Nuclear Fission Chain Reaction (2D)

This 2D top-down simulator tracks individual neutrons flying through a circular pile of uranium nuclei: enrichment decides how many nuclei are fissile, moderation decides how many neutrons start thermal (and therefore fission more readily), and each real fission releases a realistic ~2.43 new neutrons. Raise fuel density to shrink the pile's critical size, or lower and raise the control rods to absorb neutrons outright, and watch the live fission rate tip the reaction from subcritical decay through a steady critical state into a supercritical, runaway cascade.