PG, UK
3
National Nuclear Laboratory, Workington, Cumbria CA14 3YQ, UK
*
Author to whom correspondence should be addressed.
Metals 2019, 9(10), 1070; https://doi.org/10.3390/met9101070
Received: 11 July 2019 / Revised: 5 September 2019 / Accepted: 12 September 2019 / Published: 1 October 2019
(This article belongs to the Special Issue Advances in Synthesis of Metallic, Oxidic and Composite Powders)
In this study, synthetic zirconolite samples with a target composition Ca0.75Ce0.25ZrTi2O7, prepared using two different methods, were used to study the stability of zirconolite for nuclear waste immobilisation. Particular focus was on plutonium, with cerium used as a substitute. The testing of destabilisation was conducted under conditions previously applied to other highly refractory uranium minerals that have been considered for safe storage of nuclear waste, brannerite and betafite. Acid (HCl, H2SO4) leaching for up to 5 h and alkaline (NaHCO₃, Na2CO3) leaching for up to 24 h was done to enable comparison with brannerite leached under the same conditions. Ferric ion was added as an oxidant. Under these conditions, the synthetic zirconolite dissolved much slower than brannerite and betafite. While the most intense conditions were observed previously to result in near complete dissolution of brannerite in under 5 h, zirconolite was not observed to undergo significant attack over this timescale. Fine zirconolite dissolved faster than the coarse material, indicating that dissolution rate is related to surface area. This data and the long term stability of zirconolite indicate that it is a good material for long-term sequestration of radioisotopes. Besides its long term durability in the disposal environment, a wasteform for fissile material immobilisation must demonstrate proliferation resistance such that the fissile elements cannot be retrieved by leaching of the wasteform. This study, in conjunction with the previous studies on brannerite and betafite leaching, strongly indicates that the addition of depleted uranium to the wasteform, to avert long term criticality events, is detrimental to proliferation resistance. Given the demonstrated durability of zirconolite, long term criticality risks in the disposal environment seem a remote possibility, which supports its selection, above brannerite or betafite, as the optimal wasteform for the disposition of nuclear waste, including of surplus plutonium. View Full-Text
Keywords: uranium; zirconolite; brannerite; betafite; leaching; kinetics uranium; zirconolite; brannerite; betafite; leaching; kinetics
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Nikoloski, A.N.; Gilligan, R.; Squire, J.; Maddrell, E.R. Chemical Stability of Zirconolite for Proliferation Resistance under Conditions Typically Required for the Leaching of Highly Refractory Uranium Minerals. Metals 2019, 9, 1070.

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