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. In uranium-235 fission, the nucleus becomes unstable, breaks apart, and emits more neutrons that can trigger additional fissions.
This process is the basis of nuclear power plants and some nuclear weapons.
The energy comes from a small decrease in mass during the reaction, which is converted to energy according to E = mc^2. A chain reaction continues when released neutrons are absorbed by more fissile nuclei, but it depends on fuel amount, shape, density, and neutron losses. Critical mass is the minimum amount of fissile material needed to sustain a chain reaction under specific conditions.
In reactors, control rods, moderators, coolant, and careful geometry keep the reaction steady and useful rather than rapidly increasing.
Key Facts
- A common fission reaction is n + U-235 -> U-236* -> Ba-141 + Kr-92 + 3n + energy.
- Mass defect is the missing mass after a nuclear reaction: Δm = m_initial - m_final.
- Energy released by mass conversion is E = Δmc^2.
- Each U-235 fission releases about 200 MeV of energy, mostly as kinetic energy of fragments.
- A chain reaction is sustained when, on average, at least one neutron from each fission causes another fission.
- Criticality is described by the neutron multiplication factor k, where k = 1 is steady, k < 1 dies out, and k > 1 grows.
Vocabulary
- Nuclear fission
- Nuclear fission is the splitting of a heavy nucleus into smaller nuclei, neutrons, and energy.
- Fissile isotope
- A fissile isotope is a nuclide, such as uranium-235 or plutonium-239, that can sustain a neutron-driven chain reaction.
- Chain reaction
- A chain reaction is a sequence in which neutrons from one fission event cause more fission events.
- Critical mass
- Critical mass is the minimum mass of fissile material needed to maintain a self-sustaining chain reaction under given conditions.
- Control rod
- A control rod is a reactor component made of neutron-absorbing material that reduces the number of neutrons available to cause fission.
Common Mistakes to Avoid
- Thinking fission and fusion are the same process. Fission splits heavy nuclei, while fusion joins light nuclei, so they involve different fuels, conditions, and reaction products.
- Assuming every released neutron causes another fission. Many neutrons escape or are absorbed by non-fuel materials, which is why reactor design and critical mass matter.
- Treating critical mass as one fixed number for all samples. Critical mass changes with isotope purity, shape, density, reflectors, and surrounding materials.
- Forgetting that reactor control rods do not slow neutrons down. Control rods absorb neutrons, while moderators such as water or graphite slow neutrons to increase the chance of fission in many reactor designs.
Practice Questions
- 1 A U-235 fission event releases about 200 MeV. If 1 MeV = 1.60 x 10^-13 J, how many joules are released by one fission event?
- 2 In a simple chain reaction model, each fission produces 3 neutrons and exactly 1 neutron from each fission causes another fission. Starting with 1 fission, how many fissions occur in the next 5 generations, not including the original event?
- 3 A reactor operator inserts control rods farther into the core. Explain how this changes the neutron population and why it can reduce the fission rate.