Moderant

Moderant refers to a substance used to slow down a nuclear chain reaction. In nuclear reactors, neutrons are emitted when atoms of fissile material, such as uranium or plutonium, undergo fission. These neutrons can then go on to cause further fissions, creating a self-sustaining chain reaction. However, for controlled nuclear energy production, this reaction needs to be managed.

Moderants work by reducing the speed of fast neutrons to thermal energies. Thermal neutrons are more likely to be captured by fissile nuclei and induce fission, thus sustaining the chain reaction. Common moderating materials include light water (ordinary water), heavy water (deuterium oxide), and graphite. The choice of moderator depends on the specific reactor design and the type of fuel used. Light water is the most common moderator due to its availability and low cost, but it also tends to absorb neutrons, requiring enriched uranium fuel. Heavy water is a more efficient moderator as it absorbs fewer neutrons, allowing for the use of natural uranium. Graphite, while a good moderator, can be susceptible to oxidation at high temperatures. The process of moderation is crucial for the safe and efficient operation of most nuclear reactors.