Normal nuclear power plants burn up their enriched uranium fuel and leave behind useless nuclear waste. A breeder reactor is specifically designed to recycle this useless material into fresh fuel. The reactor core blasts stable heavy metal atoms with high speed neutrons.
When a stable atom absorbs a flying neutron, it becomes highly unstable. The atom quickly undergoes radioactive decay and transforms into a completely new element. For example, useless uranium-238 absorbs a fast neutron and slowly turns into valuable plutonium-239.
The reactor produces heat for electricity while constantly replacing the fuel it just burned. To make this incredible process work, the neutrons must travel at extremely fast speeds. Most normal reactors use regular water to slow down neutrons, but breeder reactors cannot.
Instead, they often use liquid metals like molten sodium to keep the hot reactor cool. Students often mistakenly think these reactors create free energy out of thin air. However, they simply unlock the hidden energy inside heavy materials that normal reactors throw away.
Natural uranium rock is mostly made of the completely useless type of heavy uranium. Breeder reactors could theoretically power human cities for thousands of years using this leftover material.