'' (1872) by
Mårten Eskil Winge;
Thor, the
Norse god of thunder, raising his hammer
Mjölnir in a battle against the
giants.
Erroneous report In 1815, the Swedish chemist
Jöns Jacob Berzelius analysed an unusual sample of
gadolinite from a copper mine in
Falun, central Sweden. He noted impregnated traces of a white mineral, which he cautiously assumed to be an earth (
oxide in modern chemical nomenclature) of an unknown element. Berzelius had already discovered two elements,
cerium and
selenium, but he had made a public mistake once, announcing a new element,
gahnium, that turned out to be
zinc oxide. and its supposed oxide "thorina" after
Thor, the
Norse god of thunder. In 1824, after more deposits of the same mineral in
Vest-Agder, Norway, were discovered, he retracted his findings, as the mineral (later named
xenotime) proved to be mostly
yttrium orthophosphate.
Discovery In 1828,
Morten Thrane Esmark found a black mineral on
Løvøya island,
Telemark county, Norway. He was a Norwegian
priest and amateur
mineralogist who studied the minerals in Telemark, where he served as
vicar. He commonly sent the most interesting specimens, such as this one, to his father,
Jens Esmark, a noted mineralogist and professor of mineralogy and geology at the
Royal Frederick University in Christiania (today called
Oslo). The elder Esmark determined that it was not a known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element. He published his findings in 1829, having isolated an impure sample by reducing (potassium pentafluorothorate(IV)) with
potassium metal. Berzelius reused the name of the previous supposed element discovery and named the source mineral thorite. , who first identified thorium as a new element Berzelius made some initial characterisations of the new metal and its chemical compounds: he correctly determined that the thorium–oxygen mass ratio of thorium oxide was 7.5 (its actual value is close to that, ~7.3), but he assumed the new element was divalent rather than tetravalent, and so calculated that the atomic mass was 7.5 times that of oxygen (); it is actually 15 times as large. He determined that thorium was a very
electropositive metal, ahead of cerium and behind zirconium in electropositivity. Metallic thorium was isolated for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger.
Initial chemical classification In the periodic table published by
Dmitri Mendeleev in 1869, thorium and the rare-earth elements were placed outside the main body of the table, at the end of each vertical period after the
alkaline earth metals. This reflected the belief at that time that thorium and the rare-earth metals were divalent. With the later recognition that the rare earths were mostly trivalent and thorium was tetravalent, Mendeleev moved cerium and thorium to group IV in 1871, which also contained the modern
carbon group (group 14) and titanium group (group 4), because their maximum oxidation state was +4. Cerium was soon removed from the main body of the table and placed in a separate lanthanide series; thorium was left with group 4 as it had similar properties to its supposed lighter congeners in that group, such as
titanium and zirconium. Starting from 1899, the New Zealand physicist
Ernest Rutherford and the American electrical engineer
Robert Bowie Owens studied the radiation from thorium; initial observations showed that it varied significantly. It was determined that these variations came from a short-lived gaseous daughter of thorium, which they found to be a new element. This element is now named
radon, the only one of the rare radioelements to be discovered in nature as a daughter of thorium rather than uranium. After accounting for the contribution of radon, Rutherford, now working with the British physicist
Frederick Soddy, showed how thorium decayed at a fixed rate over time into a series of other elements in work dating from 1900 to 1903. This observation led to the identification of the
half-life as one of the outcomes of the
alpha particle experiments that led to the disintegration theory of
radioactivity. The biological effect of radiation was discovered in 1903. The newly discovered phenomenon of radioactivity excited scientists and the general public alike. In the 1920s, thorium's radioactivity was promoted as a cure for
rheumatism,
diabetes, and
sexual impotence. In 1932, most of these uses were banned in the United States after a federal investigation into the health effects of radioactivity. , who settled thorium's location in the f-block
Further classification Up to the late 19th century, chemists unanimously agreed that thorium and uranium were 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. Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium. In 1945, when American physicist
Glenn T. Seaborg and his team had discovered the transuranic elements americium and curium, he proposed the
actinide concept, realising that thorium was the second member of an f-block actinide series analogous to the lanthanides, instead of being the heavier congener of
hafnium in a fourth d-block row.
Phasing out In the 1990s, most applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements were found. Despite its radioactivity, the element has remained in use for applications where no suitable alternatives could be found. A 1981 study by the
Oak Ridge National Laboratory in the United States estimated that using a thorium gas mantle every weekend would be safe for a person, Some manufacturers have changed to other materials, such as yttrium. As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even falsely claiming them to be non-radioactive.
Nuclear power (
Buchanan, New York, United States), home of the world's first thorium reactor Thorium has been used as a power source on a prototype scale. The earliest thorium-based reactor was built at the
Indian Point Energy Center located in
Buchanan, New York,
United States in 1962. China may be the first to have attempted to commercialise the technology. The country with the largest estimated reserves of thorium in the world is
India, which has sparse reserves of uranium. In the 1950s, India targeted achieving energy independence with their
three-stage nuclear power programme. In most countries, uranium was relatively abundant and the progress of thorium-based reactors was slow; in the 20th century, three reactors were built in India and twelve elsewhere. Large-scale research was begun in 1996 by the
International Atomic Energy Agency to study the use of thorium reactors; a year later, the
United States Department of Energy started their research.
Alvin Radkowsky of
Tel Aviv University in
Israel was the head designer of
Shippingport Atomic Power Station in Pennsylvania, the first American civilian reactor to breed thorium. He founded a consortium to develop thorium reactors, which included other laboratories:
Raytheon Nuclear Inc. and
Brookhaven National Laboratory in the United States, and the
Kurchatov Institute in Russia. In the 21st century, thorium's potential for reducing nuclear proliferation and its
waste characteristics led to renewed interest in the thorium fuel cycle. India has projected meeting as much as 30% of its electrical demands through thorium-based
nuclear power by 2050. In February 2014,
Bhabha Atomic Research Centre (BARC), in
Mumbai, India, presented their latest design for a "next-generation nuclear reactor" that burns thorium as its fuel core, calling it the
Advanced Heavy Water Reactor (AHWR). In 2009, the chairman of the Indian Atomic Energy Commission said that India has a "long-term objective goal of becoming energy-independent based on its vast thorium resources." On 16 June 2023 China's National Nuclear Safety Administration issued a licence to the Shanghai Institute of Applied Physics (SINAP) of the Chinese Academy of Sciences to begin operating the
TMSR-LF1, 2 MWt liquid fuel thorium-based molten salt experimental reactor which was completed in August 2021. China is believed to have one of the largest thorium reserves in the world. The exact size of those reserves has not been publicly disclosed, but it is estimated to be enough to meet the country's total energy needs for more than 20,000 years.
Nuclear weapons When gram quantities of
plutonium were first produced in the
Manhattan Project, it was discovered that a minor isotope (
240Pu) underwent significant
spontaneous fission, which brought into question the viability of a plutonium-fuelled
gun-type nuclear weapon. While the
Los Alamos team began work on the
implosion-type weapon to circumvent this issue, the
Chicago team discussed reactor design solutions.
Eugene Wigner proposed to use the 240Pu-contaminated plutonium to drive the conversion of thorium into 233U in a special converter reactor. It was hypothesized that the 233U would then be usable in a gun-type weapon, though concerns about contamination from 232U were voiced. Progress on the implosion weapon was sufficient, and this converter was not developed further, but the design had enormous influence on the development of nuclear energy. It was the first detailed description of a highly enriched water-cooled, water-moderated reactor similar to future naval and commercial power reactors. In 1943 the
Manhattan Project contracted two private companies,
Union Carbide and
Chevron, to quietly compile a survey of uranium and thorium deposits around the world. The primary focus was uranium but early in the process thorium was also included. Deposits of
monazite sands where identified in Brazil,
Netherlands East Indies, and
Travancore in India but none of these were pursued by the project. 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 and it fired
a test bomb in 1955. It concluded that a 233U-fired bomb would be a very potent weapon, but it bore few sustainable "technical advantages" over the contemporary uranium–plutonium bombs, especially since 233U is difficult to produce in isotopically pure form. == Production ==