Define neutron moderation in the context of nuclear fission.

Neutron moderation refers to the process of slowing down fast neutrons to increase the likelihood of these neutrons being absorbed by the reactor fuel, which typically consists of fissile materials like uranium-235 or plutonium-239. In a nuclear fission reaction, fast neutrons produced during fission can escape the fuel or not cause further reactions if they are not slowed down.

By using materials known as moderators—common examples include water, heavy water, and graphite—neutrons can lose energy through collisions with the atoms in the moderator. As the neutrons slow down and transition from fast to thermal energies, their probability of being captured by the fuel increases significantly. This process is crucial because thermal neutrons are more effective at sustaining the fission chain reaction, which is necessary for the continuous operation of a nuclear reactor.

The other choices do not accurately describe the function and importance of neutron moderation in a nuclear fission context. Increasing neutron speed isn't beneficial for sustaining fission; capturing neutrons for shielding does not address the goal of sustaining the chain reaction, and enhancing nuclear reactions without mentioning the need for slowing neutrons misses the critical role of moderation.