> but putting a reactor on each plane would probably have negative externalities.
Probably? It would be a disaster every time one crashes, would carry a huge proliferation and terrorism risk. Oof.
In the 50's some countries were that crazy and they even put reactors in space. Two of which crashed and one contaminated a huge area in Canada. Luckily common sense prevailed and these things don't happen anymore. Though nuclear ships still exist, there's only a few icebreakers in the civilian fleet AFAIK.