There are several different types of fuel that can be produced, and the type of fuel depends on the type of reactor being used. To learn more about reactors, see Reactors. Pressurized water reactors use uranium dioxide as a fuel. Fast reactors use mixed oxide fuel containing uranium dioxide and plutonium dioxide. Thorium may also by used by some reactors. For more information about thorium, see New Types of Fuel.
Uranium Fuel
Uranium is the conventional nuclear fuel. Among all the radioactive elements, why was uranium chosen as the nuclear reactor fuel? There are several reasons. The safety, economy, availability, and water-cooled thermal reactors burning uranium fuel became the basis for the development of nuclear power in the world, thus uranium is widely used (1). Uranium dioxide is operationally safe and the technology has been perfected. Yet as reactor technology advances, new types of reactors allow for different types of fuel.
Uranium dioxide fuel in a light-water reactor is relatively inefficient compared to the potential utilization of natural uranium, but it has been used because it is safe and competitive with respect to power production from fossil fuels (1). Nuclear power can exist for a long time by using the concept of a closed fuel cycle with fast reactors (see Figure 1). A closed fuel cycle is one in which the used nuclear fuel is reprocessed and developed into fuel for reuse. The system becomes much more efficient and will extend the lifetime of nuclear fuel.
Figure 1: A Closed Nuclear Fuel Cycle
Image courtesy of Japan Nuclear
After low-enriched UF6 has been enriched, it must be processed into UO2 powder. The powder is then compressed into pellets and placed into Zircaloy (an aluminum-zirconium alloy) tubes, which is made into fuel rods. The Zircaloy canisters are shown in the figure below. The fuel rods are then bundled into a fuel assembly, shown in Figure 3.
Figure 2: Zirconium fuel rods
Figure 3: A Fuel Assembly
Mixed Oxide Fuel
Mixed oxide (MOX) fuel was developed as a means to reuse the plutonium that remained in spent nuclear reactor fuel. MOX also provides a means to burn weapons-grade plutonium to generate electricity. Currently, MOX is the new fuel used in about 2% of reactors around the world, and this proportion is expected to rise to 5% by 2010 (2).
In all fission reactors, there is both fission of isotopes such as U-235, and there is also neutron capture by isotopes such as uranium-238. The neutron capture is demonstrated in Figure 4. The successive neutron capture of plutonium-239 will form plutonium-240, plutonium-241, and plutonium-242. Plutonium-239 and plutonium 241 are fissile like U-235, and some of it will burn in the reactor, giving off about 1/3 of the energy in a reactor in which the fuel is changed every three years. If a reactor has a higher “burn-up” more plutonium will be used. Approximately one percent of the spent nuclear fuel is plutonium. Two thirds of this remaining plutonium is fissile.
Figure 4: Production of plutonium 239 from uranium 238
Image courtesy of Hyperphysics
This leftover plutonium can be used again as nuclear fuel. However, it can also be used in nuclear weapons. To read more about the plutonium problem, see The Debate over Reprocessing. Although the half-life of plutonium-239 is long, Pu-238 and Pu-241 have shorter life-times that will decrease the fissile value of the plutonium over time. Due to the short lifetime of some isotopes, the plutonium should be reprocessed immediately. If the plutonium is recycled, 12%* more energy is derived from the uranium, and if the uranium is recycled as well, this percentage increases to 22 percent* (These numbers are based on a light water reactor with burn-up of 45 GWd/tU) (2).
How can the leftover plutonium and uranium be recycled? Please see Reprocessing.
Plutonium oxide is mixed with depleted uranium leftover at the enrichment plant, forming a new mixed oxide fuel (UO2+PuO2). MOX fuel consists of about 7-9% plutonium mixed with depleted uranium and is equivalent to uranium oxide fuel enriched to about 4.5% U-235, assuming that the plutonium has about two thirds fissile isotopes.** If weapons-grade plutonium is used (>90% Pu-239), only about 5% Pu would be needed in the mix.
** Reactors with higher burn-up will have a smaller percentage of fissile plutonium (2).
MOX reactors are already being used commercially in Europe. France and Japan have plans to increase their usage of MOX fuel by 2010. Many reactors use up to one-third MOX fuel. Although MOX fuel can be used in several different types of reactors, the plant must be adapted for the MOX fuel. More control rods are required in the nuclear reactor.
MOX fuel has several advantages. The fissile concentration of the fuel can be increased by simply adding more plutonium, which is much cheaper than enriching uranium. As the price of uranium itself goes up, MOX fuel will become more attractive economically. Also, by using MOX fuel, one is burning left-over plutonium that could have been used for nuclear weaponry.
MOX fuel can be compared to plutonium-thorium fuel. For more information, see New Types of Fuel.
Figure 5: MOX Fuel Production 
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