Editing Pressurized water reactor (section)

=== Control ===
In PWRs 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. (See: [[Negative temperature coefficient]].) Boron and cadmium 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 density of the primary reactor coolant water to decrease, allowing higher neutron speeds, thus less fission and decreased power output. This decrease of power will eventually result in primary system temperature returning to its previous steady-state value. The operator can control the steady state [[operating temperature]] by addition of [[boric acid]] and/or movement of control rods.

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 primary nuclear reactions in 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 and to compensate for [[nuclear fuel]] depletion. However, these effects are more usually accommodated by altering the primary coolant boric acid concentration.

In contrast, [[boiling water reactor|BWR]]s have no boron in the reactor coolant and control the reactor power by adjusting the reactor coolant flow rate.