The design and evaluation of nuclear-waste forms: Clues from mineralogy

The disposal of fission products and actinides generated by the nuclear-fue l cycle is one of the major challenges in Environmental Sciences of the 21( st) Century. Because some fission products (e.g., Tc-99, I-129, Se-79 and C s-135) and actinides (e.g., Pu-239 and Np-237) are long-lived, they have a major impact on the risk assessment of geological repositories. Thus, demon strable long-term chemical and mechanical durability are essential properti es of waste forms for the immobilization and disposal of radionuclides. Min eralogical and geological studies provide excellent candidate phases for im mobilization and a unique database that cannot be duplicated by a purely Ma terials Science approach. The "mineralogical approach" is illustrated by a discussion of zircon as a phase for the immobilization of plutonium from di smantled nuclear weapons. Other minerals, e.g., monazite, apatite, pyrochlo re, zirconolite and zeolites, also are important candidates for the immobil ization of actinides.