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

It is difficult to initiate 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 control fusion reaction

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Nuclear Power Engineering
Molten sodium (as a coolant in fast breeder reactor)

None of these
Is highly radioactive at elevated temperatures and can cause explosion, when it comes in contact with air or water
Is not at all corrosive, even at a higher temperature
Can't attain high temperature at normal pressure

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