Nuclear Power Engineering
The atomic mass of an element is fractional, because

Of uncertainty principle
It contains neutrons
It may have isobars
It may have isotopes

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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 control fusion reaction
It is difficult to initiate fusion reaction

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