Nuclear fuel cycle Facts for Kids

The nuclear fuel cycle shows how nuclear fuel is found, processed, used to make energy, and then safely managed.

The nuclear fuel cycle, also known as the nuclear fuel chain, is the journey of nuclear fuel, the material used to power nuclear power plants. Think of it like the life story of the fuel, from when it is taken from the ground to when it is used to make electricity, and finally, to how it is safely stored away.

This journey has three main parts: the front end (getting the fuel ready), the service period (using the fuel in a reactor), and the back end (managing the fuel after it has been used). When used fuel is recycled, it's called a closed fuel cycle. If it's used only once and then stored, it's called an open fuel cycle.

How Nuclear Fuel Works

All the used nuclear fuel from commercial power plants in the United States could fit on a single American football field.

Nuclear power plants create electricity by splitting atoms, a process called fission. This process releases a huge amount of energy as heat. The most common fuel used is an element called uranium.

Natural uranium is made of different types, called isotopes. The most important one for nuclear energy is Uranium-235 (U-235) because it can split easily. However, most natural uranium is Uranium-238 (U-238), which doesn't split as easily. Because of this, the uranium fuel for most reactors needs to be "enriched," which means increasing the amount of U-235.

Inside a reactor, when a U-235 atom splits, it releases tiny particles called neutrons. These neutrons hit other U-235 atoms, causing them to split too. This creates a chain reaction, which produces a steady flow of heat. This heat is used to boil water, create steam, and turn turbines to generate electricity.

After being used for a few years, the fuel can no longer produce enough heat efficiently. It is then called spent fuel. This spent fuel is highly radioactive and must be handled with great care.

The Front End: Making the Fuel

1. Uranium ore – The raw rock material that contains uranium.
2. Yellowcake – A powder made from uranium ore that is easy to transport.
3. UF₆ – A gas form of uranium used for enrichment.
4. Nuclear fuel – Small, solid pellets that go into the reactor.

Exploration and Mining

The first step is to find and mine uranium ore, which is a type of rock that contains uranium. It can be mined from deep underground or from large open pits on the surface, similar to how other metals are mined.

Milling

The uranium ore is taken to a mill where it is crushed into a fine powder. The uranium is then separated from the rock using a chemical process. The result is a bright yellow or brown powder called "yellowcake".

Conversion

To be enriched, the yellowcake powder is turned into a gas called uranium hexafluoride (UF₆). This is done because it is easier to separate the different uranium isotopes when they are in a gas form.

Enrichment

The nuclear fuel cycle shows how uranium is mined, made into fuel (1), used in a power plant, and then either reprocessed (2, 4) or sent to a final storage place (3).

Enrichment is the process of increasing the concentration of U-235 in the uranium. Most reactors need fuel that is about 3–5% U-235, while natural uranium is only about 0.7% U-235. This is often done using large, fast-spinning machines called gas centrifuges.

The leftover uranium, which has very little U-235, is called depleted uranium. It is much less radioactive and is sometimes used for other purposes, like in shielding against radiation.

Fabrication

After enrichment, the uranium gas is converted back into a solid powder, this time uranium dioxide (UO₂). This powder is pressed and baked at very high temperatures to create small, hard ceramic pellets. These pellets are about the size of a gummy bear but can produce as much energy as a ton of coal.

The pellets are then stacked inside long, thin metal tubes made of a special zirconium alloy. These sealed tubes are called fuel rods. Finally, the fuel rods are bundled together to create a fuel assembly, which is ready to be placed inside a nuclear reactor.

The Service Period: Using the Fuel

Transporting Nuclear Materials

Nuclear materials must be transported safely between the different stages of the fuel cycle. They are moved in very strong, specially designed containers called casks. These casks are built to withstand serious accidents without leaking, ensuring that the public and the environment are protected.

Inside the Nuclear Reactor

A reactor core is made up of hundreds of fuel assemblies. Inside the core, the chain reaction is carefully controlled to produce a constant amount of heat. This heat is used to generate electricity. Over time, the fuel gets used up and produces waste products that slow down the reaction. About every 1 to 2 years, the reactor is shut down, and about one-third of the oldest fuel assemblies are replaced with new ones.

The Back End: Managing Used Fuel

Interim Storage

When fuel assemblies are removed from the reactor, they are called spent fuel. They are very hot and radioactive. First, they are placed in a deep spent fuel pool of water at the power plant. The water cools the fuel and blocks the radiation. After several years of cooling in the pool, the fuel can be moved to a dry cask storage system. These are large, sealed containers made of steel and concrete that hold the fuel safely.

Reprocessing (Recycling)

Some countries choose to recycle their spent fuel through a process called nuclear reprocessing. In this process, the spent fuel is chemically treated to separate the useful materials, like leftover uranium and plutonium, from the radioactive waste products. The recovered uranium and plutonium can then be used to make new fuel, such as mixed oxide (MOX) fuel. This reduces the amount of new uranium that needs to be mined and also decreases the volume of high-level waste.

Waste Disposal

Nuclear waste remains radioactive for a very long time, so it must be disposed of in a way that keeps it away from people and the environment. The most widely accepted plan is to bury the waste deep underground in a deep geological repository. This involves placing the waste in strong containers and burying them in stable rock formations, like granite or salt, hundreds of meters below the surface. This method is designed to keep the waste safely isolated for thousands of years.

Different Types of Fuel Cycles

Once-Through Fuel Cycle

In a once-through cycle, fuel is used one time and then stored permanently as waste.

This is also called an "open" fuel cycle. The fuel is used once in a reactor and then stored as waste in a permanent repository. Countries like the United States, Canada, and Sweden use this method.

Plutonium Fuel Cycle

In a recycling fuel cycle, useful materials like plutonium are recovered from spent fuel to make new fuel.

This is a "closed" fuel cycle where spent fuel is reprocessed to recover plutonium and uranium. These materials are then used to make new fuel. This reduces the amount of waste and makes better use of uranium resources. Japan and several European countries use this approach.

Thorium Fuel Cycle

Scientists are also studying a different type of fuel cycle that uses the element thorium. Thorium is much more common in the Earth's crust than uranium. In a reactor, thorium can be turned into a type of uranium (U-233) that is a good nuclear fuel. This cycle could produce less long-lived radioactive waste than the uranium cycle.

Images for kids

Temperature inside different types of nuclear fuel pellets. Uranium dioxide (UO2) gets much hotter in the center.
A concept for an Integral Fast Reactor (IFR), which is designed to recycle its own fuel.
A diagram showing how an Integral Fast Reactor works.

Nuclear fuel cycle facts for kids

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