1. Quick Summary
A reactor splits heavy atomic nuclei such as uranium-235. Each split releases energy as heat plus a few neutrons, and those neutrons can split more nuclei — a chain reaction.
The engineering problem is not starting the chain reaction; it is keeping it exactly at one-for-one, so each fission causes on average exactly one further fission.
2. What It Means
When a uranium-235 nucleus absorbs a neutron it becomes unstable and splits into two smaller nuclei, releasing about 200 million electronvolts of energy plus two or three free neutrons.
If every neutron caused another fission, the rate would double in microseconds and the result would be a bomb. Reactors instead keep the multiplication factor at exactly one.
The heat is the product. It boils water, makes steam, spins a turbine and drives a generator — the same final stage as a coal plant, with a completely different heat source.
3. Why It Happens
Fission releases energy because the binding energy per nucleon is higher for mid-sized nuclei than for very heavy ones. Splitting a heavy nucleus leaves the fragments in a lower-energy, more tightly bound state, and the difference comes out as kinetic energy and radiation.
Fresh neutrons from fission move too fast. Most reactors use a moderator — ordinary water, heavy water or graphite — to slow them down, because slow neutrons are far more likely to cause another fission in uranium-235.
Control rods made of neutron-absorbing materials such as boron or cadmium are inserted or withdrawn to soak up the surplus neutrons and hold the rate steady.
A crucial safety feature in most designs is a negative temperature coefficient: if the core overheats, the physics itself reduces the reaction rate rather than accelerating it.
Decay heat is the counterintuitive danger. Even after fission stops, unstable fission products keep releasing heat — enough to melt fuel if cooling is lost, which is why reactors need power for cooling after shutdown.
4. Real Examples
Pressurised water reactors keep the primary loop under high pressure so water stays liquid above 300 °C, then transfer heat to a secondary loop that makes steam.
Boiling water reactors let the primary water boil directly and send that steam to the turbine, a simpler design with less separation between loops.
CANDU reactors use heavy water as moderator, which absorbs so few neutrons that they can run on natural, unenriched uranium.
Chernobyl’s 1986 accident happened during a test with control rods largely withdrawn and a positive-feedback design quirk, while Fukushima in 2011 lost cooling after the tsunami knocked out power — decay heat, not a runaway chain reaction, did the damage.
5. How It Affects Us
Nuclear power produces large amounts of electricity with very low carbon emissions and a tiny land footprint compared with wind or solar per unit of energy.
Its central problems are long-lived radioactive waste, high capital cost and long construction times.
Public acceptance depends mainly on whether safety systems can survive a total loss of power, which is what modern passive-cooling designs aim to guarantee.
6. Key Takeaways
- Reactors harness fission chain reactions held at exactly one-for-one, not runaway.
- Moderators slow neutrons; control rods absorb them; both together set the rate.
- The useful output is heat, which then follows the ordinary steam-turbine path.
- Decay heat after shutdown is the hardest safety problem and the reason cooling must never stop.