Nuclear Power Engineering
Secondary cooling circuit is a must in molten sodium cooled fast breeder reactor for

Avoiding the mixing of water with radioactive sodium, as it may cause explosion
Lowering the coolant circulation pressure
Faster heat removal rate from the core
Achieving high degree of superheat in the steam

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

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

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