Why do chain reactions happen in nuclear fission?

Answer 1

When a slow neutron is captured by a nucleus of U235 or Pu239, the nucleus fissions or splits into two fission fragments and on average 2.5 neutrons are released as well as the energy which we use as heat. In the reactor, some of these are absorbed by the moderator, some escape from the core at the boundaries, but if just one of them is then captured by another nucleus, the process is self repeating, it goes on at a steady rate which can be described as a chain reaction, a type of endless chain. For the reaction to continue at a steady rate, the number of neutrons buzzing about in the core must be constant, this is called the neutron flux, so many neutrons per sq cm per second. The fine control of neutron absorption is done by control rods of absorbing material, usually boron steel alloy, and these are adjusted so that the flux is constant, giving constant power. Note that the reaction is self starting due to the fact that U235 and Pu239 give off a small number of spontaneously produced neutrons, this happens whatever the state of the reactor and continues when it is fully shutdown, so as soon as the control rods are withdrawn the neutron flux starts to increase. You don't have to ignite the fuel to start, as you do with fossil fuels.

Answer 2

Fission of heavy nuclei can release alpha and beta particles, as well as neutrons. All of these particles are capable of initiating the fission of a heavy nucleus that they collide with.

In a large enough sample of a fissionable element (critical mass), there is a high probability that these particles, when released by spontaneous decay, will initiate a self-sustaining nuclear chain reaction.

This reaction, moderated by neutron absorbers such as carbon or gadolinium, is what releases the energy in nuclear power plants.

In an even larger (or denser) sample (supercritical mass), this reaction progresses geometrically (1, 2, 4, 16, 256, etc) and enormous amounts of energy are released.

This type of reaction is what occurs when an atomic bomb detonates.

Answer 3

The fact that for every fission of U-235, on average 2.5 neutrons are released. By slowing these down with a moderator, at least one can be captured by further U-235 nuclei, thus making the chain reaction possible.

Answer 4

The fact that when a nucleus of U-235 or Pu-239 fissions, free neutrons are emitted, about 2.5 per fission on average. So if you can arrange to absorb one of these in another nucleus, causing another fission, the process will continue and a chain reaction results.

Answer 5

This occurs through the control of the number of available neutrons through the use of high neutron capture control material.