Thorium and uranium are the only two actinide elements that are stable enough to have survived since the formation of the solar system without being destroyed by radioactive decay. They are geochemically similar. Both are strongly lithophilic, meaning that they form oxides and dissolve into silicate melts, without entering into metallic or sulfide phases. Both elements are fairly refactory, meaning that they condense earlyish in the solar nebula. And they are both incompatible during mantle melting. This means that they do not fit into the crystal structure of mantle minerals, so when a planetary mantle undergoes partial melting, the vast majority of the U and Th dissolves into the melt, and relatively little remains in the mantle mineral residue. This is because Th and U are both large +4 cations under most conditions, and mantle minerals (made mostly of Mg, Si, Fe, Al, and Ca oxides) don’t have any crystallographic sites into which such ions can fit.
Of the two, Th is slightly more refactory and incompatible, but not much, so their ratio doesn’t change a whole lot. As a result, the solar ratio (about 4) is not that different to the ratio of the lunar crust, the Martian crust, the Earth’s crust, and the Earth’s mantle. For most meteoritic, igneous, and mantle rocks, Th/U is about 4.
This number seems to have been picked up on by the more extreme proponents of nuclear fission- generated electricity, as evidence that even if we run out of uranium, there is still lots more thorium. Technically, this is true. If we were to run the entire solar system through a refining mill with a 100% recovery percentage, then at the end of the project, we would have 4 times more thorium than uranium. But the real world doesn’t work like that.
Even though the crustal Th/U ratio is also about 4, we aren’t conceivably going to dig up the entire Earth’s crust either. So the bulk crustal (or planetary) Th/U ratio is not really relevant to anything other than science fiction. What is important is the ability of geological processes to concentrate these elements into a highly enriched deposit, which we can then mine for a reasonable cost and effort. And this is where U and Th start to differ.
Although these two actinides are geochemically similar under most conditions in the solar system, there is one key difference in their chemistry. In the presence of abundant oxygen, U can oxidize from +4 to +6. For the first two billion years of Earth’s history, this was irrelevant. But about 2.4 billion years ago, free molecular oxygen first started appearing in Earth’s atmosphere and surface waters. And this changed everything.
Thorium can’t form a +6 ion, because Th +4 has the same electron configuration as the noble gas radon- all the electron shells are closed. But uranium has 2 extra electrons to lose, given enough oxygen around to take them. And the hexavalent chemistry is quite different to the tetravalent. In the +4 valence, both U and Th are generally insoluble under most hydrologic conditions. But U +6 forms a uranyl ion (UO2++), which is highly soluble in most geologically reasonable waters.
As a result, for the last 2.4 billion years, uranium has been dissolving from oxidized rocks, flowing through aquifers with the groundwater, and then reprecipitating wherever a later chemical reaction consumes the oxygen in the water. What this means is that uranium can be- and is- concentrated by a geologic process which has no effect on thorium.
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 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. 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.
So although the bulk crustal concentration of thorium is higher than that of uranium, its simpler chemistry means that it does not get concentrated into mineral deposits as easily. So while claiming that Th is more abundant is technically correct, it isn’t the sort of technicality that one should base energy policy on.
[edit: typos]
