Kinetics of uranium release from Synroc phases

This paper presents experimental studies on the kinetics of U release from near single-phase zirconolite, pyrochlore, brannerite and pyrochlore-rich t itanate ceramic materials. The dissolution tests were conducted at 20-75 de greesC with initial pHs from 2 to 12 and flow rates from 10 to 80 mi d(-1) in the open atmosphere. The U releases from these titanate materials are co ntrolled by initial fast process and then followed by linear kinetics. The close-to-stoichiometric U release from zirconolite and pyrochlore-rich mate rials and preferential U release from brannerite are consistent with the al terations observed for the natural samples. The rate constants for U releas es were determined and the effects of pH and temperature were examined. For each material, the U release vs. pH exhibits a V-shape with a minimum near pH 8. The measured activation energies suggest surface reaction controlled dissolution mechanism. Pyrochlore-rich materials and zirconolite demonstra ted higher chemical durability and more resistance to aqueous attack than b rannerite. However, impurities and minor brannerite inclusions do not appea r to have a detrimental effect on U releases from pyrochlor-erich multi-pha se ceramics. (C) 2001 Elsevier Science B.V. Ail rights reserved.