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Comparison and Modeling of Aqueous Dissolution Rates of Various Uranium Oxides

Published online by Cambridge University Press:  03 September 2012

S. A. Steward  and
E. T. Mones
S. A. Steward
Affiliation:
Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550
E. T. Mones
Affiliation:
Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550
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Abstract

The purpose of this work has been to measure and model the intrinsic dissolution rates of uranium oxides under a variety of well-controlled conditions that are relevant to a geologic repository. When exposed to air at elevated temperature, spent fuel may form the stable phase U3O8. Dehydrated schoepite, UO3H2O, has been shown to exist in drip tests on spent fuel.

Equivalent sets of U3O8 and UO3H2 dissolution experiments allowed a systematic examination of the effects of temperature (25–75°C), pH (8–10) and carbonate (2–200×10−4 molar) concentrations at atmospheric oxygen conditions.

Results indicate that UO3H2O has a much higher dissolution rate (at least ten-fold) than U3O8 under the same conditions. The intrinsic dissolution rate of unirradiated U3O8 is about twice that of UO2. Dissolution of both U3O8 and UO3.H2O shows a very high sensitivity to carbonate concentration. Present results show a 25 to 50-fold increase in room-temperature UO3H2O dissolution rates between the highest and lowest carbonate concentrations.

As with the UO2 dissolution data the classical observed chemical kinetic rate law was used to model the U3O8 dissolution rate data. The pH did not have much effect on the models, in agreement with the earlier analysis of the UO2 and spent fuel dissolution data,. However, carbonate concentration, not temperature, had the strongest effect on the U3O8 dissolution rate. The U3O8 dissolution activation energy was about 6000 cal/mol, compared with 7300 and 8000 cal/mol for spent fuel and UO2 respectively.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 465: Symposium II – Scienfific Basis for Nuclear Waste Management XX , 1996 , 557
Copyright
Copyright © Materials Research Society 1997

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References

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