AP® Physics 2: Algebra-Based · Unit 15 · Topic 15.7
Nuclear Fission
Nuclear fission is the splitting of a heavy nucleus into smaller nuclei plus free neutrons, releasing energy because the fragments weigh slightly less than the original nucleus. That missing mass becomes energy through E = mc2.
A nucleus like uranium-235 is barely holding together. Drop in one extra neutron and it wobbles, stretches, and splits into two smaller nuclei, called fission fragments, plus two or three loose neutrons and a burst of energy. Weigh everything before and after and you find the products weigh slightly less than the starting nucleus. That missing mass has not vanished; it became energy, following E = mc2. Because c2 is such a huge number, even a tiny mass loss, a fraction of an atomic mass unit, releases energy on the scale of millions of electron volts per nucleus. That is why fission dwarfs chemical burning: chemical reactions only rearrange electrons in outer shells, while fission rips apart the nuclear force itself.
The released neutrons are what make this self-sustaining. Each fission event from one absorbed neutron kicks out two or three new neutrons, and if each of those finds another uranium-235 nucleus, the number of fissions doubles, then doubles again: one event becomes two, then four, then eight, branching into a chain reaction. Left alone, that growth runs away, which is a bomb. A reactor needs the opposite: steady, controlled output.
That control is the job of control rods, made of materials like cadmium or boron that absorb neutrons without fissioning. Slide them in further and they soak up more of the free neutrons, leaving fewer available to cause new fissions, which slows the chain. Pull them out and more neutrons survive to keep the reaction going. Tuning this balance keeps the reaction rate steady instead of accelerating, connecting fission directly to how every nuclear reactor is actually run.
Key terms
- Fission
- The splitting of a heavy nucleus into two smaller nuclei plus free neutrons and energy.
- Chain reaction
- A self-sustaining sequence where neutrons from one fission event trigger further fission events.
- Mass-energy equivalence
- The principle, E = mc2, that lost mass converts directly into released energy.
- Control rod
- A neutron-absorbing rod inserted into a reactor to regulate the rate of the chain reaction.
- Fission fragment
- One of the two smaller nuclei produced when a heavy nucleus splits.
Watch it explained
- How it worksHow does one absorbed neutron split a uranium nucleus and launch two or three more neutrons?
- How it worksWatch a chain reaction branch out generation by generation from a single starting neutron.
- Worked exampleFind the energy released when a reaction loses 0.2 atomic mass units of mass.
- Going furtherWhat happens to the chain when control rods soak up a fraction of the free neutrons?
Check yourself
End of topic.