02
Inside a Nuclear Reactor
Rotate the core with your hand, slide the rods out, and feel the exact moment the chain reaction goes critical.
A reactor core is usually a diagram: gray cylinder, a few arrows, the word “fission” floating nearby. This one is a place. You orbit a glass cube holding 729 atoms of fuel, pull the control rods out, and watch neutrons streak from atom to atom until the chain reaction catches. Push the rods back in and feel it starve. Criticality stops being a vocabulary word the moment you are the one holding the rods.
Notes from the making
Every glowing dot is a real object in the simulation: U-235, U-238, and the plutonium that quietly breeds when a stray neutron is captured. Nothing is a particle effect. If a neutron hits a control rod, it is simply gone, and the reaction feels the loss.
The rods are the whole interface, on purpose. I resisted adding a big power meter; you learn criticality by feel, watching the neutron count race or starve as you slide the rods in and out.
There is a track mode that follows one atom through its whole life. Watching a single U-238 swallow a neutron and become plutonium is, I think, the quietest magic trick in physics.