nuclear fusion is a very powerful source of energy. it relies on forcing atoms together to make helium and energy, exactly as happens in a star like the sun, or in a hydrogen bomb. this needs a large amount of energy to get started, and is not easy to do yet. a big advantage over nuclear fission, which is used in today's nuclear power stations, is that it makes less nuclear waste and does not use a toxic and rare fuel like uranium. more than 600 million tons of hydrogen undergo fusion every second on the sun.