Final answer:
To sustain a nuclear chain reaction, the critical mass of fissile material such as uranium is required so that released neutrons continue the reaction.
Step-by-step explanation:
The necessary condition to maintain a nuclear chain reaction is critical mass. This is the specific amount of fissile material needed so that each neutron released during the reaction is likely to be captured by another nucleus, perpetuating the process. If the mass of uranium or another fissile element is not large enough, the probability of neutron capture is low and the chain reaction will cease. The critical mass ensures self-sustenance of the chain reaction in a controlled environment, such as a nuclear power plant, with the use of control rods and moderators to regulate the reaction and prevent an explosion.