The thing about nuclear submarines is they don't crash. If they sink they go quietly to the bottom of the ocean which is a good place to store nuclear waste.

When the X-6 flew it was always followed by a C-119 with a specially trained nuke hazmat team that would be able to immediately parachute and secure the crash site. And this was in the 1950's when nuclear explosions were seen as spectator events.

A bit impractical don't you think for your average airliner or military bomber? Especially as there are bad guys looking to shoot down the latter and hijackers for the former. Imagine 911 if the airliners had nuclear propulsion.