Nuclear fuel cycle

The nuclear fuel cycle describes the complete series of stages for nuclear fuel. This process includes the extraction, processing, use, and disposal of fuel. It is often called the nuclear fuel chain. The cycle is divided into three main parts: the front end, the service period, and the back end. The front end involves preparing the fuel for use. The service period is when the fuel operates inside a nuclear reactor. The back end is for the safe management, containment, and disposal of spent fuel.

Nuclear power relies on fissionable material to sustain a chain reaction with neutrons. Common examples include uranium and plutonium. In most reactors, a moderator is used to lower the kinetic energy of neutrons. This increases the chance that fission, or the splitting of atoms, will occur. Graphite and heavy water are very effective moderators. They slow neutrons through collisions without absorbing them. This allows some reactors to use natural uranium. Light water reactors (LWRs) use regular water as a moderator. These reactors require enriched fuel with higher concentrations of fissile isotopes.

The cycle begins with the exploration and mining of uranium. Geologists use geophysical techniques to find uranium deposits, such as uraninite. Once a deposit is found, scientists determine how much uranium can be extracted. Uranium is naturally found in two main isotopes: U-238 and U-235. About 99.28% of natural uranium is the fertile isotope U-238. Only 0.71% is the fissile isotope U-235. Fissile isotopes are special because their nuclei nearly always fission when struck by a neutron. U-238 is considered fertile because it can absorb a neutron to eventually become plutonium-239.

After mining, the ore undergoes milling to extract usable uranium. The ore is crushed or ground into a fine dust. It is then treated with chemicals, most commonly sulfuric acid. This process dissolves the uranium into a liquid solution. The solution is filtered to separate the uranium from undesirable solids called tailings. The uranium is then recovered using solvent exchange or ion exchange. Finally, the uranium is dried into a powder called "yellowcake," or U3O8. This material is then converted into uranium hexafluoride (UF6) for the next step.

Most commercial reactors require enriched uranium. This is because the natural 0.71% concentration of U-235 is too low for light water reactors. Enrichment increases this concentration to between 3% and 5%. This is often done using gas centrifuges or gaseous diffusion. This process produces a byproduct known as depleted uranium (DU). About 96% of the byproduct from enrichment is depleted uranium. As of 2008, the United States Department of Energy had 470,000 tonnes of DU in storage. This material can be used for radiation shielding or ballast.

Once enriched, the uranium hexafluoride is converted into uranium dioxide (UO2) powder. This powder is pressed into small, hard ceramic pellets. These pellets are fired in a high-temperature sintering furnace. The pellets are then ground to a uniform size and stacked into metal tubes. These tubes are called fuel rods and are made of corrosion-resistant alloys. Many fuel rods are grouped together into fuel assemblies. These assemblies form the fuel core of the power reactor.

During the service period, the fuel undergoes nuclear reactions. As fissile isotopes are consumed, fission products build up. These products are often radioactive waste. Eventually, the buildup of these products stops the reaction. The fuel is then considered "spent nuclear fuel." This spent fuel is extremely hazardous. However, nuclear fuel has a very high energy density. This means reactors produce much smaller volumes of waste than other power plants. If all the spent fuel from U.S. power stations were in one place, it would cover about one American football field.

There are two ways to manage the back end of the cycle. An open fuel cycle, or once-through cycle, means the spent fuel is not reprocessed. It is simply managed and disposed of. A closed fuel cycle involves reprocessing the spent fuel. In a closed cycle, about 95% of the spent fuel can be recycled for reuse. Reprocessing allows for the separation of plutonium and other materials from the waste. This can create new fuel types, such as mixed oxide (MOX) fuel. MOX fuel is made by blending plutonium with uranium.