 Hydrogen 's rarer isotopes also each have specific applications . Deuterium ( hydrogen @-@ 2 ) is used in nuclear fission applications as a moderator to slow neutrons , and in nuclear fusion reactions . Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects . Tritium ( hydrogen @-@ 3 ) , produced in nuclear reactors , is used in the production of hydrogen bombs , as an isotopic label in the <unk> , and as a radiation source in luminous paints . 
