I also think that actual Pu core was used in one of the worse-r tests in the pacific, rendering more than a few islanders contaminated and homeless.
Supercriticality like that is casually called the "blue flash". I'm still not sure if it's actually ionization/glowing air or Cerenkov effect (like the "glowing core" in the swimming pool photos) within the eye. Haven't studied it, would rather not experience it.
Doing criticality tests was one of the experiences that I've missed out on. My wife did a few in the early naughts - at that point remotely operated of course, but it was truly a case of "turn the knob to bring the stuff closer and watch the neutrons grow!"
All gone now from Los Alamos but for a few historic sites (The Slotin accident in particular is one of a handful of memorials in LA). It turns out that the security weenies don't like that much HEU and or Pu laying around near a (then) public highway so it's all been sent far away to Nevada.
