So the first bomb was uranium, resembling Little Boy. The enrichment process was likely the same as for early nuclear devices. Thus, no centrifuges are involved.

So the first bomb was made from uranium, resembling Little Boy. The enrichment process was likely the same as for early nuclear devices. Thus, no centrifuges are involved.

Little boy Fissile uranium mass = 64*0.8 = 50 kg

Fissile uranium needed for PWR  = 27'000 * 0.01 = 270 kg per year per GW

Total GW-year per Little boy    = 50 / 270 = 0.18

Little boy Fissile uranium mass = 64 * 0.8 = 50 kg

Fissile uranium needed for PWR  = 27'000 * 0.01 = 270 kg per year per GW

Total GW-year per Little boy    = 50 / 270 = 0.18

The solution for the second problem is the most complex one. As only one bomb design is feasible for all this to work, I need to create an environment where only one design is possible. I think a good way to do it is when such design is stolen, given, or captured.

How hard it is to fuel nuclear power station using fuel from unexploded Little-Boy-type bombs?

To summarize: It is hard to power a full-scale power station with unexploded little-boy-styled bombs. It is not impossible though.

The solution for the second problem is the most complex one. As only one bomb design is feasible for all this to work, I need to create an environment where only one design is possible. A good way to do it is when such design is stolen, given, or captured.

How hard it is to fuel a nuclear power station using fuel from unexploded Little-Boy-type bombs?

To summarize: It is hard to power a full-scale power station with unexploded little-boy-styled bombs. It is not impossible though.

P.S. If one has heavy water figured out then it is possible just to use a CANDU reactor and never think about fuel enrichment ever again :)