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Thorium

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Thorium,  90Th
Small (3 cm) ampule with a tiny (5 mm) square of metal in it
General properties
Pronunciation /ˈθɔəriəm/
THAWR-ee-əm
Appearance silvery, often with black tarnish
Standard atomic weight (Ar, std) 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
Group, period group n/a, period 7
Block f-block
Element category   actinide
Electron configuration [Rn] 6d2 7s2
Electrons per shell
2, 8, 18, 32, 18, 10, 2
Physical properties
Phase (at STP) solid
Melting point 2023 K ​(1750 °C, ​3182 °F)
Boiling point 5061 K ​(4788 °C, ​8650 °F)
Density (near r.t.) 11.7 g/cm3
Heat of fusion 13.81 kJ/mol
Heat of vaporisation 514 kJ/mol
Molar heat capacity 26.230 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 4, 3, 2, 1 ​(a weakly basic oxide)
Electronegativity Pauling scale: 1.3
Ionisation energies
  • 1st: 587 kJ/mol
  • 2nd: 1110 kJ/mol
  • 3rd: 1930 kJ/mol
Atomic radius empirical: 179.8 pm
Covalent radius 206±6 pm
Color lines in a spectral range
Miscellanea
Crystal structure face-centred cubic (fcc)
Facecentredcubic crystal structure for thorium
Speed of sound thin rod 2490 m/s (at 20 °C)
Thermal expansion 11.0 µm/(m·K) (at 25 °C)
Thermal conductivity 54.0 W/(m·K)
Electrical resistivity 157 nΩ·m (at 0 °C)
Magnetic ordering paramagnetic[2]
Magnetic susceptibility 132.0·10−6 cm3/mol (293 K)[3]
Young's modulus 79 GPa
Shear modulus 31 GPa
Bulk modulus 54 GPa
Poisson ratio 0.27
Mohs hardness 3.0
Vickers hardness 295–685 MPa
Brinell hardness 390–1500 MPa
CAS Number 7440-29-1
History
Naming after Thor, the Norse god of thunder
Discovery Jöns 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
| references | in Wikidata

Thorium is a chemical element with symbol Th and atomic number 90. Thorium metal is silvery and tarnishes black when exposed to air, forming the 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, prone to ignition on air when finely divided.

Thorium is weakly radioactive: all of its known isotopes are unstable. The most stable isotope of thorium, 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. Thorium is one of only two significantly radioactive elements that still occur naturally in large quantities as a primordial element (the other being uranium).[a] It is estimated to be about three to four times more abundant than 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's uses were diminished or annulled by concerns over its radioactivity and the availability of non-radioactive replacements.

Formerly common uses of thorium include those as an alloying element in TIG welding electrodes, as a material in high-end optics and scientific instrumentation, and as the light source in gas mantles, but these have since become marginal uses. Thorium is predicted to be able to replace uranium as nuclear fuel in nuclear reactors, but only a few thorium reactors have been completed.

Bulk properties

Thorium is a moderately hard, 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; additionally, it has two other forms at exotic conditions, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).[4]

The properties of thorium vary widely depending on the amount of impurities in the sample: the major impurity is usually thorium dioxide (ThO2). 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 intermediate 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's melting point of 1750 °C is above both that of actinium (1227 °C) and that of protactinium (~1560 °C). In the beginning 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 smooth trend downward in the melting points from thorium to plutonium where the number of f electrons increases from about 0.4 to about 6, due to the itinerance of the f-orbitals, increasing hybridisation of the 5f and 6d orbitals, and the formation of directional bonds in the metal resulting in more complex crystal structures and weakened metallic bonding.[5][6] (The f-electron count for thorium is a non-integer due to a 5f–6d overlap.)[6] Among the actinides, thorium has the highest melting and boiling points and second-lowest density; actinium is lighter. Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points, behind osmium, tantalum, tungsten, and rhenium.[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). In comparison, that of aluminium is 75.2 GPa; copper 137.8 GPa; and mild steel 160–169 GPa.[7] Thorium's hardness is similar to that of soft steel, so heated pure thorium can be rolled into sheets and pulled into wire.[5] Thorium is nearly half as dense as uranium and plutonium, but is harder than either of them.[5] Thorium becomes superconductive below 1.4 K.[4]

Thorium can also form alloys with many other metals. Addition of small amounts 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. With chromium and uranium, it forms eutectic mixtures, and thorium 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 generally accepted age of the universe (about 13.8 billion years). As such, primordial 232Th still occurs: four-fifths of the thorium present at Earth's formation has survived to the present.[8][9][10] It is the only isotope of thorium occurring in significant quantities in nature today, and thus thorium is usually considered to be a mononuclidic element.[8] Its stability is attributed to its closed nuclear shell with 142 neutrons.[11][12] Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377(4). It is one of only three significantly radioactive elements (the others being protactinium and uranium) that occur in large enough quantities on Earth for this to be possible.[1]

Thorium nuclei are susceptible to alpha decay because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons.[13] The alpha decay of 232Th decay initiates the so-called 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 stable 208Pb.[8] Any sample of thorium or its compounds contains traces of these daughters, which are isotopes of thallium, lead, bismuth, polonium, radon, radium, and actinium.[8] As such, natural thorium samples can be chemically purified to extract useful daughter nuclides, such as 212Pb, which is used in nuclear medicine for cancer therapy.[14][15] 232Th also very occasionally undergoes spontaneous fission rather than alpha decay, and has left evidence in 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.[16][17]

Ball-and-arrow presentation of the thorium decay series
The 4n decay chain of 232Th, commonly called the "thorium series"

Thirty radioisotopes have been characterised, which range in mass number from 209[18] to 238.[16] The most stable of them (after 232Th) are 230Th with a half-life of 75,380 years, 229Th with a half-life of 7,340 years, 228Th with a half-life of 1.92 years, 234Th with a half-life of 24.10 days, and 227Th with a half-life of 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.[8]

In deep seawaters the isotope 230Th becomes more significant and can make 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.[16] The International Union of Pure and Applied Chemistry (IUPAC) reclassified thorium as a binuclidic element in 2013.[1]

Thorium has three known nuclear isomers (or metastable states), 216m1Th, 216m2Th, and 229mTh. 229mTh has the lowest known excitation energy of any isomer,[19] 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.[20][21][b]

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.[23] 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.[23][c] 232Th is not fissionable, but it is fertile as it can be converted to fissile 233U by neutron capture and subsequent beta decay.[23][24]

Radiometric dating

Two radiometric dating methods involve thorium isotopes: uranium–thorium dating, involving the decay of 234U to 230Th, and ionium–thorium dating, which measures the ratio of 232Th to 230Th. (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.) 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.[25] 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.[25][26] 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.[27][28] 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.[27][28]

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.[29] 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. Such unusual behaviour 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 result 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, for which +4 is the highest possible state but +3 also 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 properties such as 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.[30][31]

Crystal structure of fluorite
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.[6] 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.[32]

Tetravalent thorium compounds are usually colourless or yellow, like those of silver or lead, as the Th4+ ion has no 5f or 6d electrons.[5] 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.[33][d] 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.[32]

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.[34] 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 eventually occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.[4] Such samples slowly tarnish in air, 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][35] It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride or fluorosilicate ions;[4][36] if these are not present, passivation can occur, as with uranium and plutonium.[4][37]

Crystal structure of thorium tetrafluoride
Crystal structure of thorium tetrafluoride
Th4+: __  /  F: __

Inorganic compounds

Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.[38] In air, thorium burns to form ThO2, which has the fluorite structure.[39] Thorium dioxide is a refractory material, with the highest melting point (3390 °C) of any known oxide.[40] It is somewhat hygroscopic and reacts readily with water and many gases;[41] it dissolves easily in concentrated nitric acid in the presence of fluoride.[42] 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.[41] Several binary thorium chalcogenides and oxychalcogenides are also known with sulfur, selenium, and tellurium.[43]

All four thorium tetrahalides are known, as are some low-valent bromides and iodides:[44] the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.[45] Additionally, many related polyhalide ions are also known.[44] 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.[44] The other tetrahalides instead have dodecahedral geometry.[45] Lower iodides ThI3 (black) and ThI2 (gold) 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.[44] Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.[44] For example, when treated with potassium fluoride and hydrofluoric acid, Th4+ forms the complex anion ThF2−
6
, which precipitates as an insoluble salt, K2ThF6.[36]

Thorium borides, carbides, silicides, and nitrides are refractory materials, as are those of uranium and plutonium, and have thus received attention as possible nuclear fuels.[38] All four heavier pnictogens (phosphorus, arsenic, antimony, and bismuth) also form binary thorium compounds. Thorium germanides are also known.[46] 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.[47] The hydrides are thermally unstable and readily decompose upon exposure to air or moisture.[48]

Molecule structure of thorocene
Sandwich molecule structure of thorocene

Coordination compounds

In acidic aqueous solution, thorium occurs as the tetrapositive aqua ion [Th(H2O)9]4+, which has tricapped trigonal prismatic molecular geometry:[49][50] at pH < 3, the solutions of thorium salts are dominated by this cation.[49] 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.[49] 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 significantly just before the precipitation).[51] 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.[30]

Large 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.[51] Thorium salts are known for their high solubility in water and polar organic solvents.[5]

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.[52] 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.[52] 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.[52] Thorium complexes with organic ligands, such as oxalate, citrate, and EDTA, are much stronger and tend to occur naturally in natural thorium-containing waters in concentrations orders of magnitude higher than the inorganic complexes.[49]

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

Organothorium compounds

Most of the work on organothorium compounds has focused on the cyclopentadienyls and cyclooctatetraenyls. Like many of the early and middle actinides (up to americium, and also expected for curium), thorium forms the yellow cyclooctatetraenide complex Th(C8H8)2, thorocene. It is isotypic with the better-known analogous uranium compound uranocene.[53] 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.[53] It is unstable in air and decomposes in water or at 190 °C.[53] 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.[30]

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;[53][54] a formal +2 oxidation state occurs in a derivative.[55] 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.[54]

Other organothorium compounds are not well-studied. Tetrabenzylthorium, Th(CH2C6H5), and tetraallylthorium, Th(C3H5)4, are known, but their structures have not been determined and 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.[30]

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.[56] 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.[57] Because of the abrupt loss of stability past 209Bi, the r-process is the only process of stellar nucleosynthesis that can create isotopes of thorium and uranium, because all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.[56][58][59]

Histogram of estimated abundances of the 83 primordial elements in the Solar system
Estimated abundances of the 83 primordial elements in the Solar system. Thorium is one of the rarest elements.

In the universe, thorium is among the rarest of the primordial elements: it achieves this position not only because it is one of the two elements that can be produced only in the r-process, but also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are uranium, the only other element produced only in the r-process, as well as thulium, lutetium, tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals.[56][57][e] Furthermore, 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.[57] In the distant past the abundances of thorium and uranium were still being enriched by the decay of extinct plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of extinct 236U to 232Th and the natural depletion of 235U, but these sources have long since decayed and no longer contribute.[60]

On Earth, thorium is much more abundant: with an abundance of 8.1 parts per million (ppm) in the Earth's crust, it is one of the most abundant of the heavy elements, almost as abundant as lead (13 ppm) and significantly more abundant than tin (2.1 ppm).[61] This is because thorium is likely to form oxide minerals that do not sink into the core; as such, it is classified as a lithophile. Furthermore, thorium compounds are also poorly soluble in water. Thus, even though the whole of the Earth contains the same abundances of the elements as the Solar System as a whole, there is significantly more accessible thorium than there are accessible heavy platinum group metals in the crust alone.[62]

Heat produced by the decay of K-40, Th-232, U-235, U-238 within the Earth over time
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 be the most abundant as the other ones decayed faster than thorium.[63][64][65][66]

On Earth

Natural thorium is essentially isotopically pure 232Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe.[16] 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 size of contribution. (At the time of the Earth's formation, 40K and 235U contributed much more by virtue of their short half-lives, but by the same token they have also decayed more quickly, leaving the almost constant contribution from 232Th and 238U predominant.)[67] 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.[16][f]

On Earth, thorium is not a rare element as was previously thought, having a crustal abundance comparable to that of lead and molybdenum, twice that of arsenic, and thrice that of tin.[69] Thorium only occurs as a minor constituent of most minerals.[69] Soil normally contains about 6 ppm of thorium.[70]

In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), zirconium(IV), hafnium(IV), and cerium(IV), but also with scandium, yttrium, and the trivalent lanthanides which have similar ionic radii.[69] Because of thorium's radioactivity, minerals containing significant quantities of thorium are often metamict, their crystal structure having been partially or totally destroyed by the alpha radiation produced in the radioactive decay of thorium.[71] An extreme example is ekanite, (Ca,Fe,Pb)2(Th,U)Si8O20, which almost never occurs in nonmetamict form due to thorium being an essential part of its chemical composition.[72]

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% thorium.[69][73] Monazite is a chemically unreactive phosphate mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it.[69] Allanite (chiefly silicates-hydroxides of various metals) can have 0.1–2% thorium and zircon (chiefly zirconium silicate, ZrSiO4) up to 0.4% thorium.[69]

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.[69][g] Thorite (chiefly thorium silicate, ThSiO4), also has a high thorium content and is the mineral in which thorium was first discovered.[69] In thorium silicate minerals, the Th4+ and SiO4−
4
ions are often replaced with M3+ (M = Sc, Y, Ln) and phosphate (PO3−
4
) ions respectively.[69] Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released in significant quantities. The Th4+ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can reach 40 ppm.[74]

History

Thor raising his hammer in a battle against the giants
Tor's Fight with the Giants (1872) by Mårten Eskil Winge, depicting an artist's perception of Thor, the Norse god of thunder, raising his hammer in a battle against the giants.[75]

Erroneous report

In 1815, the Swedish chemist Jöns Jakob 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. (By that time, 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 simply zinc oxide.)[76] Berzelius privately named the supposed tentative element "thorium" in 1817[77] and its supposed oxide "thorina" after Thor, the Norse god of thunder.[78] In 1824, after more deposits of the same mineral in Vest-Agder, Norway, were discovered, he retracted his findings, as the mineral in question proved to actually be an yttrium mineral, primarily composed of yttrium orthophosphate.[24][76][79][80] As the yttrium in this mineral was initially mistaken as being a new element, the mineral was named kenotime by the French mineralogist François Sulpice Beudant as a rebuke of Berzelius, from the Greek words κενός (kenos; "empty") and τιμή (timē; "honour"). This became "xenotime" as a misprint from the beginning, blunting the criticism.[76][80][81] This misspelt form was later explained as being from ξένος (xenos; "alien") and τιμή, supposedly referencing the small, rare and easily overlooked crystals that xenotime occurs as.[82]

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 University of Oslo.[83] The elder Esmark determined that it was not any known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element.[24] He published his findings in 1829, having isolated an impure sample for the first time by reducing KThF5 with potassium metal.[84][85][86] Berzelius reused the name of the previous supposed element discovery[84][87] and named the source mineral thorite.[24]

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

Berzelius also made some initial characterisation 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 as such assumed that the atomic mass was 7.5 times that of oxygen (120 amu); it is actually 15 times as large.[h] He determined that thorium was a very electropositive metal, that he placed ahead of cerium and behind zirconium in electropositivity.[88] (Berzelius did not isolate the element in its metallic state; thorium was isolated for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger.)[i]

Initial chemical classification

In the periodic table published by Russian chemist 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 contained the modern carbon group (group 14), titanium group (group 4), cerium, and thorium, because their maximum oxidation state was +4.[91][92] Cerium was soon removed from the main body of the table and placed in a separate lanthanide series, while thorium remained with group 4 as it had similar properties to its supposed lighter congeners in that group, such as titanium and zirconium.[93][j]

First uses

Thorium was discovered in 1828; it had no applications until 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.[24] After that, many applications were found for thorium and its compounds, such as in ceramics, carbon arc lamps, heat-resistant crucibles, and as catalysts for industrial chemical reactions such as the oxidation of ammonia to nitric acid.

Gas mantle
World War II thorium dioxide gas mantle

Radioactivity

In the late 19th century onward, the atomic theory underwent significant improvements, which shaped the further history of thorium. Thorium was first observed to be radioactive in 1898, by the German chemist Gerhard Carl Schmidt and later that year, independently, 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.[94][95][96] Between 1900 and 1903, British physicists Ernest Rutherford and Frederick Soddy showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half-life as one of the outcomes of the alpha particle experiments that led to their disintegration theory of radioactivity.[97] The biological effect of radiation was discovered in 1903.[98] Nonetheless, the newly discovered effect of radioactivity was a thrill to scientists and general public alike. In the 1920s, thorium's radioactivity was promoted as a cure for rheumatism, diabetes, and sexual impotence among others. In 1932, most of these uses were banned in the United States after a federal investigation on health effects from radioactivity. Public interest in radioactivity had declined by the end of the 1930s.[99]

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

Further classification

Up to 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 1922, 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.[91] 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,[100] such that Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.[91]

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.[101] 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 and filling a fourth d-block row.[93][k]

Phasing out

Despite thorium's radioactivity, the element has remained in use for a long time for applications not exploiting the effect as no suitable alternatives could be found. A 1981 study by the Oak Ridge National Laboratory in the United States estimated that a dose from using a thorium mantle every weekend would be safe for a person,[104] but this was not the case for the dose received by people manufacturing the mantles (and thus contacting many) or for the soils around some factory sites.[105] A major shift occurred in the 1990s, when most of these applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements have been found.[24][106] Due to concerns, some manufacturers have switched to other materials, such as yttrium, although these are usually either more expensive or less efficient. Other manufacturers continued to make thorium mantles, but moved their factories to developing countries.[104] As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even claiming them to be non-radioactive while using significant quantities of thorium, up to 259 milligrams per mantle.[106][107]

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

Nuclear power

The United States explored the possibility to use 232Th as a source for 233U, which would be used in a nuclear bomb, in the wake of the Cold War; they fired a test bomb in 1955.[108] They soon recognised that a 233U-fired bomb would be a very potent weapon, but it bore few sustainable "technical advantages" over the contemporary method of uranium–plutonium bombs.[109] In particular, this nuclide is difficult to produce without introducing other isotopes of uranium as impurities; these impurities, as well as its decay products, complicate handling it and make it more easily recognizable.[108]

Usage of thorium as a power source has been explored; the earliest thorium-based reactor was made in the United States: the first core at the Indian Point Energy Center (Buchanan, New York, United States) in 1962.[110] India has one of the largest supplies of thorium in the world but does not have much uranium used elsewhere, and targeted in the 1950s at achieving energy independence for the country with their three-stage nuclear power programme.[111][112] On the other hand, in most countries, the progress stalled because uranium was relatively abundant and the progress of thorium-based reactors was therefore slow (in the 20th century, 3 reactors were opened in India and 12 elsewhere[113]). 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 on the matter. Nuclear scientist Alvin Radkowsky of Tel Aviv University in Israel, the head designer of the American first civilian nuclear power plant in Shippingport, Pennsylvania, whose third core bred thorium,[114] founded a consortium to develop thorium reactors, which included other laboratories: Raytheon Nuclear Inc. (Cambridge, Massachusetts, United States), Brookhaven National Laboratory (Upton, New York, United States) and the Kurchatov Institute (Moscow, Russia).[115] In the 21st century, thorium's potential for improving proliferation resistance and waste characteristics led to renewed interest in the thorium fuel cycle.[116][117][118]

Production

Lower-bound estimates of thorium reserves in thousand tonnes, 2014[108]
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

Worldwide production of thorium is low, at a few tens of tons per year.[60] Such low demands make working mines for extraction of thorium alone not profitable; as a result, it is almost always extracted with the rare earths, which themselves may be by-products of production of other minerals.[119] 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.[120] Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.[121]

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.[122]

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 a 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.[34] Initial concentration varies with the type of deposit.[122]

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 dissolution with by reaction with hydrogen chloride; then follow thickening, filtration, and calcination. The result is concentrate of a thorium with rare-earth content up to 90%.[122] Secondary materials (such as coastal sands) undergo gravity separation. Then follows magnetic separation with a series of magnets of increasing strength. Monazite obtained by this method can be as pure as 98%.[122]

Industrial production in the twentieth 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 heavily relied on the techique specifics 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 higher purity of thorium; in particular, it removes phosphates from the concentrate.[122]

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 as it is so dense, 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.[122]

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 consequently require finely grained monazite sand, with 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 then 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 then air may oxidise cerium from the +3 to the +4 oxidation state, and the cerium(IV) formed can then liberate free chlorine from the hydrochloric acid. The rare earths again precipitate out at higher pH. The precipitates are then neutralised by the original sodium hydroxide solution, although most of the phosphate must be removed beforehand 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.[122]

Purification

High thorium concentrations are needed in nuclear applications; particularly, 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 these high purities. Today, liquid solvent extraction procedures involving selective complexation of Th4+ is 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.[122]

Modern applications

Non-radioactivity-related uses have been on decline since the 1950s.[123] Many applications of thorium are becoming obsolete due to environmental concerns largely stemming from the radioactivity of thorium and its decay products. Thorium is thus being phased out of many of its uses.[24][106]

Most thorium applications use its dioxide (sometimes called "thoria" in the industry), rather than the metal. One particular characteristic of this compound is its high melting point, 3300 °C (6000 °F) – the highest of all known oxides; only a few substances have higher melting points.[40] In particular, that helps the compound remain solid when introduced into a flame, and when it is, it considerably increases the luminacy of the flame; this is the main reason thorium is used in gas mantles.[124] Although all substances emit energy (glow) when heated to high temperatures, light emitted by thorium is almost exclusively located in the visible spectrum, which explains the brightness of thorium mantles.[125] Energy, some of it in the form of the visible light, is emitted when thorium is exposed to a source of energy itself, such as a cathode ray, heat or ultraviolet light. Generally, this effect is shared by cerium dioxide, which converts ultraviolet light into visible light more efficiently, but thorium dioxide gives a higher temperature of the flame, emitting less infrared light.[124] Thorium in mantles, though still common, have been being replaced with a different rare-earth element, yttrium, since the late 1990s.[126] According to the 2005 review by the United Kingdom's National Radiological Protection Board, "although they were widely available a few years ago, they are not any more."[127]

Three lenses from yellowed to transparent left-to-right
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.[123] A small addition of thorium to tungsten thermocathodes considerably reduced the work function of electrons; as the result, electrons are emitted at considerably lower temperatures.[24] (Thorium forms a one-atom-thin layer of tungsten. The work function from a thorium surface is lowered possibly because of the electric field on the surface between thorium and tungsten formed due to thorium's greater electropositivity.)[128] 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 atmospherial oxygen and nitrogen, thorium also marks impurities in the evacuated tubes. The introduction of transistors in the 1950s significantly diminished this use, though not entirely.[123] Thorium dioxide is used in gas tungsten arc welding (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.[24] Because of safety concerns, thorium oxide is being replaced in this use with other oxides, such as those of zirconium, cerium, and lanthanum.[129][130]

Thorium dioxide is a material for heat-resistant ceramics, as used in high-temperature laboratory crucibles,[24] as a main material 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 likewise been replaced by an alloy of 95% platinum and 5% rhodium because of its better mechanical properties and greater durability.[123]

When added to glass, thorium dioxide helps increase refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments.[35] 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.[131] Yellowed lenses may be restored to their original colourless state with 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.[123]

Thorium tetrafluoride is used as an antireflection material in multilayered optical coatings. It has an optical transparency in the range of 0.35–12 µm, and its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.[132] Replacements for thorium tetrafluoride are being developed as of the 2010s.[133]

Potential use for nuclear energy

Thorium has been suggested as a potent nuclear power source and a possible replacement to the currently used uranium and plutonium. India, which has little uranium but much thorium, is a big proponent of development of thorium-based power technologies and has prioritised developing a thorium fuel cycle.[112]

The main nuclear power source in a reactor is the spontaneous fission of a certain nuclide; of the synthetic fissile[c] nuclei, 233U and 239Pu can be bred from neutron capture by the naturally occurring quantity nuclides 232Th and 238U (note that 235U occurs naturally and is also fissile).[134][135][l] 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:[24]

232
90
Th
+ n → 233
90
Th
+ γ β21.8 min 233
91
Pa
β27.0 days 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 hence can be used as a nuclear fuel in much the same way as the more-commonly used 235U or 239Pu. When 233U undergoes nuclear fission, the neutrons emitted can strike further 232Th nuclei, restarting the cycle.[24] This closely 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.[136]

Advantages

Thorium is more abundant than uranium and hence can satisfy world energy demands for longer.

232Th also absorbs neutrons more readily than 238U, and not only does 233U have a higher probability of fission upon neutron capture (92.0%) than 235U (85.5%) or 239Pu (73.5%),[137] it also releases more neutrons upon fission on average.[138] A single neutron capture by 238U would produce 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.[139]

Thorium fuels also result in a safer and better-performing reactor core[24] because thorium dioxide has a higher melting point, higher thermal conductivity, 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).[140]

Disadvantages

The used fuel is difficult and dangerous to reprocess because many of the daughters of 232Th and 233U are strong gamma emitters.[138] Additionally, all 233U production methods other than mercury fluorescence always result in significant impurities of the very dangerous 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:[141]

230
90
Th
+ n → 231
90
Th
+ γ β25.5 h 231
91
Pa
α3.28 × 104
y
­ 231
91
Pa
+ n → 232
91
Pa
+ γ β1.3 d 232
92
U

232U by itself is not particularly harmful, but it quickly decays to produce significant quantities of 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 essentially negligible.)[142] These impurities of 232U make 233U very easy to detect and very dangerous to work on, and the impracticality of their separation limits the possibilities of nuclear proliferation using 233U as the fissile material.[141] Additionally, 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.[143]

The need to irradiate 232Th with neutrons and process it come before these advantages become real, and this requires more advanced technology than the presently used fuels based on uranium and plutonium; advances are being made in this area.[24] Another common criticism centres around the low commercial viability of the thorium fuel cycle:[144][145][146] some entities like the Nuclear Energy Agency go further and predict that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which may persist "in the coming decades".[147] Furthermore, though the isotopes produced in the thorium fuel cycle are mostly not transuranic, some of them are still very dangerous, such as 231Pa, which has a long half-life of 32,760 years and is a major contributor to the long-term radiotoxicity of spent nuclear fuel.[143]

Hazards

Thorium mantle installed over a small sprout of grass
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, owning and handling small amounts of thorium, such as those in a gas mantle, is considered safe, although usage of such items may pose some risks.[148] 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.[148] Exposure to thorium internally leads to increased risk of liver diseases.[149]

The decay products of 232Th include more dangerous radionuclides such as radium and radon. Although relatively little of those products is created as the result of the faint 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,[150] and which are built up quickly following the initial decay of 232Th due to the absence of long-lived nuclides along the decay chain.[151] As the dangerous daughters of thorium have much lower melting points than thorium dioxide, they would be volatilised every time the mantle is heated for use. During burning, significant fractions of the thorium daughters 224Ra, 228Ra, 212Pb, and 212Bi are released in the first hour of use alone.[152] 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.[153]

Some nuclear safety agencies make recommendations about use of thorium mantles and have raised some 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.[149]

Biological

Thorium is odourless and tasteless.[154] The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water,[155] precipitating out before entering the body as the hydroxide.[156] (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.)[151] If thorium is ingested, 0.4% of it and 90% of its dangerous daughters are leached into the body.[107] Out of the thorium that does remain in the body, three quarters of it accumulates in the skeleton. Absorption through the skin is possible, but is not a likely means of thorium exposure.[148] Thorium's low solubility in water also means that excretion of thorium by the kidneys and faeces is rather slow.[151]

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.[156] 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.[157]

Chemical

Powdered thorium metal is pyrophoric and often ignites spontaneously in air.[4] In 1964, the United States Department of the Interior listed thorium as "severe" on the table titled "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.[158]

In 1956, reprocessing and burning of thorium sludge at the Sylvania Electric Products' Metallurgical Laboratory in New York City, New York, United States, resulted in a chemical explosion. Nine people were injured; one died of complications caused by third-degree burns.[159][160][161]

Exposure routes

Thorium exists in very small quantities everywhere on Earth: the average human contains about 100 micrograms of thorium and typically consumes three micrograms per day.[157] Most thorium exposure occurs through dust inhalation; some thorium comes with food and water, but because of its low solubility, this exposure is negligible.[151]

Exposure is raised for people who live near thorium deposits, 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 industries.[162] 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.[163]

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. ^ 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.[22]
  3. ^ 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 a mass of a ball of a material which could undergo a sustained nuclear chain reaction.
  4. ^ 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.[33]
  5. ^ 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.[8]
  6. ^ 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.[16] 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).[16] 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.[68] 229Th is mostly produced as a daughter of artificial 233U, itself produced from neutron irradiation of 232Th, due to its extreme rarity in nature.[16]
  7. ^ 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.[69]
  8. ^ At the time, the rare-earth elements, among which thorium was found and with which it is closely associated in the nature, were thought to be divalent; this is shown by the fact that the rare earths are there given atomic weight values two-thirds of their actual ones, and thorium and uranium are given values half of their actual ones.
  9. ^ 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.[89] Lely and Hamburger obtained 99% pure thorium metal by reducing thorium chloride with sodium metal.[90] 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.[90]
  10. ^ 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.[91]
  11. ^ 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.[102] Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group 4 element".[103]
  12. ^ 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 single neutron capture.[135]

References

  1. ^ a b c d Meija, J.; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)" (PDF). Pure and Applied Chemistry. 88 (3): 265–291. doi:10.1515/pac-2015-0305. 
  2. ^ Lide, D. R., ed. (2005). "Magnetic susceptibility of the elements and inorganic compounds". CRC Handbook of Chemistry and Physics (PDF) (86th ed.). CRC Press. p. 4-135. ISBN 978-0-8493-0486-6. 
  3. ^ Weast, R. (1984). CRC, Handbook of Chemistry and Physics. Chemical Rubber Company Publishing. p. E110. ISBN 978-0-8493-0464-4. 
  4. ^ a b c d e f g h i j k l m n o Wickleder, Fourest & Dorhout 2006, pp. 61–63.
  5. ^ a b c d e Tretyakov, Yu. D., ed. (2007). Non-organic chemistry in three volumes. Chemistry of transition elements. 3. Academy. ISBN 978-5-7695-2533-9. 
  6. ^ a b c Johansson, B.; Abuja, R.; Eriksson, O.; et al. (1995). "Anomalous fcc crystal structure of thorium metal.". Physical Review Letters. 75 (2): 280–283. Bibcode:1995PhRvL..75..280J. doi:10.1103/PhysRevLett.75.280. 
  7. ^ Gale, W. F.; Totemeier, T. C. (2003). [Thorium at Google Books Smithells Metals Reference Book] Check |url= value (help). Butterworth-Heinemann. pp. 15–2–15–3. ISBN 978-0-08-048096-1. 
  8. ^ a b c d e f Audi, G.; Bersillon, O.; Blachot, J.; et al. (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. Archived from the original (PDF) on 24 July 2013. 
  9. ^ De Laeter, J. R.; Böhlke, J. K.; De Bièvre, P.; Hidaka, H.; Peiser, H. S.; Rosman, K. J. R.; Taylor, P. D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)" (PDF). Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683. 
  10. ^ International Union of Pure and Applied Chemistry (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)" (PDF). Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. Retrieved 27 July 2017. 
  11. ^ Nagy, S. (2009). Radiochemistry and Nuclear Chemistry. 2. EOLSS Publications. p. 374. ISBN 978-1-84826-127-3. 
  12. ^ Griffin, H. C. (2010). "Natural Radioactive Decay Chains". In Vértes, A.; Nagy, S.; Klencsár, Z.; et al. Handbook of Nuclear Chemistry. Springer Science+Business Media. p. 668. ISBN 978-1-4419-0719-6. 
  13. ^ Beiser, A. (2003). "Nuclear Transformations". Concepts of Modern Physics (PDF) (6 ed.). McGraw-Hill Education. pp. 432–434. ISBN 978-0-07-244848-1. 
  14. ^ "AREVA Med launches production of lead-212 at new facility" (Press release). Areva. 2013. Retrieved 1 January 2017. 
  15. ^ "Mineral Yearbook 2012" (PDF). United States Geological Survey. Retrieved 30 September 2017. 
  16. ^ a b c d e f g h Wickleder, Fourest & Dorhout 2006, pp. 53–55.
  17. ^ Bonetti, R.; Chiesa, C.; Guglielmetti, A.; et al. (1995). "First observation of spontaneous fission and search for cluster decay of 232Th". Phys. Rev. C. 51 (5): 2530. Bibcode:1995PhRvC..51.2530B. doi:10.1103/PhysRevC.51.2530. 
  18. ^ Ikezoe, H.; Ikuta, T.; Hamada, S.; et al. (1996). "alpha decay of a new isotope of 209Th". Physical Review C. 54 (4): 2043–2046. Bibcode:1996PhRvC..54.2043I. doi:10.1103/PhysRevC.54.2043. 
  19. ^ Ruchowska, E.; Płóciennik, W. A.; Żylicz, J.; et al. (2006). "Nuclear structure of 229Th". Physical Review C. 73 (4): 044326. Bibcode:2006PhRvC..73d4326R. doi:10.1103/PhysRevC.73.044326. 
  20. ^ Beck, B. R.; Becker, J. A.; Beiersdorfer, P.; et al. (2007). "Energy splitting in the ground state doublet in the nucleus 229Th". Physical Review Letters. 98 (14): 142501. Bibcode:2007PhRvL..98n2501B. PMID 17501268. doi:10.1103/PhysRevLett.98.142501. 
  21. ^ von der Wense, L.; Seiferle, B.; Laatiaoui, M.; et al. (2016). "Direct detection of the 229Th nuclear clock transition". Nature. 533 (7601): 47–51. Bibcode:2016Natur.533...47V. doi:10.1038/nature17669. 
  22. ^ Feynman, R.; Leighton, R.; Sands, M. (1963). The Feynman Lectures on Physics. 1. Addison-Wesley. pp. 2–5. ISBN 978-0-201-02116-5. 
  23. ^ a b c "Evaluation of nuclear criticality safety data and limits for actinides in transport" (PDF). Institut de radioprotection et de sûreté nucléaire. p. 15. Archived from the original (PDF) on 10 July 2007. Retrieved 20 December 2010. 
  24. ^ a b c d e f g h i j k l m n Wickleder, Fourest & Dorhout 2006, pp. 52–53.
  25. ^ a b "3–6: Uranium Thorium Dating" (PDF). Institute for Structure and Nuclear Astrophysics, University of Notre Dame. Retrieved 7 October 2017. 
  26. ^ Davis, O. "Uranium-Thorium Dating". Department of Geosciences, University of Arizona. Archived from the original on 28 March 2017. Retrieved 7 October 2017. 
  27. ^ a b Rafferty, J. P. (2010), [Thorium, p. 150, at Google Books Geochronology, Dating, and Precambrian Time: The Beginning of the World As We Know It] Check |url= value (help), The Geologic History of Earth, Rosen Publishing, p. 150, ISBN 978-1-61530-125-6 
  28. ^ a b Vértes, A. (2010), Nagy, S.; Klencsár, Z.; Lovas, R. G.; et al., eds., [Thorium, p. 800, at Google Books Handbook of Nuclear Chemistry] Check |url= value (help), 5 (2nd ed.), Springer Science+Business Media, p. 800, ISBN 978-1-4419-0719-6 
  29. ^ Wickleder, Fourest & Dorhout 2006, pp. 59–60.
  30. ^ a b c d Cotton, S. (2006). Lanthanide and Actinide Chemistry. John Wiley & Sons. 
  31. ^ Martin, W. C.; Hagan, L.; Reader, J.; et al. (1974). "Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions" (PDF). J. Phys. Chem. Ref. Data. 3 (3): 771–779. Bibcode:1974JPCRD...3..771M. doi:10.1063/1.3253147. Archived from the original (PDF) on 4 March 2016. Retrieved 19 October 2013. 
  32. ^ a b Greenwood & Earnshaw 1997, p. 1262.
  33. ^ a b King, R. Bruce (1995). Inorganic Chemistry of Main Group Elements. Wiley-VCH. ISBN 978-0-471-18602-1. 
  34. ^ a b Stoll 2005, p. 6.
  35. ^ a b Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC Press. ISBN 978-0-8493-0485-9. 
  36. ^ a b Hyde, E. K. (1960). The radiochemistry of thorium (PDF). National Academy of Sciences. Retrieved 29 September 2017. 
  37. ^ Greenwood & Earnshaw 1997, p. 1264.
  38. ^ a b Greenwood & Earnshaw 1997, p. 1267.
  39. ^ Yamashita, Toshiyuki; Nitani, Noriko; Tsuji, Toshihide; Inagaki, Hironitsu (1997). "Thermal expansions of NpO2 and some other actinide dioxides". J. Nucl. Mat. 245 (1): 72–78. Bibcode:1997JNuM..245...72Y. doi:10.1016/S0022-3115(96)00750-7. 
  40. ^ a b Emsley, John (2001). Nature's Building Blocks (Hardcover, First ed.). Oxford University Press. p. 441. ISBN 0-19-850340-7. 
  41. ^ a b Wickleder, Fourest & Dorhout 2006, pp. 70–77.
  42. ^ Greenwood & Earnshaw 1997, p. 1269.
  43. ^ Wickleder, Fourest & Dorhout 2006, pp. 95–97.
  44. ^ a b c d e Wickleder, Fourest & Dorhout 2006, pp. 78–94.
  45. ^ a b Greenwood & Earnshaw 1997, p. 1271.
  46. ^ Wickleder, Fourest & Dorhout 2006, pp. 97–101.
  47. ^ Wickleder, Fourest & Dorhout 2006, pp. 64–66.
  48. ^ Greenwood & Earnshaw 1997, p. 127.
  49. ^ a b c d Wickleder, Fourest & Dorhout 2006, pp. 117–134.
  50. ^ Persson, I. (2010). "Hydrated metal ions in aqueous solution: How regular are their structures?" (PDF). Pure and Applied Chemistry. 82 (10): 1901–1917. doi:10.1351/PAC-CON-09-10-22. Retrieved 23 August 2014. (Registration required (help)). 
  51. ^ a b Greenwood & Earnshaw 1997, pp. 1275–1277.
  52. ^ a b c Wickleder, Fourest & Dorhout 2006, pp. 101–115.
  53. ^ a b c d Wickleder, Fourest & Dorhout 2006, pp. 116–117.
  54. ^ a b Greenwood & Earnshaw 1997, pp. 1278–1280.
  55. ^ Langeslay, Ryan R.; Fieser, Megan E.; Ziller, Joseph W.; Furche, Philip; Evans, William J. (2015). "Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C5H3(SiMe3)2]3Th}1− anion containing thorium in the formal +2 oxidation state". Chem. Sci. 6 (6): 517–521. doi:10.1039/C4SC03033H. Retrieved 16 July 2016. 
  56. ^ a b c Cameron, A. G. W. (1973). "Abundance of the Elements in the Solar System" (PDF). Space Science Reviews. 15: 121–146. Bibcode:1973SSRv...15..121C. doi:10.1007/BF00172440. Archived from the original (PDF) on 21 October 2011. 
  57. ^ a b c Roederer, I. U.; Kratz, K.-L.; Frebel, A.; et al. (2009). "The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy". The Astrophysical Journal. American Astronomical Society. 698 (2): 1963–1980. Bibcode:2009ApJ...698.1963R. arXiv:0904.3105Freely accessible. doi:10.1088/0004-637X/698/2/1963. Retrieved 18 July 2016. 
  58. ^ Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; et al. (1957). "Synthesis of the Elements in Stars" (PDF). Reviews of Modern Physics. 29 (4): 547. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. 
  59. ^ Clayton, D. D. (1968). Principles of Stellar Evolution and Nucleosynthesis. McGraw-Hill Education. pp. 577–591. ISBN 978-0-226-10953-4. 
  60. ^ a b Stoll 2005, p. 2.
  61. ^ Greenwood & Earnshaw 1997, p. 1294.
  62. ^ Albarède, F. (2003). [Thorium, p. 17, at Google Books Geochemistry: an introduction] Check |url= value (help). Cambridge University Press. p. 17. ISBN 978-0-521-89148-6. 
  63. ^ Trenn, T. J. (1978). "Thoruranium (U-236) as the extinct natural parent of thorium: The premature falsification of an essentially correct theory". Annals of Science. 35 (6): 581–597. doi:10.1080/00033797800200441. 
  64. ^ Diamond, H.; Friedman, A. M.; Gindler, J. E.; et al. (1956). "Possible Existence of Cm247 or Its Daughters in Nature". Physical Review. 105 (2): 679–680. Bibcode:1957PhRv..105..679D. doi:10.1103/PhysRev.105.679. 
  65. ^ Rao, M. N.; Gopalan, K. (1973). "Curium-248 in the Early Solar System". Nature. 245 (5424): 304–307. Bibcode:1973Natur.245..304R. doi:10.1038/245304a0. 
  66. ^ Rosenblatt, D. B. (1953). "Effects of a Primeval Endowment of U236". Physical Review. 91 (6): 1474–1475. Bibcode:1953PhRv...91.1474R. doi:10.1103/PhysRev.91.1474. 
  67. ^ Gando, A.; Gando, Y.; Ichimura, K.; et al. (2011). "Partial radiogenic heat model for Earth revealed by geoneutrino measurements". Nature Geoscience. 4 (9): 647–651. Bibcode:2011NatGe...4..647K. doi:10.1038/ngeo1205. 
  68. ^ Peppard, D. F.; Mason, G. W.; Gray, P. R.; et al. (1952). "Occurrence of the (4n + 1) Series in Nature". J. Am. Chem. Soc. 74 (23): 6081–6084. doi:10.1021/ja01143a074. 
  69. ^ a b c d e f g h i j Wickleder, Fourest & Dorhout 2006, pp. 55–56.
  70. ^ Agency for Toxic Substances and Disease Registry (2016). Thorium (PDF) (Report). Retrieved 30 September 2017. 
  71. ^ Woodhead, J. A. (1991). "The metamictization of zircon: Radiation dose-dependent structural characteristics" (PDF). American Mineralogist. 76: 74–82. 
  72. ^ Szymański, J. T. (1982). "A mineralogical study and crystal-structure determination of nonmetamict ekanite, ThCa2Si8O20" (PDF). Canadian Mineralogist. 20: 65–75. 
  73. ^ Greenwood & Earnshaw 1997, p. 1255.
  74. ^ Emsley, J. (2011). Nature's building blocks: an A-Z guide to the elements. Oxford University Press. p. 547. ISBN 978-0-19-960563-7. 
  75. ^ "Tor's Fight with the Giants". Google Arts & Culture. Retrieved 26 June 2016. 
  76. ^ a b c Fontani, M.; Costa, M.; Orna, V. (2014). [Thorium at Google Books The Lost Elements: The Periodic Table's Shadow Side] Check |url= value (help). Oxford University Press. p. 73. ISBN 978-0-19-938334-4. 
  77. ^ Ryabchikov, D. I.; Gol'braikh, E. K. (2013). The Analytical Chemistry of Thorium: International Series of Monographs on Analytical Chemistry. Elsevier. p. 1. ISBN 978-1-4831-5659-0. 
  78. ^ Thomson, T. (1831). A System of Chemistry of Inorganic Bodies. 1. Baldwin & Cradock and William Blackwood. p. 475. 
  79. ^ Berzelius, J. J. (1824). "Undersökning af några Mineralier. 1. Phosphorsyrad Ytterjord." [Examining some minerals. 1st phosphoric yttria.]. Kungliga Svenska Vetenskapsakademiens Handlingar (in Swedish). 2: 334–338. 
  80. ^ a b "Xenotime-(Y)". Mindat database. Retrieved 7 October 2017. 
  81. ^ Anthony, J. W.; Bideaux, R. A.; Bladh, K. W.; et al., eds. (2000). "Xenotime-(Y)". Handbook of Mineralogy (PDF). IV (Arsenates, Phosphates, Vanadates). Mineralogical Society of America. ISBN 978-0-9622097-3-4. 
  82. ^ Dana, J. D.; Brush, G. J. (1875). [Thorium at Google Books A System of Mineralogy: Descriptive Mineralogy, Comprising the Most Recent Discoveries] Check |url= value (help). John Wiley & Sons. p. 529. 
  83. ^ Selbekk, R. S. (2007). "Morten Thrane Esmark". Store norske leksikon (in Norwegian). Kunnskapsforlaget. Retrieved 16 May 2009. 
  84. ^ a b Weeks, M. E. (1932). "The discovery of the elements. XI. Some elements isolated with the aid of potassium and sodium: Zirconium, titanium, cerium, and thorium". Journal of Chemical Education. 9 (7): 1231. Bibcode:1932JChEd...9.1231W. doi:10.1021/ed009p1231. 
  85. ^ Berzelius, J. J. (1829). "Untersuchung eines neues Minerals und einer darin erhalten zuvor unbekannten Erde" [Investigation of a new mineral and of a previously unknown earth contained therein]. Annalen der Physik und Chemie (in German). 16 (7): 385–415. Bibcode:1829AnP....92..385B. doi:10.1002/andp.18290920702.  (modern citation: Annalen der Physik, vol. 92, no. 7, pp. 385–415).
  86. ^ Berzelius, J. J. (1829). "Undersökning af ett nytt mineral (Thorit), som innehåller en förut obekant jord" [Investigation of a new mineral (thorite), as contained in a previously unknown earth]. Kungliga Svenska Vetenskaps Akademiens Handlingar (in Swedish): 1–30. 
  87. ^ Schilling, J. (1902). "Die eigentlichen Thorit-Mineralien (Thorit und Orangit)" [The actual thoritic minerals (thorite and orangite)]. Zeitschrift für Angewandte Chemie (in German). 15 (37): 921–929. doi:10.1002/ange.19020153703. 
  88. ^ Leach, M. R. "The Internet Database of Periodic Tables: Berzelius' Electronegativity Table". Retrieved 16 July 2016. 
  89. ^ Nilson, L. F. (1882). "Über metallisches Thorium" [About metallic thorium]. Berichte der deutschen chemischen Gesellschaft (in German). 15 (2): 2537–2547. doi:10.1002/cber.188201502213. 
  90. ^ a b Meister, G. (1948). Production of Rarer Metals (PDF) (Report). United States Atomic Energy Commission. Retrieved 22 September 2017. 
  91. ^ a b c d Leach, M. R. "The INTERNET Database of Periodic Tables". Retrieved 14 May 2012. 
  92. ^ Jensen, William B. (2003). "The Place of Zinc, Cadmium, and Mercury in the Periodic Table" (PDF). Journal of Chemical Education. American Chemical Society. 80 (8): 952–961. Bibcode:2003JChEd..80..952J. doi:10.1021/ed080p952. 
  93. ^ a b Masterton, W. L.; Hurley, C. N.; Neth, E. J. Chemistry: Principles and reactions (7th ed.). Cengage Learning. p. 173. ISBN 978-1-111-42710-8. 
  94. ^ Curie, M. (1898). "Rayons émis par les composés de l'uranium et du thorium" [Rays emitted by compounds of uranium and thorium]. Comptes Rendus (in French). 126: 1101–1103. OL 24166254M. 
  95. ^ Schmidt, G. C. (1898). "Über die vom Thorium und den Thoriumverbindungen ausgehende Strahlung" [On the radiation emitted by thorium and thorium compounds]. Verhandlungen der Physikalischen Gesellschaft zu Berlin (Proceedings of the Physical Society in Berlin) (in German). 17: 14–16. 
  96. ^ Schmidt, G. C. (1898). "Über die von den Thorverbindungen und einigen anderen Substanzen ausgehende Strahlung" [On the radiation emitted by thorium compounds and some other substances]. Annalen der Physik und Chemie (in German). 65 (5): 141–151. Bibcode:1898AnP...301..141S. doi:10.1002/andp.18983010512.  (modern citation: Annalen der Physik, vol. 301, pages 141–151 (1898)).
  97. ^ Simmons, J. G. (1996). The Scientific 100: A Ranking of the Most Influential Scientists, Past and Present. Carol. p. 19. ISBN 978-0-8065-2139-8. 
  98. ^ Fröman, N. (1996). "Marie and Pierre Curie and the Discovery of Polonium and Radium". nobelprize.org. Nobel Media AB. Retrieved 11 May 2017. 
  99. ^ Burns, M. (1987). [Thorium, p. 24, at Google Books Low-Level Radioactive Waste Regulation-Science, Politics and Fear] Check |url= value (help). CRC Press. pp. 24–25. ISBN 978-0-87371-026-8. 
  100. ^ van Spronsen, J. W. (1969). The periodic system of chemical elements. Elsevier. pp. 315–316. ISBN 978-0-444-40776-4. .
  101. ^ Rhodes, R. (2012). The Making of the Atomic Bomb (25th Anniversary ed.). Simon & Schuster. pp. 221–222, 349. ISBN 978-1-4516-7761-4. 
  102. ^ Türler, A.; Buklanov, G. V.; Eichler, B.; et al. (1998). "Evidence for relativistic effects in the chemistry of element 104". Journal of Alloys and Compounds. 271–273: 287. doi:10.1016/S0925-8388(98)00072-3. 
  103. ^ Kratz, J. V.; Nagame, Y. (2014). "Liquid-Phase Chemistry of Superheavy Elements". In Schädel, M.; Shaughnessy, D. The Chemistry of Superheavy Elements (2nd ed.). Springer-Verlag. p. 335. ISBN 978-3-642-37465-4. doi:10.1007/978-3-642-37466-1. 
  104. ^ a b Adams, C. (2003). "The Straight Dope: Are camp lanterns radioactive?". www.straightdope.com. Retrieved 7 October 2017. 
  105. ^ New Jersey Department of Health (1996). "Health and hazardous waste" (PDF). A Practitioner's Guide to Patients' Environmental Exposures. 1 (3): 1–8. Archived from the original (PDF) on 15 April 2016. 
  106. ^ a b c Furuta, E.; Yoshizawa, Y.; Aburai, T. (2000). "Comparisons between radioactive and non-radioactive gas lantern mantles". J. Radiol. Prot. 20 (4): 423–431. Bibcode:2000JRP....20..423F. PMID 11140713. doi:10.1088/0952-4746/20/4/305. 
  107. ^ a b Poljanc, K.; Steinhauser, G.; Sterba, J. H.; et al. (2007). "Beyond low-level activity: on a "non-radioactive" gas mantle". Sci. Total. Environ. 374 (1): 36–42. PMID 17270253. doi:10.1016/j.scitotenv.2006.11.024. 
  108. ^ a b c "Thorium". World Nuclear Association. 2017. Retrieved 21 June 2017. 
  109. ^ Woods, W. K. (1966). "LRL Interest in U-233". Battelle Memorial Institute. doi:10.2172/79078. 
  110. ^ Kazimi, M. (2003). "Thorium Fuel for Nuclear Energy". American Scientist. Archived from the original on 1 January 2017. Retrieved 29 September 2017. 
  111. ^ Majumdar, S.; Purushotham, D. S. C. (1999). "Experience of thorium fuel development in India". Thorium fuel utilization: Options and trends (PDF) (Report). International Atomic Energy Agency. Retrieved 7 October 2017. 
  112. ^ a b "Nuclear Power in India". World Nuclear Association. 2017. Retrieved 29 September 2017. 
  113. ^ "IAEA-TECDOC-1450 Thorium Fuel Cycle-Potential Benefits and Challenges" (PDF). International Atomic Energy Agency. 2005. Retrieved 23 March 2009. 
  114. ^ Shippingport Atomic Power Station. "Historic Achievement Recognized: Shippingport Atomic Power Station, A National Engineering Historical Landmark" (PDF). p. 4. Archived from the original (PDF) on 17 July 2015. Retrieved 24 June 2006. 
  115. ^ Educational Foundation for Nuclear Science, Inc. (1997). [Thorium, p. 19, at Google Books Bulletin of the Atomic Scientists] Check |url= value (help). Educational Foundation for Nuclear Science, Inc. pp. 19–20. ISSN 0096-3402. 
  116. ^ "IAEA-TECDOC-1349 Potential of thorium-based fuel cycles to constrain plutonium and to reduce the long-lived waste toxicity" (PDF). International Atomic Energy Agency. 2002. Retrieved 24 March 2009. 
  117. ^ Evans, B. (2006). "Scientist urges switch to thorium". ABC News. Archived from the original on 28 March 2010. Retrieved 17 September 2011. 
  118. ^ Martin, R. (2009). "Uranium is So Last Century — Enter Thorium, the New Green Nuke". Wired. Retrieved 19 June 2010. 
  119. ^ Stoll 2005, p. 7.
  120. ^ United States Geological Survey (2012). "Thorium" (PDF). Retrieved 12 May 2017. 
  121. ^ Jayaram, K. M. V. (1987). An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future (PDF) (Report). Department of Atomic Energy. Archived from the original (PDF) on 28 June 2011. 
  122. ^ a b c d e f g h Stoll 2005, p. 8.
  123. ^ a b c d e Stoll 2005, p. 32.
  124. ^ a b Stoll 2005, p. 31.
  125. ^ Ivey, H. F. (1974). "Candoluminescence and radical-excited luminescence". Journal of Luminescence. 8 (4): 271–307. Bibcode:1974JLum....8..271I. doi:10.1016/0022-2313(74)90001-5. 
  126. ^ Matson, Tim (2011). [Thorium, p. 60, at Google Books The Book of Non-electric Lighting: The Classic Guide to the Safe Use of Candles, Fuel Lamps, Lanterns, Gaslights & Fire-View Stoves] Check |url= value (help). Countryman Press. p. 60. ISBN 978-1-58157-829-4. 
  127. ^ Shaw, J.; Dunderdale, J.; Paynter, R. A. "A Review of Consumer Products Containing Radioactive Substances in the European Union" (PDF). NRPB Occupational Services Department. 
  128. ^ Pridham, G. J. (2016). Electronic Devices and Circuits: The Commonwealth and International Library: Electrical Engineering Division. Elsevier. p. 105. ISBN 978-1-4831-3979-1. 
  129. ^ Uttrachi, J. (2015). Weld Like a Pro: Beginning to Advanced Techniques. CarTech Inc. p. 42. ISBN 978-1-61325-221-5. 
  130. ^ Jeffus, L. (2016). Welding: Principles and Applications. Cengage Learning. p. 393. ISBN 978-1-305-49469-5. 
  131. ^ Oak Ridge Associated Universities (1999). "Thoriated Camera Lens (ca. 1970s)". Retrieved 29 September 2017. 
  132. ^ Rancourt, J. D. (1996). [Thorium, p. 196, at Google Books Optical thin films: user handbook] Check |url= value (help). SPIE Press. p. 196. ISBN 978-0-8194-2285-9. 
  133. ^ Kaiser, N.; Pulker, H. K. (2013). [Thorium, p. 111, at Google Books Optical Interference Coatings] Check |url= value (help). Springer. p. 111. ISBN 978-3-540-36386-6. 
  134. ^ Ronen, Y. (2006). "A rule for determining fissile isotopes". Nuclear Science and Engineering. 152 (3): 334–335. ISSN 0029-5639. doi:10.13182/nse06-a2588. 
  135. ^ a b Ronen, Y. (2010). "Some remarks on the fissile isotopes". Annals of Nuclear Energy. 37 (12): 1783–1784. doi:10.1016/j.anucene.2010.07.006. 
  136. ^ "Plutonium". World Nuclear Association. 2017. Retrieved 29 September 2017. 
  137. ^ "Interactive Chart of Nuclides". Brookhaven National Laboratory. Retrieved 12 August 2013. 
  138. ^ a b Greenwood & Earnshaw 1997, p. 1259.
  139. ^ "Thorium test begins". World Nuclear News. 2013. Retrieved 21 July 2013. 
  140. ^ "IAEA-TECDOC-1450 Thorium Fuel Cycle-Potential Benefits and Challenges" (PDF). International Atomic Energy Agency. 2005. Retrieved 23 March 2009. 
  141. ^ a b Langford, R. E. (2004). Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological. John Wiley & Sons. p. 85. ASIN 0471465607. ISBN 978-0-471-46560-7. 
  142. ^ Stoll 2005, p. 30.
  143. ^ a b Nakajima, Ts.; Groult, H. (2005). [Thorium, p. 562, at Google Books Fluorinated Materials for Energy Conversion] Check |url= value (help). Elsevier. pp. 562–565. ISBN 978-0-08-044472-7. 
  144. ^ Rees, E. (2011). "Don't believe the spin on thorium being a greener nuclear option". The Guardian. Retrieved 29 September 2017. 
  145. ^ Sovacool, B. K.; Valentine, S. V. (2012). The National Politics of Nuclear Power: Economics, Security, and Governance. Routledge. p. 226. ISBN 978-1-136-29437-2. 
  146. ^ "Nuclear Energy FAQs" (PDF). Argonne National Laboratory. 2014. 
  147. ^ Findlay, T. (2011). Nuclear Energy and Global Governance: Ensuring Safety, Security and Non-proliferation. Routledge. p. 9. ISBN 978-1-136-84993-0. 
  148. ^ a b c "Thorium: Radiation Protection". United States Environmental Protection Agency. Archived from the original on 1 October 2006. Retrieved 27 February 2016. 
  149. ^ a b "Radioactivity in Lantern Mantles". Australian Radiation Protection and Nuclear Safety Agency. Archived from the original on 14 October 2007. Retrieved 29 September 2017. 
  150. ^ "Natural Decay Series: Uranium, Radium, and Thorium" (PDF). Argonne National Laboratory. 2005. Archived from the original (PDF) on 17 August 2016. Retrieved 30 September 2017. 
  151. ^ a b c d Stoll 2005, p. 35.
  152. ^ Luetzelschwab, J. W.; Googins, S. W. (1984). "Radioactivity released from burning gas lantern mantles". Health Phys. 46 (4): 873–881. PMID 6706595. 
  153. ^ Huyskens, C. J.; Hemelaar, J. T.; Kicken, P. J. (1985). "Dose estimates for exposure to radioactivity in gas mantles". Sci. Total Environ. 45: 157–164. PMID 4081711. 
  154. ^ "Toxicological Profile for Thorium" (PDF). Agency for Toxic Substances and Disease Registry U.S. Public Health Service. 1990. p. 4. 
  155. ^ Merkel, B.; Dudel, G.; et al. (1988). Untersuchungen zur radiologischen Emission des Uran-Tailings Schneckenstein (PDF) (Report) (in German). Sächsisches Staatsministerium für Umwelt und Landesentwicklung. Archived from the original (PDF) on 8 January 2013. 
  156. ^ a b Stoll 2005, p. 34.
  157. ^ a b Emsley, J. (2011). Nature's building blocks: an A-Z guide to the elements. Oxford University Press. pp. 544–545. ISBN 978-0-19-960563-7. 
  158. ^ Jacobson, M.; Cooper, A. R.; Nagy, J. (1964). Explosibility of metal powders (PDF) (Report). United States Department of the Interior. Retrieved 29 September 2017. 
  159. ^ Associated Press (1956). "Nine Injured In Atomic Lab Blasts". Pittsburgh Post-Gazette. p. 2. Retrieved 29 September 2017. 
  160. ^ Associated Press (1956). "No Radiation Threat Seen In A-laboratory Blast". St. Petersburg Times. p. 2. Retrieved 29 September 2017. 
  161. ^ Harrington, M. (2003). "Sad Memories of '56 Sylvania Explosion". New York Newsday. Archived from the original on 4 February 2012. Retrieved 29 September 2017. 
  162. ^ "Thorium ToxFAQs" (PDF). Agency for Toxic Substances and Disease Registry. Retrieved 29 September 2017. 
  163. ^ "Compendium Of Policy And Statutory Provisions Relating To Exploitation Of Beach Sand Minerals". Department of Atomic Energy. Archived from the original on 4 December 2008. Retrieved 19 December 2008. 

Bibliography

Further reading

  • Jordan, B. W.; Eggert, R.; Dixon, B.; et al. (2014). "Thorium: Does Crustal Abundance Lead to Economic Availability?" (PDF). Colorado School of Mines. Archived from the original (PDF) on 30 June 2017. Retrieved 29 September 2017. 
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