In a nuclear reactor, the part that absorbs excess neutrons from a chain reaction is the moderator.
Explanation:
1. The moderator in a nuclear reactor plays a crucial role in slowing down fast neutrons produced during the fission process.
2. By slowing down the neutrons, the moderator increases the likelihood of neutron capture by fissile isotopes, such as uranium-235 or plutonium-239, which sustain the chain reaction.
3. If there are excess neutrons in the reactor core, the moderator helps regulate and control the rate of the chain reaction by absorbing some of the neutrons.
4. Common moderators used in nuclear reactors include water, graphite, and heavy water (deuterium oxide).
Therefore, the correct answer to the question is C. Moderator.
