"what if nuclear power stations had been targeted by Al Qaeda during 9/11 instead of the Pentagon, the WTC and the failed attack on Capitol Hill "
Not much. Even if the containment building had been breached the reactor pressure vessel inside is quite small and well protected.
'It's almost as if they were designed to withstand an airliner crashing into them from the outset.....'
"Hate to think of a plane crashing into a sodium cooled FBR and the subsequent sodium fire."
Sodium FBRs have a habit of catching fire without needing any assistance from external factors, Look at Monju. However as they don't need to be designed to cater to a steam explosion the containment buildings are much smaller and even tougher than PWR reactors.