Maybe because of the difficult recovery of Th? [quote="Chuck"] The result is that uranium forms deposits more frequently, and of higher grade, than does thorium, which is distributed much more evenly across a wide variety of rock types. Today the world has a [size=15]uranium reserve of 4 million tonnes, with a resource maybe ten times larger. This is despite not actively exploring for the substance since the cold war ended. Thorium, strictly speaking, doesn't have reserves at all; it is currently only recovered as a byproduct of rare earth element mining.[/size] But based on known occurrences of monazite (LREE)PO4, which can contain a few percent of Th), the estimated resource is about 1.5 million tons.