Question

What is nuclear fission?

Hint:

Remember: fusion joins light nuclei; fission splits heavy nuclei—both release energy due to increased binding energy of the products.

Answer 1

Step 1: Mechanism. A thermal neutron captured by \(^{235}\mathrm{U}\) produces an excited compound nucleus \(^{236}\mathrm{U}^{*}\) which deforms and splits, e.g. \[ ^{235}\mathrm{U}+n \rightarrow\^{141}\mathrm{Ba}+^{92}\mathrm{Kr}+3n+Q \] with \(Q\approx 200\) MeV.

Step 2: Energy origin. Products have higher total binding energy per nucleon than the parent; the mass defect converts to energy (\(E=mc^{2}\)), mostly as kinetic energy of fragments plus prompt \(\gamma\)-rays and delayed \(\beta\)-decays. 

Step 3: Chain reaction control. The average neutrons per fission sustain a chain reaction. Reactors keep the multiplication factor \(k_{\mathrm{eff}}=1\) using moderators (to slow neutrons) and control rods (B, Cd) that absorb excess neutrons; coolant removes heat to run turbines. If \(k_{\mathrm{eff}}>1\) and uncontrolled, the neutron population grows explosively.