In the fission process, two or three neutrons are formed (Figure 7.4), 99% of which are emitted within a very short time. These are called “prompt neutrons,” and their mean half-life is about 10 4 s. Some fission products also emit neutrons (via neu­tron decay, as discussed in Section 4.4.4), and these are delayed neutrons. The amount of the delayed neutrons is less than 1% of the total numbers of the neu­trons. Delayed neutrons play a role in the neutron balance of the reactor. If all the neutrons were produced as prompt neutrons, the whole fission products would be complete within a very short time and could not be controlled. Therefore, the nuclear reactors are planned in such a way the controlled chain reaction can be ini­tiated in the same time as the delayed neutrons.

The reactors are controlled by control rods. They are fabricated from very good neutron absorbers, such as boron (as boron carbide) or cadmium. The control rods are inserted among the fuel rods (Figure 7.3). The reactivity is controlled by the movement of the control rods. When the reactor starts, the control rods are raised. The power is measured by neutron detectors. When the power reaches the desired value, the control rods are stopped. So, the power does not continue to increase, the reactor becomes critical.

There are three different types of control rods:

1. Safety rods, whose function is the fast stop of the reactor in an emergency. In normal operation, they are totally raised.

2. Shim rods, which are used for coarse control and/or to change reactivity in relatively large amounts. They equalize the changes of the reactivity resulting in burn-up, poison­ing, or breeding. Another tool for the equalization of reactivity is that neutron absorbers (usually boric acid) are dissolved in the coolant, whose concentration can be varied as required. Of course, this method can be applied only in the water reactors.

3. Regulating rods, which are used for fine adjustments and to maintain the desired power or temperature.