The US very nearly built a 60MW nuclear reactor for use in airplanes after their scaled down design at 2.5MW was successfully built, ran, and tested. This was done in the 1950s and, incidentally, required inventing molten salt reactors. https://en.m.wikipedia.org/wiki/Aircraft_Reactor_Experiment

(They actually planned to go all the way to 350MW, which could theoretically run a transatlantic passenger jet with 2,000 passengers, assuming it’s even possible to build such an airframe.)