Nuclear fusion is the fusing of atoms, thus causing a great release of energy. This is what powers (albeit somewhat partially, due to the fact that a fission bomb is needed to start the reaction) fusion bombs (thermonuclear) and the sun. Nuclear fusion as to reactors is being researched around the globe, and results are expected mostly in 2050 or so. It is being researched mainly at the Large Helical Device in Japan and the Tokamak Reactor in the US.