Editing Control rod (section)

==Operating principle==
[[File:Nuclear Reactor Uranium Pile (30502443888).jpg|thumb|upright=1.25|1943 Reactor diagram using boron control rods]]
Control rods are inserted into the [[nuclear reactor core|core of a nuclear reactor]] and adjusted in order to [[process control|control]] the [[reaction rate|rate]] of the [[nuclear chain reaction]] and, thereby, the [[thermal power]] output of the reactor, the rate of [[steam]] production, and the [[electrical power]] output of the power station.

The number of control rods inserted, and the distance to which they are inserted, strongly influence the [[reactivity of a nuclear reactor|''reactivity'']] of the reactor. When reactivity (as [[effective neutron multiplication factor]]) is above 1, the rate of the [[nuclear chain reaction]] increases exponentially over time. When reactivity is below 1, the rate of the reaction decreases exponentially over time. When all control rods are fully inserted, they keep reactivity barely above 0, which quickly slows a running reactor to a stop and keeps it stopped (in [[shutdown (nuclear reactor)|shutdown]]). If all control rods are fully removed, reactivity is significantly above 1, and the reactor quickly runs hotter and hotter, until some other factor (such as [[Fuel temperature coefficient of reactivity|temperature reactivity feedback]]) slows the reaction rate. Maintaining a constant power output requires keeping the long-term average neutron multiplication factor close to 1.

A new reactor is assembled with its control rods fully inserted. Control rods are partially removed from the core to allow the [[nuclear chain reaction]] to start up and increase to the desired power level. [[Neutron flux]] can be measured, and is roughly proportional to reaction rate and power level. To increase power output, some control rods are pulled out a small distance for a while. To decrease power output, some control rods are pushed in a small distance for a while. Several other factors affect the reactivity; to compensate for them, an automatic control system adjusts the control rods small amounts in or out, as-needed in some reactors. Each control rod influences some part of the reactor more than others; calculated adjustments to fuel distribution can be made to maintain similar reaction rates and temperatures in different parts of the core.

Typical [[shutdown (nuclear reactor)|shutdown]] time for modern reactors such as the [[European Pressurized Reactor]] or [[Advanced CANDU reactor]] is two seconds for 90% reduction, limited by [[decay heat]].
{{anchor|control rod drive mechanism}}
Control rods are usually used in control rod assemblies (typically 20 rods for a commercial PWR assembly) and inserted into guide tubes within the fuel elements. Control rods often stand vertically within the core. In PWRs they are inserted from above, with the control rod drive mechanisms mounted on the reactor [[pressure vessel]] head. In BWRs, due to the necessity of a steam dryer above the core, this design requires insertion of the control rods from beneath.