Nuclear fusion reactions, computer artwork. These three reactions are being investigated for use in fusion power. Protons are red and neutrons blue. The relative amount of energy produced by a reaction is shown by the size of the pink flare at right. The top reaction is the D-T fuel cycle. A deuterium (D) nucleus fuses with a tritium (T) nucleus, forming a helium (He-4) nucleus and a free neutron. At centre is the D-He3 fuel cycle, in which D fuses with a He-3 nucleus, forming a helium nucleus and a proton. At bottom is the D-D fuel cycle, where two D nuclei fuse to produce He-3 and a neutron. D-T is the easiest, D-He3 produces the most energy. These reactions will be tested by the ITER project, scheduled to begin in 2016.