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

The disposal of fission products and actinides generated by the nuclear-fuel 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 Cs-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, demonstrable long-term chemical and mechanical durability are essential properties of waste forms for the immobilization and disposal of radionuclides. Mineralogical and geological studies provide excellent candidate phases for immobilization and a unique database that cannot be duplicated by a purely Materials Science approach. The "mineralogical approach" is illustrated by a discussion of zircon as a phase for the immobilization of plutonium from dismantled nuclear weapons. Other minerals, e.g., monazite, apatite, pyrochlore, zirconolite and zeolites, also are important candidates for the immobilization of actinides.