Details about the type of thorium deposition, and mineralization. Indian occurrences, Reserves etc

1. Thorium
 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, 232
Th decays through a number of stages to eventually form 208
Pb, 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.
 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 oxidised zones of mineral deposits.
Deposit Type of Thorium:
Thorium occurs in a large variety of rock and deposit types as follows:
 Placer deposits (as monazite heavy mineral in sands found in the coastal region of the India and
Australia).
 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).
Resource of Monazite in India:
As per IBM year book 2018 57th
edition The resource estimates of monazite in the beach and inland
placer deposits have been enhanced from 11.935 million tonnes in 2012 to 12.47 million tonnes in 2016.

2. How from thorium uses as fuel:
Thorium can be used as a nuclear fuel, through breeding to 233U. 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.
Resources of Monazite
State Resources (Mt)
Andra Pradesh 3.69
Gujarat 0.003
Jharkhand 0.21
Kerala 1.84
Maharastra 0.004
Odisha 3.06
Tamilnadu 2.46
West Bengal 1.20
All India 12.47
Source: Department of Atomic Energy, Mumbai.

3. Thorium is therefore called fertile, whereas U-233 is called fissile. Reactors that use thorium are
operating on what’s called the Thorium-Uranium (Th-U) fuel cycle.
Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor. However,
it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233, which is an excellent fissile
fuel material. In this regard it is similar to uranium-238 (which transmutes to plutonium-239). All
thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the
necessary neutron dosing to produce protactinium-233. The Pa-233 that is produced can either be
chemically separated from the parent thorium fuel and the decay product U-233 then recycled into new
fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form, especially in molten salt reactors (MRS).
Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain can be maintained. The only
fissile driver options are U-233, U-235 or Pu-239. (None of these is easy to supply).