💥 Fission chain reaction — k below 1 dies, k above 1 runs away
A slow neutron strikes ²³⁵U; it splits into two fragments and throws out two or three fast neutrons plus about 200 MeV. If, on average, k of those neutrons trigger a new fission, the g-th generation has kᵍ fissions — so k alone decides whether the chain fades or explodes.
A slow neutron (blue) strikes a ²³⁵U nucleus. It splits into two fragments (red + brown) and throws out two or three fast neutrons, releasing about 200 MeV.
Chain reaction Ng = kg ·
n + ²³⁵U → 2 fragments + ~2.5 n + ~200 MeV
k = — · generation g = — ·
fissions this generation = — · —