Editing Fast-neutron reactor (section)

==Control==
Like thermal reactors, fast-neutron reactors are controlled by keeping the [[critical mass|criticality]] of the reactor reliant on [[delayed neutron]]s, with gross control from neutron-absorbing control rods or blades.

They cannot, however, rely on changes to their moderators because there is no moderator. So [[Doppler broadening]] in the moderator, which affects [[thermal neutron]]s, does not work, nor does a negative [[void coefficient]] of the moderator. Both techniques are common in ordinary [[light-water reactor]]s.

Doppler broadening from the molecular motion of the fuel, from its heat, can provide rapid negative feedback. The molecular movement of the fissionables themselves can tune the fuel's relative speed away from the optimal neutron speed. Thermal expansion of the fuel can provide negative feedback. Small reactors as in submarines may use Doppler broadening or thermal expansion of neutron reflectors.