A nucleus is bombarded with a high speed neutron so that resulting nucleus is a radioactive one. This phenomenon is called

Fission

When atoms of an element are bombarded by neutrons, the atomic nuclei are (artificially) disintegrated and emit lighter particle ( $eg$ . $\alpha -particle,\beta -particle$ ,proton etc.).
Sometimes a neutron is observed by the nucleus which is converted into its heavier isotope and energy is emitted in the form of $\gamma$ -photons.
This process in which heavy nucleus is broken into two nearly equal fragments is called nuclear fission.