Assume that .015% of the hydrogen atoms in seawater H2O are deuterium. The atomic mass number of oxygen is 16. About how many kilograms of seawater would be needed to provide the hydrogen fuel for fusion reactors to meet 10^20 J? The number of deuterium atoms necessary to provide this magnitude of energy = 2.89 x 10^31 atoms, and the amount of energy released in a single fusion reaction = 3.46 x 10^-12 J.