In the context of nuclear reactors the most important reactions are those where the incident particle is a neutron. Also, neutrons are electrically neutral and are able to interact with nuclei much more readily than charged particles due to the absence of any electrostatic barrier.

In a nuclear reaction the neutron may undergo a scattering collision or it may be absorbed by the tar­get nucleus. These possible neutron reactions will now be discussed in some detail here and in the following Sections but the general principles apply also to re­actions induced by particles other than neutrons.