Uranium is a naturally-occurring element in the Earth’s crust. Traces of it occur almost everywhere, although mining takes place in locations where it is naturally concentrated. Uranium mines operate in some twenty countries, though about half of world production comes from just ten mines in six countries, in Canada, Australia, Niger, Kazakhstan, Russia and Namibia.
Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight. 235U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons. For example The very first uranium bomb, Little Boy dropped by the United States on Hiroshima in 1945, used 64 kilograms of 80% enriched uranium. The weapon grade uranium is enriched more than 20%.
To make nuclear fuel from the uranium ore requires first for the uranium to be extracted from the rock in which it is found, then enriched in the uranium-235 isotope, before being made into pellets that are loaded into the nuclear fuel assembly.
At conventional mines, the ore goes through a mill where it is first crushed. It is then ground in water to produce a slurry of fine ore particles suspended in the water. The slurry is leached with sulphuric acid to dissolve the uranium oxides, leaving the remaining rock and other minerals undissolved.
However, nearly half the world’s mines now use a mining method is in sit leaching (ISL). This means that the mining is accomplished without any major ground disturbance. Groundwater with a lot of oxygen injected into it is circulated through the uranium ore, extracting the uranium. The solution with dissolved uranium is pumped to the surface.
Both mining methods produce a liquid with uranium dissolved in it. This is filtered and the uranium then separated by ion exchange, precipitated from the solution, filtered and dried to produce a uranium oxide concentrate (U3O8), which is then sealed in drums. This concentrate is a bright yellow colour, and is known as ‘yellowcake’.
When you finish processing uranium ore, what you have is uranium oxide. Uranium oxide contains two types (or isotopes) of uranium: U-235 and U-238. U-235 is what you need if you want to make a bomb or fuel a nuclear power plant. But the uranium oxide from the mine is about 99 percent U-238. So you need to somehow separate the U-235 from the U-238 and increase the amount of U-235. The process of concentrating the U-235 is called enrichment.
Methods of Nuclear Enrichment:
- Diffusion techniques
- Centrifuge techniques
- Molecular laser isotope separation (MLIS)
- Separation of Isotopes by Laser Excitation (SILEX)
- Aerodynamic processes
- Electromagnetic isotope separation
- Plasma separation
Most widely used method is Centrifuge: U-235 weighs slightly less than U-238. By exploiting this weight difference, you can separate the U-235 and the U-238. The first step is to react the uranium with hydrofluoric acid, an extremely powerful acid. After several steps, you create the gas uranium hexafluoride.
Now that the uranium is in a gaseous form, it is easier to work with. You can put the gas into a centrifuge and spin it up. The centrifuge creates a force thousands of times more powerful than the force of gravity. Because the U-238 atoms are slightly heavier than the U-235 atoms, they tend to move out toward the walls of the centrifuge. The U-235 atoms tend to stay more toward the center of the centrifuge.
Although it is only a slight difference in concentrations, when you extract the gas from the center of the centrifuge, it has slightly more U-235 than it did before. You place this slightly concentrated gas in another centrifuge and do the same thing. If you do this thousands of times, you can create a gas that is highly enriched in U-235. At a uranium enrichment plant, thousands of centrifuges are chained together in long cascades.
At the end of a long chain of centrifuges, you have uranium hexafluoride gas containing a high concentration of U-235 atoms.
The creation of the centrifuges is a huge technological challenge. The centrifuges must spin very quickly — in the range of 100,000 rpm. To spin this fast, the centrifuges must have:
- very light, yet strong, rotors
- well-balanced rotors
- high-speed bearings, usually magnetic to reduce friction
Meeting all three of these requirements has been out of reach for most countries. The recent development of inexpensive, high-precision computer-controlled machining equipment has made things somewhat easier. This is why more countries are learning to enrich uranium in recent years.
Now you need to turn the uranium hexafluoride gas back into uranium metal. You do this by adding calcium. The calcium reacts with the fluoride to create a salt, and the pure uranium metal is left behind. With this highly concentrated U-235 metal, you can either make a nuclear bomb or power a nuclear reactor.