Nuclear Fission

What to observe

1. Turn the time rate right down and follow one neutron. It flies, it hits a nucleus, and one of three things happens. Orange means it split the nucleus and two or three new neutrons burst out. Grey means a nucleus swallowed it. Blue at the rim means it leaked away. That is the entire simulation. k is just the score: how many new neutrons each departing one leaves behind. Every scenario loads at its own sensible speed, and you can always slow it down further to watch, or speed it up to see the chain.
2. Inert lump is natural uranium, 0.7% U-235. Almost every neutron is captured or leaks before it finds a nucleus worth splitting. k stays near zero and nothing catches. This is why uranium ore is safe to hold.
3. Raise enrichment, then raise the core radius. Enrichment gives a neutron more targets. Size gives it less edge to escape through: a bigger core leaks proportionally less. Either one pushes k up, and where k reaches one you have the critical mass. With weapons-grade metal and no reflector that happens at about 52 kg, the real figure.
4. Add a reflector. It bounces escaping neutrons back in, so a smaller and lighter core goes critical. Push in the control rods and they eat neutrons instead, pulling k back down. Those two sliders are the brake and the accelerator of every reactor on earth.
5. Press Chernobyl and read the card under the simulator: that is the state Reactor 4 was actually left in at 01:23 on 26 April 1986. Then walk into the accident yourself, because it is only four moves.
   
   1. Load Power reactor first, and let it settle. Rods a third in, auto control on, k locked on 1.000, population graph flat. This is a station making electricity.
   2. Uncheck Auto control. Nothing corrects you now. Nothing changes yet either, which is the trap: the core looks exactly as safe.
   3. Drag Control rods down toward 0, the way the operators did to fight the xenon. k lifts off one, to about 1.14. Only 14% per generation, and it looks like nothing.
   4. Watch. The graph bends, then goes vertical, and the core floods with neutrons. 14% compounded over forty generations multiplies the power nearly two hundred times, and the generations take a heartbeat each.
   5. Try to save it: shove the rods back in. Early and k drops under one and the core settles. A few seconds late and there is nothing left to do, which is exactly the position the operators found themselves in.
   
   Now notice what the panel refuses to give you: no nuclear yield. Reactor fuel is far too dilute to detonate. It boiled, burst its pipes and set its graphite alight, which was catastrophe enough.
6. Hiroshima is the other side of the line: weapons-grade metal, compact, reflected, nothing absorbing. k leaps to about two, so the population doubles every generation instead of creeping. The graph runs off the top, the core detonates, and the yield strip lands on 15 kilotons from 64 kg of uranium, which is what the real bomb held.
7. Four sliders moved between a paperweight, a power station and a weapon. The physics never changed. Only k did.