The reaction in which the nuclear energy is obtained by breaking a heavy nucleus into two or more light nuclei is called as nuclear fission reaction. Highly fissionable material like are not found in nature. However, the natural Uranium contains 0.7% of which has high probability of absorbing a slow neutron and thus forming. Thus, a heavy nucleus is bombarded by thermal neutrons which results in formation of. This highly fissionable material now breaks up and nuclear fission reaction occurs.