What is the role of a moderator in reactors?

A moderator in reactors slows down fast neutrons, enabling them to cause further fission reactions.

In nuclear reactors, a moderator plays a crucial role in sustaining the nuclear chain reaction. The primary function of a moderator is to reduce the speed of fast neutrons produced during nuclear fission. These fast neutrons are too energetic to cause further fission in the fuel. By slowing them down to thermal (or slow) neutrons, the probability of these neutrons causing further fission in the fuel nuclei increases, thus sustaining the chain reaction.

The choice of a moderator is critical and depends on two main properties: low neutron-absorption cross-section and high scattering cross-section. The former ensures that the moderator does not absorb the neutrons, while the latter ensures that the neutrons are effectively slowed down. Commonly used moderators include light water, heavy water, and graphite.

Light water, which is ordinary water, is the most commonly used moderator in reactors. It is effective in slowing down neutrons but has a relatively high neutron absorption cross-section. This means that more fuel is needed to sustain the chain reaction. Heavy water, or deuterium oxide, is a more efficient moderator as it has a lower neutron absorption cross-section. However, it is more expensive and not as readily available as light water. Graphite, a form of carbon, is also used as a moderator, particularly in gas-cooled and some types of advanced reactors.

The moderator also plays a role in the safety of a nuclear reactor. In the event of a power surge, the moderator can help to control the reaction. For example, in a light water reactor, if the temperature rises too high, the water can boil and turn into steam. Since steam is less dense than water, it is less effective as a moderator. This reduces the number of slow neutrons, thus slowing down the chain reaction and reducing the power output. This is known as a negative feedback mechanism and is one of the ways a reactor can be made inherently safe.

In summary, the moderator in a reactor is a vital component that enables the nuclear chain reaction to be sustained and controlled.