Switching Nuclear Fuel: A Step-By-Step Guide To Success

Nuclear reactors use uranium as their main fuel source. Uranium is mined, refined, and enriched before being loaded into a nuclear reactor. The uranium is formed into pellets, which are then inserted into thin tubes known as fuel rods. These fuel rods are then grouped together to form fuel assemblies, which are loaded into the reactor core. Once the uranium has been used up, the fuel assemblies become highly radioactive and must be removed and stored in a spent fuel pool to cool down. This process of changing the fuel in a nuclear reactor involves several steps and ensures the safe and efficient operation of the reactor.

Uranium-235 is the most common nuclear fuel

Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its fission cross-section for slow thermal neutrons is about 584.3±1 barns. For fast neutrons, it is on the order of 1 barn. Most neutron absorptions induce fission, though a minority (about 15%) result in the formation of uranium-236.

The fission of one atom of uranium-235 releases 202.5 MeV (3.24×10−11 J) inside the reactor. That corresponds to 19.54 TJ/mol, or 83.14 TJ/kg. Another 8.8 MeV escapes the reactor as anti-neutrinos. When uranium-235 nuclei are bombarded with neutrons, one of the many fission reactions that can occur is the following:

1 0n + 235 92U → 141 56Ba + 92 36Kr + 3 1 0n

Uranium-235 has many uses, including fuel for nuclear power plants and nuclear weapons such as the atomic bomb dropped on Hiroshima in 1945. It is also used in artificial satellites, such as the SNAP-10A and the RORSATs.

Most commercial nuclear power reactors require uranium to be 'enriched' in the uranium-235 isotope for their fuel. The commercial process for this enrichment involves gaseous uranium in centrifuges. Uranium found in nature consists largely of two isotopes, uranium-235 and uranium-238. The production of energy in nuclear reactors comes from the 'fission' or splitting of uranium-235 atoms, a process that releases energy in the form of heat. Uranium-235 is the main fissile isotope of uranium.

Natural uranium contains 0.7% of the uranium-235 isotope. The remaining 99.3% is mostly the uranium-238 isotope, which does not contribute directly to the fission process but does so indirectly by the formation of fissile isotopes of plutonium. Isotope separation is a physical process to concentrate ('enrich') one isotope relative to others. Most reactors are light water reactors and require uranium to be enriched from 0.7% to 3-5% uranium-235 in their fuel. This is normal low-enriched uranium (LEU).

Uranium is mined and refined

The first step in refining uranium is called milling, which changes raw ore into uranium concentrate, either black oxide (uranium oxide, U3O8) or soda salt (sodium diuranate, Na2U2O7), also known as "yellow cake". The uranium concentrate then undergoes a number of chemical processes in various refineries, plants, and laboratories to prepare it for use. The milling process involves several steps of physical and chemical conversion, including mechanically crushing and grinding the raw ore into a fine sandy material, treating it with acid to extract the uranium content, and treating the extract with caustic and other chemicals to precipitate the impurities and uranium. A final roasting or drying operation produces the refined material as either black oxide or soda salt.

Uranium oxide is converted to uranium hexafluoride

Uranium hexafluoride is a chemical compound that contains one atom of uranium and six atoms of fluorine. It is a white crystalline solid that becomes a gas at fairly low temperatures. Uranium hexafluoride is used in the uranium enrichment process, which produces fuel for nuclear reactors.

The UO2 is then reacted with hydrogen fluoride (HF) to form uranium tetrafluoride (UF4). This can be done using either a wet or dry process. In the wet process, the UO2 is reacted with aqueous HF, while in the dry process, it is reacted with gaseous HF.

Finally, the UF4 is fed into a fluidized bed reactor or flame tower with gaseous fluorine to produce uranium hexafluoride (UF6). The UF6 is then condensed and stored.

Uranium hexafluoride is enriched

Uranium hexafluoride, sometimes called "hex", is a chemical compound with the formula UF6. It is a volatile, toxic white solid that becomes a gas at fairly low temperatures. It is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.

Uranium hexafluoride is used in both of the main uranium enrichment methods: gaseous diffusion and the gas centrifuge method. Uranium hexafluoride is convenient to process because it can be used as a gas for processing, but can also be transported and stored as a solid or liquid without the use of extreme pressures or temperatures.

In the gaseous diffusion process, mined uranium ore is sent to a plant that produces uranium oxide, or "yellowcake". Uranium oxide is then combined with hydrogen fluoride and fluorine gas to chemically form uranium hexafluoride. For gaseous diffusion to take place, uranium hexafluoride must be heated as it is solid under normal conditions, but transforms into a gas if the temperature is raised or the pressure is lowered. The 235UF6 molecules are slightly lighter than the 238UF6 molecules, so they move more quickly as a gas through diffusion. Therefore, if uranium hexafluoride is passed through a very long pipe, the gas that emerges at the far end will have a slightly higher percentage of 235U. However, the pipe must be extremely long as the lighter 235UF6 diffuses only 0.43% faster than 238UF6.

The gas centrifuge method is much more energy-efficient than gaseous diffusion. Gaseous diffusion-produced nuclear fuel produces 25 times more energy than is used in the diffusion process, while centrifuge-produced fuel produces 1,500 times more energy than is used in the centrifuge process.

Uranium dioxide powder is pressed into fuel pellets

Uranium dioxide powder is pressed into small, hard pellets, each about the size of a sugar cube. These fuel pellets are then heated to form a hard ceramic material.

The pressing process is a critical step in the fabrication of nuclear fuel. The uranium dioxide powder is compacted under high pressure to form the small pellets. This process ensures that the pellets have a uniform density and desired shape. The density of the pellets is crucial as it directly affects their dimensional behaviour. A higher density also helps to improve the fuel's performance in the reactor.

To enhance the pressing capabilities, the uranium dioxide powder may be mixed with a binder, such as zinc stearate, and compacted with a lubricant. This mixture helps to improve the flow of the powder during pressing and reduces friction, resulting in a more uniform pellet.

After pressing, the pellets are typically heated to a high temperature of around 1700°C in a hydrogen atmosphere. This process, known as sintering, further increases the density of the pellets, making them harder and more durable. The high temperature is necessary to achieve the required density for optimal performance in a nuclear reactor.

The uranium dioxide fuel pellets are then ready to be inserted into thin tubes called fuel rods, which are then grouped together to form fuel assemblies. These fuel assemblies are then loaded into the nuclear reactor, where they can stay for several years, generating a significant amount of low-carbon electricity.

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