The Role of Control Rods in a Thermal Fission Reactor (8.4.5)

The chain reaction in a nuclear power stations must be controlled, which means that the number of neutrons must be continuously regulated to stop the chain reaction diverging or closing down.

The control rod, made of boron, steel or cadmium, which absorb neutrons, can be raised or lowered into the reactor core to control the rate of the chain reactions. The further into the core a control rod is lowered, the more neutrons it will absorb and the more chain reactions it will stop. The number of control rods inserted and the distance by which they are inserted can be varied to control the reactivity of the reactor.

The control rods also control the multiplication factor k which is the ratio of the number of neutrons present at the beginning of a particular generation to the number present at the beginning of the next generation. For k=1, the operation of the reactor is said to be exactly critical, which is the optimal level for a steady-power operation. Reactors are designed so that they are naturally supercritical (k>1); the multiplication factor is then adjusted to the critical operation by inserting the control rods.

In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, due to the force of gravity, fully into the pile to stop the reaction.


Inside the Reactor, showing how the control rods are utilized.