There were airplane nuclear engine designs built and tested and the concept was discontinued. You can view one of the test engines in Idaho at the EBR-1 Historical exhibit. The reasons for abandoning the idea are many. I think the Russians may have found out why our attempts are a museum exhibit now. The fuel was graphite moderated, some was never used. The fuel was recovered to some extent. If you crash a nuclear plane how do you clean up the mess. Just saying.
They did the very same in 1950s and abandoned it too. Probably a second thought now but I have no idea.
Messes dont matter ever since Fukushima. So party on Garth. Apparently... Expecting India and Pakistan to light em up soon.
There are regimes out there which would not care about side effects, as long as they get a military advantage.