THE KINETICS OF ALPHA-DECAY-INDUCED AMORPHIZATION IN ZIRCON AND APATITE CONTAINING WEAPONS-GRADE PLUTONIUM OR OTHER ACTINIDES

Zircon and apatite form as actinide host phases in several high-level waste forms and have been proposed as host phases for the disposition of excess weapons-grade Pu and other actinides. Additionally, closely- related structure types appear as actinide-bearing phases among the co rrosion products of spent nuclear fuel and high-level waste glasses. S elf-radiation damage from alpha-decay of the incorporated Pu or other actinides can affect the durability and performance of these actinide- bearing phases. For both zircon and apatite, these effects can be mode led as functions of storage time and repository temperature and valida ted by comparison with data from natural occurrences. Natural zircons and apatites, with ages up to 4 billion years, provide abundant eviden ce for their long-term durability because of their wide spread use in geochronology and fission-track dating. Detailed studies of natural zi rcons and apatites, Pu-238-containing zircon, a Cm-244-containing sili cate apatite, and ion-irradiated zircon, natural apatite and synthetic silicate apatites provide a unique basis for the analysis of alpha-de cay effects over broad time scales. Models for alpha-decay effects in zircon and apatite are developed that show alpha-decay of Pu and other actinides will lead to a crystalline-to-amorphous transformation in z ircon, but not in apatite, under conditions typical of a repository, s uch as the Yucca Mountain site. (C) 1997 Elsevier Science B.V.