Terrence P. Mernagh and Yanis Miezitis reported in 2008 that thorium is a naturally occurring radioactive element which is found in the Earth mainly in oxides, silicates, carbonates and phosphates. Thorium exists almost entirely as 232Th which has a half-life of 14,050 million years. From its natural state, 232Th decays through a number of stages to eventually form 208Pb, which is stable.Thorium occurs in nature either within minerals or as a tetravalent ion. The large, highly charged Th4+ ion has a marked tendency to form complex ions in solution. The major ones of interest are the complex chloride ions, the complex fluoride ions, the complex nitrate ions, the complex sulphate ions and the complex carbonate ions. In addition, there are a number of hydroxide complexes, silicate complexes, phosphate complexes, various citrate, oxalate, tartrate, and acetate complexes and numerous chelate and other organic complexes of thorium, all of which render the element relatively soluble (and mobile) under certain natural conditions.
Thorium and uranium are chemically similar due to the similarities in ionic size and bond character of these elements. This explains why they tend to occur together in igneous rocks and hydrothermal ore deposits that form at high temperature. However, in the surficial environment, uranium is chemically more mobile. This is due to the limited stability of Th4+ complexes, especially those with carbonate, and the marked insolubility of Th4+ salts, compared with those of U4+ and U6+ at low temperatures.The average abundance of thorium in the Earth’s crust is about 5.6 ppm, but thorium is highly enriched in the upper crust with an average concentration of 10.5 ppm, while the middle crust has an average of 6.5 ppm, and the lower crust an average of 1.2 ppm. The most important thorium minerals are monazite, thorianite, thorite, and thorogummite. Other minerals that contain lesser amounts of thorium are allanite, bastnäsite, pyrochlore, xenotime, fluorapatite and zircon. Many of the thorium-bearing minerals are remarkably resistant to oxidation and tend to become enriched in the oxidized zones of mineral deposits.
Thorium occurs in a large variety of rock and deposit types as follows:
• Placer deposits of which heavy mineral sands are the most important Australian source of thorium.
• Carbonatites (e.g. Mount Weld, Cummins Range and Mud Tank).
• Alkaline and peralkaline igneous complexes (e.g. the Brockman deposit, Yangibana ironstones, Toongi, and the Narraburra Complex).
• Granites (e.g. Yilgarn craton, Arunta region, and Crockers Well).
• Pegmatites (e.g. Greenbushes, the Entia pegmatite field, and Wodgina).
• Iron oxide copper-gold-(uranium) deposits (e.g. Olympic Dam, Ernest Henry, Mt Painter)
• Thorium-bearing veins and lodes (e.g. Nolans Bore, the Peaked Hill Shear Zone, Pine Hill,and the West Bore Shear Zone near Alice Springs).
• Pitchblende (uraninite) veins, lodes, and disseminated bodies in or near major faults, shear zones, or unconformities (e.g. Rum Jungle, the Alligator Rivers, and South Alligator Rivers uranium fields).
• Pyritic quartz-pebble conglomerates (e.g. Witwatersrand and the Blind River-Elliot Lake deposits).
• Skarns and hornfels deposits (e.g. the Mary Kathleen deposit).
• Phosphates (e.g. the Georgina Basin, northern Australia).
• Coal, peat, and carbonaceous material.
Most of the known thorium resources in Australia are held in the monazite component of heavy mineral sand deposits, which are mined for their ilmenite, rutile, leucoxene and zircon content.Australia’s monazite resources are estimated to be of the order of 5.2 million tonnes. Therefore,assuming an average thorium content in monazite of about 7%, Australia’s thorium resources in heavy mineral sand deposits could amount to about 364,000 tonnes. Other significant sources of thorium include the Nolans Bore rare earth phosphate uranium deposit, which is in fluorapatite veins and dykes (with about 53,300 tonnes of thorium) and the Toongi zirconia project (with about 35,000 tonnes of thorium).
Consumption of thorium in non-energy applications occurs mainly in chemical catalysts, lighting,and welding electrodes. Worldwide thorium consumption is very small and is estimated at a few hundred tonnes annually. The production of thorium from rare-earth bearing minerals continues to create an oversupply of thorium. Because of the cost of disposal of thorium bearing products, non-radioactive substitutes are being developed for the limited thorium applications resulting in depressed demand for thorium.
The main factors generating current interest in thorium resources are its possible use as fuel in nuclear reactors in place of a perceived shortage of cheap uranium, and the thorium fuel cycle produces much less plutonium and other transuranic elements, compared with uranium fuel cycles and is considered by some to present less problems in waste disposal and to be more proliferation resistant than uranium fueled reactors.Thorium can be used as a nuclear fuel, through breeding to 233U.
Several reactor concepts based on thorium-fuel cycles are under consideration, but much development work is still required before the thorium-fuel cycles can be commercialized. India is currently testing components for a 300 MWe (Megawatt electric) technology demonstrator thorium-fueled reactor and may commence construction some time during the period 2007 to 2012. A collaborative research program involving Moscow's Kurchatov Institute and the U.S. company Thorium Power (with some additional funding from the U.S. government) is aimed at developing thorium-uranium fuel for the existing Russian Vodo-Vodyanoi Energetichesky (VVER)-1000 reactors. At one stage the program was planned for the disposal of weapons grade plutonium by using it as Th-plutonium fuel. Whereas normal fuel uses enriched uranium oxide, the new design has a demountable center portion and blanket arrangement, with plutonium fuel in the center and the Th(with uranium) around it.
The 232Th becomes 233U, which is fissile - as is the core 239Pu. The blanket material remains in the reactor for 9 years but the center portion is burned for only three years (as in a normal VVER) (see World Nuclear Association Information Paper – Thorium, May 2007: http://www.world-nuclear.org/info/info.html).
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