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= Thorium =
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Introduction
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Thorium is a chemical element; it has symbol Th and atomic number 90.
Thorium is a weakly radioactive light silver metal which tarnishes
olive grey when it is exposed to air, forming thorium dioxide; it is
moderately soft, malleable, and has a high melting point. Thorium is
an electropositive actinide whose chemistry is dominated by the +4
oxidation state; it is quite reactive and can ignite in air when
finely divided.
All known thorium isotopes are unstable. The most stable isotope,
232Th, has a half-life of 14.05 billion years, or about the age of the
universe; it decays very slowly via alpha decay, starting a decay
chain named the thorium series that ends at stable 208Pb. On Earth,
thorium and uranium are the only elements with no stable or
nearly-stable isotopes that still occur naturally in large quantities
as primordial elements. Thorium is estimated to be over three times as
abundant as uranium in the Earth's crust, and is chiefly refined from
monazite sands as a by-product of extracting rare-earth elements.
Thorium was discovered in 1828 by the Swedish chemist Jöns Jacob
Berzelius, who named it after Thor, the Norse god of thunder and war.
Its first applications were developed in the late 19th century.
Thorium's radioactivity was widely acknowledged during the first
decades of the 20th century. In the second half of the 20th century,
thorium was replaced in many uses due to concerns about its
radioactive properties.
Thorium is still used as an alloying element in TIG welding electrodes
but is slowly being replaced in the field with different compositions.
It was also material in high-end optics and scientific
instrumentation, used in some broadcast vacuum tubes, and as the light
source in gas mantles, but these uses have become marginal. It has
been suggested as a replacement for uranium as nuclear fuel in nuclear
reactors, and several thorium reactors have been built. Thorium is
also used in strengthening magnesium, coating tungsten wire in
electrical and welding equipment, controlling the grain size of
tungsten in electric lamps, high-temperature crucibles, and glasses
including camera and scientific instrument lenses. Other uses for
thorium include heat-resistant ceramics, aircraft engines, and in
light bulbs. Ocean science has utilised 231Pa/230Th isotope ratios to
understand the ancient ocean.
Bulk properties
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Thorium is a moderately soft, paramagnetic, bright silvery radioactive
actinide metal that can be bent or shaped. In the periodic table, it
lies to the right of actinium, to the left of protactinium, and below
cerium. Pure thorium is very ductile and, as normal for metals, can be
cold-rolled, swaged, and drawn. At room temperature, thorium metal has
a face-centred cubic crystal structure; it has two other forms, one at
high temperature (over 1360 °C; body-centred cubic) and one at high
pressure (around 100 GPa; body-centred tetragonal).
Thorium metal has a bulk modulus (a measure of resistance to
compression of a material) of 54 GPa, about the same as tin's (58.2
GPa). Aluminium's is 75.2 GPa; copper's 137.8 GPa; and mild steel's is
160-169 GPa. Thorium is about as hard as soft steel, so when heated it
can be rolled into sheets and pulled into wire.
Thorium is nearly half as dense as uranium and plutonium and is harder
than both. Thorium has a magnetic susceptibility of 0.412 × 4π ×
10E-9 m3 kg -1 at room temperature. This susceptibility is mostly
temperature-independent, however impurities and dopants can affect
this value. It becomes superconductive below 1.4 K. Thorium's melting
point of 1750 °C is above both those of actinium (1227 °C) and
protactinium (1568 °C). At the start of period 7, from francium to
thorium, the melting points of the elements increase (as in other
periods), because the number of delocalised electrons each atom
contributes increases from one in francium to four in thorium, leading
to greater attraction between these electrons and the metal ions as
their charge increases from one to four. After thorium, there is a new
downward trend in melting points from thorium to plutonium, where the
number of f-electrons increases from about 0.4 to about 6: this trend
is due to the increasing hybridisation of the 5f and 6d orbitals and
the formation of directional bonds resulting in more complex crystal
structures and weakened metallic bonding. (The f-electron count for
thorium metal is a non-integer due to a 5f-6d overlap.) Among the
actinides up to californium, which can be studied in at least
milligram quantities, thorium has the highest melting and boiling
points and second-lowest density; only actinium is lighter. Thorium's
boiling point of 4788 °C is the fifth-highest among all the elements
with known boiling points.
The properties of thorium vary widely depending on the degree of
impurities in the sample. The major impurity is usually thorium
dioxide (); even the purest thorium specimens usually contain about a
tenth of a per cent of the dioxide. Experimental measurements of its
density give values between 11.5 and 11.66 g/cm3: these are slightly
lower than the theoretically expected value of 11.7 g/cm3 calculated
from thorium's lattice parameters, perhaps due to microscopic voids
forming in the metal when it is cast. These values lie between those
of its neighbours actinium (10.1 g/cm3) and protactinium (15.4 g/cm3),
part of a trend across the early actinides.
Thorium can form alloys with many other metals. Addition of small
proportions of thorium improves the mechanical strength of magnesium,
and thorium-aluminium alloys have been considered as a way to store
thorium in proposed future thorium nuclear reactors. Thorium forms
eutectic mixtures with chromium and uranium, and it is completely
miscible in both solid and liquid states with its lighter congener
cerium.
Isotopes
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There are seven naturally occurring isotopes of Thorium but none are
stable. 232Th is one of the two nuclides beyond bismuth (the other
being 238U) that have half-lives measured in billions of years; its
half-life is 14.05 billion years, about three times the age of the
Earth, and slightly longer than the age of the universe. Four-fifths
of the thorium present at Earth's formation has survived to the
present. 232Th is the only isotope of thorium occurring in quantity in
nature. Its stability is attributed to its closed nuclear subshell
with 142 neutrons. Thorium has a characteristic terrestrial isotopic
composition, with atomic weight . It is one of only four radioactive
elements (along with bismuth, protactinium and uranium) that occur in
large enough quantities on Earth for a standard atomic weight to be
determined.
Thorium nuclei are susceptible to alpha decay because the strong
nuclear force cannot overcome the electromagnetic repulsion between
their protons. The alpha decay of 232Th initiates the 4'n' decay chain
which includes isotopes with a mass number divisible by 4 (hence the
name; it is also called the thorium series after its progenitor). This
chain of consecutive alpha and beta decays begins with the decay of
232Th to 228Ra and terminates at 208Pb. Any sample of thorium or its
compounds contains traces of these daughters, which are isotopes of
thallium, lead, bismuth, polonium, radon, radium, and actinium.
Natural thorium samples can be chemically purified to extract useful
daughter nuclides, such as 212Pb, which is used in nuclear medicine
for cancer therapy. 227Th (alpha emitter with an 18.68 days half-life)
can also be used in cancer treatments such as targeted alpha
therapies. 232Th also very occasionally undergoes spontaneous fission
rather than alpha decay, and has left evidence of doing so in its
minerals (as trapped xenon gas formed as a fission product), but the
partial half-life of this process is very large at over 1021 years and
alpha decay predominates.
In total, 32 radioisotopes have been characterised, which range in
mass number from 207 to 238. After 232Th, the most stable of them
(with respective half-lives) are 230Th (75,380 years), 229Th (7,917
years), 228Th (1.92 years), 234Th (24.10 days), and 227Th (18.68
days). All of these isotopes occur in nature as trace radioisotopes
due to their presence in the decay chains of 232Th, 235U, 238U, and
237Np: the last of these is long extinct in nature due to its short
half-life (2.14 million years), but is continually produced in minute
traces from neutron capture in uranium ores. All of the remaining
thorium isotopes have half-lives that are less than thirty days and
the majority of these have half-lives that are less than ten minutes.
233Th (half-life 22 minutes) occurs naturally as the result of neutron
activation of natural 232Th. 226Th (half-life 31 minutes) has not yet
been observed in nature, but would be produced by the still-unobserved
double beta decay of natural 226Ra.
In deep seawaters the isotope 230Th makes up to of natural thorium.
This is because its parent 238U is soluble in water, but 230Th is
insoluble and precipitates into the sediment. Uranium ores with low
thorium concentrations can be purified to produce gram-sized thorium
samples of which over a quarter is the 230Th isotope, since 230Th is
one of the daughters of 238U. The International Union of Pure and
Applied Chemistry (IUPAC) reclassified thorium as a binuclidic element
in 2013; it had formerly been considered a mononuclidic element.
Thorium has three known nuclear isomers (or metastable states),
216m1Th, 216m2Th, and 229mTh. 229mTh has the lowest known excitation
energy of any isomer, measured to be . This is so low that when it
undergoes isomeric transition, the emitted gamma radiation is in the
ultraviolet range. The nuclear transition from 229Th to 229mTh is
being investigated for a nuclear clock.
Different isotopes of thorium are chemically identical, but have
slightly differing physical properties: for example, the densities of
pure 228Th, 229Th, 230Th, and 232Th are respectively expected to be
11.5, 11.6, 11.6, and 11.7 g/cm3. The isotope 229Th is expected to be
fissionable with a bare critical mass of 2839 kg, although with steel
reflectors this value could drop to 994 kg. 232Th is not fissionable,
but it is fertile as it can be converted to fissile 233U by neutron
capture and subsequent beta decay.
Radiometric dating
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Two radiometric dating methods involve thorium isotopes:
uranium-thorium dating, based on the decay of 234U to 230Th, and
ionium-thorium dating, which measures the ratio of 232Th to 230Th.
These rely on the fact that 232Th is a primordial radioisotope, but
230Th only occurs as an intermediate decay product in the decay chain
of 238U. Uranium-thorium dating is a relatively short-range process
because of the short half-lives of 234U and 230Th relative to the age
of the Earth: it is also accompanied by a sister process involving the
alpha decay of 235U into 231Th, which very quickly becomes the
longer-lived 231Pa, and this process is often used to check the
results of uranium-thorium dating. Uranium-thorium dating is commonly
used to determine the age of calcium carbonate materials such as
speleothem or coral, because uranium is more soluble in water than
thorium and protactinium, which are selectively precipitated into
ocean-floor sediments, where their ratios are measured. The scheme has
a range of several hundred thousand years. Ionium-thorium dating is a
related process, which exploits the insolubility of thorium (both
232Th and 230Th) and thus its presence in ocean sediments to date
these sediments by measuring the ratio of 232Th to 230Th. Both of
these dating methods assume that the proportion of 230Th to 232Th is a
constant during the period when the sediment layer was formed, that
the sediment did not already contain thorium before contributions from
the decay of uranium, and that the thorium cannot migrate within the
sediment layer.
Chemistry
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A thorium atom has 90 electrons, of which four are valence electrons.
Four atomic orbitals are theoretically available for the valence
electrons to occupy: 5f, 6d, 7s, and 7p. The 7p orbitals are not
occupied in the ground state of Thorium, however, due to being greatly
destabilized. Despite thorium's position in the f-block of the
periodic table, it has an anomalous [Rn]6d27s2 electron configuration
in the ground state, as the 5f and 6d subshells in the early actinides
are very close in energy, even more so than the 4f and 5d subshells of
the lanthanides: thorium's 6d subshells are lower in energy than its
5f subshells, because its 5f subshells are not well-shielded by the
filled 6s and 6p subshells and are destabilised. This is due to
relativistic effects, which become stronger near the bottom of the
periodic table, specifically the relativistic spin-orbit interaction.
The closeness in energy levels of the 5f, 6d, and 7s energy levels of
thorium results in thorium almost always losing all four valence
electrons and occurring in its highest possible oxidation state of +4.
This is different from its lanthanide congener cerium, in which +4 is
also the highest possible state, but +3 plays an important role and is
more stable. Thorium complexes in the trivalent and divalent oxidation
states are known, however. Thorium is much more similar to the
transition metals zirconium and hafnium than to cerium in its
ionization energies and redox potentials, and hence also in its
chemistry: this transition-metal-like behaviour is the norm in the
first half of the actinide series, from actinium to americium.
Despite the anomalous electron configuration for gaseous thorium
atoms, metallic thorium shows significant 5f involvement. A
hypothetical metallic state of thorium that had the [Rn]6d27s2
configuration with the 5f orbitals above the Fermi level should be
hexagonal close packed like the group 4 elements titanium, zirconium,
and hafnium, and not face-centred cubic as it actually is. The actual
crystal structure can only be explained when the 5f states are
invoked, proving that thorium is metallurgically a true actinide.
Tetravalent thorium compounds are usually colourless or yellow, like
those of silver or lead, as the ion has no 5f or 6d electrons.
Thorium chemistry is therefore largely that of an electropositive
metal forming a single diamagnetic ion with a stable noble-gas
configuration, indicating a similarity between thorium and the main
group elements of the s-block. Thorium and uranium are the most
investigated of the radioactive elements because their radioactivity
is low enough not to require special handling in the laboratory.
Reactivity
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Thorium is a highly reactive and electropositive metal. With a
standard reduction potential of −1.90 V for the /Th couple, it is
somewhat more electropositive than zirconium or aluminium. Finely
divided thorium metal can exhibit pyrophoricity, spontaneously
igniting in air. When heated in air, thorium turnings ignite and burn
with a brilliant white light to produce the dioxide. In bulk, the
reaction of pure thorium with air is slow, although corrosion may
occur after several months; most thorium samples are contaminated with
varying degrees of the dioxide, which greatly accelerates corrosion.
Such samples slowly tarnish, becoming grey and finally black at the
surface.
At standard temperature and pressure, thorium is slowly attacked by
water, but does not readily dissolve in most common acids, with the
exception of hydrochloric acid, where it dissolves leaving a black
insoluble residue of ThO(OH,Cl)H. It dissolves in concentrated nitric
acid containing a small quantity of catalytic fluoride or
fluorosilicate ions; if these are not present, passivation by the
nitrate can occur, as with uranium and plutonium.
Inorganic compounds
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Most binary compounds of thorium with nonmetals may be prepared by
heating the elements together. In air, thorium burns to form , which
has the fluorite structure. Thorium dioxide is a refractory material,
with the highest melting point (3390 °C) of any known oxide. It is
somewhat hygroscopic and reacts readily with water and many gases; it
dissolves easily in concentrated nitric acid in the presence of
fluoride.
When heated in air, thorium dioxide emits intense blue light; the
light becomes white when is mixed with its lighter homologue cerium
dioxide (, ceria): this is the basis for its previously common
application in gas mantles. A flame is not necessary for this effect:
in 1901, it was discovered that a hot Welsbach gas mantle (using with
1% ) remained at "full glow" when exposed to a cold unignited mixture
of flammable gas and air. The light emitted by thorium dioxide is
higher in wavelength than the blackbody emission expected from
incandescence at the same temperature, an effect called
candoluminescence. It occurs because : Ce acts as a catalyst for the
recombination of free radicals that appear in high concentration in a
flame, whose deexcitation releases large amounts of energy. The
addition of 1% cerium dioxide, as in gas mantles, heightens the effect
by increasing emissivity in the visible region of the spectrum; but
because cerium, unlike thorium, can occur in multiple oxidation
states, its charge and hence visible emissivity will depend on the
region on the flame it is found in (as such regions vary in their
chemical composition and hence how oxidising or reducing they are).
Several binary thorium chalcogenides and oxychalcogenides are also
known with sulfur, selenium, and tellurium.
All four thorium tetrahalides are known, as are some low-valent
bromides and iodides: the tetrahalides are all 8-coordinated
hygroscopic compounds that dissolve easily in polar solvents such as
water. Many related polyhalide ions are also known. Thorium
tetrafluoride has a monoclinic crystal structure like those of
zirconium tetrafluoride and hafnium tetrafluoride, where the ions are
coordinated with ions in somewhat distorted square antiprisms. The
other tetrahalides instead have dodecahedral geometry. Lower iodides
(black) and (gold-coloured) can also be prepared by reducing the
tetraiodide with thorium metal: they do not contain Th(III) and
Th(II), but instead contain and could be more clearly formulated as
electride compounds. Many polynary halides with the alkali metals,
barium, thallium, and ammonium are known for thorium fluorides,
chlorides, and bromides. For example, when treated with potassium
fluoride and hydrofluoric acid, forms the complex anion
(hexafluorothorate(IV)), which precipitates as an insoluble salt,
(potassium hexafluorothorate(IV)).
Thorium borides, carbides, silicides, and nitrides are refractory
materials, like those of uranium and plutonium, and have thus received
attention as possible nuclear fuels. All four heavier pnictogens
(phosphorus, arsenic, antimony, and bismuth) also form binary thorium
compounds. Thorium germanides are also known. Thorium reacts with
hydrogen to form the thorium hydrides and , the latter of which is
superconducting below 7.5-8 K; at standard temperature and pressure,
it conducts electricity like a metal. The hydrides are thermally
unstable and readily decompose upon exposure to air or moisture.
Coordination compounds
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In an acidic aqueous solution, thorium occurs as the tetrapositive
aqua ion , which has tricapped trigonal prismatic molecular geometry:
at pH < 3, the solutions of thorium salts are dominated by this
cation. The ion is the largest of the tetrapositive actinide ions,
and depending on the coordination number can have a radius between
0.95 and 1.14 Å. It is quite acidic due to its high charge, slightly
stronger than sulfurous acid: thus it tends to undergo hydrolysis and
polymerisation (though to a lesser extent than ), predominantly to in
solutions with pH 3 or below, but in more alkaline solution
polymerisation continues until the gelatinous hydroxide forms and
precipitates out (though equilibrium may take weeks to be reached,
because the polymerisation usually slows down before the
precipitation). As a hard Lewis acid, favours hard ligands with
oxygen atoms as donors: complexes with sulfur atoms as donors are less
stable and are more prone to hydrolysis.
High coordination numbers are the rule for thorium due to its large
size. Thorium nitrate pentahydrate was the first known example of
coordination number 11, the oxalate tetrahydrate has coordination
number 10, and the borohydride (first prepared in the Manhattan
Project) has coordination number 14. These thorium salts are known for
their high solubility in water and polar organic solvents.
Many other inorganic thorium compounds with polyatomic anions are
known, such as the perchlorates, sulfates, sulfites, nitrates,
carbonates, phosphates, vanadates, molybdates, and chromates, and
their hydrated forms. They are important in thorium purification and
the disposal of nuclear waste, but most of them have not yet been
fully characterised, especially regarding their structural properties.
For example, thorium nitrate is produced by reacting thorium hydroxide
with nitric acid: it is soluble in water and alcohols and is an
important intermediate in the purification of thorium and its
compounds. Thorium complexes with organic ligands, such as oxalate,
citrate, and EDTA, are much more stable. In natural thorium-containing
waters, organic thorium complexes usually occur in concentrations
orders of magnitude higher than the inorganic complexes, even when the
concentrations of inorganic ligands are much greater than those of
organic ligands.
In January 2021, the aromaticity has been observed in a large metal
cluster anion consisting of 12 bismuth atoms stabilised by a center
thorium cation. This compound was shown to be surprisingly stable,
unlike many previous known aromatic metal clusters.
Organothorium compounds
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Most of the work on organothorium compounds has focused on the
cyclopentadienyl complexes and cyclooctatetraenyls. Like many of the
early and middle actinides (up to americium, and also expected for
curium), thorium forms a cyclooctatetraenide complex: the yellow ,
thorocene. It is isotypic with the better-known analogous uranium
compound uranocene. It can be prepared by reacting potassium
cyclooctatetraenide with thorium tetrachloride in tetrahydrofuran
(THF) at the temperature of dry ice, or by reacting thorium
tetrafluoride with . It is unstable in air and decomposes in water or
at 190 °C. Half sandwich compounds are also known, such as
{{chem2|(η^{8}\-C8H8)ThCl2(THF)2}}, which has a piano-stool structure
and is made by reacting thorocene with thorium tetrachloride in
tetrahydrofuran.
The simplest of the cyclopentadienyls are and : many derivatives are
known. The former (which has two forms, one purple and one green) is a
rare example of thorium in the formal +3 oxidation state; a formal +2
oxidation state occurs in a derivative. The chloride derivative is
prepared by heating thorium tetrachloride with limiting used (other
univalent metal cyclopentadienyls can also be used). The alkyl and
aryl derivatives are prepared from the chloride derivative and have
been used to study the nature of the Th-C sigma bond.
Other organothorium compounds are not well-studied. Tetraallylthorium,
, is known, but its structures has not been determined. The molecular
structure of tetrabenzylthorium, , without ancillary ligands has been
reported. They decompose slowly at room temperature. Thorium forms the
monocapped trigonal prismatic anion , heptamethylthorate(IV), which
forms the salt (tmeda = ). Although one methyl group is only attached
to the thorium atom (Th-C distance 257.1 pm) and the other six connect
the lithium and thorium atoms (Th-C distances 265.5-276.5 pm), they
behave equivalently in solution. Tetramethylthorium, , is not known,
but its adducts are stabilised by phosphine ligands.
Formation
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232Th is a primordial nuclide, having existed in its current form for
over ten billion years; it was formed during the r-process, which
probably occurs in supernovae and neutron star mergers. These violent
events scattered it across the galaxy. The letter "r" stands for
"rapid neutron capture", and occurs in core-collapse supernovae, where
heavy seed nuclei such as 56Fe rapidly capture neutrons, running up
against the neutron drip line, as neutrons are captured much faster
than the resulting nuclides can beta decay back toward stability.
Neutron capture is the only way for stars to synthesise elements
beyond iron because of the increased Coulomb barriers that make
interactions between charged particles difficult at high atomic
numbers and the fact that fusion beyond 56Fe is endothermic. Because
of the abrupt loss of stability past 209Bi, the r-process is the only
process of stellar nucleosynthesis that can create thorium and
uranium; all other processes are too slow and the intermediate nuclei
alpha decay before they capture enough neutrons to reach these
elements.
Abundance
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In the universe, thorium is among the rarest of the primordial
elements at rank 77th in cosmic abundance because it is one of the two
elements that can be produced only in the r-process (the other being
uranium), and also because it has slowly been decaying away from the
moment it formed. The only primordial elements rarer than thorium are
thulium, lutetium, tantalum, and rhenium, the odd-numbered elements
just before the third peak of r-process abundances around the heavy
platinum group metals, as well as uranium. In the distant past the
abundances of thorium and uranium were enriched by the decay of
plutonium and curium isotopes, and thorium was enriched relative to
uranium by the decay of 236U to 232Th and the natural depletion of
235U, but these sources have long since decayed and no longer
contribute.
In the Earth's crust, thorium is much more abundant: with an abundance
of 8.1 g/tonne, it is one of the most abundant of the heavy elements,
almost as abundant as lead (13 g/tonne) and more abundant than tin
(2.1 g/tonne). This is because thorium is likely to form oxide
minerals that do not sink into the core; it is classified as a
lithophile under the Goldschmidt classification, meaning that it is
generally found combined with oxygen. Common thorium compounds are
also poorly soluble in water. Thus, even though the refractory
elements have the same relative abundances in the Earth as in the
Solar System as a whole, there is more accessible thorium than heavy
platinum group metals in the crust.
On Earth
==========
Natural thorium is usually almost pure 232Th, which is the
longest-lived and most stable isotope of thorium, having a half-life
comparable to the age of the universe. Its radioactive decay is the
largest single contributor to the Earth's internal heat; the other
major contributors are the shorter-lived primordial radionuclides,
which are 238U, 40K, and 235U in descending order of their
contribution. (At the time of the Earth's formation, 40K and 235U
contributed much more by virtue of their short half-lives, but they
have decayed more quickly, leaving the contribution from 232Th and
238U predominant.) Its decay accounts for a gradual decrease of
thorium content of the Earth: the planet currently has around 85% of
the amount present at the formation of the Earth. The other natural
thorium isotopes are much shorter-lived; of them, only 230Th is
usually detectable, occurring in secular equilibrium with its parent
238U, and making up at most 0.04% of natural thorium.
Thorium only occurs as a minor constituent of most minerals, and was
for this reason previously thought to be rare. In fact, it is the 37th
most abundant element in the Earth's crust with an abundance of 12
parts per million. In nature, thorium occurs in the +4 oxidation
state, together with uranium(IV), zirconium(IV), hafnium(IV), and
cerium(IV), and also with scandium, yttrium, and the trivalent
lanthanides which have similar ionic radii. Because of thorium's
radioactivity, minerals containing it are often metamict (amorphous),
their crystal structure having been damaged by the alpha radiation
produced by thorium. An extreme example is ekanite, , which almost
never occurs in nonmetamict form due to the thorium it contains.
Monazite (chiefly phosphates of various rare-earth elements) is the
most important commercial source of thorium because it occurs in large
deposits worldwide, principally in India, South Africa, Brazil,
Australia, and Malaysia. It contains around 2.5% thorium on average,
although some deposits may contain up to 20%. Monazite is a chemically
unreactive mineral that is found as yellow or brown sand; its low
reactivity makes it difficult to extract thorium from it. Allanite
(chiefly silicates-hydroxides of various metals) can have 0.1-2%
thorium and zircon (chiefly zirconium silicate, ) up to 0.4% thorium.
Thorium dioxide occurs as the rare mineral thorianite. Due to its
being isotypic with uranium dioxide, these two common actinide
dioxides can form solid-state solutions and the name of the mineral
changes according to the content. Thorite (chiefly thorium silicate,
), also has a high thorium content and is the mineral in which thorium
was first discovered. In thorium silicate minerals, the and ions are
often replaced with (where M = Sc, Y, or Ln) and phosphate () ions
respectively. Because of the great insolubility of thorium dioxide,
thorium does not usually spread quickly through the environment when
released. The ion is soluble, especially in acidic soils, and in such
conditions the thorium concentration can be higher.
Erroneous report
==================
In 1815, the Swedish chemist Jöns Jacob Berzelius analysed an unusual
sample of gadolinite from a copper mine in Falun, central Sweden. He
noted impregnated traces of a white mineral, which he cautiously
assumed to be an earth (oxide in modern chemical nomenclature) of an
unknown element. Berzelius had already discovered two elements, cerium
and selenium, but he had made a public mistake once, announcing a new
element, 'gahnium', that turned out to be zinc oxide. Berzelius
privately named the putative element "thorium" in 1817 and its
supposed oxide "thorina" after Thor, the Norse god of thunder. In
1824, after more deposits of the same mineral in Vest-Agder, Norway,
were discovered, he retracted his findings, as the mineral (later
named xenotime) proved to be mostly yttrium orthophosphate.
Discovery
===========
In 1828, Morten Thrane Esmark found a black mineral on Løvøya island,
Telemark county, Norway. He was a Norwegian priest and amateur
mineralogist who studied the minerals in Telemark, where he served as
vicar. He commonly sent the most interesting specimens, such as this
one, to his father, Jens Esmark, a noted mineralogist and professor of
mineralogy and geology at the Royal Frederick University in
Christiania (today called Oslo). The elder Esmark determined that it
was not a known mineral and sent a sample to Berzelius for
examination. Berzelius determined that it contained a new element. He
published his findings in 1829, having isolated an impure sample by
reducing (potassium pentafluorothorate(IV)) with potassium metal.
Berzelius reused the name of the previous supposed element discovery
and named the source mineral thorite.
Berzelius made some initial characterisations of the new metal and its
chemical compounds: he correctly determined that the thorium-oxygen
mass ratio of thorium oxide was 7.5 (its actual value is close to
that, ~7.3), but he assumed the new element was divalent rather than
tetravalent, and so calculated that the atomic mass was 7.5 times that
of oxygen (120 amu); it is actually 15 times as large. He determined
that thorium was a very electropositive metal, ahead of cerium and
behind zirconium in electropositivity. Metallic thorium was isolated
for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and
Lodewijk Hamburger.
Initial chemical classification
=================================
In the periodic table published by Dmitri Mendeleev in 1869, thorium
and the rare-earth elements were placed outside the main body of the
table, at the end of each vertical period after the alkaline earth
metals. This reflected the belief at that time that thorium and the
rare-earth metals were divalent. With the later recognition that the
rare earths were mostly trivalent and thorium was tetravalent,
Mendeleev moved cerium and thorium to group IV in 1871, which also
contained the modern carbon group (group 14) and titanium group (group
4), because their maximum oxidation state was +4. Cerium was soon
removed from the main body of the table and placed in a separate
lanthanide series; thorium was left with group 4 as it had similar
properties to its supposed lighter congeners in that group, such as
titanium and zirconium.
First uses
============
While thorium was discovered in 1828 its first application dates only
from 1885, when Austrian chemist Carl Auer von Welsbach invented the
gas mantle, a portable source of light which produces light from the
incandescence of thorium oxide when heated by burning gaseous fuels.
Many applications were subsequently found for thorium and its
compounds, including ceramics, carbon arc lamps, heat-resistant
crucibles, and as catalysts for industrial chemical reactions such as
the oxidation of ammonia to nitric acid.
Radioactivity
===============
Thorium was first observed to be radioactive in 1898, by the German
chemist Gerhard Carl Schmidt and later that year, independently, by
the Polish-French physicist Marie Curie. It was the second element
that was found to be radioactive, after the 1896 discovery of
radioactivity in uranium by French physicist Henri Becquerel. Starting
from 1899, the New Zealand physicist Ernest Rutherford and the
American electrical engineer Robert Bowie Owens studied the radiation
from thorium; initial observations showed that it varied
significantly. It was determined that these variations came from a
short-lived gaseous daughter of thorium, which they found to be a new
element. This element is now named radon, the only one of the rare
radioelements to be discovered in nature as a daughter of thorium
rather than uranium.
After accounting for the contribution of radon, Rutherford, now
working with the British physicist Frederick Soddy, showed how thorium
decayed at a fixed rate over time into a series of other elements in
work dating from 1900 to 1903. This observation led to the
identification of the half-life as one of the outcomes of the alpha
particle experiments that led to the disintegration theory of
radioactivity. The biological effect of radiation was discovered in
1903. The newly discovered phenomenon of radioactivity excited
scientists and the general public alike. In the 1920s, thorium's
radioactivity was promoted as a cure for rheumatism, diabetes, and
sexual impotence. In 1932, most of these uses were banned in the
United States after a federal investigation into the health effects of
radioactivity. 10,000 individuals in the United States had been
injected with thorium during X-ray diagnosis; they were later found to
suffer health issues such as leukaemia and abnormal chromosomes.
Public interest in radioactivity had declined by the end of the 1930s.
Further classification
========================
Up to the late 19th century, chemists unanimously agreed that thorium
and uranium were the heaviest members of group 4 and group 6
respectively; the existence of the lanthanides in the sixth row was
considered to be a one-off fluke. In 1892, British chemist Henry
Bassett postulated a second extra-long periodic table row to
accommodate known and undiscovered elements, considering thorium and
uranium to be analogous to the lanthanides. In 1913, Danish physicist
Niels Bohr published a theoretical model of the atom and its electron
orbitals, which soon gathered wide acceptance. The model indicated
that the seventh row of the periodic table should also have f-shells
filling before the d-shells that were filled in the transition
elements, like the sixth row with the lanthanides preceding the 5d
transition metals. The existence of a second inner transition series,
in the form of the actinides, was not accepted until similarities with
the electron structures of the lanthanides had been established; Bohr
suggested that the filling of the 5f orbitals may be delayed to after
uranium.
It was only with the discovery of the first transuranic elements,
which from plutonium onward have dominant +3 and +4 oxidation states
like the lanthanides, that it was realised that the actinides were
indeed filling f-orbitals rather than d-orbitals, with the
transition-metal-like chemistry of the early actinides being the
exception and not the rule. In 1945, when American physicist Glenn T.
Seaborg and his team had discovered the transuranic elements americium
and curium, he proposed the actinide concept, realising that thorium
was the second member of an f-block actinide series analogous to the
lanthanides, instead of being the heavier congener of hafnium in a
fourth d-block row.
Phasing out
=============
In the 1990s, most applications that do not depend on thorium's
radioactivity declined quickly due to safety and environmental
concerns as suitable safer replacements were found. Despite its
radioactivity, the element has remained in use for applications where
no suitable alternatives could be found. A 1981 study by the Oak Ridge
National Laboratory in the United States estimated that using a
thorium gas mantle every weekend would be safe for a person, but this
was not the case for the dose received by people manufacturing the
mantles or for the soils around some factory sites. Some manufacturers
have changed to other materials, such as yttrium. As recently as 2007,
some companies continued to manufacture and sell thorium mantles
without giving adequate information about their radioactivity, with
some even falsely claiming them to be non-radioactive.
Nuclear power
===============
Thorium has been used as a power source on a prototype scale. The
earliest thorium-based reactor was built at the Indian Point Energy
Center located in Buchanan, New York, United States in 1962. China may
be the first to have attempted to commercialise the technology. The
country with the largest estimated reserves of thorium in the world is
India, which has sparse reserves of uranium. In the 1950s, India
targeted achieving energy independence with their three-stage nuclear
power programme. In most countries, uranium was relatively abundant
and the progress of thorium-based reactors was slow; in the 20th
century, three reactors were built in India and twelve elsewhere.
Large-scale research was begun in 1996 by the International Atomic
Energy Agency to study the use of thorium reactors; a year later, the
United States Department of Energy started their research. Alvin
Radkowsky of Tel Aviv University in Israel was the head designer of
Shippingport Atomic Power Station in Pennsylvania, the first American
civilian reactor to breed thorium. He founded a consortium to develop
thorium reactors, which included other laboratories: Raytheon Nuclear
Inc. and Brookhaven National Laboratory in the United States, and the
Kurchatov Institute in Russia.
In the 21st century, thorium's potential for reducing nuclear
proliferation and its waste characteristics led to renewed interest in
the thorium fuel cycle. India has projected meeting as much as 30% of
its electrical demands through thorium-based nuclear power by 2050. In
February 2014, Bhabha Atomic Research Centre (BARC), in Mumbai, India,
presented their latest design for a "next-generation nuclear reactor"
that burns thorium as its fuel core, calling it the Advanced Heavy
Water Reactor (AHWR). In 2009, the chairman of the Indian Atomic
Energy Commission said that India has a "long-term objective goal of
becoming energy-independent based on its vast thorium resources."
On 16 June 2023 China's National Nuclear Safety Administration issued
a licence to the Shanghai Institute of Applied Physics (SINAP) of the
Chinese Academy of Sciences to begin operating the TMSR-LF1, 2 MWt
liquid fuel thorium-based molten salt experimental reactor which was
completed in August 2021. China is believed to have one of the largest
thorium reserves in the world. The exact size of those reserves has
not been publicly disclosed, but it is estimated to be enough to meet
the country's total energy needs for more than 20,000 years.
Nuclear weapons
=================
When gram quantities of plutonium were first produced in the Manhattan
Project, it was discovered that a minor isotope (240Pu) underwent
significant spontaneous fission, which brought into question the
viability of a plutonium-fuelled gun-type nuclear weapon. While the
Los Alamos team began work on the implosion-type weapon to circumvent
this issue, the Chicago team discussed reactor design solutions.
Eugene Wigner proposed to use the 240Pu-contaminated plutonium to
drive the conversion of thorium into 233U in a special converter
reactor. It was hypothesized that the 233U would then be usable in a
gun-type weapon, though concerns about contamination from 232U were
voiced. Progress on the implosion weapon was sufficient, and this
converter was not developed further, but the design had enormous
influence on the development of nuclear energy. It was the first
detailed description of a highly enriched water-cooled,
water-moderated reactor similar to future naval and commercial power
reactors.
During the Cold War the United States explored the possibility of
using 232Th as a source of 233U to be used in a nuclear bomb; they
fired a test bomb in 1955. They concluded that a 233U-fired bomb would
be a very potent weapon, but it bore few sustainable "technical
advantages" over the contemporary uranium-plutonium bombs, especially
since 233U is difficult to produce in isotopically pure form.
Thorium metal was used in the radiation case of at least one nuclear
weapon design deployed by the United States (the W71).
Production
======================================================================
The low demand makes working mines for extraction of thorium alone not
profitable, and it is almost always extracted with the rare earths,
which themselves may be by-products of production of other minerals.
The current reliance on monazite for production is due to thorium
being largely produced as a by-product; other sources such as thorite
contain more thorium and could easily be used for production if demand
rose. Present knowledge of the distribution of thorium resources is
poor, as low demand has led to exploration efforts being relatively
minor. In 2014, world production of the monazite concentrate, from
which thorium would be extracted, was 2,700 tonnes.
The common production route of thorium constitutes concentration of
thorium minerals; extraction of thorium from the concentrate;
purification of thorium; and (optionally) conversion to compounds,
such as thorium dioxide.
Concentration
===============
There are two categories of thorium minerals for thorium extraction:
primary and secondary. Primary deposits occur in acidic granitic
magmas and pegmatites. They are concentrated, but of small size.
Secondary deposits occur at the mouths of rivers in granitic mountain
regions. In these deposits, thorium is enriched along with other heavy
minerals. Initial concentration varies with the type of deposit.
For the primary deposits, the source pegmatites, which are usually
obtained by mining, are divided into small parts and then undergo
flotation. Alkaline earth metal carbonates may be removed after
reaction with hydrogen chloride; then follow thickening, filtration,
and calcination. The result is a concentrate with rare-earth content
of up to 90%. Secondary materials (such as coastal sands) undergo
gravity separation. Magnetic separation follows, with a series of
magnets of increasing strength. Monazite obtained by this method can
be as pure as 98%.
Industrial production in the 20th century relied on treatment with
hot, concentrated sulfuric acid in cast iron vessels, followed by
selective precipitation by dilution with water, as on the subsequent
steps. This method relied on the specifics of the technique and the
concentrate grain size; many alternatives have been proposed, but only
one has proven effective economically: alkaline digestion with hot
sodium hydroxide solution. This is more expensive than the original
method but yields a higher purity of thorium; in particular, it
removes phosphates from the concentrate.
Acid digestion
================
Acid digestion is a two-stage process, involving the use of up to 93%
sulfuric acid at 210-230 °C. First, sulfuric acid in excess of 60% of
the sand mass is added, thickening the reaction mixture as products
are formed. Then, fuming sulfuric acid is added and the mixture is
kept at the same temperature for another five hours to reduce the
volume of solution remaining after dilution. The concentration of the
sulfuric acid is selected based on reaction rate and viscosity, which
both increase with concentration, albeit with viscosity retarding the
reaction. Increasing the temperature also speeds up the reaction, but
temperatures of 300 °C and above must be avoided, because they cause
insoluble thorium pyrophosphate to form. Since dissolution is very
exothermic, the monazite sand cannot be added to the acid too quickly.
Conversely, at temperatures below 200 °C the reaction does not go fast
enough for the process to be practical. To ensure that no precipitates
form to block the reactive monazite surface, the mass of acid used
must be twice that of the sand, instead of the 60% that would be
expected from stoichiometry. The mixture is then cooled to 70 °C and
diluted with ten times its volume of cold water, so that any remaining
monazite sinks to the bottom while the rare earths and thorium remain
in solution. Thorium may then be separated by precipitating it as the
phosphate at pH 1.3, since the rare earths do not precipitate until pH
2.
Alkaline digestion
====================
Alkaline digestion is carried out in 30-45% sodium hydroxide solution
at about 140 °C for about three hours. Too high a temperature leads to
the formation of poorly soluble thorium oxide and an excess of uranium
in the filtrate, and too low a concentration of alkali leads to a very
slow reaction. These reaction conditions are rather mild and require
monazite sand with a particle size under 45 μm. Following filtration,
the filter cake includes thorium and the rare earths as their
hydroxides, uranium as sodium diuranate, and phosphate as trisodium
phosphate. This crystallises trisodium phosphate decahydrate when
cooled below 60 °C; uranium impurities in this product increase with
the amount of silicon dioxide in the reaction mixture, necessitating
recrystallisation before commercial use. The hydroxides are dissolved
at 80 °C in 37% hydrochloric acid. Filtration of the remaining
precipitates followed by addition of 47% sodium hydroxide results in
the precipitation of thorium and uranium at about pH 5.8. Complete
drying of the precipitate must be avoided, as air may oxidise cerium
from the +3 to the +4 oxidation state, and the cerium(IV) formed can
liberate free chlorine from the hydrochloric acid. The rare earths
again precipitate out at higher pH. The precipitates are neutralised
by the original sodium hydroxide solution, although most of the
phosphate must first be removed to avoid precipitating rare-earth
phosphates. Solvent extraction may also be used to separate out the
thorium and uranium, by dissolving the resultant filter cake in nitric
acid. The presence of titanium hydroxide is deleterious as it binds
thorium and prevents it from dissolving fully.
Purification
==============
High thorium concentrations are needed in nuclear applications. In
particular, concentrations of atoms with high neutron capture
cross-sections must be very low (for example, gadolinium
concentrations must be lower than one part per million by weight).
Previously, repeated dissolution and recrystallisation was used to
achieve high purity. Today, liquid solvent extraction procedures
involving selective complexation of are used. For example, following
alkaline digestion and the removal of phosphate, the resulting nitrato
complexes of thorium, uranium, and the rare earths can be separated by
extraction with tributyl phosphate in kerosene.
Modern applications
======================================================================
Non-radioactivity-related uses of thorium have been in decline since
the 1950s due to environmental concerns largely stemming from the
radioactivity of thorium and its decay products.
Most thorium applications use its dioxide (sometimes called "thoria"
in the industry), rather than the metal. This compound has a melting
point of 3300 °C (6000 °F), the highest of all known oxides; only a
few substances have higher melting points. This helps the compound
remain solid in a flame, and it considerably increases the brightness
of the flame; this is the main reason thorium is used in gas lamp
mantles. All substances emit energy (glow) at high temperatures, but
the light emitted by thorium is nearly all in the visible spectrum,
hence the brightness of thorium mantles.
Energy, some of it in the form of visible light, is emitted when
thorium is exposed to a source of energy itself, such as a cathode
ray, heat, or ultraviolet light. This effect is shared by cerium
dioxide, which converts ultraviolet light into visible light more
efficiently, but thorium dioxide gives a higher flame temperature,
emitting less infrared light. Thorium in mantles, though still common,
has been progressively replaced with yttrium since the late 1990s.
According to the 2005 review by the United Kingdom's National
Radiological Protection Board, "although [thoriated gas mantles] were
widely available a few years ago, they are not any more." Thorium is
also used to make cheap permanent negative ion generators, such as in
pseudoscientific health bracelets.
During the production of incandescent filaments, recrystallisation of
tungsten is significantly lowered by adding small amounts of thorium
dioxide to the tungsten sintering powder before drawing the filaments.
A small addition of thorium to tungsten thermocathodes considerably
reduces the work function of electrons; as a result, electrons are
emitted at considerably lower temperatures. Thorium forms a
one-atom-thick layer on the surface of tungsten. The work function
from a thorium surface is lowered possibly because of the electric
field on the interface between thorium and tungsten formed due to
thorium's greater electropositivity. Since the 1920s, thoriated
tungsten wires have been used in electronic tubes and in the cathodes
and anticathodes of X-ray tubes and rectifiers.The reactivity of
thorium with atmospheric oxygen required the introduction of an
evaporated magnesium layer as a getter for impurities in the evacuated
tubes, giving them their characteristic metallic inner coating. The
introduction of transistors in the 1950s significantly diminished this
use, but not entirely. Thorium dioxide is used in gas tungsten arc
welding (GTAW) to increase the high-temperature strength of tungsten
electrodes and improve arc stability. Thorium oxide is being replaced
in this use with other oxides, such as those of zirconium, cerium, and
lanthanum.
Thorium dioxide is found in refractory ceramics, such as
high-temperature laboratory crucibles, either as the primary
ingredient or as an addition to zirconium dioxide. An alloy of 90%
platinum and 10% thorium is an effective catalyst for oxidising
ammonia to nitrogen oxides, but this has been replaced by an alloy of
95% platinum and 5% rhodium because of its better mechanical
properties and greater durability.
When added to glass, thorium dioxide helps increase its refractive
index and decrease dispersion. Such glass finds application in
high-quality lenses for cameras and scientific instruments. The
radiation from these lenses can darken them and turn them yellow over
a period of years and it degrades film, but the health risks are
minimal. Yellowed lenses may be restored to their original colourless
state by lengthy exposure to intense ultraviolet radiation. Thorium
dioxide has since been replaced in this application by rare-earth
oxides, such as lanthanum, as they provide similar effects and are not
radioactive.
Thorium tetrafluoride is used as an anti-reflection material in
multilayered optical coatings. It is transparent to electromagnetic
waves having wavelengths in the range of 0.350-12 μm, a range that
includes near ultraviolet, visible and mid infrared light. Its
radiation is primarily due to alpha particles, which can be easily
stopped by a thin cover layer of another material. Replacements for
thorium tetrafluoride are being developed as of the 2010s, which
include Lanthanum trifluoride.
Mag-Thor alloys (also called thoriated magnesium) found use in some
aerospace applications, though such uses have been phased out due to
concerns over radioactivity.
Potential use for nuclear energy
======================================================================
The main nuclear power source in a reactor is the neutron-induced
fission of a nuclide; the synthetic fissile nuclei 233U and 239Pu can
be bred from neutron capture by the naturally occurring quantity
nuclides 232Th and 238U. 235U occurs naturally in significant amounts
and is also fissile. In the thorium fuel cycle, the fertile isotope
232Th is bombarded by slow neutrons, undergoing neutron capture to
become 233Th, which undergoes two consecutive beta decays to become
first 233Pa and then the fissile 233U:
:{^{232}_{90}Th} ->[\text{(n,}\gamma\text{)}]
{^{233}_{90}Th}->[\beta^-][\text{21.8 min}] {^{233}_{91}Pa}
->[\beta^-][\text{27 days}] {^{233}_{92}U} \ (->[\alpha][1.60
\times 10^5\text{years}])
233U is fissile and can be used as a nuclear fuel in the same way as
235U or 239Pu. When 233U undergoes nuclear fission, the neutrons
emitted can strike further 232Th nuclei, continuing the cycle. This
parallels the uranium fuel cycle in fast breeder reactors where 238U
undergoes neutron capture to become 239U, beta decaying to first 239Np
and then fissile 239Pu.
The fission of produces 2.48 neutrons on average.
One neutron is needed to keep the fission reaction going. For a
self-contained continuous breeding cycle, one more neutron is needed
to breed a new atom from the fertile . This leaves a margin of 0.45
neutrons (or 18% of the neutron flux) for losses.
Advantages
============
Thorium is more abundant than uranium, and can satisfy world energy
demands for longer. It is particularly suitable for being used as
fertile material in molten salt reactors.
232Th absorbs neutrons more readily than 238U, and 233U has a higher
probability of fission upon neutron capture (92.0%) than 235U (85.5%)
or 239Pu (73.5%). It also releases more neutrons upon fission on
average. A single neutron capture by 238U produces transuranic waste
along with the fissile 239Pu, but 232Th only produces this waste after
five captures, forming 237Np. This number of captures does not happen
for 98-99% of the 232Th nuclei because the intermediate products 233U
or 235U undergo fission, and fewer long-lived transuranics are
produced. Because of this, thorium is a potentially attractive
alternative to uranium in mixed oxide fuels to minimise the generation
of transuranics and maximise the destruction of plutonium.
Thorium fuels result in a safer and better-performing reactor core
because thorium dioxide has a higher melting point, higher thermal
conductivity, and a lower coefficient of thermal expansion. It is more
stable chemically than the now-common fuel uranium dioxide, because
the latter oxidises to triuranium octoxide (), becoming substantially
less dense.
Disadvantages
===============
The used fuel is difficult and dangerous to reprocess because many of
the daughters of 232Th and 233U are strong gamma emitters. All 233U
production methods result in impurities of 232U, either from parasitic
knock-out (n,2n) reactions on 232Th, 233Pa, or 233U that result in the
loss of a neutron, or from double neutron capture of 230Th, an
impurity in natural 232Th:
: + n → + ( )
: + n → +
232U by itself is not particularly harmful, but quickly decays to
produce the strong gamma emitter 208Tl. (232Th follows the same decay
chain, but its much longer half-life means that the quantities of
208Tl produced are negligible.) These impurities of 232U make 233U
easy to detect and dangerous to work on, and the impracticality of
their separation limits the possibilities of nuclear proliferation
using 233U as the fissile material. 233Pa has a relatively long
half-life of 27 days and a high cross section for neutron capture.
Thus it is a neutron poison: instead of rapidly decaying to the useful
233U, a significant amount of 233Pa converts to 234U and consumes
neutrons, degrading the reactor efficiency. To avoid this, 233Pa is
extracted from the active zone of thorium molten salt reactors during
their operation, so that it does not have a chance to capture a
neutron and will only decay to 233U.
The irradiation of 232Th with neutrons, followed by its processing,
need to be mastered before these advantages can be realised, and this
requires more advanced technology than the uranium and plutonium fuel
cycle; research continues in this area. Others cite the low commercial
viability of the thorium fuel cycle: the international Nuclear Energy
Agency predicts that the thorium cycle will never be commercially
viable while uranium is available in abundance--a situation which may
persist "in the coming decades". The isotopes produced in the thorium
fuel cycle are mostly not transuranic, but some of them are still very
dangerous, such as 231Pa, which has a half-life of 32,760 years and is
a major contributor to the long-term radiotoxicity of spent nuclear
fuel.
Radiological
==============
Natural thorium decays very slowly compared to many other radioactive
materials, and the emitted alpha radiation cannot penetrate human
skin. As a result, handling small amounts of thorium, such as those in
gas mantles, is considered safe, although the use of such items may
pose some risks. Exposure to an aerosol of thorium, such as
contaminated dust, can lead to increased risk of cancers of the lung,
pancreas, and blood, as lungs and other internal organs can be
penetrated by alpha radiation. Internal exposure to thorium leads to
increased risk of liver diseases.
The decay products of 232Th include more dangerous radionuclides such
as radium and radon. Although relatively little of those products are
created as the result of the slow decay of thorium, a proper
assessment of the radiological toxicity of 232Th must include the
contribution of its daughters, some of which are dangerous gamma
emitters, and which are built up quickly following the initial decay
of 232Th due to the absence of long-lived nuclides along the decay
chain. As the dangerous daughters of thorium have much lower melting
points than thorium dioxide, they are volatilised every time the
mantle is heated for use. In the first hour of use large fractions of
the thorium daughters 224Ra, 228Ra, 212Pb, and 212Bi are released.
Most of the radiation dose by a normal user arises from inhaling the
radium, resulting in a radiation dose of up to 0.2 millisieverts per
use, about a third of the dose sustained during a mammogram.
Some nuclear safety agencies make recommendations about the use of
thorium mantles and have raised safety concerns regarding their
manufacture and disposal; the radiation dose from one mantle is not a
serious problem, but that from many mantles gathered together in
factories or landfills is.
Biological
============
Thorium is odourless and tasteless. The chemical toxicity of thorium
is low because thorium and its most common compounds (mostly the
dioxide) are poorly soluble in water, precipitating out before
entering the body as the hydroxide. Some thorium compounds are
chemically moderately toxic, especially in the presence of strong
complex-forming ions such as citrate that carry the thorium into the
body in soluble form. If a thorium-containing object has been chewed
or sucked, it loses 0.4% of thorium and 90% of its dangerous daughters
to the body. Three-quarters of the thorium that has penetrated the
body accumulates in the skeleton. Absorption through the skin is
possible, but is not a likely means of exposure. Thorium's low
solubility in water also means that excretion of thorium by the
kidneys and faeces is rather slow.
Tests on the thorium uptake of workers involved in monazite processing
showed thorium levels above recommended limits in their bodies, but no
adverse effects on health were found at those moderately low
concentrations. No chemical toxicity has yet been observed in the
tracheobronchial tract and the lungs from exposure to thorium. People
who work with thorium compounds are at a risk of dermatitis. It can
take as much as thirty years after the ingestion of thorium for
symptoms to manifest themselves. Thorium has no known biological role.
Chemical
==========
Powdered thorium metal is pyrophoric: it ignites spontaneously in air.
In 1964, the United States Department of the Interior listed thorium
as "severe" on a table entitled "Ignition and explosibility of metal
powders". Its ignition temperature was given as 270 °C (520 °F) for
dust clouds and 280 °C (535 °F) for layers. Its minimum explosive
concentration was listed as 0.075 oz/cu ft (0.075 kg/m3); the minimum
igniting energy for (non-submicron) dust was listed as 5 mJ.
In 1956, the Sylvania Electric Products explosion occurred during
reprocessing and burning of thorium sludge in New York City, United
States. Nine people were injured; one died of complications caused by
third-degree burns.
Exposure routes
=================
Thorium exists in very small quantities everywhere on Earth although
larger amounts exist in certain parts: the average human contains
about 40 micrograms of thorium and typically consumes three micrograms
per day. Most thorium exposure occurs through dust inhalation; some
thorium comes with food and water, but because of its low solubility,
this exposure is negligible.
Exposure is raised for people who live near thorium deposits or
radioactive waste disposal sites, those who live near or work in
uranium, phosphate, or tin processing factories, and for those who
work in gas mantle production. Thorium is especially common in the
Tamil Nadu coastal areas of India, where residents may be exposed to a
naturally occurring radiation dose ten times higher than the worldwide
average. It is also common in northern Brazilian coastal areas, from
south Bahia to Guarapari, a city with radioactive monazite sand
beaches, with radiation levels up to 50 times higher than world
average background radiation.
Another possible source of exposure is thorium dust produced at
weapons testing ranges, as thorium is used in the guidance systems of
some missiles. This has been blamed for a high incidence of birth
defects and cancer at Salto di Quirra on the Italian island of
Sardinia.
See also
======================================================================
*Thorium Energy Alliance
Further reading
======================================================================
*
* International Atomic Energy Agency (2005).
[
http://www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf Thorium
fuel cycle - Potential benefits and challenges]
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=========
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http://en.wikipedia.org/wiki/Thorium
