Power Systems
Critical mass of the fuel is the amount required to make the multiplication factor

 0.5.
  equal to factor.
 more than 1.
 less than 1.

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Power Systems
A sodium graphite reactors use

 a mixture of sodium and graphite as coolant.
  sodium as coolant and graphite as moderator.
 sodium as moderator and graphite as coolant.
 a mixture of sodium and graphite as moderator.

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