Thorium heat source

More controversial (although sometimes cited as proven fact) have been claims (e.g., Taylor and Jakes, 1974 Taylor, 1982) that the bulk Moon is enriched roughly twofold in the cosmochemically refractory lithophile elements (a class that includes the REEs, the heat sources thorium and uranium, and the major elements aluminum, calcium, and titanium), and that compared to Earth s primitive mantle, the Moon s silicate mg ratio is much lower, i.e., its EeO concentration is much higher. Neither of these claims has been confirmed by recent lunar science developments, which include the advent of global thorium and samarium maps (Lawrence et al., 2002a Prettyman et al., 2002), data from lunar meteorites, and some radically changed interpretations of the Apollo seismic database. 

The iron would not be molten and flow, however, without a continuous heat source to keep it from solidifying. Radioactive fission energy from uranium and thorium decay has long been the assumed source of the heat in the core, and still provides that heat, but not enough to have sustained a magnetic field for 4 billion years. 

The homogeneous reactor experiment-2 (HRE-2) was tested as a power-breeder in the late 1950s. The core contained highly enriched uranyl sulfate in heavy water and the reflector contained a slurry of thorium oxide [1314-20-1J, Th02, in D2O. The reactor thus produced fissile uranium-233 by absorption of neutrons in thorium-232 [7440-29-1J, the essentially stable single isotope of thorium. Local deposits of uranium caused reactivity excursions and intense sources of heat that melted holes in the container (18), and the project was terrninated. 

The main sources of infrared radiation used in spectrophotometers are (1) a nichrome wire wound on a ceramic support, (2) the Nernst glower, which is a filament containing zirconium, thorium and cerium oxides held together by a binder, (3) the Globar, a bonded silicon carbide rod. These are heated electrically to temperatures within the range 1200- 2000 °C when they will glow and produce the infrared radiation approximating to that of a black body. 

Heating the ore with sulfuric acid converts neodymium to its water soluble sulfate. The product mixture is treated with excess water to separate neodymium as soluble sulfate from the water-insoluble sulfates of other metals, as well as from other residues. If monazite is the starting material, thorium is separated from neodymium and other soluble rare earth sulfates by treating the solution with sodium pyrophosphate. This precipitates thorium pyrophosphate. Alternatively, thorium may be selectively precipitated as thorium hydroxide by partially neutralizing the solution with caustic soda at pH 3 to 4. The solution then is treated with ammonium oxalate to precipitate rare earth metals as their insoluble oxalates. The rare earth oxalates obtained are decomposed to oxides by calcining in the presence of air. Composition of individual oxides in such rare earth oxide mixture may vary with the source of ore and may contain neodymium oxide, as much as 18%. 

Large thorium deposits have heen found in many parts of the world. It occurs in minerals thorite, ThSi04, and thorianite, Th02"U02. Thorium also is found in mineral monazite which contains between 3 to 9% Th02. Th02 is the principal source of commercial thorium. Abundance of thorium in earth s crust is estimated at about 9.6 mg/kg. Thorium and uranium are believed to have contributed much of the internal heat of the earth due to their radioactive emanations since earth s formation. 

About one-fifth of our annual exposure to radiation comes from nonnatural sources, primarily medical procedures. Television sets, fallout from nuclear testing, and the coal and nuclear power industries are minor but significant nonnatural sources. Interestingly, the coal industry far outranks the nuclear power industry as a source of radiation. The global combustion of coal annually releases into the atmosphere about 13,000 tons ol radioactive thorium and uranium. Worldwide, the nuclear power industries generate about 10,000 tons of radioactive waste each year. Most of this waste is contained, however, and is not released into the environment. As we explore in Chapter 19, where to bury this contained radioactive waste is a heated issue yet to be resolved. 

Thorium occurs in monazite sand in Brazil, India, North and South Carolina this ore contains 3-9% thorium oxide, and is the chief source thorium is also found in thorite containing about 60% oxide and in thorianite. about 80% oxide. When heated with concentrated H2SO4 the minerals form thonum sulfate, from which, by a senes of reactions, thonum nitrate, the chief commercial compound, is obtained. 

Uranium and thorium may well become important sources of heat and energy in the world of the future. There are large amounts of these elements available—the amount of uranium in the earth s crust has been estimated as 4 parts per million and the amount of thorium... 

The principal source of thorium is monazite (p. 425), a phosphate of cerium and lanthanum with up to 15% of thoria. It is dissolved in concentrated sulphuric acid and the thorium phosphate precipitated with magnesium oxide. The washed phosphate heated with sodium carbonate gives crude thoria, ThOg, which is converted to the soluble oxalate and separated from the insoluble oxalates of cerium and lanthanum. After ignition to oxide the nitrate is made, purified by recrystallisation, and again calcined to thoria. 

Named after Thor, the Scandinavian god of war, thorium was discovered by Jons Jakob Berzelius (1779-1848) in 1828 in a mineral sample from Norway. Although thorium does not occur in elemental form, thorium ores are thought to be about as abundant as lead in the Earth s crust. Thorium has limited uses as a commercial product, primarily in specialty electronics and Welsbach mantles (portable gas lights). Because thorium is radioactive and more plentiful than uranium, it may be used as a power source in the future. The internal heating of the Earth may be a result of the action of thorium. 

Ceramic infrared light sources are used in some spectrometers. A ceramic stick is heated by a metallic conductor, made from platinum or a platinum alloy, and wound around the ceramic stick. The conductor is surrounded with a sintered layer of aluminum, thorium oxide, zirconium silicate or a similar material. The heating conductors made from chrome nickel or tungsten wire are preferably suitable for short-wave spectral analysis. 

The Nemst glower is a cylindrical bar composed of zirconium oxide, cerium oxide, and thorium oxide that is heated electrically to a temperature between 1500 and 2000 K. The source is generally about 20 mm long and 2 mm in diameter. The rare earth oxide ceramic is an electrical resistor passing current through it causes it to heat and glow. 

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