The IAEA-TECDOC-#1892 indicates that thorium and uranium can both be used as nuclear fuels. Interest in the use of thorium as a source of nuclear fuel has re-emerged in early 21st century. Full commercialization of nuclear power based on thorium is yet to materialize because pilot test reactors and projects intended to appraise thorium have not yet found it a feasible reactor fuel. Several decades of research in various countries have demonstrated that designing a thorium-based nuclear reactor will take about as long as the history of reactors fueled by uraniumÍŸ-plutonium i.e many decades. For comparison with thorium fuel cycles, the various concepts for advanced uranium reactors currently being researched and developed include:
ï‚· High temperature gas-cooled reactors (HTGRs);
ï‚· Molten salt reactors (MSRs); [... with variations in the form of SMRs will likely appear in the next few years, which may have a major impact.]
ï‚· CANDU-type reactors;
ï‚· Advanced heavy water reactors (AHWRs);
ï‚· Fast breeder reactors (FBRs). Both Russia and India have units ready for operation
Government supported projects have recently focused research towards the development of thorium-based nuclear power. These projects build upon past pilot scale reactor projects in Canada, Germany, India, the United States of America and the United Kingdom. On the basis of current activities, it is expected that, in the not too distant future, thorium will be utilized as a reactor fuel.
The advantages of thorium as a source of nuclear fuel include:
- Thorium’s average abundance (6–10 g/t) in the Earth’s crust (it is 3–4 times more abundant than uranium);
- Most significant thorium deposits contain other valuable minerals; for example, niobium, rare earth elements (REE), tantalum and/or titanium;
- The types of waste product arising from thorium fission are not useful in nuclear weaponry (in contrast to plutonium, which is a by-product of nuclear power generation using uranium). Additionally, fuels based on thorium are efficient for reduction of current plutonium stocks whereby the fission chain reaction can be initiated using plutonium:
- Thorium-rich spent fuels have fewer radioactive elements and have smaller mass, volume and half-life compared with traditional uranium-based nuclear wastes.
In the past, there were more non-nuclear applications for thorium compounds, especially in gas mantles in light bulbs (thorium oxide produces a bright light), welding electrodes, heat resistant ceramics (thorium oxide melting point 3300+°C), arc lamps, camera lenses (high refractive index glass), aerospace components and many other applications. Radiological concerns over the use of thorium has led to increased use of alternatives, which have adversely impacted demand for thorium. For example, in an industrial context, yttrium compounds have replaced thorium compounds in incandescent lamp mantles and a magnesium alloy containing lanthanides, yttrium and zirconium has replaced magnesium–thorium alloys in aerospace applications. However, thorium continues to be used in certain catalysts, high temperature ceramics and welding electrodes.
The IAEA report emphasized the possibilities for the production of thorium as a co/by-product from REE-bearing minerals. There is a significant potential to produce thorium compounds during the production of REE. Thorium often also occurs in nature associated with uranium. However, under specific circumstances, thorium may form deposits that do not contain uranium (or only in very small amounts) and is associated with other elements. Deposits of major importance, where thorium can be available as a by-product of REE and rare metal production, are found in specific geological environments.
Thorium deposits can be classified into:
ï‚· Carbonatites;
ï‚· Alkaline/peralkaline rocks;
ï‚· Vein-type;
ï‚· Metamorphic type;
ï‚· Placers.
Carbonatites
The majority of carbonatites comprise 50% or more primary carbonate minerals (dolomite, calcite and/or ankerite). A characteristic of carbonatites is their enrichment in volatiles (CO2) and in HDSE, especially REE with associated enrichments in Th. Examples occur in Australia, Brazil, Canada, the Russian Federation, Scandinavia, South Africa and the USA.
Alkaline/Peralkaline Rocks
Alkaline and peralkaline rock types containing thorium resources are typified by higher contents of alkali elements (Na and K) than Al (i.e. Na2O + K2O > Al2O3). HFSE, especially REE, Y, Nb, Ta, Zr, U and Th, are frequently enriched in these rocks with concentrations locally reaching economic grades. Deposits occur in alkaline plutons, magmatic layered complexes, and associated veins and/or dykes. Examples are mainly polymetallic, such as the Thor Lake deposit in Canada or the Kvanefjeld deposit in Greenland.
Vein-type
Vein-type mineralization is described as discordant mineralization mostly inclined in various country rocks occupying faults, joints or fissures. Vein-type deposits, including dykes, often show a genetic relationship to magmatic intrusions (granites, alkaline rocks, carbonatites). They can be monometallic or polymetallic, with concentrations of base metals, rare metals, uranium, thorium and REE. Examples include the Lemhi Pass deposit in the USA (uranothorianite) and the Steenkampskraal deposit in South Africa (monazite vein), Nolans Bore in Australia (rich in REE, fluorapatite, uranium and thorium). At Steenkampskraal mine in South Africa, the
monazite–apatite veins are associated with charnockites and leucotonalites.
Metamorphic type
This type consists of thorium concentrations in metamorphosed or metasomatized rocks. Thorium-bearing minerals are mostly in finely dispersed form, on joints, schistosity planes, etc. Host rocks of this type can be anatexites, migmatites, pyroxenites, gneisses and schists, as well as contact metamorphic rocks such as skarn, hornfels and marbles. Examples include deposits in southern Kerala in India and the Mary Kathleen deposit in Australia.
Placer Deposits
Resistant sand-sized grains of hard, high specific gravity minerals accumulate along with sand, silt and clay in coastal, stream and river environments, locally forming economic concentrations of HMS. Monazite is the dominant REE- and Th-bearing mineral in HMS, from which such elements may be recovered. The ages of placer-type deposits vary from Archaean (e.g. the palaeo-quartz-pebble conglomerates in the Witwatersrand Basin in South Africa) to Tertiary. Recent aged HMS deposits (black sands) exist chiefly in coastal areas in Australia, Brazil, India, Mozambique, South Africa and the USA.
Global thorium resources were estimated in 2013 at 6.2 Mt, although uncertainties in resources exist in several countries, and thus the world total may be more than 7 Mt Th.Total resources of thorium in placer-type deposits are estimated worldwide to be ~2.2 Mt, which represents 35% of total thorium resources.
Carbonatite-hosted deposits are estimated to amount 1.8 Mt of Th, which represents 29% of the total resources. Typically, the carbonatite-hosted deposits are enriched in REE and/or Nb/Ta. Vein-type deposits are estimated to amount to 1.5 Mt of Th,
representing 25% of the total resources.
Alkaline and peralkaline rock-type deposits contain 6 Mt of Th, representing 9% of the total resources. Currently, recovery of thorium as a by-product of REE production from monazite is the most likely option. Current world production of REE is estimated at around 110 000 t/y.
Thorium concentration in the monazite varies depending on the deposit, and thus the quantities of thorium that might be available through separate recovery is unknown. India is currently the sole country where thorium is extracted from monazite obtained by separation from other heavy minerals in placer deposits.
Global (Brazil, India, Malaysia, Vietnam) monazite production in 2013 had a theoretical Th content of around 400t Th at an average grade of 6% Th. The recovery of thorium from monazite is possibly the most realistic option in the near term. However, if demand for thorium were to occur, other sources of recovery of by-product thorium may become available.
The IAEA report concludes by stating that thorium resources as co- and by-products of uranium, REE, rare metals and base metals are significant, and given a renewed interest in the use of thorium as a possible nuclear fuel, then these resources could be important in the future. Careful considerations of the mineralogy will open up possibilities for co/by-product recovery of thorium. Thorium is an important energy source and could be used as a fuel in various nuclear power designs. However, due to the challenges in processing thorium compounds, the need for initial fissile material stock such as enriched uranium or plutonium, and the relatively easier availability of uranium, thorium is not currently widely used as a nuclear fuel.
Countries like India and China have ambitious thorium utilization programs underway. As technology improves, it is expected that some of the challenges in using thorium will be overcome and its use may become widespread. Therefore, it will be of economic prudence not to lose thorium to the waste stream when processing thorium-bearing minerals. Apart from creating a radiological waste problem, it is also wasting valuable material that has a dense energy value. As the world faces an unprecedented climate crisis, the role of low-carbon fuel sources such as thorium will gain importance. A more pragmatic approach will be to store thorium safely so that the stockpiles will be available as a source of sustainable, low-carbon energy for the future.
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