Theoretically possible I suppose, though you require a critcal mass of nuclear matter to begin the reaction. In a power plant this creates heat which is used to produce steam, which turns turbines etc. If the aircrafts power was produced in this way then a substansial amount of water would be required. Secondly, think about what would happen if things went wrong, say a plane exploded in the air, thereby spreading nuclear material for hundreds of miles (unlike in submarines where it would probably just contimate the nearby ocean).