Introduction
The primary aim of this article is to explain Iran’s nuclear fuel cycle and the uranium based nuclear enrichment technology at the level of a non-specialist. Familiar and rudimentary examples are used so that interested readers can have a better understanding of this critical and controversial technology which is at the core of the contentious US-Iran relation.
Undoubtedly among many issues that Iran and US do not agree on, Iran’s desire to develop a nuclear enrichment technology and have a nascent enrichment facility in the city of Natanz is at the top of that list. Most certainly Iran’s access and mastering of this technology will impact the hegemony of US in that part of the world more than any other issue. If US were to embark on yet another illegal and immoral war in the Middle East, no other issue is more important than Iran’s stubbornness to exercise its rights under established international agreements.
Bush administration’s decision to make suspension of enrichment activity as a precondition for any future diplomatic discussions with Iran, except for Iraq related security discussions, speaks for the importance of this issue. By domestically developing and acquiring this highly sophisticated industry, Iran’s position to be a technologically advanced nation in the Middle East will be established, at least in the area of nuclear fuel cycle.
Uranium Fuel
The purity or level of enriched uranium (U-235) needed for running a nuclear reactor, such as Iran’s Bushehr power plant, is about 5%. This is called Low Enriched Uranium (LEU) but for making a nuclear weapon the U-235 material has to be enriched and purified to the level of 90% or more, commonly referred to as Highly Enriched Uranium (HEU). The uranium found in nature has very little light weight uranium, and it is mostly the heavier uranium (U-238 isotope). Thus, one has to find ways of separating the heavy isotopes from lighter ones. There are various techniques to accomplish this separation task such as using lasers, specialized filters (diffusion), or using magnetic fields, but the gas centrifuge method discussed later is considered the best approach economically.
Nuclear Fuel Cycle
The nuclear fuel cycle consists of four major steps to process natural occurring uranium ore from mines into fuel rods useable for a nuclear power plant. These four steps are: (1) uranium mining and production of yellow cake, (2) conversion, (3) enrichment, and (4) fuel manufacturing.
1- Uranium Mining and Making Yellow Cake:
In this step uranium ore, which is mined, is crushed and processed into a yellowish colored powder that is radioactive and contains uranium oxide in the form of U3O8. Uranium in this form is 99.3% U-238 and 0.7% U-235 isotopes. The latter is what is usually needed for a standard modern nuclear reactor, although reactors based on the natural occurring uranium (U-238) also do exist, but these are generally less safe and have the potential of proliferation because of their weapon grade plutonium byproduct. It is believed that Iran has yellow cake processing fully established in Saghand and Gachin, near its uranium mine facilities.
Figure 2. Yellow cake
2- Uranium Conversion:
For the most reliable and cost effective enrichment techniques, it is customary to use uranium in gaseous form, because the yellow cake cannot directly be enriched. After several steps yellow cake is chemically processed and converted into a gaseous form called the uranium hexafluoride gas (UF6) --this conversion takes place at the Uranium Conversion Facility (UCF) of Isfahan. UF6 has the unique property of having the lowest melting point of any uranium compound; making it a perfect choice as uranium feed for gaseous centrifuge machines. This means it is easier to produce uranium based gaseous feed for a centrifuge using UF6 than other compounds, such as UF4which has a melting point 10 times higher than UF6. The gas produced in this facility is stored in containers for delivery to the Natanz Fuel Enrichment Plant (FEP). The most economical method for enrichment is to spin the UF6gas and collect the light weight UF6 molecules and repeat the process until the desired purity is achieved. It is important that the injected UF6 into the centrifuge is of the highest purity; otherwise it can disturb the enrichment process. Initially Western media reported that the UF6from UCF contained heavy metals such as molybdenum and therefore was of inferior quality for any kind of enrichment, but this was rejected by the Iranian officials. Even in its pure form, UF6 is a very corrosive material that has to be kept in optimal temperature; otherwise it can corrode the pipes or clog the pipes if the temperature is lower than the expected norm.
3- Enrichment (Centrifuge Technique):
The enrichment process is to separate heavier uranium (U-238) isotopes from lighter ones (U-235). The number designation is directly related to the weight of the atom, meaning U-238 is heavier than U-235. As it turns out the light uranium atoms are better suited for fueling a nuclear reactor to generate electricity. In order to separate these isotopes, the UF6 should be fed into a series of centrifuge machines. A centrifuge is designed to turn at a very high speed; in some designs it could reach higher than the speed of sound. Centrifuge operation can best be described as the way a dryer works in the laundry room. By spinning around, a standard dryer “separates” water molecules from clothes. The same centrifugal forces when applied at very high speeds enable separation of U-238 molecules of UF6 from U-235 molecules.
4- Fuel Manufacturing:
The final step in the fuel cycle is to take the enriched UF6and create the uranium oxide UO2. This means that the fluoride has to be removed from the UF6 molecules and uranium oxide has to be turned into a metal shaped tablet (similar to a hockey puck in shape and color). These tablets will be stacked in fuel rod tubes made out of zirconium alloy. Although Iran is one of the few countries that claims to have a working zirconium plant, the Fuel Manufacturing Plant (FMP) in Isfahan is not complete and it is planned to be finished within a year. Clearly unless Natanz enrichment facility is fully operational at the industrial level producing tons of enriched uranium, it is not urgent to have FMP completed.
Gas Centrifuge– A complex and challenging technology [1-3]
By far using gas centrifuge technology for enriching uranium is the most complicated step in the uranium fuel cycle, and as such we steer the rest of this article to better explain this crucial and key step in the process.
A measure of how good a uranium enrichment centrifuge operates (its “efficiency” factor) is defined by an engineering concept and a term called Separative Work Unit (SWU) which means the amount of enriched uranium separated from the input mix. Its units are usually in Kg or tons referring to the amount of mass produced. The higher the SWU for a centrifuge design, the better and more efficient it is for enriching uranium gas and it is a function of certain features in the design and operation of the centrifuge machine. For instance, the centrifuge design that Iran has acquired for the Natanz plant from A Q Khan--the nuclear technology broker from Pakistan-- is purported to have efficiency (i.e., SWU) of about 2. The exact number has not been publicly announced. A commercial design developed by the European firm URENCO is reported to be about 40, and the US latest deign is expected to be 300 or more. This efficiency is directly related to the maximum speed that a centrifuge can spin as well as the height of the centrifuge. The speed has a major impact on the performance of the centrifuge, for instance, if the speed of the centrifuge is doubled the efficiency will go up by a factor of 16. This intuitively implies that using our dryer example for comparison, if the dryer spins twice as fast as before its ability to dry clothes will increase 16 fold. Thus, it is desirable to design a centrifuge that can spin as fast as possible.
The types of centrifuges utilized in Natanz are mentioned to spin at the rate of about 64,000 revolutions per minute (RPM), or 350 m/s. This is a little over the speed of sound (344 m/s), but the latest designs from Europe are expected to have a speed of 90,000 RPM or more (500 m/s), an increase of about 50%. To appreciate the speed requirement, let us use the car engine for comparison. A typical car engine has a turning speed of about 8000 RPM, when the gas pedal is fully pressed down. So a Natanz centrifuge spins 8 times faster than that. Another difference is that these centrifuge machines have to operate non-stop for months or longer to purify uranium gas, but one can not expect to run the car engine for more than a few seconds at that speed before it overheats. This clearly explains the technological challenges and the complexity of the design for centrifuge in order to maintain operation for months without any interruptions
The maximum spinning speed of a centrifuge depends directly on the strength and inversely related on the weight of the material used to make its major moving parts. The most advanced designs should have the strongest material with the lightest possible weight. For instance, the earlier designs from 30 years ago, similar to what Iran has in Natanz, are based on Aluminum. This metal is very light as it has been used for airplanes, but the strength is not as good as certain steel alloy (maraging) which is relatively heavier, however, the ratio of strength to weight which decides on the maximum speed is in favor of this type of steel. Hence, the second generation centrifuge systems have relied on this technology. The picture below shows a diagram for a typical centrifuge. The light blue circles denote the movement of light uranium molecules needed for fuel. By heating up the bottom of this machine the process of separation is enhanced. The dark blue are the heavier molecules that are not contributing to the fuel enrichment process.
Figure 4. Gas Centrifuge
Even better than maraging steel for centrifuge design is carbon fiber. This has the highest strength with the least weight among alternative designs. The most advanced designs use this technology, such as the ones used for US models. Except for US and some European countries, no other country has the technology to reach this level of sophistication for centrifuge design. In this article we have only focused on the turning speed of a centrifuge machine, there are other important parameters, but none have the impact on improving performance as the centrifugal speed does, except for the length of centrifuge and temperature of the UF6 gas spinning inside. Requiring centrifuges to spin at very high speed when it is very tall has major engineering issues related to stability and maintaining balance. The efficiency of a centrifuge directly increases with increase in the height of the design as well with decrease in gas temperature. Both of these methods are very difficult to manage because the height will impact the stability of the design and the lower gas temperature will add to the problems associated with clogging of the pipes.
In order to improve the throughput of enriched uranium production, it is common to cascade centrifuge machines. The uranium feed (NF) into a centrifuge machine after spinning results into two outputs. One is called tail assay (NT) or the depleted uranium and the other one is called the product (NP). The product contains the enriched uranium which is used for making the fuel for a reactor.
Figure 5. Centrifuge Operation
In order to establish a cascade, the tail output becomes the feed for the next centrifuge machine. Using our dryer example, although this is not commonly done, but one could use two dryers to handle a very large load. This can be done by taking not fully dried clothes from the first dryer, after it was running for a while, and transfer them to the second dryer. Then, load the first dryer, that is now empty with a fresh set of wet clothes in order to dry them again for a while before transferring to the second dryer, and the process continues until all the work is done. This approach will effectively improve the throughput of the enrichment process.
The product delivery rate for Natanz centrifuge machines is estimated to be about 12%. This means if one feeds 80 grams of uranium to 164-cacacded machines, the product is 10 grams per hour of low enriched uranium (LEU) with appropriate purity for a reactor. If the 164-cascade machines work for 12 months without any interruptions, it needs 700Kg of feed and will produce 87 Kg of product per year.
Generating Fuel for a Light Water Reactor (LWR)
A LWR’s function is to split the light uranium isotopes (U-235) in a controlled manner to generate heat and produce the necessary energy to boil water and subsequently spin the turbines that generate electricity. The amount of energy produced has to be under strict control at all times. If this energy is released too fast, it may result in a melt down and other calamities that are quite dangerous. In order to produce fuel for a nuclear reactor such as the Bushehr LWR, Iran’s Natanz enrichment facility is designed to have 164 cascaded centrifuge machines as the basic unit of enrichment. It is reported that so far 8 164-cascaded systems are installed and working for fuel enrichment, adding up to 1312 centrifuges, in preparation for 18 164-cascaded modules. The final goal for this facility is to have 18 modules with the total capacity of 53136 centrifuges, in order to provide for the annual fuel requirements of at least one power plant per year when fully operational. Using our previous calculations, when fully completed, Natanz could produce around 30 tons of LEU annually.
Conclusion
Iran has been actively pursuing a uranium fuel cycle technology to provide domestically produced LEU for future nuclear power plants. The process of fuel enrichment is complicated and requires numerous high technology steps. Ostensibly, Iran has mastered almost all of these steps and is in the process of producing fuel at the industrial level.
References:
1. David Albright Testimony to Congress (March 15, 2007)
3. Would Air Strike Work? Oxford Research Group, (March 2007
0 comments:
Post a Comment