I have already written about thorium under the general subject of nuclear reactors. I have decided to cover thorium in more depth because of its possible use as a nuclear fuel.
Thorium is a radioactive element with the symbol Th and the atomic number 90. It was discovered in 1828 by the Swedish chemist Jons Jacob Berzelious. The element was named for Thor, the hammer wielding Norse god.
Thorium is a silvery-white metal that is soft and ductile. It oxidizes slowly and the chemical properties are strongly affected by the degree of oxidation. Powdered thorium is pyrophoric which means that it can spontaneously burst into flame when exposed to open air. Thorium can form compounds with oxygen, hydrated nitrogen and fluoride, carbon, and phosphate.
Thorium has 33 isotopes, all of them radioactive. They range in atomic weight from 209 to 238. Their half-lives vary from Th-220 at 9 millionths of a second to Th-232 at 14 billion years, about the current age of the universe. Six of the isotopes of thorium occur in nature, mostly Th-232. Traces of Th-230 occur as a result of the decay of U-238. Most isotopes of thorium emits positrons and decays to radium. In rare cases some isotopes emit alpha particles and decay to actinium. Th-232 can decay to radium, uranium, ytterbium and neon. Isotopes above 232 emit electrons and decay to protactinium-233. When bombarded with a neutron source, Th-232 can absorb neutrons and then decay to protactinium. Protactinium in turn decays to uranium-233.
Natural thorium is found in most soil and rock on Earth. It about four times as abundant in the earth’s crust as uranium, three times as abundant as tin and about the same abundance as lead. It occurs in several minerals including thorite with oxygen and silicon, thorianite combined with oxygen and monazite which is a phosphate mineral containing rare earth metals. Rare earth metal mining and extraction produce thorium as a byproduct.
A common use of thorium was as a component in alloys such as a magnesium alloy called Mag-Thor that was used in the aerospace industry for engines because of its stability at high temperatures. It has also been used in electronic applications and welding rods. Thorium dioxide has a very high melting point and was used in the mantles of gas lamps and as an additive for high temperature ceramics and laboratory glassware Recent concerns over radioactivity have ultimately made thorium and thorium dioxide unattractive for these application.
Breathing large amounts of thorium dust has been shown to lead to lung disease or lung cancer. Thorium in the bloodstream can cause liver cancer, pancreatic cancer, leukemia, bone cancer, kidney cancer, and cancer of the spleen. Being around mining and processing facilities for uranium, phosphate or tin ore or nuclear waste can result in thorium exposure. Injection of thorium compounds for contrast enhancement in x-rays has been shown to be a health threat.
Thorium crystal:
