Alpha-recoil damage in natural zirconolite and perovskite

Zirconolite (CaZrTi2O7) and perovskite (CaTiO3) are the key minerals in SYNROC, a ceramic material developed for the immobilization of high level nuclear reactor wastes. When these minerals are incorporated in the SYNROC, long-lived radioactive actinide elements are preferentially partitioned into zirconolite and perovskite, consequently these are subjected to the effect of alpha-recoil resulting from the decay of the elements. This effect has been studied by X-ray and electron diffraction investigations of the natural samples of zirconolite and perovskite of varying ages and varying uranium and thorium contents. The samples studied have received the cumulative alpha dose ranging from 1.0 x 1018 to 1.1 x 1020 α/g. These studies showed that zirconolite has remained to be crystalline up to and beyond the alpha dose of 2 x 1019 α/g. The density change produced by the irradiation was as small as up to 3 %. The density of perovskite samples decreased by 1.8%, and their X-ray powder patterns were essentially unaffected. The results demonstrated that zirconolite and perovskite were extremely resistant to the effect of nuclear radiation, and provide stable crystal structures for the containment of radioactive waste elements during the time required for the decay of the radioactivity to safe level (typically 105 - 106 years). (Kako, I.)