They shot an F4 target drone at a block of "reactor grade" concrete wall back in the 80s and they took measurements and did science on the resulting lack of damage and concluded that a reactor can shrug off one of anything. They didn't change containment buildings to be plane proof. It's just a side effect of the design required to contain a melting down reactor with a sufficient safety factor.

video of the test https://www.youtube.com/watch?v=F4CX-9lkRMQ

A whole lot of concrete, mostly.

Yes. A NRC requirement for utility scale reactors after 9/11. One wonders why nuclear reactors are so expensive.

It's a good reason to design reactors to be as small as possible, with a small containment building. Much easier to build.