Since I didn't know anything about nuclear fuel production for a reactor and the difference between that kind of fuel and the fuel used for weapons, I thought it would a good idea to know, so I looked it up. The effort was worth it, since it sorted out the current bullshit clammor over Iran. For listmembers interested Here's an outline.
Natural uranium ore contains U-235 (less than 1%) and U-238 (more than 99%). The numbers indicate the atomic weight or the number of protons and neutrons in the nucleus. The U-235 nucleus is unstable because it contains more many more neutrons than protons (143, 92) and can be broken apart with high speed neutrons into lighter elements, accompanied by a release of more neutrons. If the concentration is sufficient and the number of U-235 nuclei hit are sufficient, the released neutrons will hit other U-235 nuclei and cause a chain reaction. Fuel rods used to produce heat as a product of fission require low U-235 concentrations but these concentration are above those found in nature.
On the other hand, U-238 is stable and will not break apart if bombarded with neutrons. Instead the nucleus will absorb the neutrons in fixed steps (quantum nuclear chemistry) to produce heavier trans-uranium elements, including plutonium and its isotopes. A mix of U-238 and concentrated U-235 can be used to produce fuel for a breeder reactor that will in turn produce more plutonium from U-238 through the neutron addition.
Refining the raw uranium ore starts with crushing and water-acid leaching process to extract the uranium from its mineral base. This is usually done at a mining sight. The extracted uranium ore is then acid leached or put through some other crude chemical processing again in a milling plant to produce uranates, uranyl nitrate, and a uranium oxide U308 also called Yellow Cake.
The next refinement step is a conversion of the uranates and uranyl nitrate into uranium tetrafluoride (UF4, green salts). UF4 serves as a base to produce uranium metal or uranium hexafluoride (UF6). An alternative is use Yellow Cake and directly
The conversion from UF4 to UF6 requires a sophisticated and advanced refining facility that includes high pressure and high temperature processing with gaseous ammonia, hydrogen, and fluorine. For example UF4 is transformed into UF6 by fluorination in a flame reactor where UF4 spontaneously bursts into flame on contact with fluorine gas. The UF6 is cooled and crystallized into a container.
This uranium refinement and conversion process is at the heart of the Iran controversy over the uses of their conversion plant in Isfahan and a newer facility at Natanz.
The above uranium refinement process is a weak point in Iran's nuclear industrial capacity since it requires various high tech refining facilities including buying or manufacture and fabrication with high grade materials to build the flame reactor and cooling condenser system. These materials must be resistant to the high temperatures, pressures and extremely toxic and reactive processing of converting UF4 into UF6.
This conversion-refinement process is also considered a critical step towards the production of greater uranium purity for use in advanced reactors---and a necessary first step towards manufacturing more highly refined uranium.
More background. China negotiated to build a UF4 to UF6 plant for Iran in the early 90s. But the US persuaded the Chinese to stop assistance in 1997. So, the Iranians bought the blueprints to finish their plant(s). The project was more or less a failure and needed help from Chinese engineers between 1998 and 2003. The difficulties (I guess) were in the construction of the flame reactor and cooling, crystallizing system.
The Iranians claimed to have improved on the Chinese version, using their own design, which is supposed to be less complex to build. The two different versions are called a pulse column and mixer-settler. I think these terms refer to the flame reactor chamber configurations. In any case it appears to be a point of controversy as to which of these designs is Chinese and which is Iranian. (I can't tell which is which from limited web searches.)
An older and cruder refinement process converts UF4 to uranium metal, which with sufficient U-235 concentration, can be used to make a fuel rod for a natural uranium graphite gas reactor. Heavy water reactors can also use low-enriched uranium (0.7%). Light water reactors on the other hand require a minium of 3.5% U-235 usually in the form of a uranium oxide fuel, U02.
Each of these versions of reactors, as far as I can tell produce higher or lower quality reactor performance, or heat efficiencies, with the graphite version as the least efficient or lowest performance and light water as the greatest efficiency with highest performance. There are other negative and positive offsets in each design. For example the heavy water type reactors require a production facility to produce heavy water or the purchase of heavy water which is expensive in either case. Heavy water refers to deuterium oxide, or water in which the hydrogen atoms have a neutron added to their normal one proton nucleus.
In any event UF6 requires further enrichment and conversion to be useful as reactor fuel where UF6 is converted to U02, uranium oxide.
There are several methods of enriching UF6. The French use the gas diffusion method. The process vaporizes UF6 which also separates it into its three isotopes U-234, U-235, and U-238. Under high pressure and temperature the gas of isotopes is forced through a series of diffusion barriers where the lighter isotope pass through the barriers more easily than the heavier isotope U-238. The system is built in a cascade array in which the same process is repeated over and over for further and further refinement. Enriched UF6 with a greater percentage of U-235 comes out the other end.
The other method is called centrifugation. This is evidently the method Iran is using. UF6 is vaporized to separate its isotopes and pumped into an array of gas centrifuges. The centrifugal forces compress the heaviest isotope on the internal surface of a spinning cylinder while the lighter isotopes float above this layer where they are siphoned off and moved to the next spinning cylinder.
There is a formula for measuring the potential purity output per unit input of gas centrifugation given in separative work units (SWUs). For example 3.5% U-235 purity requires 4.3 SWUs. Weapons grade or 90% U-235 requires 4,600 SWUs.
Depending on the story you read, Iran recently claimed to achieve between 3% to 9% purity of UF6 the base used to produce U02 or uranium oxide.
Using the SWUs measure and the 3.5% figure, Iran must develop a centrifuge system with one thousand times greater efficiency to get to weapons grade U-235 purity.
Both the 3.5% and 9% figures fall far below the highly enriched uranium or HEU category minimum of 20%. The 20% minimum comes from the US DOE in its Reduced Enrichment for Research and Test Reactors or RERTR. This program was started in 1978 to promote research and development of civilian reactors that use low enrichment fuel, below 20% and hence support the non-proliferation of highly enriched uranium. Part of the project was devoted to re-processing spent nuclear fuel from HEU based reactors to low enriched uranium or LEU reactors. The preferred LEU level was in the 3.5% range. What is most interesting about this program was a take back system in which the country of origin takes back the spent HEU fuels, with the USA and FSU as the principle countries of origin. The range of purity of these HEU fuels is from 36% purity from FSU and Eastern Europe to 90% by the USA. The purpose of the take back program was putatively to develop recycling methods for production of LEU for new reactor fuel.
If you contemplate the above, it is obvious that the management of HEU fuels in the 36-90% range is a far more dangerous problem of international management of non-proliferation for the IAEA than any Iranian processing plant capable of producing a boasted LEU fuel of 3.5% which falls squarely in IAEA protocols and guidelines.
The information for all this comes from a French website (http://www.francenuc.org/toc_e.htm) various Wikipedia sites and several US government sites. For those willing to follow a chemical description (http://chemcases.com/nuclear/nc-06.htm). I would suggest taking a look because its descriptive (high school level) and it illustrates the whole process at a glance.