Nuclear Fission

Nuclear fission is a process in which a heavy nucleus disintegrates into two or more lighter nuclei of different sizes, or fission fragments, and neutrons. Remarkably, the fission fragments and number of neutrons are not the same for every fission. 

However, the sums of the mass and atomic numbers are always the same on both sides of fission equations. In addition to the ‘prompt’ neutrons produced by fission, additional ‘delayed’ neutrons may be produced after beta decay of the high-energy fission fragments.

In fission reactions, the sum of the binding energies of the daughter nuclides is greater than the binding energy of the parent nuclide. The difference accounts for the huge amount of energy released during fission.

The neutrons released by fission are typically ‘fast’ neutrons, which have high kinetic energies and move through most large nuclei without interacting with them. 

Neutrons lose substantial energy upon colliding with similarly-sized nuclei. Those that approach equilibrium with their surroundings are ‘slow’ or ‘thermal’ neutrons. Fissionable nuclides that undergo fission by absorbing thermal neutrons are called ‘fissile’. 

Not all neutrons produced in a fission react