It turns out that the ore that contains natural Uranium (often called Yellowcake) actually contains TWO different types of Uranium. The primary one is an isotope called U-238, which is generally about 99.3% of all the Uranium in natural ore. The other 0.7% is a different isotope, called U-235.
Individual atoms of U-235 and U-238 are remarkably similar. They behave identically in chemical reactions. They are both naturally radioactive, where the Uranium atoms will eventually spontaneously Decay into two large (other) atoms, along with a few small fragments.
This was discovered to be important during the 1930s, when it was realized that U-235 Decays give off TWO neutrons, while U-238 Decays usually result in a single released neutron.
Physicists in the 1930s realized that if a single neutron was used as a billiards cue ball to hit and shatter a U-235 atom, then TWO neutrons were created in the process. There are some complications to this reasoning, but if those two neutrons could be 'slowed down' (don't ask!) then they could hit and shatter other U-235 atoms, which would give off a lot of energy but also now FOUR neutrons! Those four neutrons might cause four other U-235 atoms to Decay and to also create 8 neutrons.
This is referred to as a Self-sustaining Nuclear Chain-Reaction. Once started, by even a single neutron, a moment later, an enormous number of U-235 atoms have also Decayed, with tremendous release of energy. THIS reasoning was what caused the Manhattan Project to try to learn how to cause this. By July 1945, the United States had accomplished this and they had proven it with the first Atomic Test in the New Mexico desert. Only that single test had been done before the second and third atomic bombs were dropped on Nagasaki and Hiroshima Japan on August 6th and 9th a few weeks later.
There are many technical accomplishments that need to be done in the creation of an atomic bomb. I do not intend to teach anyone how they might build one, but I do now want to focus on a single step of the process.
The step is to try to separate the U-238 from the U-235. There are several methods which were found and tried during the Manhattan Project. Most are very 'elegant' but also extremely expensive to do. Probably the very best is the use of a Mass Spectrometer. But that process is not efficient enough for people who want results! A Mass Spectrometer first creates some gaseous compound with the Uranium. The most common is UF6, Uranium hexafluoride, which is a yellowish gas. For the Mass Spectrometer, that gas is then ionized to electrically charge it. Then the charged gas is blown through a strong magnetic field. What this does is cause the charged molecules to be REDIRECTED due to the electrostatic effect. A Mass Spectrometer normally then sends those deflected gaseous molecules at detectors. Different molecules which have different masses are deflected different amounts and so they arrive at different locations on the detectors.
A familiar use of a Mass Spectrometer is in detecting Carbon-14 atoms in finding ages of organic objects from antiquity. Molecules that contain the standard Carbon-12 atoms are deflected more than the heavier molecules that contain the Carbon-14 atoms. But a mass (weight) difference of 14 to 12 is significant, which results in obvious different locations of the Mass Spectrometer beams.
In the case of the UF6, the molecule mass difference is a very small fraction. When one molecule has a total mass of around 400 and the other has a mass that is only three different, the distinction of spacing between the two deflected beams is quite small. In Research efforts, such differences are sufficient, but for mass separation, it is very time consuming and expensive to do.
There are other methods of separating different isotopes, but the most popular one became the use of Centrifuges.
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Where the initial percentage of U-235 is only 0.7%, after a substantial time in a Centrifuge, you can get that collection of Uranium to have had some of the U-238 removed where you are now at 1.0% concentration.
Nuclear Reactors that produce electricity must have at least 4.0% concentration, and some Reactors can accept fuel that has as high as 7.0% concentration.
There is absolutely NO point in trying to go to the trouble to concentrate the U-235 to any higher concentration than 7.0%.
The OTHER use that has been made of Uranium is in atomic bombs, and the 'Weapons Grade' U-235 needs to be at least 95% for that use (as discussed above when the chain-reaction was described.)
There IS NO other known use for Uranium than these two purposes. In one, a concentration of 4% to 7% is desired. In the other, the concentration needs to be at least 95%.
So when Iran admitted (or bragged) that they had achieved 20% concentration, EVERYONE should have immediately realized the importance of that statement! It would be idiotic to go to all the trouble to concentrate it to 20% just to then 'thin it out' down to 4% for an electricity-producing nuclear power plant. So any such claims are clearly absolute lies. But Iran seems to get away with such claims of 'concentrating uranium for peaceful purposes in generating electricity'. No one either cares or understands the subject (which is part of the point of this explanation by a Physicist).
Having 20% U-235 in itself is of no benefit. But THAT concentrated Uranium mixture can be sent through Centrifuges over and over and over and over, to increase its concentration even more. THAT is actually how the United States had gotten the 95% concentration that they used in their thousands of atomic bombs. It is likely that North Korea, India, Pakistan, Israel, France, Great Britain, Russia and other countries also achieved the very high concentration to build their own atomic bombs. Given enough Centrifuges and enough time, nearly any desired purity of U-235 is possible. For Iran to get their Uranium supplies up to a 95% purity is therefore simply a matter of time, using the many thousands of Centrifuge devices that they already have and are using.
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