Investigation of microscopic radiation damage in waste forms using ODNMR and AEM techniques

This report summarizes work done after 2 years of a 3-year project. Using analytic electron microscopic and selective laser spectroscopic techniques, the authors first examined lanthanide orthophosphate crystals of YPO{sub 4} and LuPO{sub 4} that accumulated self-radiation damage from dopant (1 wt.%) {sup 244}Cm ions for 17 years. Although the accumulated dose of radiation (> 10{sup 18} alpha-decay events/mg) is significantly high, the samples that they examined physically remain crystalline on a macroscopic scale. Amorphization is not evident, even though isolated defects of various sizes were observed. Microscopic radiation effects in the crystals were manifested by (1) individual defect clusters of 2 to 5 nm size, which resemble disordered fission tracks, and (2) bubbles of 5 to 20 nm that are attributed to accumulation of He atoms generated during alpha decay events. These bubbles are relatively mobile and easily coalesce in electron microscopy studies due to enhanced diffusion arising from electron irradiation. They have observed that, when exposed to an electron beam, the bubbles aggregated as a function of exposure time. This observation thus provides additional evidence that the bubbles developed from the aggregation of helium atoms that were created from alpha-decay of Cm-244, and the local lattice recovered from radiation damage. In addition to bubbles and fission tracks of nanometer sizes, there exist smaller scale structural defects and lattice strains that were revealed from selective laser excitation and fluorescence spectra. These defects are attributed to alpha-decay induced structural damage that occurred randomly throughout the lattice. Annealing of the samples at 773 K for 12 hours removed more of the residual defects