Nuclear fission is the splitting of a heavy atomic nucleus into smaller nuclei after it absorbs a neutron. It matters because a tiny amount of nuclear fuel can release a very large amount of energy compared with chemical fuels. Fission powers nuclear reactors, and the same basic process can also produce the explosive energy of nuclear weapons.
Understanding fission helps explain both useful energy production and the need for careful safety controls.
In a typical uranium-235 fission event, an incoming neutron is absorbed by a U-235 nucleus, forming an unstable U-236 nucleus for a very short time. The nucleus then splits into two fission fragments, such as Ba-141 and Kr-92, releases energy, and emits more neutrons. If these neutrons cause more U-235 nuclei to split, a chain reaction can grow.
Reactors control this chain reaction with moderators, control rods, coolant, and fuel design, while bombs are designed to make the reaction grow extremely fast.
Key Facts
- A common fission reaction is n + U-235 -> Ba-141 + Kr-92 + 3n + energy.
- Energy released per U-235 fission is about 200 MeV, or 3.2 x 10^-11 J.
- Mass energy relation: E = mc^2, where lost mass becomes released energy.
- A chain reaction occurs when emitted neutrons cause additional fission events.
- Criticality factor: k = neutrons in one generation / neutrons in previous generation.
- If k = 1 the reaction is steady, if k < 1 it dies out, and if k > 1 it grows.
Vocabulary
- Nuclear fission
- Nuclear fission is the splitting of a heavy nucleus into smaller nuclei, neutrons, and energy.
- Uranium-235
- Uranium-235 is a fissile isotope of uranium that can split after absorbing a slow neutron.
- Chain reaction
- A chain reaction is a repeating process in which neutrons from one fission event trigger more fission events.
- Critical mass
- Critical mass is the minimum amount and arrangement of fissile material needed to sustain a chain reaction.
- Control rod
- A control rod is a reactor component that absorbs neutrons to slow or stop the fission chain reaction.
Common Mistakes to Avoid
- Confusing fission with fusion is wrong because fission splits heavy nuclei, while fusion joins light nuclei.
- Thinking all emitted neutrons cause more fission is wrong because many neutrons escape, are absorbed by nonfuel materials, or have the wrong energy.
- Assuming a reactor works like a nuclear bomb is wrong because reactors are engineered to keep k near 1, while bombs are designed for a rapid supercritical reaction.
- Ignoring energy units is wrong because nuclear energies are often given in MeV per nucleus, but power calculations usually need joules and seconds.
Practice Questions
- 1 A U-235 nucleus releases about 3.2 x 10^-11 J in one fission event. How much energy is released by 1.0 x 10^20 fission events?
- 2 In one neutron generation a sample has 500 fission-causing neutrons, and in the next generation it has 575. Calculate k and state whether the chain reaction is subcritical, critical, or supercritical.
- 3 Explain why control rods can reduce reactor power, and describe how this differs from simply removing heat with coolant.