**2. History**

Thorium was identified as an element in the mineral thorite in 1828 by the Swedish chemist Berzelius. Newly discovered element was named for Thor, the Scandinavian God of thunder and lighting, because of its use in energy. In 1885, thorium came into commercial use when it was discovered that a fabric mantle impregnated with a thorium compounds would give a steady, bright white light when heated. This discovery led to the development of the Welsh mantle, which was adopted in gas lighting and later in kerosene lamps. Thorium derived from monazite occurring in the Brazilian beach sands was produced as early as 1885. In 1911, monazite from the Indian beach sand deposits mastered world monazite markets.

During this time, the German manufacturers organized a monopoly of the thorium nitrate industry. World War I restricted German supplies of thorium compounds and enabled US production of thorium nitrate to expand. In the early 1920s, electricity began to replace gas and kerosene for general lighting purposes, and the need for thorium mantles declined. Up to the end of World War II, dominate monazite producers were India and Brazil. Since 1945 some other countries have started with their monazite production (e.g., Australia and Malaysia). During World War II started new using of thorium as a component in a hightemperature alloys.

After the war, monazite was processed largely for its nuclear fuel potential. The discovery in 1946 that 232Th could be transmuted into 233U increased the interest in thorium. However, the decision to develop nuclear reactors based on uranium fuels slowed development of thorium-fuelled reactors and reduced thorium demand. During the 1950s, some became new producers of thorium, namely Canada and South Africa, where uranium ores from uraniumenriched quartz-pebble conglomerates contain also some thorium. At this time distinctly increased interest in the rare earth elements (REE) and monazite was mined in the first place for its REE content. Some other thorium was also acquired from REE bearing bastnaesite, occurring in carbonate-enriched magmatic rocks (carbonatites). Much of thorium contained in residues is being stockpiled by private industry [1].

New interest about using thorium as nuclear fuels started in 1960s together with ideas in the development of Fast Breeder Reactors (FBR). Basic research of thorium fuels cycles are being undertaken by Brazil, Germany, the USA, India, Italy, Australia, Canada, China, France, USSR, Romania, and some other countries. Several experimental and prototype nuclear power reactors were successfully operated from the mid-1950s to the mid-1970s using (Th, U)O2 , (Th, U)C2 , and LiF/BeF2 /ThF<sup>4</sup> /UF<sup>4</sup> fuel. The activity of the Nuclear Cycle Division of the IAEA in this area was supported mainly by organizing some technical committee meetings [2–5]. However, thorium fuels have not been introduced commercially because the estimated uranium resources turned out to be sufficient. On the other hand, using thorium in nuclear energy cycle has some significant precedence: (i) the intrinsic proliferation resistance of thorium fuel cycle, (ii) better thermophysical properties and chemical stability of ThO2 , as compared to UO2 , (iii) lesser long-lived minor actinides than the traditional uranium fuel cycle, (iv) superior plutonium incineration in (Th, Pu)O2 fuel as compared to (U, Pu)O2 , and (v) attractive features of thorium related to accelerated-driven system and energy amplifier. However, there are several challenges in the form and back end of the thorium fuel cycles. Irradiated ThO2 and spent ThO2 -based fuels are difficult to dissolve in HNO3 because of the inertness of ThO2 . The high gamma radiation associated with the short-lived daughter products of 232U, which is always associated with 233U, necessitates remote reprocessing and refabricating of fuel. The protactinium formed in thorium fuel cycle also causes some problems, which need to be suitably resolved. Consequently, recently the various experimental nuclear reactors based on thorium fuel cycle are operated only in India. Some other basic research on thorium fuel cycle continued in China, France, Japan, Norway, Russia, and the USA [6].

The other thorium's commercial uses included catalysts, high-temperature ceramics, and welding electrodes. Other no energy uses of thorium are in electron tubes, special use lighting such as airport runway lighting, high-refractive glass, radiation detectors, computer memory components, photoconductive films, target material for X-ray tubes, and fuel cell elements. Its use in most of these products is generally limited because of concerns over its naturally occurring radioactivity. Consequently, no radioactive substitutes have been developed for many applications of thorium. Beryllium, aluminium, and yttrium oxides can be substituted for thorium oxide as a refractory. Yttrium compounds have replaced thorium compounds in incandescent lamp mantles. Magnesium alloys containing Zn, Al, REE, Y, and Zr can substitute for magnesium-thorium alloys in aerospace applications. Research is being conducted to find a replacement for thorium in lamp mantles. These substitutions for thorium in no energy uses are expected to increase because of growing public concern and governmental regulations on radioactive materials [7].
