ABSTRACTThe 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.
Comparison and Modeling of Aqueous Dissolution Rates of Various
Uranium Oxides
