Nuclear Reactor

A reactor sustains fission at a steady rate by holding the neutron multiplication factor at unity, so that each fission event produces on average exactly one further fission. Achieving that balance requires several components acting together. Fuel supplies the fissile nuclei; a moderator, typically water or graphite, slows fast neutrons to the low energies at which fission is most probable; control rods of a neutron-absorbing material such as boron or cadmium are inserted or withdrawn to trim the reactivity; and a coolant carries away the heat, which in a power reactor raises steam for a turbine. Delayed neutrons emitted by fission fragments a few seconds after the event are what make the process controllable on a human timescale. Research reactors serve isotope production and neutron scattering rather than generation.