Alpha-decay damage and aqueous durability of actinide host phases in natural systems

This paper provides an overview of the geochemical alteration and alpha-decay damage effects observed in the titanate mineral phases brannerite, perovskite, pyrochlore, zirconolite, and the silicate mineral zircon. The available data show that brannerite, pyrochlore. zircon, and zirconolite are all highly durable in natural systems, often surviving the complete destruction of the host rock during weathering. In comparison, perovskite is prone to dissolution and conversion to anatase and other secondary alteration products at moderate to low temperatures. The mineralogical studies provide quantitative data on the crystalline-amorphous transformation and the structure of the metamict state. These studies indicate that the titanate and silicate phases of interest become metamict (amorphous) as a result of the gradual accumulation of alpha-recoil collision cascades. When compared to the data for synthetic pyrochlore and zirconolite doped with Pu-238 and Cm-244, dose-age data indicate that the critical dose values for geologically young samples of natural pyrochlore and zirconolite are higher by a factor of approximately 2-3. The available geological data indicate that most of the natural samples occur in rock types that cooled very rapidly to 300 degreesC or lower, followed by slow cooling over geological time periods at lower temperatures without subsequent reheating (metamorphism). Average storage temperatures are on the order of 100-200 degreesC, indicating that the higher critical dose values of the natural samples are due to thermal annealing over geological time periods. The dose-age data also provide evidence for the long-term annealing of isolated alpha-recoil collision cascades in perovskite, pyrochlore, zirconolite, and zircon. Rate constants obtained from the current data sets suggest similar long-term annealing rates of 1-2 x 10(-9) yr(-1) for all four minerals. Additional recovery of damage from long-term annealing at ambient conditions will not be significant for time periods up to approximately 10(7) yr. (C) 2001 Elsevier Science B.V. All rights reserved.