More importantly, 9/11 hit the WTC. When it did everyone immediately said "imagine if they'd hit a nuclear reactor".

But they didn't. They could've, but they didn't (and it also wouldn't have done very much damage). The reality is that even at the largest conceivable scales, nuclear security as implemented is not threatened by violent terrorism or direct action, so much as leaky side-channels (scientists selling off isotopes on the side).