Microstructure and mineral chemistry of Synroc-d

Synroc-D is a polyphase nuclear waste ceramic optimally consisting of zirconolite, perovskite, an ulvospinel-rich solid solution, a hercynitic spinel solid solution, and nepheline. A continuous intergranular glass phase is also present, which may aid in densification, but decreases the waste form's durability in aqueous environments, as does the presence of nepheline. The major radionuclides are distributed in the following manner: Uranium is immobilized in zirconolite and perovskite, the lanthanides and strontium are immobilized in perovskite, cesium is distributed between nepheline and glass. The partitioning of tetravalent uranium and trivalent actinides may prove useful as surrogates for tetravalent and trivalent actinides, respectively. The spinels are free of radionuclides and provide a degree of microencapsulation for the radionuclide-bearing phases. Values of mineral-mineral partition coefficients are largely dependent on powder preparation; the choice of zirconia reagent is of particular importance