Of course, but those externalities are for more expensive/less cost efficient when discussing uranium and nuclear power than anything involving diesel. Personnel training, fuel production, equipment maintenance, pick an angle to inspect and you'll find the nuclear solution is far more expensive than traditional power plants.
Cost of operation isn't an externality. Externalities are costs we don't directly pay for, like carbon emissions causing global warming. Fossil fuels have incalculably higher externalities in this regard.
You are talking about direct costs that are priced in, and I actually disagree about including personnel and training costs in this specific instance because they are all - to an extent - fungible in the military or navy, as there is no shortage of souls and people will be trained for something for some duration of time.