Nuclear Fission

Nuclear fission: heavy nucleus (e.g., U-235, Pu-239) splits into two smaller fragments, plus 2-3 free neutrons, plus energy.

Discovered by Hahn and Strassmann (1938); explained by Meitner and Frisch using liquid-drop nuclear model.

Typical fission of U-235: ²³⁵U + n → ¹⁴⁴Ba + ⁸⁹Kr + 3n + ~200 MeV.

Energy comes from mass defect: products have higher B/A than parent.

Released neutrons (~2.5 on average) can induce further fissions — basis of CHAIN REACTION.

Critical mass: minimum amount of fissile material to sustain chain reaction (e.g., ~50 kg of U-235 bare-sphere, much less if reflected).

Controlled chain reaction = NUCLEAR REACTOR. Uncontrolled = ATOMIC BOMB.

Moderators (water, graphite, heavy water) slow neutrons to thermal speeds — thermal U-235 fission has much larger cross-section than fast.