Nuclear Power Engineering
A heterogeneous reactor is the one, in which the moderator and the

Coolant are different materials
Fuel are present in different phases (e.g. uranium and heavy water)
Coolant are present in different phases (e.g., heavy water and graphite)
None of these

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Nuclear Power Engineering
Commercial power generation from fusion reactor is not yet possible, because

It is difficult to control fusion reaction
Quantity of fuel required for initiating fusion reaction is prohibitively high
The fuel required (e.g. deuterium and tritium) is scarce
It is difficult to initiate fusion reaction

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