8.1.3 The front-end of the thorium fuel cycle

Mining and milling

Since no thorium-based fuel is being used at an industrial or commercial scale in the world today, there is no international market for thorium and it can be assumed that a supporting mining industry will not develop until the demand for thorium fuel increases. Nevertheless, thorium is still being used today for various specific applications,[16] and is generally obtained as a by-product of uranium and, more especially, of rare-earth mining. As such, there is no real need to develop a specific thorium mining industry given that enough thorium is, for the time being, generated as a by-product. The available mining experience allows one to make some of the following observations.

The primary source of thorium is the rare-earth and thorium phosphate mineral, monazite. Without demand for the rare earths, monazite would probably not be recovered for its thorium content. Other ore minerals with higher thorium contents, such as thorite, would be more likely sources. However, mining of monazite deposits is easier than that of uranium-bearing ores. Very little overburden is removed as monazite is produced from beach sands or placer deposits.

Thorium is found in a number of minerals. Hence there are several process alternatives, like physical and magnetic separation and heavy-metal chemical extraction. Monazite, the chief commercial ore from which thorium is extracted, is chemically inert and any chemical treatment for extracting thorium must initially be very severe to achieve the complete dissolution necessary for the separation of the rare-earth elements, uranium and phosphates. The most common dissolution processes use highly concentrated sulphuric acid or highly concentrated sodium hydroxide.

Monazite is pulverized and leached in a 50-70% solution of hot sodium hydroxide and undergoes solvent extraction, stripping operations and ion exchange to obtain thorium nitrate, which is ultimately converted to thorium oxide powder. Those operations are necessary to obtain the level of purity required for nuclear applications. Like uranium, thorium is naturally radioactive but the ‘radon impact’ from processing thorium ores is easier to handle because its radioactive daughter thoron (Rn-220) is shorter lived (its half-life is 56 s) than its radon counterpart from uranium milling operations (Rn-222 with a half-life of 3.8 days).