Nuclear Fission
The process by which a heavy atomic nucleus (typically U-235 or Pu-239) splits into two lighter nuclei when struck by a neutron, releasing enormous energy and additional neutrons that can trigger further fissions — a self-sustaining chain reaction.
The Discovery (1938)
The discovery of fission was the culmination of four decades of nuclear science:
- 1895–1934: Röntgen, Becquerel, Curie, Rutherford, and Chadwick progressively uncover radioactivity, nuclear transmutation, and the neutron.
- 1935: Fermi discovers neutron bombardment creates the widest variety of artificial radionuclides, including heavy elements.
- December 1938: Otto Hahn and Fritz Strassmann in Berlin bombard uranium with neutrons and detect barium — roughly half the mass of uranium. This could only happen if the uranium nucleus had split.
- Lise Meitner and Otto Frisch (working with Niels Bohr) explain the mechanism: a neutron is captured by the nucleus, inducing severe vibration; the nucleus splits into two roughly equal parts, releasing ~200 million electron volts (MeV) — orders of magnitude more energy than any chemical reaction. This directly confirmed Einstein’s E = mc².
The Chain Reaction
Hahn and Strassmann also showed fission releases additional neutrons (~2–3 per event). If those neutrons cause further fissions, which release more neutrons, a self-sustaining chain reaction results — and with it, an enormous release of energy.
Key parameters:
- Critical mass: The minimum amount of fissile material needed to sustain a chain reaction. Below critical mass, too many neutrons escape without hitting nuclei. For pure U-235 metal, this is ~15–50 kg (adjustable by geometry and neutron reflectors). The Frisch-Peierls Memorandum estimated ~5 kg — spectacularly low, enough to alarm British policymakers into action.
- Moderator: Slow (thermal) neutrons are far more effective at inducing fission in U-235 than fast neutrons. Materials that slow neutrons — heavy water, graphite, light water — are called moderators and are essential to reactor design.
- Enrichment: Natural uranium is 99.3% U-238 (which absorbs neutrons without fissioning efficiently) and only 0.7% U-235. Enriching — increasing the proportion of U-235 — was one of the hardest technical challenges of the manhattan-project. Three methods: electromagnetic separation, gaseous diffusion, centrifuge.
Fission vs. Fusion
Nuclear weapons use both:
- Fission bombs (“atomic bombs”): U-235 or Pu-239 assembled past critical mass → uncontrolled chain reaction. Hiroshima and Nagasaki were fission bombs.
- Hydrogen bombs (“thermonuclear”): A fission primary ignites a fusion secondary (hydrogen isotopes), releasing far greater energy. The US H-bomb (1952) was 500× more powerful than Nagasaki; the Tsar Bomba (1961) reached 58 megatons.
Controlled vs. Uncontrolled
The same physics has two applications:
- Uncontrolled (weapons): chain reaction proceeds at maximum speed → explosion.
- Controlled (reactors): neutron-absorbing control rods limit the reaction rate → sustained heat generation for electricity. Fermi’s Chicago Pile-1 (December 1942) was the first demonstration of a controlled chain reaction.
This dual-use nature is why nuclear energy and nuclear weapons proliferation are inseparable — the enrichment and reactor technologies that produce electricity also produce weapons-grade fissile material.
Plutonium Pathway
U-238 (the common isotope) does not fission efficiently, but when it absorbs a neutron in a reactor, it transmutes into Pu-239 (plutonium) — which does fission efficiently and is also a potent bomb material. This was discovered by Bretscher and Feather (Cambridge, 1940) and independently confirmed in the US by McMillan and Abelson. Plutonium is chemically separable from uranium, making it easier to extract than enriched U-235 — hence the plutonium bomb path (Nagasaki, Soviet RDS-1).