How can one fission lead to another?
Some heavy nuclei can split after absorbing a neutron. They release energy and new neutrons. This is nuclear fission. If a new neutron triggers another fission, a chain reaction can continue.
Follow a neutron. It might trigger another reaction, be absorbed without fission, or leave the material. These outcomes together determine whether the reaction decreases, holds steady or grows on average.
A sustained reaction does not mean an immediate explosion. Nuclear power plants use engineering controls to maintain a desired level. Nuclear weapons release energy extremely rapidly and cause severe harm. This film explains nuclear physics, not weapon construction.
Follow neutron outcomes, then compare the trend
- Pause one fission and find the original nucleus, fragments and new neutrons.
- Follow different neutron outcomes. Which really leads to another fission?
- Try another random run and distinguish one path from an average trend. Revisit why power plants and weapons cannot be distinguished by size alone.
Energy, averages and model limits
Further controls and numerical checks
- Pause one fission and identify the original nucleus, the two fragments and the new neutrons. Their displayed sizes, colors and speeds are illustrative.
- Compare tracks with the record. Blue dots mark escape. Absorption need not cause another fission, and not every collision is a fission.
- Compare the film’s two teaching examples, then change the random seed. Individual tracks change; an average trend and the survival of one chain are different questions.
- In the reactor comparison, notice how a small delayed-neutron contribution spreads events over time. This helps explain the possibility of control; it is not a full reactor-control simulation.
- Watch the expansion chapter. Distributing the same mass over a larger space changes collision opportunities. The model does not calculate real temperature, pressure or expansion speed, so it does not predict the duration of a real event.
Want to check the technical details?
An atomic nucleus is a tiny object containing protons and neutrons. Some heavy nuclei can absorb a neutron and split into smaller nuclei, releasing energy and new neutrons. This is nuclear fission. If the new neutrons cause further fissions, a chain reaction can continue from one generation to the next.
Follow one uranium-235 event: a neutron is absorbed, the excited nucleus stretches, and it separates. The drop shape helps illustrate a change; a nucleus is not literally a drop of water. Barium and krypton are just one possible pair of products. This particular example releases about 173 MeV; other fission channels have different products and immediate energies.
Now follow a newly released neutron. It might cause another fission, be absorbed without causing fission, or leave the material. Whether the chain continues depends on all these possible outcomes.
Critical means self-sustaining on average, not about to explode. Here k is the average ratio of neutron populations in successive generations. Below 1 they decrease on average; at 1 they remain level; above 1 they increase on average. An individual random chain starting from one neutron may still die out. A trend is not a promise about every generation. The US Nuclear Regulatory Commission’s definition also connects criticality with normal reactor operation.
A reactor uses engineering controls to keep the reaction at a chosen level. A weapon releases energy extremely rapidly and uncontrollably, causing severe harm and lasting consequences. The difference cannot be reduced to the size of a sphere. A piece of uranium does not inevitably explode just because it exceeds a particular size.
Where does the energy come from?
The total mass after the reaction is slightly smaller. The mass difference corresponds to released energy. Binding energy offers another description: these products are more tightly bound overall. The curve shows a trend, not a rule that splitting any nucleus releases energy.
The example is n + U-235 → Ba-141 + Kr-92 + 3n. Its approximately 173 MeV comes from this channel’s immediate mass difference. Later fragment decays release further energy. The commonly quoted value of about 200 MeV per uranium fission is a broader approximation across channels and subsequent processes; these numbers describe different quantities. OpenStax’s fission chapter explains the energy accounting.
Why can a growing average still include early extinction?
k averages many possible histories. A single starting neutron might escape or be absorbed before leaving descendants. Different random seeds give different outcomes. An average capable of growth does not make every chain grow, and reaching the display limit does not imply an unlimited energy supply.
A small fraction of neutrons appears later, when fission fragments decay. These delayed neutrons provide time for reactor control. The film compares simplified average generation times; it does not solve full neutron kinetics, temperature feedback or control systems.
Which numbers belong only to the teaching model?
The tracks are two-dimensional projections of three-dimensional random walks. Lengths use an illustrative mean free path. Collision probabilities are teaching choices, not data for a particular material. Fixed random seeds preserve the same history when pausing, replaying or seeking.
Spherical geometry, random walks and a diffusion approximation compare continuation, absorption and escape. They do not provide weapon designs, real critical dimensions or explosion predictions. Heat, pressure, harm distances and manufacturing steps are not modeled.
References: MIT neutron science and reactor physics notes, OpenStax: nuclear binding energy.