Study of Uranium Incorporation Into Zircon: Solubility Limits and Durability of Fixation
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SOLUBILITY LIMITS AND DURABILITY OF FIXATION S.V. IOUDINTSEV, B.I. OMELIANENKO, and M.I. LAPINA Institute of Geology of Ore Deposits, Staromonetny 35, 109017 Moscow, Russia
ABSTRACT Research on synthetic zircons fabricated in system ZrO 2-UO 2-SiO 2 at T = 400 - 7500 C, 100-200 MPa, and Ni-NiO oxygen buffer was made in order to estimate the limits of uranium incorporation into the phase's structure. It was shown that up to 27 wt.% of uranium may locally enter into the synthetic zircons. SEM/EDS study of the samples has showed that the element is uniformly distributed within the zircon crystals. Incorporation of U into zircon increases its unit-cell dimensions up to a = 6.70 A and c = 6.00 A relative to 6.60 and 5.98 A for a pure phase without impurities. After thermal high temperature treatment of the samples under 14000 C, in air, for 6 hours inhomogeneous distribution of uranium within the zircon grains was revealed. Very small uranium rich spots probably corresponding to a newly formed phase were observed in the zircon crystals. Contents of the element throughout the grains decreased to about 10 wt.%. It is probably caused by destruction of zircon-coffinite solid solution formed at hydrothermal conditions in the course of its re-heating. Study of interaction of natural zircons with alkaline carbonate solution at 1000 C shows that U release from the phase can reach up to 6 % of total uranium in the samples even under relatively short term interaction. Loss of uranium rises with the element content (or amorphisation degree) and run duration, as well as with grain size reduction. Research on PH20 =
uranium distribution
in natural zircons picked up from altered
and weathered
rocks is
characterised by strong depletion of U at grain margins relative to the centre of the crystals. These results indicate that zircon may lose some uranium and, probably, the other actinides with time due to its lattice destruction under radioactive decay followed by hot water attack. INTRODUCTION Zirconium orthosilicate - zircon (ZrSiO 4) - has been recently [1, 2] proposed to be used as a matrix for conservation of surplus weapons plutonium. The choice is based on good isomorphic properties of the mineral and its high stability under various natural processes. Earlier it was reported [2] that zircon with up to 10 wt.% Pu had been synthesised. Nevertheless the limits of the actinides solubility have not been accurately determined. There are some other questions that require further study, e.g. durability of fixation of actinides in the zircon structure. Experimental study of natural zircons has shown that the phase, especially when metamict, is transformed with new phases forming under the influence of thermal alkaline solutions [3]. EXPERIMENTAL METHODS
Research on zircons produced in a system ZrO2 -UO 2-SiO 2-H20-HCI under T = 400-7500 C, P = 100-200 MPa, and Ni-NiO oxygen buffer was made. Runs were performed in welded platinum tubes placed in externally sealed pressure vessels. Three sets of runs differing in initial 185 Mat. Re
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