PHASE COMPOSITIONS AND LEACH RESISTANCE OF ACTINIDE-BEARING MURATAITE CERAMICS

摘要

Phase composition of the murataite-based ceramics containing 10 wt.% ThO_2, UO_2, NpO_2 or PuO_2 and leaching of actinides using a MCC-1 procedure were studied. The ceramics were prepared by melting of oxide mixtures in Pt ampoules in air at ～ 1500?. They are composed of predominant murataite-type phases and contain traces of extra phases (rutile, crichtonite, perovskite). At least two murataite-related phases with five- and eight-fold elementary fluorite unit cell (5C and 8C) were simultaneously observed. Minor phase 3C (murataite) in the ceramics doped with ThO_2 and UO_2 was also found. In the Th-bearing sample the 5C phase prevails over the 8C phase. In the U-bearing ceramic they co-exist in comparable amounts. The sample produced at 1500℃ contains crichtonite whereas the ceramic produced at lower temperature (1400 ℃) contained rutile. Higher temperature favors further rutile reactions with formation of crichtonite. The Np- and Pu-doped ceramics are also composed of major the 5C and the 8C phases and minor rutile and crichtonite (in Pu-loaded sample only). Unlike the sample prepared under slightly reducing conditions (in glassy carbon crucible) the Pu-doped ceramic produced under neutral conditions (in Pt ampoule) doesn't contain perovskite-type phase. Occurrence of perovskite in the first sample was supposed to be due to reduction of some Pu(IV) to Pu(IIT) during experiment. Lach rates (7-day MCC-1 test, 90 ℃) of the actinide elements from all the ceramics studied are at the level of 10~(-6) - 10~(-7) g/(m~2·day).