Natural thorium, which, as noted above, has only one isotope, is a relatively abundant element with an average concentration of 7.2 ppm in the earth’s crust. This is significantly higher than uranium (2.5 to 3 ppm), reflecting the longer half-life of Th-232 (1.4 x 101 0 years) compared to 4.5 * 109 years for U-238. Nevertheless, it does not mean at all that the exploitable reserves of thorium are two or three times larger than uranium, as many would assert. In fact, because of its limited uses so far, extensive prospecting of thorium has not yet been conducted so that reliable estimates of the world wide reserves of thorium are not currently available. The famous IAEA ‘red book’ on uranium resources, published periodically, included detailed data on thorium resources until its edition of 1981 but, since then, only global data have been provided. For example, in the last edition published in 2009, a figure of 6.038 tons is given for the total world thorium resource. However, the IAEA has launched recently a small programme specifically intended to estimate thorium resources in the world.

The largest source of thorium is the mineral monazite (phosphate), also a primary source of rare earth elements. It is also found in the mineral thorianite (thorium dioxide) and some has been recovered from igneous veins and igneous carbonate deposits called carbonatites. Significant deposits of thorium are found in Australia, Brazil, Canada, Greenland, India, South Africa and the United States. More generally, the world’s reasonably assured reserves (RAR) are known to be at least as large as those of uranium, and quite probably higher.

In any event, should a closed thorium cycle be deployed on a large industrial scale it must be underscored here that thorium reserves are not a real issue since, like U-238, it is a fertile isotope, that, when deployed with U-233 recycling, would be able to sustain nuclear energy development for a very long time. To provide an explanation of what we mean, let us suppose for example that thorium reserves are only those identified as easily available today U. S. Geological Society ([USGS]) (let us say between 1 and 2 million tons). If one transforms all these reserves into U-233 in nuclear reactors, the complete fission of this uranium-233 would be enough to produce energy equivalent to that produced annually by all the existing nuclear power plants for several thousands of years. Therefore, the problem is not that of the amount of available thorium reserves but that of the quantities of fissile materials necessary to initiate and then sustain a cycle with thorium. Exactly the same may be said of U-238 and the availability of plutonium or U-235.