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| Thorium rods. Photo: Department of Energy |
Thorium is a source of nuclear power. There is probably more untapped energy available for use from thorium in the minerals of the earth's crust than from combined uranium and fossil fuel sources. Much of the internal heat the earth has been attributed to thorium and uranium.
When pure, thorium is a silvery white metal which is air-stable and retains its lustre for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and finally black. Thorium oxide has a melting point of 3300°C, 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.
Thorium is named for Thor, the Scandinavian god of war. It is found in thorite and thorianite in New England (USA) and other sites.
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Data Zone
| Classification: | Thorium is an actinide metal |
| Color: | silvery |
| Atomic weight: | 232.0381, no stable isotopes |
| State: | solid |
| Melting point: | 1750 oC, 2023 K |
| Boiling point: | 4790 oC, 5063 K |
| Electrons: | 90 |
| Protons: | 90 |
| Neutrons in most abundant isotope: | 142 |
| Electron shells: | 2,8,18,32,18,10,2 |
| Electron configuration: | [Rn] 6d2 7s2 |
| Density @ 20oC: | 11.7 g/cm3 |
| Atomic volume: | 19.9 cm3/mol |
| Structure: | face-centered cubic |
Uses of Thorium
An exciting possibility for the future is fueling nuclear reactors with thorium. Not only is thorium more abundant on Earth than uranium, but 1 ton of mined thorium can produce as much energy as 200 tons of mined uranium. (8)
The difference in the energy output of the two elements arises because most uranium mined is uranium-238, which is not fissile. (Naturally occurring uranium is over 99% uranium-238 with only about 0.7% of the fissile uranium-235.) Nearly all mined thorium, however, can easily be made into the fissile uranium isotope uranium-233 through neutron bombardment (as shown above).
Waste from a thorium reactor is expected to lose its dangerous radioactivity after about 400-500 years, compared with many thousands of years for nuclear waste produced today. (8)
Thorium fuel research is continuing in several countries including the USA and India. (9)
Most non-nuclear uses of thorium are driven by the unique properties of its oxide.
Thorium dioxide was used in Welsbach gas mantles in the 19th century and today these mantles may still be found in camping lanterns. (Thorium dioxide’s very high melting point ensures it stays solid, glowing with an intense, bright white light at the temperature of the lantern’s burning gas.)
Thorium dioxide is used for heat resistant ceramics.
Glass that contains thorium dioxide has a high refractive index and low dispersion, so thorium dioxide is added to glass for use in high quality lenses and scientific equipment.
Thorium-magnesium alloys are used in the aerospace industry for aircraft engines. These alloys are lightweight and have excellent strength and creep resistance at high temperatures.
Thorium is used to coat tungsten filaments in light bulbs.
The demand for thorium in non-nuclear applications is decreasing because of environmental and health concerns due to its radioactivity.
Discovery of Thorium
Thorium was discovered by JΓΆns Jacob Berzelius in 1828, in Stockholm, Sweden after he received a sample of an unusual black mineral from Hans Esmark found on an island close to Brevik, Norway.
The mineral contained a large number of known elements including iron, manganese, lead, tin and uranium plus another substance Berzelius could not identify.
He concluded that the mineral contained a new element.
He called the black mineral thorite, in honor of the Scandinavian god Thor.
His analysis indicated that 57.91% of thorite was an oxide of the proposed new element, which he called thorium. (1)
To isolate thorium metal, Berzelius found the most effective method was to react thorium chloride with potassium, to yield potassium chloride and thorium. (Berzelius made thorium chloride by mixing thorium oxide with carbon and heating to red-heat in a stream of chlorine gas.) (2)
Berzelius’s isolation of thorium from its chloride using potassium was similar to the approach used by WΓΆhler and Bussy to isolate beryllium in 1828 and by Γrsted to isolate aluminum in 1825.
Thorium was discovered to be radioactive by Gerhard Schmidt in 1898 – the first element after uranium to be identified as such.
Marie Curie also found this, independently, later in the same year. (3)
In the early 1900s Ernest Rutherford and Frederick Soddy found that thorium decayed into other elements at a fixed rate – a key discovery in our understanding of the radioactive elements. (4), (5)
A method for producing high purity thorium metal was discovered in 1925 by Anton Eduard van Arkel and Jan Hendrik de Boer. Thorium iodide is decomposed on a white hot tungsten filament creating a crystal bar of pure thorium. (6)
Prior to his discovery of thorium, Berzelius had discovered two other elements, cerium in 1803 and selenium in 1817.
credit/source: http://www.chemicool.com/elements/thorium.html
Where does thorium come from?
Almost all thorium is natural, but, thorium isotopes can be artificially produced. Thorium occurs at very low levels in virtually all rock, soil, and water, and therefore is found in plants and animals as well. Minerals such as monazite, thorite and thorianite are rich in thorium and may be mined for the metal. Generally, artificial isotopes come from decay of other man-made radionuclides, or absorption in nuclear reactions.
What are the properties of thorium?
Thorium is a soft, silvery white metal. Pure thorium will remain shiny for months in air, but if it contains impurities it tarnishes to black when exposed to air. When heated, thorium oxide glows bright white, a property that makes it useful in lantern mantles. It dissolves slowly in water. Thorium-232 has a half-life of 14 billion (14x109) years, and decays by alpha emission, with accompanying gamma radiation. Thorium-232 is the top of a long decay series that contains key radionuclides such as radium-228, its direct decay product, and radon-220. Two other isotopes of thorium, which can be significant in the environment, are thorium-230 and thorium-228. Both belong to other decay series. They also decay by alpha emission, with accompanying gamma radiation, and have half-lives of 75,400 years and 1.9 years, respectively.
What is thorium used for?
Thorium has coloring properties that has made it useful in ceramic glazes. But, it has been most widely used in lantern mantles for the brightness it imparts (though alternatives are replacing it), and in welding rods, which burn better with small amounts of added thorium. Thorium improves the properties of ophthalmic lenses, and is an alloying agent in certain metals used in the aerospace industry. More than 30 years ago, thorium oxides were used in hospitals to make certain kinds of diagnostic X-ray photographs. But, this practice has been discontinued.
credit/source: http://www.epa.gov/radiation/radionuclides/thorium.html#discovered
Thorium as a nuclear fuel
Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233)a, which is an excellent fissile fuel materialb. In this regard it is similar to uranium-238 (which transmutes to plutonium-239). All thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the necessary neutron dosing. The U-233 that is produced can either be chemically separated from the parent thorium fuel and recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form, especially in molten sale reactors (MSR).
Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain reaction (and thus supply of surplus neutrons) can be maintained. The only fissile driver options are U-233, U-235 or Pu-239. (None of these is easy to supply)
It is possible – but quite difficult – to design thorium fuels that produce more U-233 in thermal reactors than the fissile material they consume (this is referred to as having a fissile conversion ratio of more than 1.0 and is also called breeding). Thermal breeding with thorium requires that the neutron economy in the reactor has to be very good (ie, there must be low neutron loss through escape or parasitic absorption). The possibility to breed fissile material in slow neutron systems is a unique feature for thorium-based fuels and is not possible with uranium fuels.
Another distinct option for using thorium is as a ‘fertile matrix’ for fuels containing plutonium that serves as the fissile driver while being consumed (and even other transuranic elements like americium). Mixed thorium-plutonium oxide (Th-Pu MOX) fuel is an analog of current uranium-MOX fuel, but no new plutonium is produced from the thorium component, unlike for uranium fuels in U-Pu MOX fuel, and so the level of net consumption of plutonium is high. Production of all actinides is lower than with conventional fuel, and negative reactivity coefficient is enhanced compared with U-Pu MOX fuel.
In fresh thorium fuel, all of the fissions (thus power and neutrons) derive from the driver component. As the fuel operates the U-233 content gradually increases and it contributes more and more to the power output of the fuel. The ultimate energy output from U-233 (and hence indirectly thorium) depends on numerous fuel design parameters, including: fuel burn-up attained, fuel arrangement, neutron energy spectrum and neutron flux (affecting the intermediate product protactinium-233, which is a neutron absorber). The fission of a U-233 nucleus releases about the same amount of energy (200 MeV) as that of U-235.
An important principle in the design of thorium fuel systems is that of heterogeneous fuel arrangement in which a high fissile (and therefore higher power) fuel zone called the seed region is physically separated from the fertile (low or zero power) thorium part of the fuel – often called the blanket. Such an arrangement is far better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U-233, in fact all thermal breeding fuel designs are heterogeneous. This principle applies to all the thorium-capable reactor systems.
Th-232 is fissionable with fast neutrons of over 1 MeV energy. It could therefore be used in fast molten salt and other Gen IV reactors with uranium or plutonium fuel to initiate fission. However, Th-232 fast fissions only one tenth as well as U-238, so there is no particular reason for using thorium in fast reactors, given the huge amount of depleted uranium awaiting use.
Reactors able to use thorium
There are seven types of reactor into which thorium can be introduced as a nuclear fuel. The first five of these have all entered into operational service at some point. The last two are still conceptual:
Heavy Water Reactors (PHWRs): These are well suited for thorium fuels due to their combination of: (i) excellent neutron economy (their low parasitic neutron absorption means more neutrons can be absorbed by thorium to produce useful U-233), (ii) slightly faster average neutron energy which favours conversion to U-233, (iii) flexible on-line refueling capability. Furthermore, heavy water reactors (especially CANDU) are well established and widely-deployed commercial technology for which there is extensive licensing experience.
There is potential application to Enhanced Candu 6 (EC6) and ACR-1000 reactors fueled with 5% plutonium (reactor grade) plus thorium. In the closed fuel cycle, the driver fuel required for starting off is progressively replaced with recycled U-233, so that on reaching equilibrium 80% of the energy comes from thorium. Fissile drive fuel could be LEU, plutonium, or recycled uranium from LWR. Fleets of PHWRs with near-self-sufficient equilibrium thorium fuel cycles could be supported by a few fast breeder reactors to provide plutonium.
High-Temperature Gas-Cooled Reactors (HTRs): These are well suited for thorium-based fuels in the form of robust ‘TRISO’ coated particles of thorium mixed with plutonium or enriched uranium, coated with pyrolytic carbon and silicon carbide layers which retain fission gases. The fuel particles are embedded in a graphite matrix that is very stable at high temperatures. Such fuels can be irradiated for very long periods and thus deeply burn their original fissile charge. Thorium fuels can be designed for both ‘pebble bed’ and ‘prismatic’ types of HTR reactors.
Boiling (Light) Water Reactors (BWRs): BWR fuel assemblies can be flexibly designed in terms of rods with varying compositions (fissile content), and structural features enabling the fuel to experience more or less moderation (eg, half-length fuel rods). This design flexibility is very good for being able to come up with suitable heterogeneous arrangements and create well-optimised thorium fuels. So it is possible, for example, to design thorium-plutonium BWR fuels that are tailored for ‘burning’ surplus plutonium. And importantly, BWRs are a well-understood and licensed reactor type.
Pressurised (Light) Water Reactors (PWRs): Viable thorium fuels can be designed for a PWR, though with less flexibility than for BWRs. Fuel needs to be in heterogeneous arrangements in order to achieve satisfactory fuel burn-up. It is not possible to design viable thorium-based PWR fuels that convert significant amounts of U-233. Even though PWRs are not the perfect reactor in which to use thorium, they are the industry workhorse and there is a lot of PWR licensing experience. They are a viable early-entry thorium platform.
Fast Neutron Reactors (FNRs): Thorium can serve as a fuel component for reactors operating with a fast neutron spectrum – in which a wider range of heavy nuclides are fissionable and may potentially drive a thorium fuel. There is, however, no relative advantage in using thorium instead of depleted uranium (DU) as a fertile fuel matrix in these reactor systems due to a higher fast-fission rate for U-238 and the fission contribution from residual U-235 in this material. Also, there is a huge amount of surplus DU available for use when more FNRs are commercially available, so thorium has little or no competitive edge in these systems.
Molten Salt Reactors (MSRs): These reactors are still at the design stage but are likely to be very well suited for using thorium as a fuel. The unique fluid fuel can incorporate thorium and uranium (U-233 and/or U-235) fluorides as part of a salt mixture that melts in the range 400-700ΒΊC, and this liquid serves as both heat transfer fluid and the matrix for the fissioning fuel. The fluid circulates through a core region and then through a chemical processing circuit that removes various fission products (poisons) and/or the valuable U-233. The level of moderation is given by the amount of graphite built into the core. Certain MSR designsc will be designed specifically for thorium fuels to produce useful amounts of U-233.
Accelerator Driven Reactors (ADS): The sub-critical ADS system is an unconventional nuclear fission energy concept that is potentially ‘thorium capable’. Spallation neutrons are producedd when high-energy protons from an accelerator strike a heavy target like lead. These neutrons are directed at a region containing a thorium fuel, eg, Th-plutonium which reacts to produce heat as in a conventional reactor. The system remains subcritical ie, unable to sustain a chain reaction without the proton beam. Difficulties lie with the reliability of high-energy accelerators and also with economics due to their high power consumption. (See also information page onAccelerator-Driven Nuclear Energy.)
A key finding from thorium fuel studies to date is that it is not economically viable to use low-enriched uranium (LEU – with a U-235 content of up to 20%) as a fissile driver with thorium fuels, unless the fuel burn-up can be taken to very high levels – well beyond those currently attainable in LWRs with zirconium cladding.
With regard to proliferation significance, thorium-based power reactor fuels would be a poor source for fissile material usable in the illicit manufacture of an explosive device. U-233 contained in spent thorium fuel contains U-232 which decays to produce very radioactive daughter nuclides and these create a strong gamma radiation field. This confers proliferation resistance by creating significant handling problems and by greatly boosting the detectability (traceability) and ability to safeguard this material.
Prior Thorium Fueled Electricity Generation
There have been several significant demonstrations of the use of thorium-based fuels to generate electricity in several reactor types. Many of these early trials were able to use high-enriched uranium (HEU) as the fissile ‘driver’ component, and this would not be considered today.
The 300 MWe Thorium High Temperature Reactor (THTR) in Germany operated with thorium-HEU fuel between 1983 and 1989. Over half of its 674,000 pebbles contained Th-HEU fuel particles (the rest comprised graphite moderator and some neutron absorbers). These were continuously moved through the reactor as it operated, and on average each fuel pebble passed six times through the core.
The 40 MWe Peach Bottom HTR in the USA was a demonstration thorium-fuelled reactor that ran from 1967-74.2 It used a thorium-HEU fuel in the form of microspheres of mixed thorium-uranium carbide coated with pyrolytic carbon. These were embedded in annular graphite segments (not pebbles). This reactor produced 33 billion kWh over 1349 equivalent full-power days with a capacity factor of 74%.
The 330 MWe Fort St Vrain HTR in Colorado, USA, was a larger-scale commercial successor to the Peach Bottom reactor and ran from 1976-89. It also used thorium-HEU fuel in the form of microspheres of mixed thorium-uranium carbide coated with silicon oxide and pyrolytic carbon to retain fission products. These were embedded in graphite ‘compacts’ that were arranged in hexagonal columns ('prisms'). Almost 25 tonnes of thorium was used in fuel for the reactor, much of which attained a burn-up of about 170 GWd/t.
A unique thorium-fuelled Light Water Breeder Reactor operated from 1977 to 1982 at Shippingport in the USA3 – it used uranium-233 as the fissile driver in special fuel assemblies that had movable ‘seed’ regions which allowed the level of neutron moderation to be gradually increased as the fuel agede. The reactor core was housed in a reconfigured early PWR. It operated with a power output of 60 MWe (236 MWt) and an availability factor of 86% producing over 2.1 billion kWh. Post-operation inspections revealed that 1.39% more fissile fuel was present at the end of core life, proving that breeding had occurred.
Indian heavy water reactors (PHWRs) have for a long time used thorium-bearing fuel bundles for power flattening in some fuel channels – especially in initial cores when special reactivity control measures are needed.
credit/source: http://www.world-nuclear.org/info/Current-and-Future-Generation/Thorium/
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My Note: Today is my 5th Anniversary in Nature's Eye: naomispenny.blogspot.com
Once more please allow me to thank you Everyone who searches for an answer and they chose my article. I have tried so many types of topics and websites for my references hope you enjoyed all the journey with me that comes with it.
