Thorium

Two radiometric dating methods involve thorium isotopes: uranium–thorium dating, based on the decay of ²³⁴U to ²³⁰Th, and ionium–thorium dating, which measures the ratio of ²³²Th to ²³⁰Th.[lower-alpha 5] 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.[38] 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.[38][39] 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.[40][41] 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.[40][41]

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.[48] Finely divided thorium metal can exhibit pyrophoricity, spontaneously igniting in air.[10] When heated in air, thorium turnings ignite and burn with a brilliant white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.[10] Such samples slowly tarnish, becoming grey and finally black at the surface.[10]

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.[10][49] It dissolves in concentrated nitric acid containing a small quantity of catalytic fluoride or fluorosilicate ions;[10][50] if these are not present, passivation by the nitrate can occur, as with uranium and plutonium.[10][51][52]

Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.[53] In air, thorium burns to form ThO₂, which has the fluorite structure.[54] Thorium dioxide is a refractory material, with the highest melting point (3390 °C) of any known oxide.[55] It is somewhat hygroscopic and reacts readily with water and many gases;[56] it dissolves easily in concentrated nitric acid in the presence of fluoride.[57]

When heated in air, thorium dioxide emits intense blue light; 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.[56] A flame is not necessary for this effect: in 1901, it was discovered that a hot Welsbach gas mantle (using ThO₂ with 1% CeO₂) remained at "full glow" when exposed to a cold unignited mixture of flammable gas and air.[58] The light emitted by thorium dioxide is higher in wavelength than the blackbody emission expected from incandescence at the same temperature, an effect called candoluminescence. It occurs because ThO₂ : Ce acts as a catalyst for the recombination of free radicals that appear in high concentration in a flame, whose deexcitation releases large amounts of energy. The addition of 1% cerium dioxide, as in gas mantles, heightens the effect by increasing emissivity in the visible region of the spectrum; but because cerium, unlike thorium, can occur in multiple oxidation states, its charge and hence visible emissivity will depend on the region on the flame it is found in (as such regions vary in their chemical composition and hence how oxidising or reducing they are).[58]

Several binary thorium chalcogenides and oxychalcogenides are also known with sulfur, selenium, and tellurium.[59]

All four thorium tetrahalides are known, as are some low-valent bromides and iodides:[60] the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.[61] Many related polyhalide ions are also known.[60] 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.[60] The other tetrahalides instead have dodecahedral geometry.[61] Lower iodides ThI₃ (black) and ThI₂ (gold-coloured) can also be prepared by reducing the tetraiodide with thorium metal: they do not contain Th(III) and Th(II), but instead contain Th⁴⁺ and could be more clearly formulated as electride compounds.[60] Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.[60] For example, when treated with potassium fluoride and hydrofluoric acid, Th⁴⁺ forms the complex anion [ThF₆]²⁻ (hexafluorothorate(IV)), which precipitates as an insoluble salt, K₂[ThF₆] (potassium hexafluorothorate(IV)).[50]

Thorium borides, carbides, silicides, and nitrides are refractory materials, like those of uranium and plutonium, and have thus received attention as possible nuclear fuels.[53] All four heavier pnictogens (phosphorus, arsenic, antimony, and bismuth) also form binary thorium compounds. Thorium germanides are also known.[62] 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.[63] The hydrides are thermally unstable and readily decompose upon exposure to air or moisture.[64]

In an acidic aqueous solution, thorium occurs as the tetrapositive aqua ion [Th(H₂O)₉]⁴⁺, which has tricapped trigonal prismatic molecular geometry:[65][66] at pH < 3, the solutions of thorium salts are dominated by this cation.[65] 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.[65] It is quite acidic due to its high charge, slightly stronger than sulfurous acid: thus it tends to undergo hydrolysis and polymerisation (though to a lesser extent than Fe³⁺), predominantly to [Th₂(OH)₂]⁶⁺ in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide Th(OH)₄ forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation).[67] 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.[43]

High coordination numbers are the rule for thorium due to its large size. Thorium nitrate pentahydrate was the first known example of coordination number 11, the oxalate tetrahydrate has coordination number 10, and the borohydride (first prepared in the Manhattan Project) has coordination number 14.[67] These thorium salts are known for their high solubility in water and polar organic solvents.[12]

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.[68] They are important in thorium purification and the disposal of nuclear waste, but most of them have not yet been fully characterized, especially regarding their structural properties.[68] 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.[68] Thorium complexes with organic ligands, such as oxalate, citrate, and EDTA, are much more stable. In natural thorium-containing waters, organic thorium complexes usually occur in concentrations orders of magnitude higher than the inorganic complexes, even when the concentrations of inorganic ligands are much greater than those of organic ligands.[65]

In January 2021, the aromaticity has been observed in a large metal cluster anion consisting of 12 bismuth atoms stabilised by a center thorium cation.[69] This compound was shown to be surprisingly stable, unlike many previous known aromatic metal clusters.

Most of the work on organothorium compounds has focused on the cyclopentadienyl complexes and cyclooctatetraenyls. Like many of the early and middle actinides (up to americium, and also expected for curium), thorium forms a cyclooctatetraenide complex: the yellow Th(C₈H₈)₂, thorocene. It is isotypic with the better-known analogous uranium compound uranocene.[70] 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.[70] It is unstable in air and decomposes in water or at 190 °C.[70] 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.[43]

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;[70][71] a formal +2 oxidation state occurs in a derivative.[72] The chloride derivative [Th(C₅H₅)₃Cl] is prepared by heating thorium tetrachloride with limiting KC₅H₅ 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.[71]

Other organothorium compounds are not well-studied. Tetrabenzylthorium, Th(CH₂C₆H₅)₄, and tetraallylthorium, Th(CH₂CH=CH₂)₄, are known, but their structures have not been determined. They decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion [Th(CH₃)₇]³⁻, heptamethylthorate(IV), which forms the salt [Li(tmeda)]₃[Th(CH₃)₇] (tmeda = (CH₃)₂NCH₂CH₂N(CH₃)₂). 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.[43]

²³²Th is a primordial nuclide, having existed in its current form for over ten billion years; it was formed during the r-process, which probably occurs in supernovae and neutron star mergers. These violent events scattered it across the galaxy.[73][74] 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.[75] Because of the abrupt loss of stability past ²⁰⁹Bi, the r-process is the only process of stellar nucleosynthesis that can create thorium and uranium; all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.[73][76][77]

In the universe, thorium is among the rarest of the primordial elements at rank 77th in cosmic abundance[73][78] because it is one of the two elements that can be produced only in the r-process (the other being uranium), and also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are thulium, lutetium, tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals, as well as uranium.[73][75][lower-alpha 7] In the distant past the abundances of thorium and uranium were enriched by the decay of plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of ²³⁶U to ²³²Th and the natural depletion of ²³⁵U, but these sources have long since decayed and no longer contribute.[79]

In the Earth's crust, thorium is much more abundant: with an abundance of 8.1 g/tonne, it is one of the most abundant of the heavy elements, almost as abundant as lead (13 g/tonne) and more abundant than tin (2.1 g/tonne).[80] This is because thorium is likely to form oxide minerals that do not sink into the core; it is classified as a lithophile under the Goldschmidt classification, meaning that it is generally found combined with oxygen. Common thorium compounds are also poorly soluble in water. Thus, even though the refractory elements have the same relative abundances in the Earth as in the Solar System as a whole, there is more accessible thorium than heavy platinum group metals in the crust.[81]

Natural thorium is usually almost pure ²³²Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe.[26] Its radioactive decay is the largest single contributor to the Earth's internal heat; the other major contributors are the shorter-lived primordial radionuclides, which are ²³⁸U, ⁴⁰K, and ²³⁵U in descending order of their contribution. (At the time of the Earth's formation, ⁴⁰K and ²³⁵U contributed much more by virtue of their short half-lives, but they have decayed more quickly, leaving the contribution from ²³²Th and ²³⁸U predominant.)[86] Its decay accounts for a gradual decrease of thorium content of the Earth: the planet currently has around 85% of the amount present at the formation of the Earth.[55] 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.[26][lower-alpha 8]

Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare.[88][89] In fact, it is the 37th most abundant element in the Earth’s crust with an abundance of 12 parts per million.[90] In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), zirconium(IV), hafnium(IV), and cerium(IV), and also with scandium, yttrium, and the trivalent lanthanides which have similar ionic radii.[88] Because of thorium's radioactivity, minerals containing it are often metamict (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium.[91] An extreme example is ekanite, (Ca,Fe,Pb)₂(Th,U)Si₈O₂₀, which almost never occurs in nonmetamict form due to the thorium it contains.[92]

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%.[88][93] Monazite is a chemically unreactive mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it.[88] Allanite (chiefly silicates-hydroxides of various metals) can have 0.1–2% thorium and zircon (chiefly zirconium silicate, ZrSiO₄) up to 0.4% thorium.[88]

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.[88][lower-alpha 9] Thorite (chiefly thorium silicate, ThSiO₄), also has a high thorium content and is the mineral in which thorium was first discovered.[88] In thorium silicate minerals, the Th⁴⁺ and SiO4−4 ions are often replaced with M³⁺ (where M = Sc, Y, or Ln) and phosphate (PO3−4) ions respectively.[88] Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released. The Th⁴⁺ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can be higher.[55]

In 1815, the Swedish chemist Jöns Jacob Berzelius analysed an unusual sample of gadolinite from a copper mine in Falun, central Sweden. He noted impregnated traces of a white mineral, which he cautiously assumed to be an earth (oxide in modern chemical nomenclature) of an unknown element. Berzelius had already discovered two elements, cerium and selenium, but he had made a public mistake once, announcing a new element, gahnium, that turned out to be zinc oxide.[95] Berzelius privately named the putative element "thorium" in 1817[96] and its supposed oxide "thorina" after Thor, the Norse god of thunder.[97] In 1824, after more deposits of the same mineral in Vest-Agder, Norway, were discovered, he retracted his findings, as the mineral (later named xenotime) proved to be mostly yttrium orthophosphate.[37][95][98][99]

In 1828, Morten Thrane Esmark found a black mineral on Løvøya island, Telemark county, Norway. He was a Norwegian priest and amateur mineralogist who studied the minerals in Telemark, where he served as vicar. He commonly sent the most interesting specimens, such as this one, to his father, Jens Esmark, a noted mineralogist and professor of mineralogy and geology at the Royal Frederick University in Christiania (today called Oslo).[100] The elder Esmark determined that it was not a known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element.[37] He published his findings in 1829, having isolated an impure sample by reducing K[ThF₅] (potassium pentafluorothorate(IV)) with potassium metal.[101][102][103] Berzelius reused the name of the previous supposed element discovery[101][104] and named the source mineral thorite.[37]

Berzelius made some initial characterizations of the new metal and its chemical compounds: he correctly determined that the thorium–oxygen mass ratio of thorium oxide was 7.5 (its actual value is close to that, ~7.3), but he assumed the new element was divalent rather than tetravalent, and so calculated that the atomic mass was 7.5 times that of oxygen (120 amu); it is actually 15 times as large.[lower-alpha 10] He determined that thorium was a very electropositive metal, ahead of cerium and behind zirconium in electropositivity.[105] Metallic thorium was isolated for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger.[lower-alpha 11]

In the periodic table published by Dmitri Mendeleev in 1869, thorium and the rare-earth elements were placed outside the main body of the table, at the end of each vertical period after the alkaline earth metals. This reflected the belief at that time that thorium and the rare-earth metals were divalent. With the later recognition that the rare earths were mostly trivalent and thorium was tetravalent, Mendeleev moved cerium and thorium to group IV in 1871, which also contained the modern carbon group (group 14) and titanium group (group 4), because their maximum oxidation state was +4.[108][109] Cerium was soon removed from the main body of the table and placed in a separate lanthanide series; thorium was left with group 4 as it had similar properties to its supposed lighter congeners in that group, such as titanium and zirconium.[110][lower-alpha 12]

While thorium was discovered in 1828 its first application dates only from 1885, when Austrian chemist Carl Auer von Welsbach invented the gas mantle, a portable source of light which produces light from the incandescence of thorium oxide when heated by burning gaseous fuels.[37] Many applications were subsequently found for thorium and its compounds, including ceramics, carbon arc lamps, heat-resistant crucibles, and as catalysts for industrial chemical reactions such as the oxidation of ammonia to nitric acid.[111]

Thorium was first observed to be radioactive in 1898, by the German chemist Gerhard Carl Schmidt and later that year, independently, by the Polish-French physicist Marie Curie. It was the second element that was found to be radioactive, after the 1896 discovery of radioactivity in uranium by French physicist Henri Becquerel.[112][113][114] Starting from 1899, the New Zealand physicist Ernest Rutherford and the American electrical engineer Robert Bowie Owens studied the radiation from thorium; initial observations showed that it varied significantly. It was determined that these variations came from a short-lived gaseous daughter of thorium, which they found to be a new element. This element is now named radon, the only one of the rare radioelements to be discovered in nature as a daughter of thorium rather than uranium.[115]

After accounting for the contribution of radon, Rutherford, now working with the British physicist Frederick Soddy, showed how thorium decayed at a fixed rate over time into a series of other elements in work dating from 1900 to 1903. This observation led to the identification of the half-life as one of the outcomes of the alpha particle experiments that led to the disintegration theory of radioactivity.[116] The biological effect of radiation was discovered in 1903.[117] The newly discovered phenomenon of radioactivity excited scientists and the general public alike. In the 1920s, thorium's radioactivity was promoted as a cure for rheumatism, diabetes, and sexual impotence. In 1932, most of these uses were banned in the United States after a federal investigation into the health effects of radioactivity.[118] 10,000 individuals in the United States had been injected with thorium during X-ray diagnosis; they were later found to suffer health issues such as leukaemia and abnormal chromosomes.[55] Public interest in radioactivity had declined by the end of the 1930s.[118]

Up to the late 19th century, chemists unanimously agreed that thorium and uranium were the heaviest members of group 4 and group 6 respectively; the existence of the lanthanides in the sixth row was considered to be a one-off fluke. In 1892, British chemist Henry Bassett postulated a second extra-long periodic table row to accommodate known and undiscovered elements, considering thorium and uranium to be analogous to the lanthanides. In 1913, Danish physicist Niels Bohr published a theoretical model of the atom and its electron orbitals, which soon gathered wide acceptance. The model indicated that the seventh row of the periodic table should also have f-shells filling before the d-shells that were filled in the transition elements, like the sixth row with the lanthanides preceding the 5d transition metals.[108] 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;[119] Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.[108]

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.[120] In 1945, when American physicist Glenn T. Seaborg and his team had discovered the transuranic elements americium and curium, he proposed the actinide concept, realising that thorium was the second member of an f-block actinide series analogous to the lanthanides, instead of being the heavier congener of hafnium in a fourth d-block row.[110][lower-alpha 13]

In the 1990s, most applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements were found.[37][123] Despite its radioactivity, the element has remained in use for applications where no suitable alternatives could be found. A 1981 study by the Oak Ridge National Laboratory in the United States estimated that using a thorium gas mantle every weekend would be safe for a person,[123] but this was not the case for the dose received by people manufacturing the mantles or for the soils around some factory sites.[124] Some manufacturers have changed to other materials, such as yttrium.[125] As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even falsely claiming them to be non-radioactive.[123][126]

Thorium has been used as a power source on a prototype scale. The earliest thorium-based reactor was built at the Indian Point Energy Center located in Buchanan, New York, United States in 1962.[127] China may be the first to have a shot at commercializing the technology.[128] The country with the largest estimated reserves of thorium in the world is India, which has sparse reserves of uranium. In the 1950s, India targeted achieving energy independence with their three-stage nuclear power programme.[129][130] In most countries, uranium was relatively abundant and the progress of thorium-based reactors was slow; in the 20th century, three reactors were built in India and twelve elsewhere.[131] Large-scale research was begun in 1996 by the International Atomic Energy Agency to study the use of thorium reactors; a year later, the United States Department of Energy started their research. Alvin Radkowsky of Tel Aviv University in Israel was the head designer of Shippingport Atomic Power Station in Pennsylvania, the first American civilian reactor to breed thorium.[132] He founded a consortium to develop thorium reactors, which included other laboratories: Raytheon Nuclear Inc. and Brookhaven National Laboratory in the United States, and the Kurchatov Institute in Russia.[133]

In the 21st century, thorium's potential for reducing nuclear proliferation and its waste characteristics led to renewed interest in the thorium fuel cycle.[134][135][136] India has projected meeting as much as 30% of its electrical demands through thorium-based nuclear power by 2050. In February 2014, Bhabha Atomic Research Centre (BARC), in Mumbai, India, presented their latest design for a "next-generation nuclear reactor" that burns thorium as its fuel core, calling it the Advanced Heavy Water Reactor (AHWR). In 2009, the chairman of the Indian Atomic Energy Commission said that India has a "long-term objective goal of becoming energy-independent based on its vast thorium resources."

On 16 June 2023 China's National Nuclear Safety Administration issued a license to the Shanghai Institute of Applied Physics (SINAP) of the Chinese Academy of Sciences to begin operating the TMSR-LF1, 2 MWt liquid fuel thorium-based molten salt experimental reactor which was completed in August 2021.[137] China is believed to have one of the largest thorium reserves in the world. The exact size of those reserves has not been publicly disclosed, but it is estimated to be enough to meet the country's total energy needs for more than 20,000 years.[138]

When gram quantities of plutonium were first produced in the Manhattan Project, it was discovered that a minor isotope (²⁴⁰Pu) underwent significant spontaneous fission, which brought into question the viability of a plutonium-fueled gun-type nuclear weapon. While the Los Alamos team began work on the implosion-type weapon to circumvent this issue, the Chicago team discussed reactor design solutions. Eugene Wigner proposed to use the ²⁴⁰Pu-contaminated plutonium to drive the conversion of thorium into ²³³U in a special converter reactor. It was hypothesized that the ²³³U would then be usable in a gun-type weapon, though concerns about contamination from ²³²U were voiced. Progress on the implosion weapon was sufficient, and this converter was not developed further, but the design had enormous influence on the development of nuclear energy. It was the first detailed description of a highly enriched water-cooled, water-moderated reactor similar to future naval and commercial power reactors.[139]

During the Cold War the United States explored the possibility of using ²³²Th as a source of ²³³U to be used in a nuclear bomb; they fired a test bomb in 1955.[140] They concluded that a ²³³U-fired bomb would be a very potent weapon, but it bore few sustainable "technical advantages" over the contemporary uranium–plutonium bombs,[141] especially since ²³³U is difficult to produce in isotopically pure form.[140]

Thorium metal was used in the radiation case of at least one nuclear weapon design deployed by the United States (the W71).[142]

There are two categories of thorium minerals for thorium extraction: primary and secondary. Primary deposits occur in acidic granitic magmas and pegmatites. They are concentrated, but of small size. Secondary deposits occur at the mouths of rivers in granitic mountain regions. In these deposits, thorium is enriched along with other heavy minerals.[48] Initial concentration varies with the type of deposit.[147]

For the primary deposits, the source pegmatites, which are usually obtained by mining, are divided into small parts and then undergo flotation. Alkaline earth metal carbonates may be removed after reaction with hydrogen chloride; then follow thickening, filtration, and calcination. The result is a concentrate with rare-earth content of up to 90%.[147] Secondary materials (such as coastal sands) undergo gravity separation. Magnetic separation follows, with a series of magnets of increasing strength. Monazite obtained by this method can be as pure as 98%.[147]

Industrial production in the 20th century relied on treatment with hot, concentrated sulfuric acid in cast iron vessels, followed by selective precipitation by dilution with water, as on the subsequent steps. This method relied on the specifics of the technique and the concentrate grain size; many alternatives have been proposed, but only one has proven effective economically: alkaline digestion with hot sodium hydroxide solution. This is more expensive than the original method but yields a higher purity of thorium; in particular, it removes phosphates from the concentrate.[147]

Acid digestion is a two-stage process, involving the use of up to 93% sulfuric acid at 210–230 °C. First, sulfuric acid in excess of 60% of the sand mass is added, thickening the reaction mixture as products are formed. Then, fuming sulfuric acid is added and the mixture is kept at the same temperature for another five hours to reduce the volume of solution remaining after dilution. The concentration of the sulfuric acid is selected based on reaction rate and viscosity, which both increase with concentration, albeit with viscosity retarding the reaction. Increasing the temperature also speeds up the reaction, but temperatures of 300 °C and above must be avoided, because they cause insoluble thorium pyrophosphate to form. Since dissolution is very exothermic, the monazite sand cannot be added to the acid too quickly. Conversely, at temperatures below 200 °C the reaction does not go fast enough for the process to be practical. To ensure that no precipitates form to block the reactive monazite surface, the mass of acid used must be twice that of the sand, instead of the 60% that would be expected from stoichiometry. The mixture is then cooled to 70 °C and diluted with ten times its volume of cold water, so that any remaining monazite sinks to the bottom while the rare earths and thorium remain in solution. Thorium may then be separated by precipitating it as the phosphate at pH 1.3, since the rare earths do not precipitate until pH 2.[147]

Alkaline digestion is carried out in 30–45% sodium hydroxide solution at about 140 °C for about three hours. Too high a temperature leads to the formation of poorly soluble thorium oxide and an excess of uranium in the filtrate, and too low a concentration of alkali leads to a very slow reaction. These reaction conditions are rather mild and require monazite sand with a particle size under 45 μm. Following filtration, the filter cake includes thorium and the rare earths as their hydroxides, uranium as sodium diuranate, and phosphate as trisodium phosphate. This crystallises trisodium phosphate decahydrate when cooled below 60 °C; uranium impurities in this product increase with the amount of silicon dioxide in the reaction mixture, necessitating recrystallisation before commercial use. The hydroxides are dissolved at 80 °C in 37% hydrochloric acid. Filtration of the remaining precipitates followed by addition of 47% sodium hydroxide results in the precipitation of thorium and uranium at about pH 5.8. Complete drying of the precipitate must be avoided, as air may oxidise cerium from the +3 to the +4 oxidation state, and the cerium(IV) formed can liberate free chlorine from the hydrochloric acid. The rare earths again precipitate out at higher pH. The precipitates are neutralised by the original sodium hydroxide solution, although most of the phosphate must first be removed to avoid precipitating rare-earth phosphates. Solvent extraction may also be used to separate out the thorium and uranium, by dissolving the resultant filter cake in nitric acid. The presence of titanium hydroxide is deleterious as it binds thorium and prevents it from dissolving fully.[147]

High thorium concentrations are needed in nuclear applications. In particular, concentrations of atoms with high neutron capture cross-sections must be very low (for example, gadolinium concentrations must be lower than one part per million by weight). Previously, repeated dissolution and recrystallisation was used to achieve high purity. Today, liquid solvent extraction procedures involving selective complexation of Th⁴⁺ are used. For example, following alkaline digestion and the removal of phosphate, the resulting nitrato complexes of thorium, uranium, and the rare earths can be separated by extraction with tributyl phosphate in kerosene.[147]

Thorium is more abundant than uranium, and can satisfy world energy demands for longer.[163] It is particularly suitable for being used as fertile material in molten salt reactors.

²³²Th absorbs neutrons more readily than ²³⁸U, and ²³³U has a higher probability of fission upon neutron capture (92.0%) than ²³⁵U (85.5%) or ²³⁹Pu (73.5%).[164] It also releases more neutrons upon fission on average.[163] A single neutron capture by ²³⁸U produces transuranic waste along with the fissile ²³⁹Pu, but ²³²Th only produces this waste after five captures, forming ²³⁷Np. This number of captures does not happen for 98–99% of the ²³²Th nuclei because the intermediate products ²³³U or ²³⁵U undergo fission, and fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide fuels to minimise the generation of transuranics and maximise the destruction of plutonium.[165]

Thorium fuels result in a safer and better-performing reactor core[37] because thorium dioxide has a higher melting point, higher thermal conductivity, and a lower coefficient of thermal expansion. It is more stable chemically than the now-common fuel uranium dioxide, because the latter oxidises to triuranium octoxide (U₃O₈), becoming substantially less dense.[166]

The used fuel is difficult and dangerous to reprocess because many of the daughters of ²³²Th and ²³³U are strong gamma emitters.[163] All ²³³U production methods result in impurities of ²³²U, either from parasitic knock-out (n,2n) reactions on ²³²Th, ²³³Pa, or ²³³U that result in the loss of a neutron, or from double neutron capture of ²³⁰Th, an impurity in natural ²³²Th:[167]

²³⁰
₉₀Th
+ n → ²³¹
₉₀Th
+ γ β⁻→25.5 h ²³¹
₉₁Pa
( α→3.28 × 10⁴
y ²²⁷
₈₉Ac
)

²³¹
₉₁Pa
+ n → ²³²
₉₁Pa
+ γ β⁻→1.3 d ²³²
₉₂U
α→69 y

²³²U by itself is not particularly harmful, but quickly decays to produce the strong gamma emitter ²⁰⁸Tl. (²³²Th follows the same decay chain, but its much longer half-life means that the quantities of ²⁰⁸Tl produced are negligible.)[168] These impurities of ²³²U make ²³³U easy to detect and dangerous to work on, and the impracticality of their separation limits the possibilities of nuclear proliferation using ²³³U as the fissile material.[167] ²³³Pa has a relatively long half-life of 27 days and a high cross section for neutron capture. Thus it is a neutron poison: instead of rapidly decaying to the useful ²³³U, a significant amount of ²³³Pa converts to ²³⁴U and consumes neutrons, degrading the reactor efficiency. To avoid this, ²³³Pa is extracted from the active zone of thorium molten salt reactors during their operation, so that it does not have a chance to capture a neutron and will only decay to ²³³U.[169]

The irradiation of ²³²Th with neutrons, followed by its processing, need to be mastered before these advantages can be realised, and this requires more advanced technology than the uranium and plutonium fuel cycle;[37] research continues in this area. Others cite the low commercial viability of the thorium fuel cycle:[170][171][172] the international Nuclear Energy Agency predicts that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which may persist "in the coming decades".[173] The isotopes produced in the thorium fuel cycle are mostly not transuranic, but some of them are still very dangerous, such as ²³¹Pa, which has a half-life of 32,760 years and is a major contributor to the long-term radiotoxicity of spent nuclear fuel.[169]

Natural thorium decays very slowly compared to many other radioactive materials, and the emitted alpha radiation cannot penetrate human skin. As a result, handling small amounts of thorium, such as those in gas mantles, is considered safe, although the use of such items may pose some risks.[174] 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.[174] Internal exposure to thorium leads to increased risk of liver diseases.[175]

The decay products of ²³²Th include more dangerous radionuclides such as radium and radon. Although relatively little of those products are created as the result of the slow decay of thorium, a proper assessment of the radiological toxicity of ²³²Th must include the contribution of its daughters, some of which are dangerous gamma emitters,[176] and which are built up quickly following the initial decay of ²³²Th due to the absence of long-lived nuclides along the decay chain.[177] As the dangerous daughters of thorium have much lower melting points than thorium dioxide, they are volatilised every time the mantle is heated for use. In the first hour of use large fractions of the thorium daughters ²²⁴Ra, ²²⁸Ra, ²¹²Pb, and ²¹²Bi are released.[178] 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.[179]

Some nuclear safety agencies make recommendations about the use of thorium mantles and have raised safety concerns regarding their manufacture and disposal; the radiation dose from one mantle is not a serious problem, but that from many mantles gathered together in factories or landfills is.[175]

Thorium is odourless and tasteless.[180] The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water,[181] precipitating out before entering the body as the hydroxide.[182] 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.[177] If a thorium-containing object has been chewed or sucked, it loses 0.4% of thorium and 90% of its dangerous daughters to the body.[126] Three-quarters of the thorium that has penetrated the body accumulates in the skeleton. Absorption through the skin is possible, but is not a likely means of exposure.[174] Thorium's low solubility in water also means that excretion of thorium by the kidneys and faeces is rather slow.[177]

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.[182] 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.[55] Thorium has no known biological role.[55]

Powdered thorium metal is pyrophoric: it ignites spontaneously in air.[10] In 1964, the United States Department of the Interior listed thorium as "severe" on a table entitled "Ignition and explosibility of metal powders". Its ignition temperature was given as 270 °C (520 °F) for dust clouds and 280 °C (535 °F) for layers. Its minimum explosive concentration was listed as 0.075 oz/cu ft (0.075 kg/m³); the minimum igniting energy for (non-submicron) dust was listed as 5 mJ.[183]

In 1956, the Sylvania Electric Products explosion occurred during reprocessing and burning of thorium sludge in New York City, United States. Nine people were injured; one died of complications caused by third-degree burns.[184][185][186]

Thorium exists in very small quantities everywhere on Earth although larger amounts exist in certain parts: the average human contains about 40 micrograms of thorium and typically consumes three micrograms per day.[55] Most thorium exposure occurs through dust inhalation; some thorium comes with food and water, but because of its low solubility, this exposure is negligible.[177]

Exposure is raised for people who live near thorium deposits or radioactive waste disposal sites, those who live near or work in uranium, phosphate, or tin processing factories, and for those who work in gas mantle production.[187] 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.[188] It is also common in northern Brazilian coastal areas, from south Bahia to Guarapari, a city with radioactive monazite sand beaches, with radiation levels up to 50 times higher than world average background radiation.[189]

Another possible source of exposure is thorium dust produced at weapons testing ranges, as thorium is used in the guidance systems of some missiles. This has been blamed for a high incidence of birth defects and cancer at Salto di Quirra on the Italian island of Sardinia.[190]