Nuclear reactors use a process called nuclear fission, which uses atoms like uranium or plutonium (In particular the isotope Uranium 235) and splits them apart with particles called neutrons. This converts some of the mass into energy, according to Einstein's equation E=mc2. The fissionable elements are placed into rods called 'fuel rods'. The fuel rods are submerged in water, and the energy released in the fission reaction heats up the water which turns into steam.