Uranium is used as an energy source in nuclear reactors and was used to make the first atomic bomb dropped on Hiroshima in 1945. Uranium is mined in the form of a mineral called pitchblende, and consists of numerous isotopes with different atomic weights and of different levels of radioactivity. To be used in fission reactions, the amount of the uranium isotope U235 it must be increased to a level that allows fission in a reactor or pump. This process is called uranium enrichment and there are several ways to carry it out.
Method 1 of 7: The Basic Process for Enriching Uranium
Step 1. Decide how you will use the uranium
Most of the mined uranium contains only about 0.7 percent U235, finding its remaining part (most of it) composed of the most stable isotope of U238. Depending on the type of fission reaction in which the uranium is going to be used, the level at which the U235 it must be increased, so that it is used efficiently.
- The uranium used in most power plants must be enriched to a level between 3 and 5 percent U235. A few nuclear reactors, such as CANDU in Canada or the Magnox reactor in the UK, have been designed to enrich uranium.
- In contrast, the uranium used in atomic bombs and warheads requires to be enriched to 90 percent of U235.
Step 2. Convert the uranium ore into a gas
Most of the methods that exist today for the enrichment of uranium require that the mineral be transformed into a low temperature gas. To that end, the usual thing will be to pump fluorine gas to a mineral conversion plant; then the uranium oxide gas will react with the fluorine gas and this will produce uranium hexafluoride (UF6). The gas is allowed to act to separate and gather the isotopes of U235.
Step 3. Enrich the uranium
The remaining sections of this article describe the different processes that exist to enrich uranium. Of all those processes, gas diffusion and gas centrifugation processes are the two most common, however, the laser isotope separation process is expected to replace them.
Step 4. Convert the UF gas6 in uranium dioxide (UO2).
Once the uranium is enriched, it is necessary to convert it into a solid and stable form for its required use.
Uranium dioxide used as fuel in nuclear reactors is produced in the form of sintered ceramic pellets, housed in metal tubes that form rods 4 meters (13.12 feet) long
Method 2 of 7: Gaseous Diffusion Process
Step 1. Pump UF6 through the pipelines.
Step 2. Pass the gas through a porous filter or membrane
While the isotope U235 is lighter than the U isotope238, the UF6 that contains the lighter isotope will spread through the membrane more quickly than that containing heavier isotopes.
Step 3. Repeat the diffusion process until enough U has been collected235.
Repetitive diffusion is called a "cascade". It can take up to 1,400 diffusions through porous membranes to get enough U235 and thus sufficiently enrich the uranium.
Step 4. Condense the UF gas6 so that it acquires a liquid form.
Once the gas is sufficiently enriched, it must be condensed into a liquid form and then stored in tanks, where it will cool and solidify to be transported and converted into fuel pellets.
Due to the number of broadcasts required, this process consumes a lot of energy and, therefore, is out of use. In the United States, only one gaseous diffusion enrichment plant remains, which is located in Paducah, Kentucky
Method 3 of 7: Gas Centrifugation Process
Step 1. Assemble a number of high-speed rotating cylinders
These cylinders will be the centrifuges. The centrifuges are assembled in series of two and mounted in parallel.
Step 2. Pump the UF gas6 inside the centrifuges.
The centrifuge will use centripetal acceleration to send the U238 heavier than it had towards the cylinder walls and the U235 lighter towards the center of it.
Step 3. Extract the gases that have been separated in the process
Step 4. Reprocess the separated gases in different centrifuges
Gases rich in U235 are sent to a centrifuge where even more U235 will be extracted; while U gas235 exhausted goes to a different centrifuge, in order to extract the remaining U235. This allows the spinning process to extract much more U235 than the process of gaseous diffusion.
The gas centrifugation process was first developed in the 1940s, but did not become relevant until its use in the 1960s, when its low energy requirement to produce enriched uranium became important. Currently, a centrifugal gas processing plant exists in the United States, in Eunice, New Mexico. By comparison, Russia has 4 such plants, Japan and China each have two plants, while the UK, the Netherlands and Germany have one each
Method 4 of 7: Aerodynamic Separation Process
Step 1. Build a series of narrow stationary tanks
Step 2. Inject UF gas6 inside the cylinders at high speed.
The gas is injected into the cylinders, in such a way that it is induced to rotate in a cyclonic way, producing the same separation of the U235 and from the U238 than that obtained by centrifugal rotation.
A method that has been developing in South Africa injects the gas into the cylinders from a tangent. It is currently being tested with light isotopes, such as those found in silicon
Method 5 of 7: Liquid Thermal Diffusion Process
Step 1. Liquefy the UF gas6 under pressure.
Step 2. Build a pair of concentric tubes
The tubes must be high enough, because the higher the height, the greater the separation of the isotopes of U235 and U238.
Step 3. Surround the tubes with a cover of water
This will cool the outside of the tube.
Step 4. Inject the UF6 liquid between the tubes.
Step 5. Heat the inside of the tube with steam
The heat will create a convection current in the UF6, the same one that will extract the isotopes of U235 as these are lighter, and it will direct it towards the hottest center of the tube and will push the U isotopes238 heavier towards the cooler and outer part of the tube.
This process was investigated in 1940 as part of the Manhattan Project, but it was abandoned still in its initial phase when the more efficient process of gaseous diffusion was developed
Method 6 of 7: Electromagnetic Isotope Separation Process
Step 1. Ionize the UF gas6.
Step 2. Pass the gas through a strong magnetic field
Step 3. Separate the ionized uranium isotopes, identifying them by the traces they leave when passing through the magnetic field
U ions235 leave traces that form a curvature different from those left by the U238. Those ions can be isolated in order to enrich the uranium.
This method was used to process the uranium from the atomic bombs that were dropped on Hiroshima in 1945 and, likewise, it is the enrichment method used by Iraq in its 1992 nuclear weapons program. This method requires 10 times more energy than that of gaseous diffusion, making it impractical for large-scale enrichment programs
Method 7 of 7: Laser Isotope Separation Process
Step 1. Adjust the laser to a specific color
The laser light must be completely on one wavelength (monochromatic). This wavelength will only be concentrated in the U atoms235, leaving the atoms U238 intact.
Step 2. Illuminate the uranium with the laser light
Unlike other uranium enrichment processes, in this case it is not necessary to use uranium hexafluoride gas, even though most other laser processes require it. You can also use an alloy of uranium and iron as a source of uranium, which can be achieved through the Atomic Vapor Laser Isotope Separation (AVLIS) process.
Step 3. Extract the uranium atoms by electrons in the excited state
These will be atoms of 235OR.
- Some countries reprocess nuclear fuels in order to recover the depleted uranium and plutonium created during the fission process. Reprocessed uranium must be stripped of U isotopes232 and U236 formed during fission, and if they are enriched, it should be done at a level higher than that of new uranium (unused uranium), since the U236 it absorbs neutrons and thus inhibits the fission process. That is why reprocessed uranium must be kept separate from uranium that, for the first time, has been enriched.
- Uranium itself is slightly radioactive; however, when it is converted to UF gas6 It becomes a toxic chemical that reacts with water and forms a corrosive form of hydrofluoric acid (this gas is commonly referred to as "etching acid" for its use to etch on glass). Therefore, uranium enrichment plants require the same protection measures as chemical plants that work with fluorine, which includes maintaining the UF gas6 most of the time under low pressure and use an extra number of containment levels in areas requiring high pressures.
- Reprocessed uranium must be kept under strong shielding, as the U232 it contains decomposes into elements that emit considerable amounts of gamma radiation.
- Enriched uranium can normally be reprocessed for only one time.