Nuclear fission is the splitting of a heavy nucleus into two smaller nuclei, usually after it absorbs a neutron. It releases energy and additional neutrons that can sustain a chain reaction.
This is IB Physics E.4 Fission, studied by both SL and HL students.
How Nuclear Fission Works
A possible neutron-induced fission reaction involving uranium-235 is:
The exact fission products can vary. In every valid nuclear equation, however, total nucleon number and proton number are conserved. Here, the nucleon numbers total 236 on each side, while the proton numbers total 92.
The daughter nuclei have a greater binding energy per nucleon than the original heavy nucleus. Their combined mass is therefore slightly less than the initial mass. This mass defect is converted into released energy:
Each fission releases approximately , mainly as kinetic energy of the fission products. The emitted neutrons may then strike other fissile nuclei. A self-sustaining chain reaction occurs when, on average, at least one neutron from each fission produces another fission.
Controlling Fission in a Reactor
| Component | Function |
|---|---|
| Moderator | Slows neutrons, increasing the probability of further fission |
| Control rods | Absorb neutrons and regulate the reaction rate |
| Heat exchanger | Transfers thermal energy away from the reactor |
| Shielding | Absorbs ionizing radiation and protects the surroundings |
Fission products are commonly radioactive, so they require secure long-term management.
A common misconception is that fission and fusion are identical. Fission splits heavy nuclei, whereas fusion joins light nuclei.
Exam Technique
For an IB Explain question, link neutron absorption, nuclear splitting, increased binding energy per nucleon, mass defect, energy release, and further neutron production. In nuclear equations, always check conservation of nucleon number and proton number. If asked about a reactor, name each component and state its specific function rather than merely listing it.
