Thorium

Thorium is a weakly radioactive metallic chemical element with symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, 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. In the universe, thorium and uranium are the only two radioactive elements that still occur naturally in large quantities as primordial elements.[a] It 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 metals.

Thorium was discovered in 1829 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder. 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 century, thorium was replaced in many uses due to concerns about its radioactivity.

Thorium is still being used as an alloying element in TIG welding electrodes but is slowly being replaced in the field with different compositions. It was also a material in high-end optics and scientific instrumentation, 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,  90Th
Small (3 cm) ampule with a tiny (5 mm) square of metal in it
Thorium
Pronunciation/ˈθɔːriəm/ (THOHR-ee-əm)
Appearancesilvery, often with black tarnish
Standard atomic weight Ar, std(Th)232.0377(4)[1]
Thorium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Ce

Th

(Uqq)
actiniumthoriumprotactinium
Atomic number (Z)90
Groupgroup n/a
Periodperiod 7
Blockf-block
Element category  actinide
Electron configuration[Rn] 6d2 7s2
Electrons per shell
2, 8, 18, 32, 18, 10, 2
Physical properties
Phase at STPsolid
Melting point2023 K ​(1750 °C, ​3182 °F)
Boiling point5061 K ​(4788 °C, ​8650 °F)
Density (near r.t.)11.7 g/cm3
Heat of fusion13.81 kJ/mol
Heat of vaporisation514 kJ/mol
Molar heat capacity26.230 J/(mol·K)
Vapour pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2633 2907 3248 3683 4259 5055
Atomic properties
Oxidation states+1, +2, +3, +4 (a weakly basic oxide)
ElectronegativityPauling scale: 1.3
Ionisation energies
  • 1st: 587 kJ/mol
  • 2nd: 1110 kJ/mol
  • 3rd: 1930 kJ/mol
Atomic radiusempirical: 179.8 pm
Covalent radius206±6 pm
Color lines in a spectral range
Spectral lines of thorium
Other properties
Natural occurrenceprimordial
Crystal structureface-centred cubic (fcc)
Facecentredcubic crystal structure for thorium
Speed of sound thin rod2490 m/s (at 20 °C)
Thermal expansion11.0 µm/(m·K) (at 25 °C)
Thermal conductivity54.0 W/(m·K)
Electrical resistivity157 nΩ·m (at 0 °C)
Magnetic orderingparamagnetic[2]
Magnetic susceptibility132.0·10−6 cm3/mol (293 K)[3]
Young's modulus79 GPa
Shear modulus31 GPa
Bulk modulus54 GPa
Poisson ratio0.27
Mohs hardness3.0
Vickers hardness295–685 MPa
Brinell hardness390–1500 MPa
CAS Number7440-29-1
History
Namingafter Thor, the Norse god of thunder
DiscoveryJöns Jakob Berzelius (1829)
Main isotopes of thorium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
227Th trace 18.68 d α 223Ra
228Th trace 1.9116 y α 224Ra
229Th trace 7917 y α 225Ra
230Th 0.02% 75400 y α 226Ra
231Th trace 25.5 h β 231Pa
232Th 99.98% 1.405×1010 y α 228Ra
234Th trace 24.1 d β 234Pa

Bulk properties

Thorium is a moderately soft, paramagnetic, bright silvery radioactive actinide metal. 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.[4] 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).[4]

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.[5] Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire.[6]

Thorium is nearly half as dense as uranium and plutonium and is harder than either of them.[6] It becomes superconductive below 1.4 K.[4] 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.[6][7] (The f-electron count for thorium is a non-integer due to a 5f–6d overlap.)[7] 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.[b] Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points.[c]

The properties of thorium vary widely depending on the degree of impurities in the sample. The major impurity is usually thorium dioxide (ThO2); even the purest thorium specimens usually contain about a tenth of a percent of the dioxide.[4] 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.[4] 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.[4]

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.[4]

Isotopes

All but two elements up to bismuth (element 83) have an isotope that is practically stable for all purposes ("classically stable"), with the exceptions being technetium and promethium (elements 43 and 61). All elements from polonium (element 84) onward are measurably radioactive. 232Th is one of the three nuclides beyond bismuth (the other two being 235U and 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.[10][11][12] 232Th is the only isotope of thorium occurring in quantity in nature.[10] Its stability is attributed to its closed nuclear shell with 142 neutrons.[13][14] Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377(4). It is one of only three radioactive elements (along with protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.[1]

Thorium nuclei are susceptible to alpha decay because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons.[15] The alpha decay of 232Th initiates the 4n 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.[10] Any sample of thorium or its compounds contains traces of these daughters, which are isotopes of thallium, lead, bismuth, polonium, radon, radium, and actinium.[10] Natural thorium samples can be chemically purified to extract useful daughter nuclides, such as 212Pb, which is used in nuclear medicine for cancer therapy.[16][17] 227Th (alpha emitter with a 18.68 days half-life) can also be used in cancer treatments such as targeted alpha therapies.[18][19][20] 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.[21][22]

Decay Chain Thorium
The 4n decay chain of 232Th, commonly called the "thorium series"

Thirty radioisotopes have been characterised, which range in mass number from 209[23] to 238.[21] After 232Th, the most stable of them (with respective half-lives) are 230Th (75,380 years), 229Th (7,340 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.[10]

In deep seawaters the isotope 230Th makes up to 0.04% of natural thorium.[1] 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.[21] 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.[1]

Thorium has three known nuclear isomers (or metastable states), 216m1Th, 216m2Th, and 229mTh. 229mTh has the lowest known excitation energy of any isomer,[24] measured to be 7.6±0.5 eV. This is so low that when it undergoes isomeric transition, the emitted gamma radiation is in the ultraviolet range.[25][26][d]

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.[28] 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.[28][e] 232Th is not fissionable, but it is fertile as it can be converted to fissile 233U by neutron capture and subsequent beta decay.[28][29]

Radiometric dating

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.[f] 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.[30] 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.[30][31] 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.[32][33] 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.[32][33]

Chemistry

A thorium atom has 90 electrons, of which four are valence electrons. Three atomic orbitals are theoretically available for the valence electrons to occupy: 5f, 6d, and 7s.[34] 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 is much more similar to the transition metals zirconium and hafnium than to cerium in its ionisation 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.[35][36]

CaF2 polyhedra
Thorium dioxide has the fluorite crystal structure.
Th4+: __  /  O2−: __

Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement. This was first realised in 1995, when it was pointed out that 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, and not protactinium, acts as the first actinide metallurgically.[7] The 5f character of thorium is also clear in the rare and highly unstable +3 oxidation state, in which thorium exhibits the electron configuration [Rn]5f1.[37]

Tetravalent thorium compounds are usually colourless or yellow, like those of silver or lead, as the Th4+ ion has no 5f or 6d electrons.[6] 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.[38][g] 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.[39]

Reactivity

Thorium is a highly reactive and electropositive metal. With a standard reduction potential of −1.90 V for the Th4+/Th couple, it is somewhat more electropositive than zirconium or aluminium.[40] Finely divided thorium metal can exhibit pyrophoricity, spontaneously igniting in air.[4] 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.[4] Such samples slowly tarnish, becoming grey and finally black at the surface.[4]

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.[4][41] It dissolves in concentrated nitric acid containing a small quantity of catalytic fluoride or fluorosilicate ions;[4][42] if these are not present, passivation by the nitrate can occur, as with uranium and plutonium.[4][43][44]

Kristallstruktur Uran(IV)-fluorid
Crystal structure of thorium tetrafluoride
Th4+: __  /  F: __

Inorganic compounds

Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.[45] In air, thorium burns to form ThO2, which has the fluorite structure.[46] Thorium dioxide is a refractory material, with the highest melting point (3390 °C) of any known oxide.[47] It is somewhat hygroscopic and reacts readily with water and many gases;[48] it dissolves easily in concentrated nitric acid in the presence of fluoride.[49] When heated, it emits intense blue light through incandescence; the light becomes white when ThO2 is mixed with its lighter homologue cerium dioxide (CeO2, ceria): this is the basis for its previously common application in gas mantles.[48] Several binary thorium chalcogenides and oxychalcogenides are also known with sulfur, selenium, and tellurium.[50]

All four thorium tetrahalides are known, as are some low-valent bromides and iodides:[51] the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.[52] Many related polyhalide ions are also known.[51] Thorium tetrafluoride has a monoclinic crystal structure like those of zirconium tetrafluoride and hafnium tetrafluoride, where the Th4+ ions are coordinated with F ions in somewhat distorted square antiprisms.[51] The other tetrahalides instead have dodecahedral geometry.[52] Lower iodides ThI3 (black) and ThI2 (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 Th4+ and could be more clearly formulated as electride compounds.[51] Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.[51] For example, when treated with potassium fluoride and hydrofluoric acid, Th4+ forms the complex anion ThF2−
6
, which precipitates as an insoluble salt, K2ThF6.[42]

Thorium borides, carbides, silicides, and nitrides are refractory materials, like those of uranium and plutonium, and have thus received attention as possible nuclear fuels.[45] All four heavier pnictogens (phosphorus, arsenic, antimony, and bismuth) also form binary thorium compounds. Thorium germanides are also known.[53] Thorium reacts with hydrogen to form the thorium hydrides ThH2 and Th4H15, the latter of which is superconducting below 7.5–8 K; at standard temperature and pressure, it conducts electricity like a metal.[54] The hydrides are thermally unstable and readily decompose upon exposure to air or moisture.[55]

Uranocene-3D-balls
Sandwich molecule structure of thorocene

Coordination compounds

In an acidic aqueous solution, thorium occurs as the tetrapositive aqua ion [Th(H2O)9]4+, which has tricapped trigonal prismatic molecular geometry:[56][57] at pH < 3, the solutions of thorium salts are dominated by this cation.[56] The Th4+ 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.[56] 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 Fe3+), predominantly to [Th2(OH)2]6+ in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide Th(OH)4 forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation).[58] As a hard Lewis acid, Th4+ favours hard ligands with oxygen atoms as donors: complexes with sulfur atoms as donors are less stable and are more prone to hydrolysis.[35]

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.[58] These thorium salts are known for their high solubility in water and polar organic solvents.[6]

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.[59] 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.[59] 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.[59] 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.[56]

Thorium half sandwich
Piano-stool molecule structure of (η8-C8H8)ThCl2(THF)2

Organothorium compounds

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 Th(C8H8)2, thorocene. It is isotypic with the better-known analogous uranium compound uranocene.[60] It can be prepared by reacting K2C8H8 with thorium tetrachloride in tetrahydrofuran (THF) at the temperature of dry ice, or by reacting thorium tetrafluoride with MgC8H8.[60] It is unstable in air and decomposes in water or at 190 °C.[60] Half sandwich compounds are also known, such as (η8-C8H8)ThCl2(THF)2, which has a piano-stool structure and is made by reacting thorocene with thorium tetrachloride in tetrahydrofuran.[35]

The simplest of the cyclopentadienyls are Th(C5H5)3 and Th(C5H5)4: 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;[60][61] a formal +2 oxidation state occurs in a derivative.[62] The chloride derivative [Th(C5H5)3Cl] is prepared by heating thorium tetrachloride with limiting K(C5H5) 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.[61]

Other organothorium compounds are not well-studied. Tetrabenzylthorium, Th(CH2C6H5), and tetraallylthorium, Th(C3H5)4, are known, but their structures have not been determined. They decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion [Th(CH3)7]3−, heptamethylthorate, which forms the salt [Li(tmeda)]3[ThMe7] (tmeda= Me2NCH2CH2NMe2). 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, Th(CH3)4, is not known, but its adducts are stabilised by phosphine ligands.[35]

Occurrence

Formation

232Th is a primordial nuclide, having existed in its current form for over ten billion years; it was forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovae and neutron star mergers.[63][64] 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.[65] 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.[63][66][67]

Elements abundance-bars
Estimated abundances of the 83 primordial elements in the Solar system, plotted on a logarithmic scale. Thorium, at atomic number 90, is one of the rarest elements.

In the universe, thorium is among the rarest of the primordial elements, 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.[63][65][h] Neutron capture by nuclides beyond A= 209 often results in nuclear fission instead of neutron absorption, reducing the fraction of nuclei that cross the gap of instability past bismuth to become actinides such as thorium.[65] 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.[68]

In the Earth's crust, thorium is much more abundant: with an abundance of 8.1 parts per million (ppm), it is one of the most abundant of the heavy elements, almost as abundant as lead (13 ppm) and more abundant than tin (2.1 ppm).[69] This is because thorium is likely to form oxide minerals that do not sink into the core; it is classified as a lithophile. Common thorium compounds are also poorly soluble in water. Thus, even though the Earth contains the same abundances of the elements as the Solar System as a whole, there is more accessible thorium than heavy platinum group metals in the crust.[70]

Evolution of Earth's radiogenic heat-no total
The radiogenic heat from the decay of 232Th (violet) is a major contributor to the earth's internal heat budget. Of the four major nuclides providing this heat, 232Th has grown to provide the most heat as the other ones decayed faster than thorium.[71][72][73][74]

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.[21] 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.)[75] 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.[47] 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.[21][i]

Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare.[77] Soil normally contains about 6 ppm of thorium.[78]

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.[77] 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.[79] An extreme example is ekanite, (Ca,Fe,Pb)2(Th,U)Si8O20, which almost never occurs in nonmetamict form due to the thorium it contains.[80]

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%.[77][81] 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.[77] Allanite (chiefly silicates-hydroxides of various metals) can have 0.1–2% thorium and zircon (chiefly zirconium silicate, ZrSiO4) up to 0.4% thorium.[77]

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 ThO2 content.[77][j] Thorite (chiefly thorium silicate, ThSiO4), also has a high thorium content and is the mineral in which thorium was first discovered.[77] In thorium silicate minerals, the Th4+ and SiO4−
4
ions are often replaced with M3+ (where M= Sc, Y, or Ln) and phosphate (PO3−
4
) ions respectively.[77] Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released. The Th4+ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can reach 40 ppm.[47]

History

Mårten Eskil Winge - Tor's Fight with the Giants - Google Art Project
Thor's Fight with the Giants (1872) by Mårten Eskil Winge; Thor, the Norse god of thunder, raising his hammer Mjölnir in a battle against the giants.[82]

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.[83] Berzelius privately named the putative element "thorium" in 1817[84] and its supposed oxide "thorina" after Thor, the Norse god of thunder.[85] 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.[29][83][86][87]

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).[88] 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.[29] He published his findings in 1829, having isolated an impure sample by reducing KThF5 with potassium metal.[89][90][91] Berzelius reused the name of the previous supposed element discovery[89][92] and named the source mineral thorite.[29]

J J Berzelius
Jöns Jacob Berzelius, who first identified thorium as a new element

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.[k] He determined that thorium was a very electropositive metal, ahead of cerium and behind zirconium in electropositivity.[93] Metallic thorium was isolated for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger.[l]

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.[96][97] 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.[98][m]

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.[29] 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.

Old thorium dioxide gas mantle - oblong shape
World War II thorium dioxide gas mantle

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.[99][100][101] Starting from 1899, the British 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.[102]

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.[103] The biological effect of radiation was discovered in 1903.[104] 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.[105] 10,000 individuals in the United States had been injected thorium during X-ray diagnosis; they were later found to suffer health issues such as leukaemia and abnormal chromosomes.[47] Public interest in radioactivity had declined by the end of the 1930s.[105]

Seaborg in lab.jpeg
Glenn T. Seaborg, who settled thorium's location in the f-block

Further classification

Up to the late 19th century, chemists unanimously agreed that thorium and uranium were analogous to hafnium and tungsten; 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.[96] 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;[106] Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.[96]

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.[107] In 1945, when American physicist Glenn T. Seaborg and his team had discovered the transuranic elements americium and curium, he realised that thorium was the second member of the actinide series and was filling an f-block row, instead of being the heavier congener of hafnium filling a fourth d-block row.[98][n]

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.[29][110] 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,[110] but this was not the case for the dose received by people manufacturing the mantles or for the soils around some factory sites.[111] Some manufacturers have changed to other materials, such as yttrium.[112] 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.[110][113]

Nuclear power

Indian Point Nuclear Power Plant
The Indian Point Energy Center (Buchanan, New York, United States), home of the world's first thorium reactor

Thorium has been used as a power source. The earliest thorium-based reactor was built at the Indian Point Energy Center in the United States in 1962.[114] India has one of the largest supplies of thorium in the world and not much uranium, and in the 1950s targeted achieving energy independence with their three-stage nuclear power programme.[115][116] 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.[117] 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.[118] 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.[119] In the 21st century, thorium's potential for reducing nuclear proliferation and its waste characteristics led to renewed interest in the thorium fuel cycle.[120][121][122]

Nuclear weapons

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.[123] 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,[124] especially since 233U is difficult to produce isotopically pure.[123]

Production

Lower-bound estimates of thorium reserves in thousand tonnes, 2014[123]
Country Reserves
 India 846
 Brazil 632
 Australia 595
 United States 595
 Egypt 380
 Turkey 374
 Venezuela 300
 Canada 172
 Russia 155
 South Africa 148
 China 100
 Norway 87
 Greenland 86
 Finland 60.5
 Sweden 50
 Kazakhstan 50
Other countries 1,725
World total 6,355

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.[125] 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.[126] Present knowledge of the distribution of thorium resources is poor, as low demand has led to exploration efforts being relatively minor.[127] In 2014, world production of the monazite concentrate, from which thorium would be extracted, was 2,700 tonnes.[128]

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.[129]

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.[40] Initial concentration varies with the type of deposit.[129]

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%.[129] 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%.[129]

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.[129]

Acid digestion

Acid digestion is a two-stage process, involving the use of up to 93% sulfuric acid at 210–230 °C. First, 60% sulfuric acid 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.[129]

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.[129]

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 Th4+ 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.[129]

Modern applications

Non-radioactivity-related uses have been in decline since the 1950s[130] due to environmental concerns largely stemming from the radioactivity of thorium and its decay products.[29][110]

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.[47] 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 mantles.[131] 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.[132] 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.[131] Thorium in mantles, though still common, has been progressively replaced with yttrium since the late 1990s.[133] 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."[134]

Yellowing of thorium lenses
Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)

During the production of incandescent filaments, recrystallisation of tungsten is signifiantly lowered by adding small amounts of thorium dioxide to the tungsten sintering powder before drawing the filaments.[130] 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.[29] Tungsten forms a one-atom-thick layer on the surface of thorium. 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.[135] Since the 1920s, thoriated tungsten wires have been used in electronic tubes and in the cathodes and anticathodes of X-ray tubes and rectifiers. Thanks to the reactivity of thorium with atmospheric oxygen and nitrogen, thorium also marks impurities in the evacuated tubes. The introduction of transistors in the 1950s significantly diminished this use, but not entirely.[130] Thorium dioxide is used in gas tungsten arc welding (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.[29] Thorium oxide is being replaced in this use with other oxides, such as those of zirconium, cerium, and lanthanum.[136][137]

Thorium dioxide is found in heat-resistant ceramics, such as high-temperature laboratory crucibles,[29] 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.[130]

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.[41] The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal.[138] Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides in this application, as they provide similar effects and are not radioactive.[130]

Thorium tetrafluoride is used as an antireflection material in multilayered optical coatings. It is transparent to electromagnetic waves having wavelengths in the range of 0.35–12 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.[139] Replacements for thorium tetrafluoride are being developed as of the 2010s.[140]

Potential use for nuclear energy

The main nuclear power source in a reactor is the neutron-induced fission of a nuclide; the synthetic fissile[e] nuclei 233U and 239Pu can be bred from neutron capture by the naturally occurring quantity nuclides 232Th and 238U. 235U occurs naturally and is also fissile.[141][142][o] 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:[29]

232
90
Th
+ n → 233
90
Th
+ γ 233
91
Pa
233
92
U
Transmutations in the thorium fuel cycle
237Np
231U 232U 233U 234U 235U 236U 237U
231Pa 232Pa 233Pa 234Pa
230Th 231Th 232Th 233Th
(Nuclides before a yellow background in italic have half-lives under 30 days;
nuclides in bold have half-lives over 1,000,000 years;
nuclides in red frames are fissile)

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.[29] 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.[143]

Advantages

Thorium is more abundant than uranium and can satisfy world energy demands for longer.[144]

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%).[145] It also releases more neutrons upon fission on average.[144] 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.[146]

Thorium fuels also result in a safer and better-performing reactor core[29] because thorium dioxide has a higher melting point, higher thermal conductivity, and a lower coefficient of thermal expansion and is more stable chemically than the now-common fuel uranium dioxide, which can further oxidise to triuranium octoxide (U3O8).[147]

Disadvantages

The used fuel is difficult and dangerous to reprocess because many of the daughters of 232Th and 233U are strong gamma emitters.[144] 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:[148]

230
90
Th
+ n → 231
90
Th
+ γ 231
91
Pa
­ 231
91
Pa
+ n → 232
91
Pa
+ γ 232
92
U

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.)[149] 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.[148] 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 only decays to 233U.[150]

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;[29] research continues in this area. Others cite the low commercial viability of the thorium fuel cycle:[151][152][153] 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".[154] 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.[150]

Hazards

PSM V74 D233 Thorium radioactive incandescent gas mantle placed above plant seeds
Experiment on the effect of radiation (from an unburned thorium gas mantle) on the germination and growth of timothy-grass seed

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.[155] 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.[155] Exposure to thorium internally leads to increased risk of liver diseases.[156]

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,[157] and which are built up quickly following the initial decay of 232Th due to the absence of long-lived nuclides along the decay chain.[158] 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.[159] 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.[160]

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.[156]

Biological

Thorium is odourless and tasteless.[161] The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water,[162] precipitating out before entering the body as the hydroxide.[163] 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.[158] 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.[113] 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.[155] Thorium's low solubility in water also means that excretion of thorium by the kidneys and faeces is rather slow.[158]

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.[163] 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.[47] Thorium has no known biological role.[47]

Chemical

Powdered thorium metal is pyrophoric: it ignites spontaneously in air.[4] 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.[164]

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.[165][166][167]

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.[47] Most thorium exposure occurs through dust inhalation; some thorium comes with food and water, but because of its low solubility, this exposure is negligible.[158]

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.[168] 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.[169] 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.[170]

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.[171]

Notes

  1. ^ Bismuth is very slightly radioactive, but its half-life (1.9×1019 years) is so long that its decay is negligible even over geological timespans.
  2. ^ While einsteinium has been measured to have a lower density, this measurement was done on small, microgram-mass samples, and is likely because of the rapid self-destruction of the crystal structure caused by einsteinium's extreme radioactivity.[8]
  3. ^ Behind osmium, tantalum, tungsten, and rhenium;[4] higher boiling points are speculated to be found in the 6d transition metals, but they have not been produced in large enough quantities to test this prediction.[9]
  4. ^ Gamma rays are distinguished by their origin in the nucleus, not their wavelength; hence there is no lower limit to gamma energy derived from radioactive decay.[27]
  5. ^ a b A fissionable nuclide is capable of undergoing fission (even with a low probability) after capturing a high-energy neutron. Some of these nuclides can be induced to fission with low-energy thermal neutrons with a high probability; they are referred to as fissile. A fertile nuclide is one that could be bombarded with neutrons to produce a fissile nuclide. Critical mass is the mass of a ball of a material which could undergo a sustained nuclear chain reaction.
  6. ^ The name ionium for 230Th is a remnant from a period when different isotopes were not recognised to be the same element and were given different names.
  7. ^ Unlike the previous similarity between the actinides and the transition metals, the main-group similarity largely ends at thorium before being resumed in the second half of the actinide series, because of the growing contribution of the 5f orbitals to covalent bonding. The only other commonly-encountered actinide, uranium, retains some echoes of main-group behaviour. The chemistry of uranium is more complicated than that of thorium, but the two most common oxidation states of uranium are uranium(VI) and uranium(IV); these are two oxidation units apart, with the higher oxidation state corresponding to formal loss of all valence electrons, which is similar to the behaviour of the heavy main-group elements in the p-block.[38]
  8. ^ An even number of either protons or neutrons generally increases nuclear stability of isotopes, compared to isotopes with odd numbers. Elements with odd atomic numbers have no more than two stable isotopes; even-numbered elements have multiple stable isotopes, with tin (element 50) having ten.[10]
  9. ^ Other isotopes may occur alongside 232Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be 228Th, which occurs in the decay chain of 232Th (the thorium series): the ratio of 228Th to 232Th would be under 10−10.[21] If uranium is present, tiny traces of several other isotopes will also be present: 231Th and 227Th from the decay chain of 235U (the actinium series), and slightly larger but still tiny traces of 234Th and 230Th from the decay chain of 238U (the uranium series).[21] 229Th is also been produced in the decay chain of 237Np (the neptunium series): all primordial 237Np is extinct, but it is still produced as a result of nuclear reactions in uranium ores.[76] 229Th is mostly produced as a daughter of artificial 233U made by neutron irradiation of 232Th, and is extremely rare in nature.[21]
  10. ^ Thorianite refers to minerals with 75–100 mol% ThO2; uranothorianite, 25–75 mol% ThO2; thorian uraninite, 15–25 mol% ThO2; uraninite, 0–15 mol% ThO2.[77]
  11. ^ At the time, the rare-earth elements, among which thorium was found and with which it is closely associated in nature, were thought to be divalent; the rare earths were given atomic weight values two-thirds of their actual ones, and thorium and uranium are given values half of the actual ones.
  12. ^ The main difficulty in isolating thorium lies not in its chemical electropositivity, but in the close association of thorium in nature with the rare-earth elements and uranium, which collectively are difficult to separate from each other. Swedish chemist Lars Fredrik Nilson, the discoverer of scandium, had previously made an attempt to isolate thorium metal in 1882, but was unsuccessful at achieving a high degree of purity.[94] Lely and Hamburger obtained 99% pure thorium metal by reducing thorium chloride with sodium metal.[95] A simpler method leading to even higher purity was discovered in 1927 by American engineers John Marden and Harvey Rentschler, involving the reduction of thorium oxide with calcium in presence of calcium chloride.[95]
  13. ^ Thorium also appears in the 1864 table by British chemist John Newlands as the last and heaviest element, as it was initially thought that uranium was a trivalent element with an atomic weight of around 120: this is half of its actual value, since uranium is predominantly hexavalent. It also appears as the heaviest element in the 1864 table by British chemist William Odling under titanium, zirconium, and tantalum. It does not appear in the periodic systems published by French geologist Alexandre-Émile Béguyer de Chancourtois in 1862, German-American musician Gustav Hinrichs in 1867, or German chemist Julius Lothar Meyer in 1870, all of which exclude the rare earths and thorium.[96]
  14. ^ The filling of the 5f subshell from the beginning of the actinide series was confirmed in 1964 when the next element, rutherfordium, was first synthesised and found to behave like hafnium, as would be expected if the filling of the 5f orbitals had already finished by then.[108] Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group 4 element".[109]
  15. ^ The thirteen fissile actinide isotopes with half-lives over a year are 229Th, 233U, 235U, 236Np, 239Pu, 241Pu, 242mAm, 243Cm, 245Cm, 247Cm, 249Cf, 251Cf, and 252Es. Of these, only 235U is naturally occurring, and only 233U and 239Pu can be bred from naturally occurring nuclei with a single neutron capture.[142]

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Bibliography

Further reading

Actinide

The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.Strictly speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are often included in any general discussion of the chemistry of the actinide elements. Actinium is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, even though IUPAC acknowledges its inclusion based on common usage.The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium. The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any f-electrons, and curium and lawrencium have the same number as the preceding element. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.

Of the actinides, primordial thorium and uranium occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).

Decay chain

In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.

Decay stages are referred to by their relationship to previous or subsequent stages. A parent isotope is one that undergoes decay to form a daughter isotope. One example of this is uranium (atomic number 92) decaying into thorium (atomic number 90). The daughter isotope may be stable or it may decay to form a daughter isotope of its own. The daughter of a daughter isotope is sometimes called a granddaughter isotope.

The time it takes for a single parent atom to decay to an atom of its daughter isotope can vary widely, not only between different parent-daughter pairs, but also randomly between identical pairings of parent and daughter isotopes. The decay of each single atom occurs spontaneously, and the decay of an initial population of identical atoms over time t, follows a decaying exponential distribution, e−λt, where λ is called a decay constant. One of the properties of an isotope is its half-life, the time by which half of an initial number of identical parent radioisotopes have decayed to their daughters, which is inversely related to λ. Half-lives have been determined in laboratories for many radioisotopes (or radionuclides). These can range from nearly instantaneous to as much as 1019 years or more.

The intermediate stages each emit the same amount of radioactivity as the original radioisotope (i.e. there is a one-to-one relationship between the numbers of decays in successive stages) but each stage releases a different quantity of energy. If and when equilibrium is achieved, each successive daughter isotope is present in direct proportion to its half-life; but since its activity is inversely proportional to its half-life, each nuclide in the decay chain finally contributes as many individual transformations as the head of the chain, though not the same energy. For example, uranium-238 is weakly radioactive, but pitchblende, a uranium ore, is 13 times more radioactive than the pure uranium metal because of the radium and other daughter isotopes it contains. Not only are unstable radium isotopes significant radioactivity emitters, but as the next stage in the decay chain they also generate radon, a heavy, inert, naturally occurring radioactive gas. Rock containing thorium and/or uranium (such as some granites) emits radon gas that can accumulate in enclosed places such as basements or underground mines.

Fuji Molten Salt Reactor

The FUJI molten salt reactor is a proposed molten-salt-fueled thorium fuel cycle thermal breeder reactor, using technology similar to the Oak Ridge National Laboratory's Molten Salt Reactor Experiment - liquid fluoride thorium reactor. It was being developed by the Japanese company International Thorium Energy & Molten-Salt Technology (IThEMS), together with partners from the Czech Republic. As a breeder reactor, it converts thorium into the nuclear fuel uranium-233. To achieve reasonable neutron economy, the chosen single-salt design results in significantly larger feasible size than a two-salt reactor (where blanket is separated from core, which involves graphite-tube manufacturing/sealing complications). Like all molten salt reactors, its core is chemically inert and under low pressure, helping to prevent explosions and toxic releases. The proposed design is rated at 200 MWe output. The IThEMS consortium planned to first build a much smaller MiniFUJI 10 MWe reactor of the same design once it had secured an additional $300 million in funding.IThEMS closed in 2011 after it was unable to secure adequate funding. A new company, Thorium Tech Solution (TTS), was founded in 2011 by Kazuo Furukawa, the chief scientist from IThEMS, and Masaaki Furukawa. TTS acquired the FUJI design and some related patents.

Gas mantle

An incandescent gas mantle, gas mantle or Welsbach mantle is a device for generating bright white light when heated by a flame. The name refers to its original heat source in gas lights, which filled the streets of Europe and North America in the late 19th century, mantle referring to the way it is hung above the flame. Today it is still used in portable camping lanterns, pressure lanterns and some oil lamps.Gas mantles are usually sold as fabric items, which, because of impregnation with metal nitrates, form a rigid but fragile mesh of metal oxides when heated during initial use; these metal oxides produce light from the heat of the flame whenever used. Thorium dioxide was commonly a major component; being radioactive, it has led to concerns about the safety of those involved in manufacturing mantles. Normal use, however, poses minimal health risk.

Isotopes of thorium

Although thorium (90Th) has 6 naturally occurring isotopes, none of these isotopes are stable; however, one isotope, 232Th, is relatively stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium. As such, thorium is considered to be mononuclidic. However, in 2013 IUPAC reclassified thorium as binuclidic, due to large amounts in 230Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty radioisotopes have been characterized, with the most stable (after 232Th) being 230Th with a half-life of 75,380 years, 229Th with a half-life of 7,340 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, recently measured to be 7.6 ± 0.5 eV.The known isotopes of thorium range in mass number from 208 to 238.

Liquid fluoride thorium reactor

The liquid fluoride thorium reactor (acronym LFTR; often pronounced lifter) is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based, molten, liquid salt for fuel. In a typical design, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat to a steam turbine or closed-cycle gas turbine.Molten-salt-fueled reactors (MSRs) supply the nuclear fuel mixed into a molten salt. They should not be confused with designs that use a molten salt for cooling only (fluoride high-temperature reactors, FHRs) and still have a solid fuel. Molten salt reactors, as a class, include both burners and breeders in fast or thermal spectra, using fluoride or chloride salt-based fuels and a range of fissile or fertile consumables. LFTRs are defined by the use of fluoride fuel salts and the breeding of thorium into uranium-233 in the thermal spectrum.

The LFTR concept was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s, though the MSRE did not use thorium. The LFTR has recently been the subject of a renewed interest worldwide. Japan, China, the UK and private US, Czech, Canadian and Australian companies have expressed the intent to develop, and commercialize the technology.

LFTRs differ from other power reactors in almost every aspect: they use thorium that is turned into uranium, instead of using uranium directly; they are refueled by pumping without shutdown; they use a liquid salt coolant, which allows for higher operating temperatures, and which thereby allows for much lower pressure in the system. These distinctive characteristics give rise to many potential advantages, as well as design challenges.

Molten salt reactor

A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. MSRs offer multiple advantages over conventional nuclear power plants, although for historical reasons, they have not been deployed.

The concept was first established in the 1950s. The early Aircraft Reactor Experiment was primarily motivated by the small size that the technique offered, while the Molten-Salt Reactor Experiment was a prototype for a thorium fuel cycle breeder nuclear power plant. The increased research into Generation IV reactor designs renewed interest in the technology.

Monazite

Monazite is a reddish-brown phosphate mineral containing rare-earth metals. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, about 4.6 to 5.7 g/cm3. There are at least four different kinds of monazite, depending on relative elemental composition of the mineral:

monazite-(Ce), (Ce, La, Nd, Th)PO4 (the most common member),

monazite-(La), (La, Ce, Nd)PO4,

monazite-(Nd), (Nd, La, Ce)PO4,

monazite-(Sm), (Sm, Gd, Ce, Th)PO4.The elements in parentheses are listed in the order of their relative proportion within the mineral: lanthanum is the most common rare-earth element in monazite-(La), and so forth. Silica (SiO2) is present in trace amounts, as well as small amounts of uranium and thorium. Due to the alpha decay of thorium and uranium, monazite contains a significant amount of helium, which can be extracted by heating.Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. India, Madagascar, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite.

Monazite is radioactive due to the presence of thorium and, less commonly, uranium. Because of its radioactive nature, monazite is used for monazite geochronology to study geological events, such as crystallization, heating, or deformation of the rocks containing monazite.

The name monazite comes from the Greek μονάζειν (to be solitary), via German Monazit, in allusion to its isolated crystals.

Protactinium

Protactinium (formerly protoactinium) is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel.

Protactinium was first identified in 1913 by Kasimir Fajans and Oswald Helmuth Göhring and named brevium because of the short half-life of the specific isotope studied, i.e. protactinium-234. A more stable isotope of protactinium, 231Pa, was discovered in 1917/18 by Otto Hahn and Lise Meitner, and they chose the name proto-actinium, but the IUPAC finally named it "protactinium" in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant "(nuclear) precursor of actinium" and reflected that actinium is a product of radioactive decay of protactinium. John Arnold Cranston (working with Frederick Soddy and Ada Hitchins) is also credited with discovering the most stable isotope in 1915, but delayed his announcement due to being called up for service in the First World War.The longest-lived and most abundant (nearly 100%) naturally occurring isotope of protactinium, protactinium-231, has a half-life of 32,760 years and is a decay product of uranium-235. Much smaller trace amounts of the short-lived protactinium-234 and its nuclear isomer protactinium-234m occur in the decay chain of uranium-238. Protactinium-233 results from the decay of thorium-233 as part of the chain of events used to produce uranium-233 by neutron irradiation of thorium-232. It is an undesired intermediate product in thorium-based nuclear reactors and is therefore removed from the active zone of the reactor during the breeding process. Analysis of the relative concentrations of various uranium, thorium and protactinium isotopes in water and minerals is used in radiometric dating of sediments which are up to 175,000 years old and in modeling of various geological processes.

Symbol (chemistry)

In relation to the chemical elements, a symbol is a code for a chemical element. Many functional groups have their own chemical symbol, e.g. Ph for the phenyl group, and Me for the methyl group. Chemical symbols for elements normally consist of one or two letters from the Latin alphabet, but can contain three when the element has a systematic temporary name (as of March 2017, no discovered elements have such a name), and are written with the first letter capitalized.

Earlier chemical element symbols stem from classical Latin and Greek vocabulary. For some elements, this is because the material was known in ancient times, while for others, the name is a more recent invention. For example, "He" is the symbol for helium (New Latin name, not known in ancient Roman times), "Pb" for lead (plumbum in Latin), and "Hg" for mercury (hydrargyrum in Greek). Some symbols come from other sources, like "W" for tungsten (Wolfram in German, not known in Roman times).

Temporary symbols assigned to newly or not-yet synthesized elements use 3-letter symbols based on their atomic numbers. For example, "Uno" was the temporary symbol for hassium (element 108) which had the temporary name of unniloctium.

Chemical symbols may be modified by the use of prepended superscripts or subscripts to specify a particular isotope of an atom. Additionally, appended superscripts may be used to indicate the ionization or oxidation state of an element. They are widely used in chemistry and they have been officially chosen by the International Union of Pure and Applied Chemistry (IUPAC). There are also some historical symbols that are no longer officially used.

Attached subscripts or superscripts specifying a nuclide or molecule have the following meanings and positions:

The nucleon number (mass number) is shown in the left superscript position (e.g., 14N). This number defines the specific isotope. Various letters, such as "m" and "f" may also be used here to indicate a nuclear isomer (e.g., 99mTc). Alternately, the number here can represent a specific spin state (e.g., 1O2). These details can be omitted if not relevant in a certain context.

The proton number (atomic number) may be indicated in the left subscript position (e.g., 64Gd). The atomic number is redundant to the chemical element, but is sometimes used to emphasize the change of numbers of nucleons in a nuclear reaction.

If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Ca2+).

The number of atoms of an element in a molecule or chemical compound is shown in the right subscript position (e.g., N2 or Fe2O3). If this number is one, it is normally omitted - the number one is then implicit.

A radical is indicated by a dot on the right side (e.g., Cl• for a neutral chlorine atom). This is often omitted unless relevant to a certain context because it is already deducible from the charge and atomic number information values.In Chinese, each chemical element has a dedicated character, usually created for the purpose (see Chemical elements in East Asian languages). However, Latin symbols are also used, especially in formulas.

A list of current, dated, as well as proposed and historical signs and symbols is included here with its signification. Also given is each element's atomic number, atomic weight or the atomic mass of the most stable isotope, group and period numbers on the periodic table, and etymology of the symbol.

Hazard pictographs are another type of symbols used in chemistry.

THTR-300

The THTR-300 was a thorium high-temperature nuclear reactor rated at 300 MW electric (THTR-300) in Hamm-Uentrop, Germany. It started operating in 1983, synchronized with the grid in 1985, operated at full power in February 1987 and was shut down September 1, 1989.

The THTR-300 served as a prototype high-temperature reactor (HTR) to use the TRISO pebble fuel produced by the AVR, an experimental pebble bed operated by VEW. The THTR-300 cost €2.05 billion and was predicted to cost an additional €425 million through December 2009 in decommissioning and other associated costs.

The German state of North Rhine Westphalia, Federal Republic of Germany, and Hochtemperatur-Kernkraftwerk GmbH (HKG) financed the THTR-300’s construction.

Thorcon

Thorcon is a company that is designing the ThorCon Reactor, a small modular reactor (SMR) that employs molten salt technology, based on the DMSR design from Oak Ridge National Laboratory. It relies on large modules as are used in modern ship building. The ThorCon reactor is a "burner" reactor that employs liquid fuel, rather than a conventional solid fuel; this liquid contains the nuclear fuel and also serves as primary coolant.

Thorium-based nuclear power

Thorium-based nuclear power generation is fueled primarily by the nuclear fission of the isotope uranium-233 produced from the fertile element thorium. According to proponents, a thorium fuel cycle offers several potential advantages over a uranium fuel cycle—including much greater abundance of thorium on Earth, superior physical and nuclear fuel properties, and reduced nuclear waste production. However, development of thorium power has significant start-up costs. Proponents also cite the lack of easy weaponization potential as an advantage of thorium, while critics say that development of breeder reactors in general (including thorium reactors, which are breeders by nature) increases proliferation concerns. Since about 2008, nuclear energy experts have become more interested in thorium to supply nuclear fuel in place of uranium to generate nuclear power. This renewed interest has been highlighted in a number of scientific conferences, the latest of which, ThEC13 was held at CERN by iThEC and attracted over 200 scientists from 32 countries.

A nuclear reactor consumes certain specific fissile isotopes to produce energy. The three most practical types of nuclear reactor fuel are:

Uranium-235, purified (i.e. "enriched") by reducing the amount of uranium-238 in natural mined uranium. Most nuclear power has been generated using low-enriched uranium (LEU), whereas high-enriched uranium (HEU) is necessary for weapons.

Plutonium-239, transmuted from uranium-238 obtained from natural mined uranium.

Uranium-233, transmuted from thorium-232, derived from natural mined thorium, which is the subject of this article.The vision of using thorium in place of uranium was set out in the 1950s by physicist Homi Bhabha.

Some believe thorium is key to developing a new generation of cleaner, safer nuclear power. According to a 2011 opinion piece by a group of scientists at the Georgia Institute of Technology, considering its overall potential, thorium-based power "can mean a 1000+ year solution or a quality low-carbon bridge to truly sustainable energy sources solving a huge portion of mankind’s negative environmental impact."After studying the feasibility of using thorium, nuclear scientists Ralph W. Moir and Edward Teller suggested that thorium nuclear research should be restarted after a three-decade shutdown and that a small prototype plant should be built.

Thorium dioxide

Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in color. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides. Only a few elements (including tungsten and carbon) and a few compounds (including tantalum carbide) have higher melting points. All thorium compounds are radioactive because there are no stable isotopes of thorium.

Thorium fuel cycle

The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, 232Th, as the fertile material. In the reactor, 232Th is transmuted into the fissile artificial uranium isotope 233U which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as 231Th), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, 232Th absorbs neutrons to produce 233U. This parallels the process in uranium breeder reactors whereby fertile 238U absorbs neutrons to form fissile 239Pu. Depending on the design of the reactor and fuel cycle, the generated 233U either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

The thorium fuel cycle has several potential advantages over a uranium fuel cycle, including thorium's greater abundance, superior physical and nuclear properties, reduced plutonium and actinide production, and better resistance to nuclear weapons proliferation when used in a traditional light water reactor though not in a molten salt reactor.

Thorium monoxide

Thorium monoxide (thorium(II) oxide), is the binary oxide of thorium having chemical formula ThO. The covalent bond in this diatomic molecule is highly polar. The electric field between the two atoms has been calculated to be 84 gigavolts per centimeter, one of the largest known internal electric fields.

Simple combustion of thorium in air produces thorium dioxide. However, laser ablation of thorium in the presence of oxygen gives the monoxide. Additionally, exposure of a thin film of thorium to low-pressure oxygen at medium temperature forms a rapidly growing layer of thorium monoxide under a more-stable surface coating of the dioxide.

At extremely high temperatures, thorium dioxide can convert to the monoxide either in by an disproportionation reaction (equilibrium with liquid thorium metal) above 1,850 K (1,580 °C; 2,870 °F) or by simple dissociation (evolution of oxygen) above 2,500 K (2,230 °C; 4,040 °F).

Thorium tetrafluoride

Thorium(IV) fluoride (ThF4) is an inorganic chemical compound. It is a white, hygroscopic powder which can be produced by reacting thorium with fluorine gas. At temperatures above 500 °C, it reacts with atmospheric moisture to produce ThOF2.

Uranium-233

Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur, to maintain the neutron economy (if it misses the 233U window, the next fissile target is 235U, meaning a total of 4 neutrons needed to trigger fission).

233U usually fissions on neutron absorption, but sometimes retains the neutron, becoming uranium-234. The capture-to-fission ratio of uranium-233 is smaller than those of the other two major fissile fuels, uranium-235 and plutonium-239.

Uranium–thorium dating

Uranium–thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique established in the 1960s which has been used since the 1970s to determine the age of calcium carbonate materials such as speleothem or coral. Unlike other commonly used radiometric dating techniques such as rubidium–strontium or uranium–lead dating, the uranium-thorium technique does not measure accumulation of a stable end-member decay product. Instead, it calculates an age from the degree to which secular equilibrium has been restored between the radioactive isotope thorium-230 and its radioactive parent uranium-234 within a sample.

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