text: Small amounts of fission products are naturally formed as the result of either spontaneous fission of natural uranium, which occurs at a low rate, or as a result of neutrons from radioactive decay or reactions with cosmic ray particles.  The microscopic tracks left by these fission products in some natural minerals (mainly apatite and  zircon) are used in fission track dating to provide the cooling (crystallization) ages of natural rocks.  The technique has an effective dating range of 0.1 Ma to >1.0 Ga depending on the mineral used and the concentration of uranium in that mineral.
  About 1.5 billion years ago in a uranium ore body in Africa, a natural nuclear fission reactor operated for a few hundred thousand years and produced approximately 5 tonnes of fission products.  These fission products were important in providing proof that the natural reactor had occurred.
  Fission products are produced in nuclear weapon explosions, with the amount depending on the type of weapon.
  The largest source of fission products is from nuclear reactors.  In current nuclear power reactors, about 3% of the uranium in the fuel is converted into fission products as a by-product of energy generation.  Most of these fission products remain in the fuel unless there is fuel element failure or a nuclear accident, or the fuel is reprocessed.