 2H , the other stable hydrogen isotope , is known as deuterium and contains one proton and one neutron in the nucleus . All deuterium in the universe is thought to have been produced at the time of the Big Bang , and has endured since that time . Deuterium is not radioactive , and does not represent a significant toxicity hazard . Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water . Deuterium and its compounds are used as a non @-@ radioactive label in chemical experiments and in solvents for 1H @-@ NMR spectroscopy . Heavy water is used as a neutron moderator and coolant for nuclear reactors . Deuterium is also a potential fuel for commercial nuclear fusion . 
