Final answer:
A requirement for a nuclear chain reaction is neutron-induced fission that releases additional neutrons, leading to a self-sustaining process. This requires a critical mass of fissionable material, plus a moderator to slow down the neutrons, a coolant to dissipate heat, and control rods to regulate the reaction.
Step-by-step explanation:
A requirement for any nuclear chain reaction is neutron-induced fission which releases more neutrons. This means that for a chain reaction to be sustained, there must be fissionable material that is at least at a critical mass—this is the minimum amount necessary to maintain a self-sustained chain reaction. Other components necessary for a nuclear reactor to function safely and effectively include a nuclear moderator, which slows down the neutrons so they can cause more fissions; a coolant, to transfer the heat generated by the reaction; and a control system, usually involving control rods that absorb neutrons to regulate the reaction rate.
The critical mass is key because it determines whether the material will have a self-sustaining chain reaction; less than critical mass is considered sub-critical and cannot sustain a chain reaction, while more is supercritical and can lead to increasing rates of fission. Therefore, a sub-critical mass of fissionable material, such as U-235, would not be sufficient for a nuclear chain reaction, nor would the presence of only a coolant or a moderator that absorbs neutrons rather than slowing them down.