Leach rates of uranium and thorium from murataite ceramics

Samples of murataite-based ceramics with chemical composition (wt.%): 5 Al2O3, 10 CaO, 55 TiO2, 10 MnO, 5 Fe2O3, 5 ZrO2, 10 AnO(2) (An = Th, U) were produced by melting in both glassy carbon crucibles in a resistive furnace and a cold crucible followed by crystallization during sow cooling. All the ceramics were composed of predominant murataite polytypes with 5-, 8-, and 3-fold elementary fluorite unit cell, minor crichtonite and pyrophanite/ilmenite, and rare grains of perovskite and pyrochlore. More than 95% of total Th and U enter the murataite polytypes. The murataite grains have zoned structure where the 5C, 8C and 8C polytypes form their core, middle zone and rim respectively. Leach rates of Th and U were determined using a single-pass-flow-through (SPFT) procedure at pH = 2 and pH = 6.5 at a temperature of 90 +/- 2 degrees C. Average Th leach rates from the ceramics produced in the resistive furnace and cold crucible were found to be 1.00 x 10(-5) g/(m(2) x d) and 1.97 x 10(-5) g/(m(2) x d). Uranium was found to be higher leachable element than thorium: its average leach rate was 1.42 x 10(-3) g/(m(2) x d). Higher leachability of U as compared to Th may be due to either occurrence of U in minor crichtonite phase with lower chemical durability or hydrolytic processes resulting in formation of higher leachable U(VI).