Power Systems
The total number of strands(N) is concentrically stranded cable with total annular space filled with strands of uniform diameter is given by (if x is number of layers)

 N = 3x²-2x+1
 N = 3x²-3x+1
 N = 3x²-6x+1
 N = 3x²+3x+1

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Power Systems
A CANDU reactor uses

 only fertile materials.
 highly enriched uranium as fuel and water as moderator and coolant.
 enriched uranium as fuel and ordinary water as moderator and cooler.
  natural uranium as fuel and heavy water as moderator and coolant.

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