Subsolidus sintering of SYNROC: II. Materials selections, process improvements, waste form evaluations

The principal areas of research were related to materials selections and characterization, process optimizations, crystalline phase development, sinterability, resultant microstructures and evaluations of leaching behavior. With and without simulated radwaste doping, the Modified SYNROC-B formulation was found to be sinterable to technical density (D > 0.95 in the CTS mode) at temperatures in the range 11950C to 12850C, depending upon TiO2 and CaCO3 materials selections, and upon powder processing methods employed prior to firing. Of the 16 TiO2 raw materials evaluated in air-fired, undoped batches, 15 yielded technically dense compacts (D > 0.95). Three fine pigmentary grades of TiO2 were selected for further study in doped and undoped versions fired in Ar, 4% H2. When intensively milled with other well chosen matrix constituents and 10% spray-calcined simulated waste, each of them yielded sintered densities of greater than or equal to 4.2 g/cm3 (D greater than or equal to 0.96) at 12600C, 2h in Ar, 4% H2 atmosphere. Leachability studies have been carried out in triple distilled H2O according to MCC-1 and MCC-2 procedures at 250 and 1500C, respectively, and under γ-irradiation for dose rates of 2-5 x 105 rad/h at approx. 250C. The results obtained showed that freshly exposed interions of sintered Modified SYNROC-B ceramics were highly stable in the leaching environment, and were very retentive of simulated waste ions, including the most leachable species, Cs. Depending on leaching conditions, the highest Cs leach rates (after 3 days) were on the order of 10-1 g.m-2.day-1, but diminished sharply for longer times (up to 92 days) to the range 10-2 - 10-4 g.m-2.day-1