How many neutrons are usually released when a nucleus undergoes fission?

When a nucleus undergoes nuclear fission, the number of neutrons released varies depending on the specific fissioning nucleus and the individual fission event.1

While the exact count fluctuates based on the mass split of the fragments, the average number of neutrons emitted per fission—represented by the symbol $\overline{\nu }$—is typically around 2 to 4. For example, the thermal-neutron induced fission of uranium-235 results in an average of approximately 2.4 neutrons. In other cases, such as the spontaneous fission of uranium-238, the average is about 2.0, while for fermium-257, it is approximately 4.0.1

Most of these neutrons are emitted almost instantaneously as prompt neutrons, primarily by the recoiling fission fragments shortly after scission occurs. The release of these multiple neutrons is what enables the possibility of a chain reaction, which is fundamental to the operation of both nuclear reactors and nuclear weapons.2134