Nuclear fission

Nuclear fission is the process by which a heavy atomic nucleus splits into two or more smaller nuclei, releasing a substantial amount of energy and several free neutrons. Discovered in 1938 by Otto Hahn and Fritz Strassmann, with the theoretical interpretation provided by Lise Meitner and Otto Frisch, fission is the physical basis of both nuclear power and nuclear weapons. The discovery that each fission event releases multiple neutrons capable of triggering further fissions — a chain reaction — transformed nuclear physics from a laboratory curiosity into a technology with civilization-scale consequences.

The energy released in fission arises from the difference in binding energy per nucleon between the heavy parent nucleus and the lighter fission products. For uranium-235, each fission releases approximately 200 MeV — roughly 50 million times the energy released per molecule in a chemical reaction. The chain reaction requires a critical mass: a sufficient quantity of fissile material so that the neutrons released by one fission have a high probability of causing another fission before escaping the system. Enrico Fermi and Leo Szilard demonstrated that this chain reaction could be controlled in Chicago Pile-1, and later projects demonstrated that it could be weaponized.

The control of fission — whether in a reactor or a weapon — is the same physical problem viewed from different engineering constraints. A reactor sustains a chain reaction at a steady rate by moderating neutron energies and absorbing excess neutrons with control rods. A weapon achieves supercriticality in microseconds by assembling subcritical masses into a single critical geometry. The physics is identical; the politics is the distinction that matters.