PNNL

Abstract

During the past half-century, the nuclear fuel cycle has generated approximately 1,400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1,000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that zirconate pyrochlore does not become amorphous, but remains crystalline as a defect fluorite structure due to disordering of the A- and B-site cations. The zirconate pyrochlore will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, studies of natural U-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls of the pyrochlore structure on its response to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.