Chances are that the nuclear plant would have held up very well to an airplane crash. The containment buildings are designed with that in mind. Search YouTube for video of crash tests where a fighter plane was flung at a mock up of a containment building wall. It's impressive. The World Trade Center buildings were an easy, high profile target for inexperienced pilots to hit. Washington DC might have been too hard of a nut to crack, though with the strike on the Pentagon, maybe it wouldn't have been.
I don't like Sodium cooled reactors for the same reason you are pointing out. Leaking hot Sodium metal is a massive hazard. There is a reactor in Japan where they dropped a crane into the Sodium pool. That spelled the end of that reactor and I think it is still just sitting there waiting to be cleaned up.