What is the typical function of the moderator substance in a nuclear reactor?

The function of the moderator substance in a nuclear reactor is to slow down neutrons. During nuclear fission, fast neutrons are produced which can be too energetic to efficiently sustain further reactions. The moderator, made from materials like water, graphite, or heavy water, interacts with these fast neutrons, reducing their speed through elastic collisions. This slowing down is crucial because thermal (slow) neutrons are more likely to be absorbed by fissile materials like uranium-235, leading to a sustained chain reaction.

In terms of the other options, while controlling temperature is essential in reactor operations, it is not the primary function of the moderator. Structural support is typically provided by the reactor's construction materials and design, not by the moderator. Absorbing excess neutrons is more aligned with the function of control rods, which are used to manage the fission process rather than the role of the moderator.