HVEM-Tandem and EELS study of radiation damage in zirconolite

Zirconolite (CaZrTi{sub 2}O{sub 7}) is the major host phase for actinides in Synroc, a promising waste form for the immobilization of high-level radioactive waste. The effect of radiation damage on the structure and durability of zirconolite are important to predictive modeling of zirconolite`s behavior in the repository environment and risk assessment. In this study, radiation damage effects in zirconolite were investigated by irradiating samples with 1.5 MeV Kr{sup +} ions using the HVEM-Tandem at Argonne National Laboratory (ANL) and energy loss electron spectroscopy (EELS). The HVEM-Tandem consists of a modified AEI high voltage transmission electron microscope interfaced to a 2 MV tandem ion accelerator. EELS spectra were collected using a Philips 420 TEM, operated at 120 kV, fitted with a Gatan Model 607 Serial EELS. EELS data were recorded at resolutions of {approximately} 1.0 eV and at a dispersion of about {approximately} 0.25 eV. Selected area diffraction patterns (SADs) of individual grains of various zirconolites were monitored as a function of dose to establish the critical dose for amorphization (D{sub c}). The authors found that (1) D{sub c}(zirconolite) is independent of the atomic weight of dopants in zirconolite and the mean atomic weight of the sample and that (2) the Bragg reflections in SAD patterns which persist to the highest doses are firstly those resulting from the fluorite sublattice and secondly the four (110)-type reflections which lie on the innermost of the two diffuse rings representative of amorphous zirconolite