Nuclear Fission
Animated chain reaction — tune k to toggle subcritical / critical / supercritical.
Key Notes
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.
Formulas
Typical U-235 fission
Q ≈ 200 MeV — distributed among kinetic energy of fragments, neutrons, γ-rays, β-decays of fragments.
Energy per kg fissioned
~2.5 million times that of TNT (~4 × 10⁶ J/kg).
Chain-reaction criterion
k = 1 critical (steady), k > 1 supercritical (growing), k < 1 subcritical (dying).
Multiplication factor
Six-factor formula (four-factor for thermal): factors for neutron production, fast fission, resonance escape, thermal utilization, non-leakage probability.
Important Points
One fission releases ~200 MeV — ~50 million times the energy from burning a single C atom.
U-235 (0.7% natural abundance) is fissile with thermal neutrons. U-238 (99.3%) is fertile (needs fast n).
Reactor moderators: light water (PWR/BWR), heavy water (CANDU), graphite (RBMK).
Control rods (cadmium, boron) absorb neutrons to control k_eff and stop runaway.
Fission products are intensely radioactive — main concern for waste storage and accidents.
Critical mass concept: enough material so that neutrons born inside have high probability of inducing further fissions before escaping.
Nuclear Fission notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.