Q. What is the meaning of the term "critical mass"?

1. This refers to the minimum amount of fissionable material required to sustain a chain reaction
2. This is the material which is just on the verge of becoming radioactive.
3. This refers to the mass of the "critical" elements in a reactor, i.e., the uranium or plutonium
4. This is the amount of mass needed to make a power reactor economically feasible