Final answer:
The mass of fissionable material needed for a self-sustaining chain reaction is called the Critical mass.
Step-by-step explanation:
The mass of fissionable material needed for a self-sustaining chain reaction is called the Critical mass. It refers to the minimum amount of fissile material required to sustain a nuclear chain reaction. If the mass is below the critical mass, the chain reaction will not be sustained, while if the mass exceeds the critical mass, a nuclear explosion may occur.
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