Take a heavy Atom ²³⁵U, add 1 neutron to make ²³⁶U, and BANG It splits into two lighter Atoms, Barium and Krypton, plus three neutrons, the total weight of all these is less than the weight of ²³⁶U which is where the energy comes from. The three neutrons are now free to find another atom of ²³⁵U and it happens all over again! This is called FISSION This picture was taken from Our Text.