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 reaction necessarily cause fission in another nucleus. However, when such neutrons do initiate fission, it’s called a nuclear chain reaction.

Chain reactions are described with neutron ‘generations’. The neutron that starts a chain reaction is the first generation, and the resulting fission produces the second generation. The neutrons produced from the fissions induced by the second-generation neutrons are the third generation. The chain reaction continues until no more neutrons are produced. 

If the average number of fissions remains the same from one generation to the next, energy is produced at a constant rate. In most cases, this process is more likely if the neutrons slow down well before they leave the material.

A certain minimum mass, called critical mass, of fissionable material is required to ensure that the neutrons produced have enough material to induce further fission. A subcritical mass is any amount below the threshold for critical mass, and a supercritical mass is any amount above that threshold.

Critical mass is affected by temperature, shape, and the composition of the surroundings. Changes in these parameters could make a subcritical mass critical or vice versa.