Nuclear fusion. Computer artwork of a nuclear fusion reaction, showing tritium and deuterium (upper left & right) nuclei colliding to form helium (lower left) and a neutron (lower right). This process releases a huge amount of energy. Deuterium and tritium are isotopes of hydrogen. Isotopes are forms of an element that contain different numbers of neutrons (blue) in the nucleus. Protons (red) are the other constituent of the nucleus. Nuclear fusion is the joining (fusing) of light elements to form heavier elements, which releases large amounts of energy. It is hoped that fusion will be a clean, renewable energy source for the future.