The nucleus is already under enormous stress
A uranium-235 nucleus is a cluster of 92 protons and 143 neutrons packed into a space about 100,000 times smaller than the atom itself. Every one of those protons carries a positive charge, which means they are all pushing each other apart with enormous electromagnetic force.
So why doesn't the nucleus just explode on its own? Because the strong nuclear force — the most powerful force in nature — holds it together. It is roughly 100 times stronger than electromagnetism. But here is the catch: it only works at extremely short range. Stretch it too far and it simply switches off.
The Uranium-235 nucleus is big enough that this balance is already precarious. It is one of the largest stable nuclei that exists. Add one more neutron — and the whole thing tips over.
The neutron arrives and tips the balance
A free neutron, moving at just the right speed, slips into the Uranium-235 nucleus. It carries no charge, so it passes straight through the electron cloud without any electromagnetic resistance.
The moment it joins the nucleus, the nucleus briefly becomes Uranium-236. The additional mass adds energy, and the nucleus starts to wobble like a liquid drop. This is not a metaphor — the strong nuclear force really does make a nucleus behave somewhat like a droplet of liquid under tension.
The wobble stretches the nucleus into an elongated shape. As the two ends pull apart, the surface tension of the strong force can no longer counteract the electromagnetic repulsion between all those protons. The nucleus snaps in two.
Click on the fission canvas. Watch the nucleus wobble, elongate, and split. Switch to chain reaction mode and see how each split triggers two more.
Where does the energy actually come from?
When the nucleus splits, it forms two smaller nuclei — typically Barium and Krypton — plus two or three free neutrons. Now here is the part that makes nuclear physics extraordinary: if you carefully weigh all those products, they are slightly lighter than the original Uranium-235 plus the neutron.
The missing mass — about 0.1% of the total — did not disappear. It became energy. Einstein's equation E = mc² tells us exactly how much: multiply that tiny lost mass by the speed of light squared (a very large number), and you get 200 million electron-volts per split.
To give that a sense of scale: burning one atom of coal releases about 4 electron-volts. Splitting one Uranium atom releases 50 million times more energy from an object that is 50 million times smaller than a grain of sand.
The freed neutrons split more nuclei
Each fission event releases 2 to 3 fast neutrons. If enough Uranium-235 is present — the so-called critical mass — those neutrons will each strike another nucleus before escaping. Each of those splits produces 2 to 3 more neutrons. Those neutrons cause more splits.
In the first generation: 1 split. Second generation: 3 splits. Third: 9. After just 80 generations, you have more splits happening than there are atoms in the visible universe. In practice, the chain reaction runs through hundreds of generations in a microsecond.
In a nuclear reactor, control rods absorb excess neutrons and keep the reaction rate steady — exactly one split per split. Remove the control rods and the chain reaction accelerates exponentially. That is the difference between a power plant and a bomb.
Each split releases 2 free neutrons. Watch the reaction feed itself.
In the full lesson you control the reactor. Three emergencies. Fix each one using what you know about neutron flux, control rods, and critical mass.
Why Uranium-235 and not Uranium-238?
Natural uranium is 99.3% Uranium-238 — the same element, but with 3 more neutrons in the nucleus. Those three extra neutrons make all the difference. When a neutron strikes U-238, the nucleus absorbs it and becomes U-239 instead of splitting. The chain reaction cannot sustain itself.
Only U-235 — the 0.7% minority — has the right nuclear configuration to fission easily. The slightly lower neutron-to-proton ratio makes the nucleus just unstable enough that an incoming neutron pushes it past the point of no return. This is why nuclear fuel must be enriched: the proportion of U-235 is increased from 0.7% to 3–5% for a reactor, or above 90% for a weapon.
This sensitivity to isotope composition is not a special property of uranium — it applies to every element. Carbon-12 is completely stable. Carbon-14 (two extra neutrons) is radioactive and decays over thousands of years. Carbon-15 (three extra neutrons) lasts less than three seconds.
See it happen yourself
Reading about chain reactions is one thing. Clicking to fire a neutron and watching the canvas erupt — that is how it actually clicks. In the Physiworld Nuclear lesson you build a nucleus from scratch, explore isotopes one neutron at a time, and try to keep a reactor from melting down. Four missions. Eight minutes. You walk away understanding exactly why splitting an atom releases 50 million times more energy than burning one.
Fire the first neutron. Unlock fission, chain reaction, and critical mass. Then build the nucleus yourself and keep the reactor stable. Everything in one 8-minute lesson.
Nuclear fission occurs when a neutron strikes a fissile nucleus — like Uranium-235 — causing it to split into two smaller nuclei. The products weigh slightly less than the original; that missing mass converts directly into 200 million electron-volts of energy via E = mc². Each split releases 2–3 new neutrons that can trigger further splits. In a reactor, control rods keep exactly one split triggering one more. The isotope matters: U-235 fissions easily, U-238 does not — because three extra neutrons change the nuclear balance entirely.
The Nuclear section covers fission, chain reactions, isotopes, radioactive decay, and nuclear reactors through interactive simulations and challenges.