- © The Mineralogical Society Of America
The disposal of radioactive “waste” generated by the nuclear fuel cycle is among the most pressing and potentially costly environmental problems of the 21st century, a heritage from the Atomic Age of the 20th century. Proposed disposal strategies are complicated, not only because of the large volumes and activities of waste, but also because of the political and public-policy issues associated with the long times required for containment and disposal (104 to 106 years). The development and use of highly durable waste forms, materials that have a high chemical durability and resistance to radiation damage effects, can simplify the disposal strategy (Ewing 2001).
An interest in phosphate-based waste forms has developed because the high-level waste generated by reprocessing of spent nuclear fuel can contain substantial amounts of phosphates (up to 15 wt % P2O5) that result from processing technologies that utilized either a bismuth phosphate or tributylphosphate process (Bunker et al. 1995). In addition to the high phosphate content, other metal oxides may achieve significant proportions (up to 15 wt % Fe2O3; up to 30 wt % Bi2O3; up to 30 wt % UO2) (Lambert and Kim 1994).
These complex compositions have presented special challenges in developing crystalline ceramics that can accommodate the full compositional range of the waste streams. The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides. The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al. 1980, McCarthy et al. 1978, 1980). The attractive qualities of monazite as a nuclear waste form are: (1) a high solubility for actinides and rare earths (10 to 20 wt %); (2) evidence from natural …