• Tags: fission, chain-reaction, reactor
• Difficulty: Moderate

Nuclear fission is the splitting of a heavy nucleus into two medium-mass fragments plus neutrons, releasing ~200 MeV per event. U-235 + thermal neutron -> Ba-141 + Kr-92 + 3 neutrons (typical, not unique — many fission products are possible). Energy breakdown: KE of fragments (~167 MeV), KE of neutrons (~5 MeV), prompt gamma rays (~6 MeV), beta/gamma from fission products (~19 MeV), neutrinos (~12 MeV, lost). Only U-235 (0.7% of natural uranium) and Pu-239 undergo thermal neutron fission. U-238 (99.3%) requires fast neutrons (>1 MeV) for fission. Chain reaction: each fission produces 2-3 neutrons. If multiplication factor k = 1 (critical), the reaction is self-sustaining. k > 1 (supercritical) leads to explosion; k < 1 (subcritical) dies out. Nuclear reactor components: fuel rods (U-235 enriched), moderator (slows neutrons — heavy water or graphite), control rods (absorb neutrons — cadmium/boron), coolant (carries heat away).