How is nuclear energy generated ?

Nuclear energy is the energy released when the structure of nuclei inside an atom changes. This is released in two ways, nuclear fission and fusion. In nuclear fission, relatively large nuclei of, say, uranium or plutonium, are split, while in nuclear fusion, smaller nuclei of deuterium and tritium, for example, are combined. In both cases, a large and highly concentrated amount of energy is released. For instance, 1 gram of uranium-235 can produce as much energy as that generated by burning 2.5 tons of coal.