In a nuclear fission reaction

Two light nuclei combine to produce a heavier nucleus A light nucleus bombarded by thermal neutrons breaks up A heavy nucleus bombarded by thermal neutrons breaks up A heavy nucleus breaks up by itself

A heavy nucleus bombarded by thermal neutrons breaks up

Fission occurs when a neutron slams into a larger atom, forcing it to excite and split into two smaller atoms—also known as fission products. Additional neutrons are also released that can initiate a chain reaction. When each atom splits, a tremendous amount of energy is released