Alpha-recoil damage in natural zirconolite and perovskite.

Abstract

Zirconolite (CaZrTi2O7) and perovskite (CaTiO3) are key minerals in SYNROC, a ceramic material developed for the immobilization of high level nuclear reactor wastes. When these are incorporated in SYNROC, the long-lived radioactive actinide elements are preferentially partitioned into zirconolite and perovskite which are therefore subjected to the effects of alpha-recoil, resulting from the decay of these elements. These effects have been studied via X-ray and electron diffraction investigations of natural samples of zirconolite and perovskite of varying ages and varying uranium and thorium contents. The samples studied have received cumulative alpha doses ranging from 1.0 × 1018 to 1.1 × 1020α/g. The upper limit corresponds to the alpha irradiation which would be received by the zirconolite in SYNROC containing 10 percent of high level waste over a period of 5 × 108 years. These studies show that zirconolites remain crystalline up to and beyond alpha doses of 2 × 1019α/g. This dose would have accumulated in such a SYNROC zirconolite after a million years of storage. Electron microscopy revealed that the grains were composed of small crystalline domains which possessed the defect fluorite-type structure. After a dose exceeding that which would be received by SYNROC in 100 million years, zirconolites appeared metamict when studied by X-ray diffraction. However, the electron micrographs and diffraction patterns clearly demonstrate that the mineral continues to retain a large degree of short range order and in no way resembles a glass. The density changes produced in these zirconolites by irradiation are small and range from 0 to 3 % at saturation. Perovskite samples which have SYNROC ages up to 20, 000 years decrease in density by 1.8 ± 0.1 %. Their X-ray powder patterns are essentially unaffected. Comparative studies show that the perovskite lattice is even more resistant to the effects of alpha-recoil than the zirconolite lattice. The results demonstrate that zirconolite and perovskite are extremely resistant to the effects of nuclear radiation and will provide stable crystal structures for the containment of the radioactive waste elements during the time required for the radioactivity to decay to safe levels (typically 105-106 years).