A nuclear power plant is a steam turbine plant where steam is generated by a nuclear reactor. Nuclear energy is the energy that is trapped inside each atom. Nuclear energy is obtained by fission of uranium-plutonium or thorium or the fusion of hydrogen into helium.
In normal conditions, the energy released due to the disintegration of a uranium atom is significantly low. The capturing of neutrons by the uranium-235 is rather easy and to fission it for the release of the energy. In a chain reaction, particles released by the splitting of the atom strike other uranium atoms and split them. The particles released by this further split other atoms in a chain process. The chain reaction gives off heat energy. This heat energy is used to boil heavy water at the core of the reactor.