noun \ˈthr-ē-əm\

Definition of THORIUM

:  a radioactive metallic element that is obtained especially from monazite and is usually associated with rare earths — see element table

Origin of THORIUM

New Latin, from Old Norse Thōrr Thor
First Known Use: 1832


noun \ˈthōr-ē-əm, ˈthr-\   (Medical Dictionary)

Medical Definition of THORIUM

: a radioactive metallic element that occurs combined in minerals and is usually associated with rare earths—symbol Th; see element table


noun    (Concise Encyclopedia)

Metallic chemical element, chemical symbol Th, atomic number 90. One of the actinide series of elements, natural thorium is a mixture of radioactive isotopes, predominantly thorium-232 (half-life of more than 10 billion years). It is a dense metal that is silver-white in pure form but turns gray or black on prolonged exposure to air. Although not a nuclear reactor fuel itself, thorium-232 can be used in breeder reactors because, on capturing slow-moving neutrons, it decays into fissionable uranium-233. Thorium is added to magnesium and its alloys to improve their high-temperature strength. Added to glass, it yields glasses with a high refractive index, useful for specialized optical applications. It was formerly in great demand as a component of mantles for gas and kerosene lamps and has been used in the manufacture of tungsten filaments for lightbulbs and vacuum tubes.


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