A fusion reaction, on the other hand, occurs when hydrogen nuclei collide and form a heavier nucleus like helium. More energy is required to "ignite" a fusion reaction, but when ignited the excess energy is far greater than in a fission reaction. The only fuel required in a fusion reactor is the heavy hydrogen found in seawater, a virtually limitless source of energy. If controlled fusion were to become reality it would be one of the great breakthroughs for civilization. However, the scientific and engineering problems associated with a fusion reactor are great. They can be divided into three categories:   1. Confinement (How long does the plasma stay around?)   2. Heating (Does the plasma get hot enough?)   3. Technology (reactor design -- many problems remain for future research)