There are three main components to the hybrid fusion fuel cycle: deuterium, tritium, and fissionable elements.  Deuterium can be derived by the separation of hydrogen isotopes in seawater (see heavy water production).  Tritium may be generated in the hybrid process itself by absorption of neutrons in lithium bearing compounds.  This would entail an additional lithium-bearing blanket and a means of collection. Small amounts of tritium are also produced by neutron activation in nuclear fission reactors, particularly when heavy water is used as a neutron moderator or coolant. The third component is externally derived fissionable materials from demilitarized supplies of fissionables, or commercial nuclear fuel and waste streams.  Fusion driven fission also offers the possibility of using thorium as a fuel, which would greatly increase the potential amount of fissionables available.  The extremely energetic nature of the fast neutrons emitted during the fusion events (up to 0.17 the speed of light) can allow normally non-fissioning U to undergo fission directly (without conversion first to Pu), enabling refined natural Uranium to be used with very low enrichment, while still maintaining a deeply subcritical regime.