Nuclear fission and fusion
Two ways to release vast energy from nuclei: fission (splitting heavy nuclei) and fusion (joining light nuclei).
Fission
A heavy unstable nucleus (e.g. uranium-235) absorbs a slow neutron, becomes excited, and splits into two roughly-equal-sized daughter nuclei plus 2 or 3 free neutrons and a lot of energy.
Example: $^{235}{92} U + ^1_0 n \to ^{141}{56} Ba + ^{92}_{36} Kr + 3,^1_0 n + \text{energy}$.
The free neutrons can hit other U-235 nuclei, causing further fissions — a chain reaction.
Controlled chain reaction (reactors)
- Fuel rods of enriched uranium.
- Moderator (water or graphite) — slows neutrons so U-235 absorbs them more readily.
- Control rods (boron or cadmium) — absorb excess neutrons; lower to slow reaction, raise to speed it up.
- Coolant carries thermal energy to a turbine; the turbine drives a generator.
Uncontrolled chain reaction
A bomb. All control mechanisms are absent — runaway fission releases enormous energy in milliseconds.
Fusion
Fusion is the joining of two light nuclei to form one heavier nucleus. Mass is "lost" and converted to energy (E = mc²). Fusion powers the Sun and stars.
Typical reaction (in the Sun's hot core):
$^2_1 H + ^3_1 H \to ^4_2 He + ^1_0 n + \text{energy}$.
(Hydrogen fusion via the proton-proton chain in stars.)
Why fusion is hard on Earth
- Both nuclei are positive → strong electrostatic repulsion.
- Need temperatures of millions of K to give them enough kinetic energy to fuse.
- Need enormous pressure or magnetic confinement (tokamak).
- Currently no commercial fusion reactor — net energy gain has been demonstrated only briefly (NIF, 2022).
Comparing fission and fusion
| Aspect | Fission | Fusion |
|---|---|---|
| Process | Heavy splits | Light join |
| Fuel | U-235, Pu-239 | Hydrogen isotopes |
| Waste | Long-lived radioactive | Mostly helium |
| Used commercially? | Yes | Not yet |
| Chain reaction | Yes (neutrons) | No |
Issues with fission power
- Long-lived radioactive waste needs deep storage.
- Risk of meltdown if cooling fails.
- Risk of weapons proliferation.
- Energy density is huge — small amount of fuel powers a city for a long time.
- No CO₂ emitted during operation.
⚠Common mistakes
- Confusing fission (split) and fusion (join).
- Saying fusion is the bomb (it's the H-bomb that uses fusion; standard fission bombs use only fission).
- Forgetting the role of the moderator (slows neutrons) vs control rod (absorbs them).
- Saying fusion is "free clean energy" — current technology hasn't reached commercial viability.
AI-generated · claude-opus-4-7 · v3-deep-physics