Evidence and uncertainty for uranium and thorium abundance:A review

The actinides are thought to have been generated by the rapid neutron capture (r)-process in a supernova at the origin of the solar system (4.6 × 10 9 an ago) or even before. Thus, given the generation processes for thorium, Th, and uranium, U, (and their precursors) are somewhat similar, the difference in half-lives ( 232Th: 14.05 × 10 9 a, and 238U: 4.468 × 10 9 a) would suggest a larger abundance of Th than U be present today. However, the chemistry of U is very different to that of Th which could suggest different pathways to explain the different concentrations of Th and U in various formations and local deposits of UO 2. Focussing on the accessible geosphere of the Earth (i.e., the crust and hydrosphere), the evidence for the presence of these actinide elements in the liquid and solid rocks was examined. This study discusses why, in seawater, soluble Th (5 × 10 −11 M) is more depleted than U (1.4 × 10 −8 M), and why solid ores of pure UO 2 or U 3O 8 are relatively common while ThO 2 deposits are quasi-non-existent. Thorium is rather diluted in rock, relative to uranium, requiring that more mining, milling and treatment is necessary to recover it on comparable scales. This paper discusses and compares the abundances of U and Th in the universe and more specifically in the Earth's accessible geosphere as well as the extent of their known, economically-recoverable resources