Nuclear Power Engineering
The critical mass of a fissionable material can be reduced by

Surrounding it by neutron absorbing material
Heating it
Cooling it
Surrounding it by neutron reflecting material

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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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