By Nader Bagherzadeh, UC,
The primary aim of this article is
Undoubtedly among many issues that
Bush administration’s decision to
make suspension of enrichment activity as a precondition for any future
diplomatic discussions with
The purity or level of enriched
uranium (U-235) needed for running a nuclear reactor, such as
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
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
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
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 UF6 and 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
Figure 3. Uranium Fuel Cycle (http://www.uic.com.au/nfc.htm)
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
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
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,
1. David Albright Testimony to Congress (March 15, 2007)
2. Uranium Enrichment, Urenco Publication (www.urenco.de)
3. Would Air Strike Work? Oxford Research Group, (March 2007)
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