Once enriched, the $UF_6$ gas is converted back into a solid, black powder: uranium dioxide ($UO_2$). This powder is pressed into small, ceramic pellets, each about the size of a gummy bear. These pellets are then stacked inside long metal tubes called fuel rods.

Finally, hundreds of these fuel rods are bundled together to create a fuel assembly. It's these assemblies that are loaded into the nuclear reactor.

Inside the reactor, the fuel assemblies form the core. Here, a controlled nuclear chain reaction, called fission, takes place. The U-235 atoms are split, releasing an enormous amount of energy in the form of heat.

This heat is used to boil water and create high-pressure steam. The steam then spins a turbine, which is connected to a generator that produces electricity. It's the same basic principle used in coal or gas power plants, but the heat source is nuclear fission instead of burning fossil fuels.

A fuel assembly typically remains in a reactor for about three to six years. Over time, the concentration of U-235 decreases and fission byproducts build up, making the fuel less efficient. At this point, it is considered 'spent fuel' and must be removed.

Spent fuel is highly radioactive and generates a lot of heat. When it's removed from the reactor, it's immediately placed in a deep pool of water called a spent fuel pool. The water cools the fuel and acts as a shield, blocking the radiation.

After several years of cooling in the pool, the fuel can be moved to dry cask storage. These are massive, sealed containers made of steel and concrete that provide safe, long-term storage on the power plant site.

What happens next depends on a country's policy. There are two main options: reprocessing or direct disposal.

Reprocessing involves chemically separating the spent fuel into three categories: uranium, plutonium, and radioactive waste. The recovered uranium and plutonium can be recycled to create new nuclear fuel. This reduces the volume of high-level waste that needs to be permanently disposed of.

Disposal is the final step. The plan for spent fuel that isn't reprocessed, and for the waste from reprocessing, is to seal it in durable canisters and bury it deep underground in a stable geologic repository. This isolates the radioactive material from the environment for thousands of years, until its radioactivity has decayed to safe levels.

And that completes the cycle. From a rock in the ground to a concentrated power source and finally to secure storage, the nuclear fuel cycle is a carefully managed process designed to safely harness the immense energy locked inside atoms.