Nuclear Chemistry
Nuclear Fission
Splitting the atom and the chain reaction
Nuclear fission is the process where a heavy nucleus, such as Uranium-235, absorbs a neutron and splits into two smaller nuclei, releasing a massive 200 MeV of energy per atom. This reaction is the basis for both nuclear power and atomic weapons, as a single gram of U-235 can generate 82 billion Joules of energy—equivalent to roughly 20 tons of TNT. When the number of released neutrons crosses the self-sustaining threshold, the process enters a "critical mass" state, triggering a chain reaction that continues until the fuel is exhausted.
- Energy Release~200 MeV per atom
- Energy Density1g U-235 ≈ 20 tons TNT
- Critical Mass52 kg (U-235 bare sphere)
- Hiroshima YieldFrom ~1 kg of fissioned U
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How it works
Fission begins when a 'slow' (thermal) neutron hits a nucleus of U-235. The nucleus becomes unstable (U-236) and stretches like a drop of liquid until it snaps into two smaller 'fission fragments' (like Barium and Krypton). Critically, the split also releases 2 or 3 additional neutrons and a massive burst of kinetic and gamma energy. If those new neutrons hit other Uranium nuclei, you have a self-sustaining chain reaction.
Critical Mass
In a small piece of Uranium, most neutrons escape through the surface before they can hit another nucleus. To sustain a reaction, you need a specific amount of material—the critical mass. In a nuclear reactor, control rods (made of materials like Boron) absorb excess neutrons to keep the reaction at a 'steady state.' In a weapon, the material is compressed rapidly to become 'supercritical,' causing an exponential explosion in microseconds.
Common pitfalls
- Confusing with Fusion: Fission is splitting heavy atoms apart; Fusion is joining light atoms together.
- Thinking all Uranium fissions: Only U-235 is easily fissile, but it makes up only 0.7% of natural Uranium. The rest (U-238) must be 'enriched' to be useful.
- Equating radiation with fission: All fission is radioactive, but not all radioactivity is fission (most is just simple alpha/beta decay).
| Feature | Nuclear Power Plant | Nuclear Weapon |
|---|---|---|
| Neutron Ratio (k) | k = 1.0 (Steady) | k > 1.0 (Exponential) |
| Fuel Enrichment | 3–5% U-235 | 85%+ U-235 |
| Control Method | Control rods (absorbers) | Rapid compression (implosion) |
| Energy Output | Slow, steady heat | Instantaneous blast |
Frequently asked questions
What is U-235?
An isotope of Uranium with 92 protons and 143 neutrons. It is the only naturally occurring fissile isotope.
Why does fission release energy?
Because the total mass of the fission fragments and neutrons is slightly less than the mass of the original nucleus. That 'lost' mass is converted to energy (E=mc²).
What are fission fragments?
The smaller nuclei (like Barium, Krypton, or Iodine) created when a large atom splits. They are usually highly radioactive 'nuclear waste.'
Can a nuclear reactor explode like an atomic bomb?
No. The fuel is not enriched enough (only 3–5%), so it is physically impossible for it to undergo the same type of exponential explosion as a weapon.
What is a 'Moderator'?
A substance like water or graphite used in reactors to slow down neutrons, making them more likely to be absorbed and cause fission.