nuclear power!

In physics and nuclear chemistry, nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus. It is accompanied by the release or absorption of energy. Iron and nickel nuclei have the largest binding energies per nucleon of all nuclei and therefore are the most stable. The fusion of two nuclei lighter than iron or nickel generally releases energy while the fusion of nuclei heavier than iron or nickel absorbs energy; vice-versa for the reverse process, nuclear fission.
Nuclear fission—also known as atomic fission—is the split of the nucleus of an atom into two or more smaller nuclei, and usually some by-product particles. Hence, fission is a form of elemental transmutation: fission changes an element into several other elements. The by-products include free neutrons, photons usually in the form of gamma rays, and other nuclear fragments such as beta particles and alpha particles. Fission of big elements is an exothermic reaction can release substantial amounts of useful energy both as gamma rays and as kinetic energy of the fragments (heating the bulk material where fission takes place).
I hope this can help your understanding.

It is about nuclear reaction.