Thorium Th · 90

A weakly radioactive metal three to four times more common than uranium, which can be bred into nuclear fuel but is not fissile on its own.

Why does it matter?

Thorium comes up unavoidably with rare earths in monazite. Handling it is one of the reasons rare-earth processing is difficult to permit.

Where it is in the Earth

Thorium is a heavy metal that belongs to the actinide series — the same family as uranium and plutonium. In the Earth's crust it occurs mostly in trace amounts dispersed through granitic and other silica-rich rocks, but it can become concentrated into workable deposits wherever geological processes have sorted heavy minerals away from lighter ones. The principal concentrating mechanism is the same one that forms placer gold deposits: weathering breaks down ancient igneous or metamorphic rocks, rivers carry the debris to the coast or into sedimentary basins, and wave action or current sorting leaves behind a residue enriched in dense, chemically resistant minerals. One of those minerals is monazite, a phosphate that contains cerium, lanthanum, neodymium and thorium all substituting for one another in the same crystal lattice.

Monazite is durable enough to survive the journey from its parent rock to a beach or river sand deposit without breaking down, which is why the richest accumulations of thorium tend to be ancient coastal or alluvial sands rather than hard-rock veins. The thorium content of any given monazite grain depends on where in the world it formed: grains from some regions carry more thorium relative to the rare-earth elements than grains from others. This variability in composition matters greatly for downstream processing, because thorium and the rare earths must eventually be separated from each other. Beyond monazite, thorium also appears in the mineral thorite and in small amounts in zircon, but neither of these sources contributes meaningfully to supply at present.

The geological reason thorium is so closely tied to rare-earth mining is that monazite is one of the main ore minerals for cerium-group rare earths. Wherever a rare-earth project encounters a heavy-mineral sand or a weathered carbonatite that contains monazite, it inherits thorium as an inseparable companion. This is not incidental; it is the central fact that shapes the economics and the regulatory environment of thorium worldwide.

Getting it out

Because the richest thorium-bearing monazite deposits are heavy-mineral sands, the dominant extraction method is surface mining of unconsolidated sediment — essentially large-scale earth-moving and wet separation rather than drilling and blasting hard rock. A mining operation strips away overburden, excavates the sand, and feeds it through a wet concentrator that uses flowing water and shaking tables or spiral separators to sort minerals by density. This exploits the same physics that makes monazite accumulate in nature: it is denser than quartz and feldspar, so it settles faster and can be concentrated mechanically. The resulting heavy-mineral concentrate contains monazite alongside ilmenite, rutile, zircon and other minerals, which are then separated from one another by a combination of magnetic, electrostatic and gravity methods.

The concept of grade in a heavy-mineral sand deposit refers to the weight of total heavy minerals as a proportion of the bulk sand, and within that, the proportion of monazite. The thorium content of the monazite itself adds a further layer. Because the ore is loose sand rather than solid rock, very large volumes can be processed at relatively low cost per tonne, but the amount of waste material — the quartz sand that gets separated and returned to the pit — still substantially exceeds the mass of useful product. Where monazite is a by-product of mining for ilmenite or rutile, the economics of thorium recovery depend on the value of those primary minerals rather than on thorium itself.

Hard-rock deposits, such as carbonatites enriched in rare-earth minerals, can also carry thorium in their monazite fraction and are mined by conventional open-pit methods with drilling, blasting and crushing. The grade and the strip ratio — the volume of barren rock removed per unit of ore — vary considerably between deposits. In either case, the radioactivity of the monazite fraction means that workers, equipment and waste streams all fall under radiation-protection regulations from the point at which the mineral is sufficiently concentrated, and this shapes how tailings are stored and how the circuit is designed.

What pulls on it

The clearest existing use of thorium is as an alloying addition to certain high-temperature metals, particularly tungsten and magnesium, where even small additions meaningfully improve high-temperature strength or arc stability. Thorium-tungsten electrodes were for many years standard in gas-tungsten arc welding because they improve arc starting and stability. This application is shrinking: occupational health concerns about the mild radioactivity of thorium-tungsten electrodes have led many manufacturers and users to shift toward cerium-tungsten or lanthanum-tungsten alternatives that perform comparably without the regulatory overhead. The gas mantle industry — the fabric mantles used in pressurised gas lanterns — was historically a significant consumer of thorium oxide because it produces a bright white light when heated; this use has also contracted substantially as gas lanterns have become less common.

The application that draws the most attention is nuclear fuel. Thorium-232 is not itself fissile, meaning a neutron alone cannot split it and sustain a chain reaction. It is, however, fertile: when it absorbs a neutron it eventually converts to uranium-233, which is fissile and can sustain a chain reaction. In a thorium fuel cycle, a reactor must first be started with fissile material — either enriched uranium or plutonium — and the bred uranium-233 then contributes to ongoing power generation. Several national programmes, notably in India, have invested in thorium reactor research over many decades, and a small number of research and experimental reactors have operated on thorium-based fuel. Commercial deployment of thorium power reactors has not occurred, and the conditions that would be necessary for it to occur — including the development of commercially viable reactor designs and the resolution of fuel reprocessing questions — remain subjects of ongoing research rather than near-term industrial planning.

Because most thorium accumulates as a by-product of rare-earth processing and currently has limited markets, much of the thorium separated from monazite is stockpiled or disposed of rather than sold. Demand would change materially if a thorium reactor programme reached commercial scale somewhere in the world, but that outcome depends on decisions in nuclear policy and reactor engineering that are outside the materials market.