Chemical Fact Sheet
Thorium
| Chemical Abstract Number (CAS #) | 7440-29-1 |
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| Synonyms | THORIUM-232 |
| Analytical Methods | 200.8 - 6020 |
| Molecular Formula | Th |
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Synopsis
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Thorium -(Thor, Scandinavian god of war), Th; at. wt. 232.0381(1); at. no. 90; m.p. 1750 deg C; b.p. 4788 deg C, sp. gr. 11.72; valence +2(?), +3(?), +4. Discovered by Berzellus in 1828. Thorium occurs in thorite (ThSiO4) and in thorianite (ThO2 + UO2). Large deposits of thorium minerals have been reported in New England and elsewhere, but these have not yet been exploited. Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum. The metal is a source of nuclear power. There is probably more energy available for use from thorium in the minerals of the earth's crust than from both uranium and fossil fuels. Any sizable demand for thorium as a nuclear fuel is still several years in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (high- temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties. Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO2 along with rare-earth minerals. Much of the internal heat the earth produces has been attributed to thorium and uranium. Several methods are available for producing thorium metal: it can be obtained by reducing thorium oxide with calcium, by electrolysis of anhydrous thorium chloride in a fused mixture of sodium and potassium chlorides, by calcium reduction of thorium tetrachloride mixed with anhydrous zinc chloride, and by reduction of thorium tetrachloride with an alkali metal. Thorium was originally assigned a position in Group IV of the periodic table. Because of its atomic weight, valence, etc., it is now considered to be the second member of the actinide series of elements. When pure, thorium is a silvery-white metal which is air-stable and retains its luster for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. The purest specimens often contain several tenths of a percent of the oxide. High-purity thorium has been made. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400 deg C from a cubic to a body-centered cubic structure. Thorium oxide has a melting point of 3300 deg C, which is the highest of all oxides. Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points. Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. Powdered thorium metal is often pyrophoric and should be carefully handled. When heated in air, thorium turnings ignite and burn brilliantly with a white light. The principal use of thorium has been in the preparation of the Welsbach mantle, used for portable gas lights. These mantles, consisting of thorium oxide with about 1% cerium oxide and other ingredients, glow with a dazzling light when heated in a gas flame. Thorium is an important alloying element in magnesium, imparting high strength and creep resistance at elevated temperatures. Because thorium has a low work-function and high electron emission, it is used to coat tungsten wire used in electronic equipment. The oxide is also used to control the grain size of tungsten used for electric lamps; it is also used for high-temperature laboratory crucibles. Glasses containing thorium oxide have a high refractive index and low dispersion. Consequently, they find application in high quality lenses for cameras and scientific instruments. Thorium oxide has also found use as a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in producing sulfuric acid. Twenty seven isotopes of thorium are known with atomic masses ranging from 212 to 237. All are unstable. 232Th occurs naturally and has a half-life of 1.4 x 10^10 years. It is an alpha emitter. 232Th goes through six alpha and four beta decay steps before becoming the stable isotope 208Pb. 232Th is sufficiently radioactive to expose a photographic plate in a few hours. Thorium disintegrates with the production of "thoron" (220Rn), which is an alpha emitter and presents a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential. Thorium metal (99.8%) costs about $15/g. |
| Consumption Patterns | Energy, 11.8%; Refractory applications, 52.9%; Lamp mantles, 15.0%; Aero space alloys, 7.1%; Welding rods, 2.6%; Ceramics and lighting, 10.6% (1984). |
| Apparent Color | GRAYISH WHITE, LUSTROUS METAL; WHEN PURE, THORIUM IS SILVERY WHITE METAL WHICH RETAINS ITS LUSTER FOR SEVERAL MONTHS; DIMORPHIC, CHANGING @ 1400 DEG C FROM CUBIC TO BODY CENTERED CUBIC STRUCTURE |
| Boiling Point | 4500 deg C, approx |
| Melting Point | 1750 deg C |
| Molecular Weight | 232.038 |
| Density | 11.3-11.7 |
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Chemical and Physical Properties |
ATOMIC NUMBER: 90; VALENCE: 4; LONG LIVED, NATURAL ISOTOPE: 232; OTHER ISOTOPES: 224-231, 233-235; SOMEWHAT DUCTILE & MALLEABLE; REACTS WITH HALOGENS AT RED HEAT; FORMS SULFIDE WHEN HEATED WITH SULFUR; FORMS NITRIDE WHEN HEATED WITH NITROGEN. PURE THORIUM IS SOFT, VERY DUCTILE & CAN BE COLD-ROLLED, SWAGED & DRAWN; HAS LOW WORK FUNCTION & HIGH ELECTRON EMISSION. On heating burns in air with formation of the oxide; radioactive. THORIUM IS SLOWLY ATTACKED BY WATER, BUT DOES NOT DISSOLVE READILY IN MOST COMMON ACIDS, EXCEPT HYDROGEN CHLORIDE. Latent heat of fusion: 17 cal/g @ 100 deg C, 760 mm Hg; specific heat: 0.03 cal/g deg C @ 25 deg C, 760 mm Hg; thermal conductivity: 0.41 watts/cm deg C @ 25 deg C, 760 mm Hg Heat capacity: 27.32 J/mol K @ 25 deg C; heat of sublimation: 597.5 + or - 4.6 kcal/mole; crystallographic density: 11.72 g/cu m. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. |
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Environmental Impact |
The effect of airborne emissions of radionuclides from coal fired power plants on the environment was studied by measuring the concentrations of the radionuclides (232)thorium, (226)radium, (137)cesium and (40)potassium in soil samples from the local environment (0-15 km). No significant increase in concentrations was found in the area downwind of the plant as compared to the other sectors. Although these radionuclides were detected in the fly ash of the power plant, their amounts were apparently low compared with those naturally present in the soil. (137)Cesium in the soil from nuclear weapon testing does not exhibt a uniform spatial distribution, even though its deposition is fairly homogeneous with regard to the relatively small area of the site examined. The exposure rates found in the study areas in the two series of measurements were remarkably similar and were within the limits determined for Czechoslovakia as a whole. No increased radioactivity was detected in the areas of brown coal burning power stations. During a 3 to 4 yr period, concn of (238)uranium, (234)uranium, (230)thorium, (232)thorium and (228)thorium were determined in soils and native vegetables at various sites around a typical uranium mining and milling operation in Wyoming. Plant/soil concn ratios for uranium and thorium isotopes were estimated for (1) exposed, weather tailings, (2) the edge of a tailings impoundment, (3) an area downwind from exposed tailings, (4) a reclamation area and (5) several background, native range locations. The (238)uranium/(234)uranium concn ratio of 0.9 to 1.1 in soil and vegetation indicated near radioactive equilibrium of both radionuclides at all locations. Mean concn of the uranium and thorium isotopes in background soil ranged from 44 to 52 mBq/g. Concn of (238)uranium and (230)thorium in soil and vegetation were elevated above background at all sites disturbed by mining and milling activities. Uranium concn in tailing and invading vegetation were an order of magnitude greater than in the background locations, whereas (230)thorium concn were elevated above background by some two orders of magnitude. No demonstrable differences in radionuclide concn between plant groups and collected years were found. The observed concn ratios values for (238)uranium and (230)thorium of 0.81 and 0.69 for vegetation growing on exposed tailing were elevated above native range by factors of 9.0 and 3.6, respectively, and generally higher than other published values. Exceptionally high concn ratios values for (230)thorium (1.9-2.9) observed near the tailings impoundment demonstrate that under certain conditions, vegetation can accumulate (230)thorium to a much greater extent than previously reported. Vegetation concn were lower for (232)throrium relative to (230)thorium and (228)thorium at locations where they are present at similar soil concn. IN MANTLE CUTTING OR MANTLE TRIMMING OPERATIONS & ESP IN RECLAIMING OPERATIONS THERE IS, BESIDES POTENTIAL EXPOSURE TO THORON GAS, POSSIBLY MORE SERIOUS EXPOSURE TO THORIUM BEARING DUSTS. PRINCIPAL HAZARDS FROM THORIUM IN INDUSTRY ARE INHALATION OF THORIUM DUST & OF THORON GAS & ITS DECAY PRODUCTS, & EXPOSURE TO EXTERNAL BETA & GAMMA RADIATION. INHALATION HAZARD IS GREATEST IN DUSTY OPERATIONS SUCH AS GRINDING OF METALS, CERAMICS, HANDLING OF THORIUM POWDER, & CONTAMINATION FROM THORIUM FIRES. 273 men were exposed to thorium and other rare earths between 1940 and 1973 at a monazite sand refinery. Because thorium is ubiquitous, daily exposure to this element is constant & was estimated by the International Commission on Radiological Protection (ICRP) at 3 ug. Industrial exposures from stable, nonradioactive thorium occur during handling of various thorium salts in fabrication of thorium ingots from nitrate, in handling thorium salts in various industrial uses, in fume from welding with thoriated tungsten electrodes, in casting & machining of thorium alloy parts, and from fires and explosions from thorium metal powder.In the production and use of thorium and its compounds, workers may be exposed to ionizing radiation from thorium and its disintegration products such as gas and thoron.Dose estimates are given for internal and external exposure that result, due to radioactive thorium, from the use of the incandescrnt mantles for gas lanterns. The collective, effective dose equivalent for all users of gas mantles is estimated to be about 100 Sv per annum in the Netherlands. For the population involved (ca 700,000 persons) this is roughly equivalent to 5% to 10% of the collective dose equivalent associated with exposure to radiation from natural sources. The major contribution to dose estimates comes from inhalation of radium during burning of the mantles. A pessimistic approach results in individual dose estimates for inhalation of up to 0.2 mSv. |
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Environmental Fate |
OCCURS IN MINERALS THORITE, THORIANITE, ORANGITE, YTTROCRASITE; IN MONAZITE SAND; PRESENT TO EXTENT OF ABOUT 15 PPM IN CRUST OF EARTH. Monazite is the most common and commercially important thorium bearing mineral with important deposits in India, Brazil, and Sri Lanka. Other extensive deposits occur in South Africa, the Soviet Union, Scandinavia, and Australia. LARGE DEPOSITS OF THORIUM MINERALS HAVE BEEN REPORTED IN NEW ENGLAND THORIUM THOUGHT TO BE ABOUT 3 TIMES AS ABUNDANT AS URANIUM AND ABOUT AS ABUNDANT AS LEAD OR MOLYBDENUM. THERE IS PROBABLY MORE ENERGY AVAILABLE FOR USE FROM THORIUM IN MINERALS OF EARTH CRUST THAN FROM BOTH URANIUM & FOSSIL FUELS. Coal samples from Pennsylvania and Utah showed less than 0.006 ppm a USA cement sample showed 1.2 ppm thorium. Because thorium is ubiquitous, daily exposure to this element is constant & was estimated by the International Commission on Radiological Protection (ICRP) at 3 ug. The thorium isotope ratio may change from one population to another due to the different sources of contamination and to differences between the mode of absorption. One mode is by inhalation of soil or ores. The other mode of entry is via by food & fluids.Percutaneous absorption. Thorium intake by an urban group (Bombay) has been estimated using neutron activation followed by simple chemical separation. Daily intake of thorium via all the three sources: food, water and air, is reported. The major contributions of thorium to intake is through food (2.0 ug), followed by water (0.02 ug) and air (0.02 ug). The individual food ingredients such as cereals, pulses, vegetables, milk, etc were also analysed for their thorium content. The cereals were found to conribute most to the daily intake. The daily intake of long lived alpha emitting members of the uranium, thorium, and actinium series by New York City residents has been estimated from measurements of diet, water, and air samples. The total daily intakes from inhalation, food, and water consumption in mBq are 18 (234)uranium, 0.7 (235)uranium, 16 (238)uranium, 6 (230)thorium, 4 (232)thorium and 52 (226)radium. From this, it is infered that the total daily intakes of (228)thorium, and (228)radium are 4 and 35 mBq, respectively. 273 men exposed to thorium and other rare earths between 1940 and 1973 at a monazite sand refinery were studied at Argonne National Laboratory from 1976 to 1980. In vivo measurements of body burden were made by counting gamma rays emitted by daughter products of retained thorium and by measuring exhaled thoron. Health status was ascertained through questionnaire, physical exam, and clinical lab tests. Measured body burden was found to be higher in those with a history of longer exposure. All parameters of the complete blood count were examined for evidence of an effect due to thorium. Comparisons of high and low body burden groups showed that only age and cigarette smoking had an effect on complete blood count parameters. The content of the lung of a male and female resident in the USA in 1975 was 0.12 and 0.20 ppm, respectively; that of the pulmonary lymph node, 0.30 ppm. A freeze dried preparation of a coal miner's lung showed 2.0 ppm. Determination of the alpha activity of human soft tissues, mainly from the USA and Great Britain, lung, liver, kidney, spleen, muscle, and hair, have been reported to contain about 7 pCi/kg wet tissue for most, although values ranged from as low as 2 pCi/kg in brain to 33 pCi/kg in hair, to which the (232)thorium series is considered to contribute less than 50%. Bone analyses from many sources tend to have an alpha radioactivity 20 times that of soft tissues. ITS CONCN IN HUMAN SKELETON IS ABOUT 1 FEMTOCURIE/G OF ASH. |
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Drinking Water Impact |
The radiological impact of former mining activities in the Fen area in southern Norway was assessed. The area is known to have enhanced concn of thorium. Uranium and thorium analyses were performed on mine and lake water. The mining activity does not seem to have contaminated drinking water significantly. The tailings from Nb production has enriched radon and thorium concn. The tailings and the possible use of waste rock from the mining are probably the most important environmental results of the mining activities. |
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Disposal |
Disposal of wastes containing uranium should follow guidelines set forth by the Nuclear Regulatory Commission & the EPA. Recovery & recycle is the preferred route. |
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