Synroc, a ceramic made from a reactive mixture of Al, Ba, Ca, Ti, and Zr oxides, is proving to be a suitable and effective medium for immobilizing nuclear wastes.
Synroc-C, a titanate-based ceramic variant, was initially developed in 1978 by Ringwood et al. for immobilizing high-level nuclear waste (HLW) from nuclear power reactor fuel reprocessing. HLW is essentially a solution of radioactive fission products, actinides, and process contaminants in ~3 mol/L nitric acid. The developers of Synroc-C aimed to immobilize radioactive waste ions by incorporating them in a ceramic. They accomplished this by mixing the HLW solution (liquid waste) with a ceramic precursor, then forming the ceramic by drying, calcining, and hot-pressing the mixture in a metal container for two hours at 1200°C/20 MPa. The result, Synroc-C, is composed of hol-landite, zirconolite, perovskite, and rutile, together with a few percent of minor phases and metal alloys. The Synroc-C precursor has the following composition (wt%): Al203(5.4); BaO(5.6); CaO(11); TiO2(71.4); and ZrO2(6.6). Since 1984, it has been prepared by hydrolyzing a mixture of Al, Ti, and Zr alkoxides with an aqueous slurry of Ba and Ca hydroxide. The abundances of the phases, and the radionuclides contained in them in dilute solid solution, are identified in Table I.