Thorium ( , ) is a
chemical element with the symbol
Th and
atomic number
90. It is a naturally occurring, slightly
radioactive metal. Thorium
is estimated to be about three to four times more abundant than
uranium in the Earth's crust.
Thorium was successfully used as an
alternative nuclear fuel to uranium in the molten-salt
reactor experiment
(MSR) from 1964-1969 to produce thermal energy, as
well as in several light-water reactors using Th232-U233 fuel
including the Shippingport Atomic Power
Station
(operation commenced 1977, decommissioned in
1982). Currently, officials in the Republic of India
are advocating a thorium-based nuclear program, and a
seed-and-blanket fuel utilizing thorium is undergoing irradiation
testing at the Kurchatov Institute
in Moscow. Advocates of the use of thorium
as the fuel source for nuclear reactor state that they can be built
to operate significantly cleaner than uranium based power plants as
the waste products are much easier to handle.
Characteristics
Physical
Pure thorium is a silvery-white metal which is air-stable and
retains its luster for several months. When contaminated with the
oxide, thorium slowly tarnishes in air, becoming gray and finally
black. The physical properties of thorium are greatly influenced by
the degree of contamination with the oxide. The purest specimens
often contain several tenths of a percent of the oxide. Pure
thorium is soft, very ductile, and can be cold-rolled, swaged, and
drawn. Thorium is dimorphic, changing at 1400 °C from a
face-centered cubic to a
body-centered cubic structure. Powdered thorium metal is often
pyrophoric and requires careful handling. When heated in air,
thorium metal
turnings ignite and burn
brilliantly with a white light. Thorium has the largest liquid
range of any element: 2946 °C between the melting point and
boiling point.
Chemical
Thorium is slowly attacked by water, but does not dissolve readily
in most common acids, except hydrochloric. It dissolves in
concentrated nitric acid containing a small amount of catalytic
fluoride ion.
Compounds
Thorium compounds are stable in the +4 oxidation state.
Thorium dioxide has the highest
melting point (3300 °C) of all oxides.
Thorium(IV) nitrate and thorium(IV) fluoride are known in their
hydrated forms: and , respectively. The thorium center has
square planar geometry.
Thorium(IV) carbonate, , is also known.
When treated with
potassium
fluoride and
hydrofluoric
acid, Th
4+ forms the complex anion , which
precipitates as an insoluble salt, .
Thorium(IV) hydroxide, , is highly insoluble in water, and is not
amphoteric. The
peroxide of thorium is rare in being an insoluble
solid. This property can be utilized to separate thorium from other
ions in solution.
In the presence of
phosphate anions,
Th
4+ forms precipitates of various compositions, which
are insoluble in water and acid solutions.
Thorium monoxide has recently been produced through laser ablation
of Thorium in the presence of oxygen
Isotopes
Naturally occurring thorium is composed mainly of one
isotope:
232Th.
230Th occurs as the
daughter product of
238U decay. Twenty-seven
radioisotopes have been characterized, with the
most stable being
232Th with a
half-life of 14.05 billion years,
230Th
with a half-life of 75,380 years,
229Th with a half-life
of 7340 years, and
228Th with a half-life of 1.92 years.
All of the remaining
radioactive
isotopes have half-lives that are less than thirty days and the
majority of these have half-lives that are less than ten minutes.
One isotope,
229Th, has a
nuclear isomer (or metastable state) with a
remarkably low excitation energy of 7.6 eV.
The known isotopes of thorium range in
atomic weight from 210
u (
210Th) to 236 u
(
236Th).
Applications
Applications of thorium:
Applications of
thorium dioxide
(ThO
2):
- Mantles in portable gas lights. These
mantles glow with a dazzling light (unrelated to radioactivity)
when heated in a gas flame.
- Used to control the grain size of tungsten used for electric lamps.
- Used in heat-resistant
ceramics like high-temperature laboratory
crucibles.
- Added to glass, it helps create glasses of
a high refractive index and with
low dispersion. Consequently,
they find application in high-quality lens for cameras and scientific
instruments.
- Has been used as a catalyst:
- Thorium dioxide is the active ingredient of Thorotrast, which was used as part of X-ray diagnostics. This use has been abandoned due to
the carcinogenic nature of Thorotrast.
Thorium as a nuclear fuel
Thorium, as well as uranium and
plutonium,
can be used as fuel in a
nuclear
reactor. Although not
fissile itself,
232Th will absorb
slow
neutrons to produce
233U,
which is fissile. Hence, like
238U, it is
fertile. Theoretically thorium is a more
suitable fuel source than uranium. It is at least 4-5 times more
abundant in nature than all of uranium isotopes combined and is
fairly evenly spread around Earth, with many countries having large
supplies of it. Also, preparation of thorium fuel does not require
a difficult and expensive enrichment process. The thorium fuel
cycle mainly creates Uranium-233 which can be used for making
nuclear weapons, and since there are no neutrons from spontaneous
fission of U-233, U-233 can be used easily in a gun-type nuclear
bomb.
In
1977 a light-water reactor at the Shippingport
Atomic Power Station was used to establish a Th232-U233 fuel
cycle. The reactor worked untill its decommissioning in
1982. Thorium can be and has been used to power nuclear energy
plants using both the modified traditional
Generation III reactor design and
prototype
Generation IV
reactor designs.
A seed-and-blanket fuel using a core of plutonium surrounded by a
blanket of thorium/uranium has been undergoing testing at Moscow's
Kurchatov Institute, under a 1994 agreement between the institute
and McLean, Virginia-based Thorium Power Ltd. Russian
government-owned nuclear design firm Red Star formed an agreement
with Thorium Power in 2007 to continue work on scaling up the test
fuel rods to commercial use and licensing in
VVER-1000 reactors. This assembly could achieve a more
efficient disposal method of weapons-grade plutonium than the
mixed-oxide disposal method, especially with the 2009 decision by
the US to shelve the Yucca Mountain nuclear waste storage program
highlighting the issue of what to do with all the plutonium left
over from decommissioned nuclear weapons. Thorium Power, with
offices in London, Dubai, and Moscow and with Dr.
Hans Blix serving as an advisor, also advises the
United Arab Emirates on their fledgling nuclear program. They are
awaiting the finalization of the US-India nuclear 1-2-3 Agreement
to complete a joint-venture with
Punj
Lloyd, an Indian engineering firm with nuclear reactor
construction ambitions.
When using thorium in modified
light
water reactor problems include: the undeveloped technology for
fuel fabrication; in traditional, once-through
LWR designs potential problems in
recycling thorium due to highly radioactive
228Th; some
weapons proliferation risk due to production of
233U;
and the technical problems (not yet satisfactorily solved) in
reprocessing. Much development work is still required before the
thorium fuel cycle can be commercialized for use in
LWR. The effort required has not seemed
worth it while abundant uranium is available, but geopolitical
forces (e.g. India looking for indigenous fuel) as well as uranium
production issues, proliferation concerns, and concerns about the
disposal/storage of radioactive waste are starting to work in its
favor. In 2008, Senator Harry Reid (D-Nevada) and Senator Orrin
Hatch (R-Utah) introduced the Thorium Energy Independence and
Security Act of 2008, which would mandate a US Department of Energy
initiative to examine the commercial use of thorium in US reactors.
Although the bill did not go to a full Senate vote, it is expected
to be reintroduced in 2009.
The
thorium fuel cycle, with its
potential for breeding fuel without
fast neutron reactors, holds
considerable potential long-term benefits. Thorium is significantly
more abundant than uranium, and is a key factor in sustainable
nuclear energy. Perhaps more importantly, thorium produces several
orders of magnitude less long-lived radioactive waste .
An early
effort to use a thorium fuel cycle took place at Oak Ridge
National Laboratory
in the 1960s. An experimental reactor was
built based on
MSR technology to
study the feasibility of such an approach, using thorium-
fluoride salt kept
hot enough to be liquid, thus eliminating the need for fabricating
fuel elements.
This effort culminated in the Molten-Salt
Reactor Experiment
that used 232Th as the fertile material
and 233U as the fissile fuel. This reactor was
operated successfully for about five years. However, due to a lack
of funding, the MSR program was discontinued in 1976. Nowadays this
design is considered as
Generation
IV reactor.
India
's Kakrapar-1
reactor is the world's first reactor which uses
thorium rather than depleted uranium to achieve power flattening
across the reactor core. India, which has about 25% of the
world's thorium reserves, is developing a 300 MW prototype of a
thorium-based
Advanced
Heavy Water Reactor (AHWR). The prototype is expected to be
fully operational by 2011, following which five more reactors will
be constructed. Considered to be a global leader in thorium-based
fuel, India's new thorium reactor is a fast-breeder reactor and
uses a plutonium core rather than an accelerator to produce
neutrons. As accelerator-based systems can operate at
sub-criticality they could be developed too, but that would require
more research. India currently envisages meeting 30% of its
electricity demand through thorium-based reactors by 2030.
In 2007,
Norway
was debating whether or not to focus on thorium
plants because of the large deposits of thorium ores in the
country, particularly at Fensfeltet near
Ulefoss in Telemark
county.
The
primary fuel of the HT3R Project
near Odessa,
Texas
, USA
will be
ceramic-coated thorium beads.
History
M. T.
Esmark found a black mineral on Løvøy Island,
Norway and gave a sample to Professor Jens Esmark, a noted mineralogist who was not able to identify it,
so he sent a sample to the Swedish chemist Jöns Jakob Berzelius for
examination in 1828. Berzelius analyzed it and named it
after
Thor, the
Norse god of thunder. The metal had
virtually no uses until the invention of the
gas mantle in 1885.
In 1898 thorium was first observed to be radioactive,
independently, by Polish-French physicist
Marie Curie and English chemist
Gerhard Carl Schmidt. Between 1900 and
1903,
Ernest Rutherford and
Frederick Soddy showed how thorium
decayed at a fixed rate over time into a series of other elements.
This observation led to the identification of
half life as one of the outcomes of the
alpha particle experiments that led to their
disintegration theory of
radioactivity.
The
crystal bar process
(or
Iodide process) was discovered by
Anton Eduard van Arkel and
Jan Hendrik de Boer in 1925 to produce
high-purity metallic thorium.
The name ionium was given early in the study of radioactive
elements to the
230Th
isotope
produced in the
decay chain of
238U before it was realized that
ionium and thorium were chemically identical. The symbol
Io was used for this supposed element.
Occurrence
Monazite, a rare-earth-and-thorium
phosphate mineral, is the primary source of the world's
thorium
Thorium is found in small amounts in most rocks and
soils, where it is about four times more abundant than
uranium, and is about as common as
lead. Soil
commonly contains an average of around 12 parts per million (ppm)
of thorium. Thorium occurs in several
minerals including
thorite
(ThSiO
4),
thorianite
(ThO
2 + UO
2) and
monazite. The latter is most common and may contain
up to about 12% thorium oxide. Thorium-containing monazite(Ce)
occurs in Africa, Antarctica, Australia, Europe, India, North
America, and South America.
232Th decays very slowly (its
half-life is comparable to the age of the
Universe) but other thorium
isotopes occur
in the thorium and uranium decay chains. Most of these are
short-lived and hence much more radioactive than
232Th,
though on a mass basis they are negligible.
Thorium extraction
Thorium has been extracted chiefly
from monazite through a complex multi-stage process. The monazite
sand is dissolved in hot concentrated
sulfuric acid (H
2SO
4).
Thorium is extracted as an insoluble residue into an organic phase
containing an amine. Next it is separated or "stripped" using an
ion such as nitrate, chloride, hydroxide, or carbonate, returning
the thorium to an aqueous phase. Finally, the thorium is
precipitated and collected.
Several methods are available for producing thorium metal: it can
be obtained by reducing thorium oxide with calcium, byelectrolysis
of anhydrous thorium chloride in a fused mixture of sodium and
potassium chlorides, by calcium reduction of thorium tetrachloride
mixed with anhydrous zinc chloride, and by reduction of thorium
tetrachloride with an alkali metal.
Distribution
Present knowledge of the distribution of thorium resources is poor
because of the relatively low-key exploration efforts arising out
of insignificant demand. There are two sets of estimates that
define world thorium reserves, one set by the US Geological Survey
(USGS) and the other supported by reports from the OECD and the
International Atomic Energy Agency (the IAEA).
Under the USGS
estimate, Australia and India have
particularly large reserves of thorium. India and Australia
are believed to possess about 300,000 metric tonnes each; i.e. each
country possessing 25% of the world's thorium reserves. However, in
the OECD reports, estimates of Australian's Reasonably Assured
Reserves (RAR) of Thorium indicate only 19,000 metric tonnes and
not 300,000 tonnes as indicated by USGS. The two sources vary
wildly for countries such as Brazil, Turkey, and Australia.
However, both reports appear to show some consistency with respect
to India's thorium reserve figures, with 290,000 metric tonnes
(USGS) and 319,000 metric tonnes (OECD/IAEA). Furthermore the IAEA
report mentions that India possesses two thirds (67%) of global
reserves of monazite, the primary thorium ore. The IAEA also states
that recent reports have upgraded India's thorium deposits up from
approximately 300,000 metric tonnes to 650,000 metric tonnes.
Therefore, the IAEA and OECD appear to
conclude that Brazil
and India
may actually possess the lion's share of world's thorium
deposits.
- The prevailing estimate of the economically available thorium
reserves comes from the US Geological Survey, Mineral Commodity
Summaries (1997-2006):
| Country |
Th Reserves (tonnes) |
Th Reserve Base (tonnes) |
| Australia |
300,000 |
340,000 |
| India |
290,000 |
300,000 |
| Norway |
170,000 |
180,000 |
| United States |
160,000 |
300,000 |
| Canada |
100,000 |
100,000 |
| South Africa |
35,000 |
39,000 |
| Brazil |
16,000 |
18,000 |
| Malaysia |
4,500 |
4,500 |
| Other Countries |
95,000 |
100,000 |
| World Total |
1,200,000 |
1,400,000 |
Note: The Australian figures are based on assumptions and not on
actual geological surveys, therefore the figures cited for
Australia may be misleading, should be treated with caution and
could possibly indicate inflated values for Australia's actual
reserves of thorium; note the OECD estimates of Australian's
Reasonably Assured Reserves (RAR) of Thorium (listed below)
indicate only 19,000 metric tonnes and not 300,000 tonnes as listed
above.
- Another estimate of Reasonably Assured Reserves (RAR) and
Estimated Additional Reserves (EAR) of thorium comes from OECD/NEA,
Nuclear Energy, "Trends in Nuclear Fuel Cycle", Paris, France
(2001):
| Country |
RAR Th (tonnes) |
EAR Th (tonnes) |
| Brazil |
606,000 |
700,000 |
| Turkey |
380,000 |
500,000 |
| India |
319,000 |
— |
| United States |
137,000 |
295,000 |
| Norway |
132,000 |
132,000 |
| Greenland |
54,000 |
32,000 |
| Canada |
45,000 |
128,000 |
| Australia |
19,000 |
— |
| South Africa |
18,000 |
— |
| Egypt |
15,000 |
309,000 |
| Other Countries |
505,000 |
— |
| World Total |
2,230,000 |
2,130,000 |
Precautions
See
Actinides in the
environment for details of the environmental aspects of
thorium.
Powdered thorium metal will often ignite spontaneously in air (it
is
pyrophoric) and should be handled
carefully. Natural thorium decays very slowly compared to many
other radioactive materials, and the
alpha radiation emitted cannot penetrate
human skin. Owning and handling small amounts of thorium, such as a
gas mantle, is considered safe if care is
taken not to ingest the thorium—lungs and other internal organs
can be penetrated by alpha radiation. Exposure to an
aerosol of thorium can lead to increased risk of
cancers of the
lung,
pancreas and
blood. Exposure
to thorium internally leads to increased risk of
liver diseases. This element has no known biological
role. See also
Thorotrast.
See also
References
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