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Mechanisms of Actinide Isotope Leaching from Monazite

Published online by Cambridge University Press:  26 February 2011

Yehuda Eyal  and
Donald R. Olander
Yehuda Eyal
Affiliation:
Department of Nuclear Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Donald R. Olander
Affiliation:
Department of Nuclear Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA
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Extract

The remarkable geological stability of the actinide-containing rare-earth orthophosphate mineral monazite has led to consideration of synthetic analogs of this material as the wasteform for high-level nuclear wastes (1). Monazite is resistant to metamictization in spite of the alpha radiation from decaying actinides to which it has been subjected during its lifetime. This material is also very insoluble in most groundwaters. Consequently, dissolution of the mineral and leaching of its constituent actinides are very small. However, leaching experiments on natural monazite specimens, presented here, show that radioactive decay and radiation damage produce significant differences in the release rates of several isotopes into aqueous solutions. Within the duration of the present experiments, 6.3 to 6.8 years, these effects are particularly prominent in the nuclides in the decay chain of 232-Th. This series includes the alpha-recoil 228-Ra atom (a 5.8-year fT emitter formed directly by 232-Th decay), and its daughter 228-Th (a 1.9-year a emitter, produced by 3 decay). The present experiments exhibit preferential release into solution of 228-Th relative to 232-Th by a factor of 1.3 to 21. This highly enhanced 228-Th release is explained by four different isotope incongruency effects that operate during leaching. One mechanism involves the preferential etching of alpha-recoil tracks. The three other mechanisms depend upon the behaviour of 228-Ra. In particular, dissolution-released, recoil-released and diffusion-released 228-Ra atoms (the latter process is applicable to a high-temperature anneal) are observed to remain with the solid as an insoluble precipitate, possibly loosely attached to the mineral surface. Subsequent decay of this released insoluble 228-Ra immediately leads to the formation of readily dissolved 226-Th. The phenomena may be quite general, so potential wasteforms should be examined for these isotope fractionation effects in order to insure satisfactory prediction of the long-term leaching of actinides from the material.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 127: Symposium BERLIN – Scientific Basis for Nuclear Waste Management XII , 1988 , 269
Copyright
Copyright © Materials Research Society 1989

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References

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