Most currently active reactors were designed, built and certified in the sixties and seventies. All systems in those plants are 60's or 70's electronics. Most won't even have something as modern as a pdp-8 to control stuff. Go watch the China Syndrome if you need a reminder.
Interfacing 40 year old control electronics to modern computers is more than a 'airgap'. It's more like your kid trying to explain GTA4 to a stone age caveman without a computer present.