Thorium is a naturally occurring, weakly radioactive actinide metal with the symbol Th and atomic number 90. In its pure state, it is a silvery-white metal that slowly tarnishes to a dark gray or black oxide layer when exposed to air. Named after the Norse god of thunder, Thorium is significantly more abundant in the Earth's crust than uranium, making it a subject of intense interest for the future of sustainable nuclear energy. It is often discussed alongside other radioactive elements like Radium and is closely related to the Uranium Decay Series.
Thorium is a soft, ductile metal with one of the highest liquid temperature ranges of any element, melting at 1,750 °C and boiling at 4,788 °C. It is found primarily in the +4 oxidation state. Its most common oxide, Thorium Dioxide (), has the highest melting point of any known oxide (3,300 °C).
Unlike Uranium-235, naturally occurring Thorium-232 is fertile rather than fissile. This means it cannot sustain a chain reaction on its own but can be transmuted into the fissile isotope Uranium-233 through neutron capture and subsequent beta radiation.
The use of Thorium in Molten Salt Reactors is frequently cited by nuclear proponents as a "safer" alternative to traditional light-water reactors. These systems operate at atmospheric pressure and are chemically stable. Furthermore, the Thorium cycle produces significantly less long-lived transuranic waste and is inherently more resistant to nuclear proliferation, as the production of fissile is typically accompanied by , which emits high-energy gamma radiation, making the material easy to detect.
Almost all natural thorium consists of the isotope . It decays via alpha radiation with an incredibly long half-life of 14.05 billion years—comparable to the age of the universe. This slow decay makes it relatively safe to handle in bulk compared to other radioactive materials, though its daughter products present their own radiological hazards, eventually terminating at the stable isotope Lead-208.
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