Another important part of nuclear reactors is the moderator. Its function is to slow down the fast neutrons emitted in the fission reaction by reducing the energy of neutrons to the level of thermal neutrons. Light elements are suitable moderators because they can effectively decrease the energy of the fast neutrons through inelastic collisions with neutrons. According to its atomic mass, hydrogen (1H) is expected to be the most effective moderator; however, a suitable moderator must also have a small cross section for neutron captures, which hydrogen does not have. Hydrogen can capture neutrons easily, transforming to deuterium. Deuterium is the most effective moderator, but it is rather expensive. The ratio of the modera­tor effect and the neutron capture can be expressed by the moderation ratio (given in Table 7.1).

Frequently used moderators are the following:

• Water (H2O), which is an effective moderator, absorbs part of the neutrons. A water­moderated reactor is shown in Figure 7.2.

• Heavy water (D2O), which is an effective moderator, absorbs only a few neutrons, but it is expensive.

• Graphite, which is a less-effective moderator than water, absorbs only a few neutrons. Its disadvantage is that it is flammable. In the first nuclear reactor, graphite was applied as the moderator.

• Beryllium and organic solvents are also suitable as moderators.

7.1.1.1 Moderator/Fuel Ratio

The moderator decreases the velocity of the neutrons, as discussed in Section 7.1.1.2. In addition, the moderator acts as a passive controller of the operation of the nuclear reactors.

All substances, including moderators, more or less absorb neutrons, inhibiting the fission chain reaction. Thus, if the reactor contains too much moderator, the degree of neutron absorption increases. However, if the quantity of the moderator is too low, the velocity of the neutrons does not decrease enough. As a result, the effective neutron multiplication factor versus the moderator/fuel plot function has maximum. Those reactors in which the moderator/fuel ratio is below the maximum are under-moderated, and those in which this ratio is above the maximum are over­moderated (Figure 7.5).

Under — and over-moderation play an important role in the safety operation of the reactors. The under-moderated reactors are safer because the decrease of the quantity of the moderator results in the decrease of the effective neutron multiplica­tion factor, stopping the reactor from entering a subcritical state. In over-moderated reactors, however, the decrease of the quantity of the moderator can increase the effective multiplication factor, and then the reactor can become supercritical (as happened in the Chernobyl accident, discussed in detail in Section 7.2).