CaZrTi2O7 and 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 CaZrTi2O7 and which are subjected to the effects of .alpha.-recoil resulting from the decay of these elements. These effects were studied via X-ray and electron diffraction investigations of natural samples of varying ages and varying U and Th contents. The samples received cumulative .alpha.-doses ranging from 1.0 .times. 1018-1.1 .times. 1020.alpha./g. The upper limit corresponded to the .alpha.-irradiation which would be received by the CaZrTi2O7 in SYNROC containing 10% of high level waste over 5 .times. 108 yr. CaZrTi2O7 remained crystalline up to and beyond .alpha.-doses of 2 .times. 1019.alpha./g. This dose would have accumulated in such a SYNROC CaZrTi2O7 after a million years of storage. EM 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 yr, CaZrTi2O7 appeared metamict when studied by X-ray diffraction. EM and diffraction patterns demonstrated that the mineral continued to retain a large degree of short range order and in no way resembled a glass. The density changes were small and ranged from 0-3% at saturation. CaTiO3 samples which had SYNROC ages up to 20,000 yr decreased in density by 1.8 .+-. 0.1%. Their X-ray powder patterns were essentially unaffected. Comparative studies showed that the CaTiO3 lattice was more resistant to the effects of .alpha.-recoil than the zirconolite lattice. CaZrTi2O7 and CaTiO3 were resistant to the effects of nuclear radiation and provided 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 yr).