Artificial neutron flux refers to neutron flux which is man-made, either as byproducts from weapons or nuclear energy production or for a specific application such as from a research reactor or by spallation. A flow of neutrons is often used to initiate the fission of unstable large nuclei. The additional neutron(s) may cause the nucleus to become unstable, causing it to decay (split) to form more stable products. This effect is essential in fission reactors and nuclear weapons.