Re: Umm. PDP-11s? good!
"Had this discussion with some folks about why they still (!) used XP."
No, it was a different discussion. XP is a commercial operating system with many undocumented hooks between subsystems. A PDP-11 running a nuclear plant is going to be running some extremely dedicated code on an operating system which is little more than a program launcher running on bare metal. In fact, it would probably be perfectly feasible to implement the entire gubbins on an FPGA.
There is a very big difference between running commercial software on commercial hardware, and running dedicated software on dedicated hardware. One gets you to the fringes of the Solar System, one potentially gets you infested with viruses.