Editing Neutron radiation (section)

=== Neutron radiation from fission ===
The neutrons in nuclear reactors are generally categorized as [[slow neutron|slow (thermal) neutrons]] or [[fast neutron]]s depending on their energy. Thermal neutrons are similar in energy distribution (the [[Maxwell–Boltzmann distribution]]) to a gas in [[thermodynamic equilibrium]]; but are easily captured by atomic nuclei and are the primary means by which elements undergo [[nuclear transmutation]].

To achieve an effective fission chain reaction, neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the [[nuclear fuel]] is not sufficiently refined to absorb enough fast neutrons to carry on the chain reaction, due to the lower [[Cross section (physics)|cross section]] for higher-energy neutrons, so a [[neutron moderator]] must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators include [[graphite]], ordinary (light) [[water]] and [[heavy water]]. A few reactors ([[fast neutron reactor]]s) and all [[nuclear weapon]]s rely on fast neutrons.