Power Systems
A moderator material should have

 may be any of the above.
 large atomic mass.
  small atomic mass.
None of these

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Power Systems
Critical mass is

 a mass of fissionable material required to stop a chain reaction.
  minimum mass of fissionable material required to sustain chain reaction.
 maximum mass of fissionable material required to sustain a chain reaction.
 none of these.

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