Schematic representation of control rods in a nuclear reactor. When the rods are lowered, the reaction is subcritical: too many neutrons are absorbed to allow a nuclear chain reaction. When the rods are pulled out, the reactor can become critical, and the fuel rods will produce heat (and neutrons).
A rule bar or control element is a bar made of a material that absorbs neutrons, which serves to control the nuclear chain reaction in a nuclear reactor.
Suitable materials for control rods contain elements that have a large working section for the absorption of neutrons (neutron gain) without disintegrating themselves. Many applied materials (often in alloys or compounds, to obtain suitable mechanical properties) are cadmium and boron.
The control rods are one of the mechanisms for regulating the state of a nuclear reactor, and are usually also the most important switch in the event of a rapid shutdown of the reactor (a so-called emergency stop or scram). In most reactor types, the rods hang in the reactor and can function as a fault tolerant (failsafe) safety system: they are held with an electromagnet in place, and in the event of unintentional drop of the control current they automatically fall into the reactor becomes subcritical and the nuclear chain reaction stops. In certain types of reactors, such as the boiler water reactor, the rods are inserted from the bottom of the reactor and must be actively pushed into the reactor core.
In reality, the control process is more complicated: the course of the reaction also depends on the temperature in the reactor, and the presence of certain wastes of the cleavage process (which sometimes absorb many neutrons). In addition, the heat produced is also an important variable. For example, when an emergency stop pushes the control rods and a lot into the reactor, the nuclear chain reaction stops almost immediately (the reactor becomes subcritical), but the reactor continues to produce heat for some time, which has to be removed. If not, a meltdown may occur.
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