Abstract
Assessing the concept of direct disposal of used nuclear fuel in a geological vault requires a model to predict the dissolution rate of UO2 in groundwater. A solubility-limited model can be used to calculate the dissolution rate of UO2 fuel under non-oxidizing conditions. When the oxidative dissolution of UO2 is an irreversible process, a kinetic model is more suitable to describe the dissolution of UO2 under oxidizing conditions. Experimental studies were carried out using electrochemical techniques and X-ray photoelectron spectroscopy, XPS, to determine criteria for selecting the appropriate model for estimating used-fuel dissolution rates as a function of the redox conditions in the vault at the time of container failure. UO2 electrodes were subjected to prolonged (>1000 min) potentiostatic oxidation, and the rate of oxidation and dissolution of UO2 fuel was investigated as a function of the applied potential. UO2 oxidation was also carried out by the products of water radiolysis and studied as a function of dose rate, total dose and solution chemistry. These studies show that significant oxidative dissolution of UO2 appears possible for potentials more positive than -100 mV vs SCE in solutions with a pH close to that of the deep groundwaters, i.e., from 6 to 10. A kinetic model, which takes into account the mechanism of UO2 oxidation, is more appropriate to estimate dissolution rates of UO2 fuel for redox conditions more oxidizing than -100 mV vs SCE.
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Acknowledgements
The authors wish to thank L.H. Johnson and V.H. Hocking for reviewing earlier drafts of this report, and H. Christensen for helpful discussions. This work is part of the Canadian Nuclear Fuel Vaste Management Program which is jointly funded by ABCL Research and Ontario Hydro under the auspices of the CANDU Owners Group.
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Sunder, S., Shoesmith, D., Miller, N. et al. Determination of Criteria for Selecting A UO2 Fuel Dissolution Model for Nuclear Fuel Waste Management Concept Assessment. MRS Online Proceedings Library 257, 345–352 (1991). https://doi.org/10.1557/PROC-257-345
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DOI: https://doi.org/10.1557/PROC-257-345