# Nuclear Fission Nuclear fission is the splitting of a heavy atomic nucleus into two lighter nuclei, releasing enormous energy — the reaction that powers every nuclear reactor and every fission-based weapon ever built. When a neutron strikes a fissile nucleus like uranium-235 or plutonium-239, the nucleus absorbs it, becomes unstable, and splits apart, releasing roughly 200 million electron volts (MeV) per event — about 80 million times the energy released by burning a single molecule of methane. That ratio between nuclear and chemical energy is why a few kilograms of enriched [[Uranium]] can do the work of thousands of tons of conventional explosive, and why fission's discovery in 1938 by Otto Hahn and Fritz Strassmann (with theoretical explanation by Lise Meitner and Otto Frisch) immediately triggered a race to weaponize it. ## The Chain Reaction Fission's power comes not from the individual splitting but from the chain reaction it enables. Each fission event releases two to three free neutrons, and if enough of those neutrons strike other fissile nuclei before escaping the material or being absorbed by non-fissile atoms, each generation of fissions triggers a larger one. Whether this process is controlled or uncontrolled is the dividing line between a reactor and a bomb. **Critical mass** is the minimum amount of fissile material needed to sustain a chain reaction — the point where enough neutrons find targets to keep the process going. For a bare sphere of weapons-grade uranium-235, critical mass is roughly 52 kilograms; for plutonium-239, about 10 kilograms. Geometry, density, and the presence of a neutron reflector all affect the number. The concept of critical mass has applications beyond physics — it describes threshold dynamics in social movements, market adoption, and political tipping points, a connection the vault tracks through [[Arms Race]] dynamics and network effects. **Moderation** is the controlled version. In a nuclear reactor, a moderator (water, graphite, or heavy water) slows neutrons to thermal speeds where they are more likely to cause fission in U-235, while control rods absorb excess neutrons to regulate the reaction rate. The reactor operates at a carefully maintained criticality — exactly one neutron from each fission causing exactly one subsequent fission. This controlled chain reaction generates heat, which produces steam, which drives turbines. The physics is elegant; the engineering is formidable. ## Reactor vs. Weapon The difference between reactor fuel and weapons material is enrichment level. Natural uranium is 99.3% U-238 (not readily fissile) and 0.7% U-235 (fissile). Reactor fuel is enriched to 3-5% U-235 — enough to sustain a controlled chain reaction with a moderator. Weapons-grade uranium is enriched above 90% — concentrated enough for an explosive supercritical assembly. The same centrifuge technology does both jobs, which is why enrichment capacity is the central concern of nonproliferation policy and the core issue in the [[JCPOA]] dispute with [[Iran]]. Plutonium-239 offers an alternative path. It does not exist in nature but is produced when U-238 in a reactor absorbs a neutron. Separating plutonium from spent fuel (reprocessing) provides weapons-usable material without enrichment — the path North Korea took to its arsenal. The dual-use nature of reactor technology — civilian power and weapons material from the same physics — is the fundamental tension that the entire nonproliferation regime, from the [[United Nations]] treaty framework to IAEA safeguards, attempts to manage. ## The Fission Weapons Legacy The first fission weapon, "Little Boy," was dropped on Hiroshima on August 6, 1945 — a gun-type device that fired one subcritical mass of U-235 into another to achieve supercriticality. "Fat Man," dropped on Nagasaki three days later, used an implosion design to compress a plutonium core. These bombings, which killed an estimated 110,000-210,000 people, ended [[World War II]] and inaugurated the nuclear age. The [[Cold War]] arms buildup that followed saw fission devices evolve into triggers for far more powerful thermonuclear weapons — fission remained the spark even as fusion became the main charge. Today, fission's dual identity persists: it generates roughly 10% of global electricity with zero direct carbon emissions (a fact central to the [[Nuclear Energy Debate]]) while remaining the foundational technology of every nuclear weapon in the [[US Strategic Nuclear Weapons]] arsenal and the arsenals of eight other states. The physics does not distinguish between purposes; politics and engineering do. ## See Also - Nuclear Physics — the broader field - [[Uranium]] — the primary fissile element - [[US Strategic Nuclear Weapons]] — fission weapons in the American arsenal - [[JCPOA]] — arms control centered on fission-material enrichment - [[Cold War]] — the geopolitical framework fission created - [[Nuclear Energy Debate]] — civilian fission power politics - [[Weapons of Mass Destruction]] — the category fission weapons define