PHASE COMPOSITIONS AND LEACH RESISTANCE OF ACTINIDE-BEARING MURATAITE CERAMICS
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0893-JJ05-23.1
PHASE COMPOSITIONS AND LEACH RESISTANCE OF ACTINIDE-BEARING MURATAITE CERAMICS S.V. Stefanovsky1, S.V. Yudintsev2, B.S. Nikonov2, A.V. Mokhov2, S.A. Perevalov3, O.I. Stefanovsky1, A.G. Ptashkin1 1
SIA Radon, 7th Rostovskii lane 2/14, Moscow 119121 Russia Institute of Geology of Ore Deposits RAS, Staromonetnii lane 35, Moscow 119017 Russia 3 Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Kosygin st. 19, Moscow Russia 2
ABSTRACT Phase composition of the murataite-based ceramics containing 10 wt.% ThO2, UO2, NpO2 or PuO2 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 0C. 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 ThO2 and UO2 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 0C contains crichtonite whereas the ceramic produced at lower temperature (1400 0C) 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(III) during experiment. Leach rates (7-day MCC-1 test, 90 0C) of the actinide elements from all the ceramics studied are at the level of 10-6-10-7 g/(m2⋅day). INTRODUCTION Among the phases potentially suitable for immobilization of high level waste (HLW) a murataite is of great interest due to capability to incorporate simultaneously both actinide and rare earth elements (REEs) and corrosion products (iron group elements, aluminum, probably, gallium) as well as sodium [1-5]. Murataite is a cubic (F⎯43m) complex oxide with fluorite-derived structure (three-fold elementary fluorite unit cell, 3C) and simplified general formula R3M12M26O20-x (R = An, Ln, Y, Ca, Na, Zr; M1 = Mn2+/3+/4+, Fe2+/3+; M2 = Ti4+, Al3+, Fe3+, Mn3+/4+) [5]. Along with 3C another structural modification with five- (5C), seven- (7C), and eight-fold (8C) elementary fluorite unit cell were found. Murataite and structure-related pyrochlore (two-fold fluorite unit cell and formula R2M12O7-x) form a polysomatic series of the phases whose structures are built from pyrochlore (2C) and murataite (3C) modules: 5C (2C+3C), 7C (2C+3C+2C), and 8C (3C+2C+3C) respectively [6]. Pyrochlore is known to be a host phase for REEs and actinides of high level
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