Deuterium is easy to find in seawater. 1 in 5000 hydrogen atoms are deuterium. However, tritium is a radioactive isotope. If lithium is exposed to neutrons with much energy, tritium is produced. The deuterium–tritium reaction also produces one neutron. In fusion reactors, a 'breeding blanket' is used where the walls of the fusion reactor have lithium on them. This way, the neutrons produced from fusion will hit the lithium and produce more tritium.