How fission is controlled in a nuclear reactor?

Answer 1

All reactors require some form of controllable neutron absorber to accommodate power changes, which cause changes in the concentration of fission product neutron poisons such as Xenon, and to accommodate fuel burn up caused reactivity changes. The reactor must be held at criticality during steady operation and very near it during slow power changes, and moveable control rods are provided for this, usually containing boron which absorbs neutrons strongly. When the reactor is tripped or scrammed the rods drop fully in, and start up requires a slow careful approach to criticality.

The following applies to PWR's and is taken from Wikipedia

Generally, reactor power can be viewed as following steam (turbine) demand due to the reactivity feedback of the temperature change caused by increased or decreased steam flow. Boron and control rods are used to maintain primary system temperature at the desired point. In order to decrease power, the operator throttles shut turbine inlet valves. This would result in less steam being drawn from the steam generators. This results in the primary loop increasing in temperature. The higher temperature causes the reactor to fission less and decrease in power. The operator could then add boric acid and/or insert control rods to decrease temperature to the desired point. Reactivity adjustment to maintain 100% power as the fuel is burned up in most commercial PWRs is normally achieved by varying the concentration of boric acid dissolved in the primary reactor coolant. Boron readily absorbs neutrons and increasing or decreasing its concentration in the reactor coolant will therefore affect the neutron activity correspondingly. An entire control system involving high pressure pumps (usually called the charging and letdown system) is required to remove water from the high pressure primary loop and re-inject the water back in with differing concentrations of boric acid. The reactor control rods, inserted through the reactor vessel head directly into the fuel bundles, are moved for the following reasons: * To start up the reactor. * To shut down the reactor. * To accommodate short term transients such as changes to load on the turbine. The control rods can also be used: * To compensate for nuclear poison inventory. * To compensate for nuclear fuel depletion. but these effects are more usually accommodated by altering the primary coolant boric acid concentration.