O.I. Stefanovsky

Phase Composition and Leach Resistance of Actinide-Bearing Murataite Ceramics

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 C. They are composed of predominant murataite-type phases and contain traces of extra phases (rutile, crichtonite, perovskite). At least two murataite-related phases with fiveand 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 C contains crichtonite whereas the ceramic produced at lower temperature (1400 C) contained rutile. Higher temperature favors further rutile reactions with formation of crichtonite. The Npand 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 C) of the actinide elements from all the ceramics studied are at the level of 10-10 g/(m⋅day).

Phase Relations In The Systems Based On Yttrium, Gadolinium, Manganese, Titanium, and Uranium Oxides Doped with Calcium, Aluminum and Iron Oxides

Titanate-based ceramics with yttrium, gadolinium, manganese, uranium, calcium, aluminum, and iron oxides being of interest as matrices for radioactive wastes containing lanthanides, actinides and corrosion products were synthesized using cold pressing and sintering and melting routes. Major constituents of the ceramics are fluorite structure phases (pyrochlore, “murataite”, cubic oxide with fluorite-related lattice) as well as brannerite. Incorporation of aluminum and iron oxides (corrosion products) forms second phases with perovskite or/and crichtonite structure.

Rare Earth-Bearing Murataite Ceramics

Phase composition of the murataite-based ceramics containing 10 wt.% lanthanum, cerium, neodymium, europium, gadolinium, yttrium, zirconium oxides was studied. The ceramics were prepared by melting of oxide mixtures in 20 mL glass-carbon crucibles in air at {approx}1500 deg. C. They are composed of predominant murataite-type phases and minor extra phases: rutile, crichtonite, perovskite, ilmenite/pyrophanite, and zirconolite (in the Zr-bearing sample only). Three murataite-related phases with five- (5C), eight- (8C), and three-fold (3C) elementary fluorite unit cell are normally present in all the ceramics. These phases form core, intermediate zone, and rim of the murataite grains, respectively. They are predominant host phases for the rare earth elements whose concentrations are reduced in a row: 5C>8C>3C. Appreciate fraction of La and Ce may enter the perovskite phase. (authors)

Murataite Ceramics Doped with Lanthanides and Uranium

ABSTRACT Phase composition of the murataite-based ceramics containing 10 wt.% of mixed oxides simulating rare earth/actinide (REE/An) and actinide (An) fractions of high level waste (HLW) was studied. The ceramics were prepared by melting of oxide mixtures in Pt ampoules in air at ~1500 0 C. Ceramics with REE/An and An fractions surrogates are composed of predominant murataite-type phases and minor extra phases: perovskite and crichtonite. Three murataite-related phases with five- (5C), eight- (8C), and th ree-fold (3C) elementary fluorite unit cell are present in these ceramics. These phases form core, intermediate zone, and rim of the murataite grains, respectively. They are predominant host phases for the rare earth elements and uranium whose concentrations are reduced in a row: M-5C>M-8C>M-3C. Appreciate fraction of Ce, Nd, and Pu may enter the perovskite phase. In the An-Gd ceramic perovskite and murataite were found to be predominant and secondary in abundance phases respectively.

Cold Crucible Vitrification of U-bearing SRS SB4 HLW Surrogate

Savannah River Site Defense Waste Processing Facility (DWPF) Sludge Batch 4 (SB4) high level waste (HLW) simulant at 55 wt % waste loading was produced in the demountable cold crucible and cooled to room temperature in the cold crucible. Appreciable losses of Cs, S and Cl took place during the melting. A second glass sample was subjected to canister centerline cooling (CCC) regime in an alumina crucible in a resistive furnace. X-ray diffraction (XRD) study showed that the glass blocks were composed of vitreous and spinel structure phases. No separate U-bearing phases were found.