What is the primary reason for using moderators in nuclear reactors?

Using moderators in nuclear reactors is crucial primarily because they are effective at slowing down neutrons. In nuclear fission reactions, fast neutrons are produced as a byproduct when a nucleus splits. However, these fast neutrons are less likely to induce further fission events in fissile materials such as uranium-235 or plutonium-239.

Moderators, which can be made from materials like water, heavy water, or graphite, interact with these fast neutrons and reduce their speed, converting them into thermal or slow neutrons. These slowed-down neutrons are much more likely to be captured by fissile nuclei, thereby increasing the probability of sustaining a controlled chain reaction. This process allows the reactor to maintain a steady rate of fission while producing energy efficiently.

The other options relate to different aspects of nuclear reactors but do not capture the primary role of moderators. Initiating fission involves different procedures such as neutron sources, while increasing fission rates and detecting radiation pertain to different reactor designs and infrastructures. Thus, the role of moderators in slowing down neutrons is essential for ensuring the reactor operates safely and effectively.