Thorium is a chemical element with the symbol Th and atomic number 90.
Thorium is a naturally occurring, slightly radioactive metal. It is estimated to be about three to four times more abundant than uranium. It has been considered a waste product in mining rare earths, so its abundance is high and cost low.
Thorium was successfully used as a breeding (fertile) source for nuclear fuel – uranium (233) in the molten-salt reactor experiment (MSR) from 1964 to 1969 (producing thermal energy for heat exchange to air or liquids), as well as in several light water reactors using solid fuel composed of a mixture of 232Th and 233U, but a thorium-uranium mix was only used at end of life to demonstrate Th-to-U breeding. Advocates of the use of thorium as the fuel source for nuclear reactors state that they can be built to operate significantly cleaner than uranium-based power plants as the waste products are much easier to handle.