What is a moderator in a nuclear reactor?

• A. The medium used to slow down fast neutrons
• B. A device that absorbs heat from the core
• C. A structure that channels coolant to the reactor
• D. A material that combusts to sustain the reaction

Answer: (A)

A moderator is the material that slows down fast neutrons to thermal energies, which greatly increases the chance that neutrons will induce another fission in fuel like U-235. After fission, neutrons are emitted with high energy, but the fission cross section of most fuels is much larger for thermal neutrons, so slowing them to be "thermal" makes the chain reaction more efficient. This slowing comes mainly from elastic collisions with light nuclei—the lighter the nucleus, the more energy a neutron loses per collision. Common moderators are light materials such as hydrogen (in water), deuterium (heavy water), or carbon (graphite). A good moderator also minimizes neutron absorption, so neutrons stay available to cause more fission. In many reactors, the moderator also carries away heat, acting as a coolant. The other descriptions refer to cooling components or to a substance that would sustain the reaction by combustion, which isn’t how moderation works.