Splitting the Atom
In December 1938, Otto Hahn and Fritz Strassmann discovered that bombarding uranium with neutrons produced barium — the uranium nucleus had split in two. Lise Meitner and Otto Frisch quickly explained the physics: the nucleus, like an overstretched liquid drop, could be destabilized by a neutron impact and split into two fragments, releasing the enormous binding energy difference. Each fission of U-235 releases about 200 MeV — roughly 50 million times the energy of burning one carbon atom.
The Chain Reaction
The key to nuclear energy is that each fission releases 2-3 free neutrons. If at least one of these neutrons causes another fission, the reaction is self-sustaining — a chain reaction. The multiplication factor k describes this: k=1 means each generation produces exactly one new fission (critical), k>1 means exponential growth (supercritical), and k<1 means the reaction dies (subcritical). Nuclear reactors operate at k=1.000; nuclear weapons require k≈1.1 or higher with fast neutrons.
Controlling the Beast
This simulation visualizes uranium atoms as cyan circles with neutrons bouncing between them. When a neutron hits a U-235 atom, it splits with a flash animation, releasing daughter fragments and new neutrons. Control rods (gray bars descending from above) absorb neutrons. Watch how adjusting rod insertion changes the neutron population — pull them out and the population grows exponentially; push them in and it decays. The generation counter shows how quickly the reaction multiplies.
Enrichment and Critical Mass
Natural uranium is only 0.7% U-235 (the fissile isotope); the rest is U-238, which absorbs neutrons without fissioning. Reactor fuel is enriched to 3-5% U-235. Weapons-grade uranium is enriched to >90%. The critical mass — the minimum amount needed for a self-sustaining chain reaction — depends on enrichment, geometry, and whether neutron reflectors are present. For weapons-grade uranium, it is about 52 kg as a bare sphere; with a good reflector, as little as 15 kg.