Nuclear Fission Fuel
NUCLEAR FISSION FUEL
A nuclear power plant generates electricity in a manner similar to a fossil fuel plant. The fundamental difference is the source of heat to create the steam that turns the turbine-generator. A fossil plant relies on the combustion of natural resources (coal, oil) to create steam. A nuclear reactor creates steam with the heat produced from a controlled chain reaction of nuclear fission (the splitting of atoms).
Uranium is used as the primary source of nuclear energy in a nuclear reactor, although one-third to one-half of the power will be produced from plutonium before the power plant is refueled. Plutonium is created during the uranium fission cycle, and after being created will also fission, contributing heat to make steam in the nuclear power plant. These two nuclear fuels are discussed separately in order to explore their similarities and differences. Mixed oxide fuel, a combination of uranium and recovered plutonium, also has limited application in nuclear fuel, and will be briefly discussed.
Uranium (symbol: U; atomic number: 92) is the heaviest element to occur naturally on Earth. The most commonly occurring natural isotope of uranium, U-238, accounts for approximately 99.3 percent of the world's uranium. The isotope U-235, the second most abundant naturally occurring isotope, accounts for another 0.7 percent. A third isotope, U-234, also occurs naturally, but accounts for less than 0.01 percent of the total naturally occurring uranium. The isotope U-234 is actually a product of radioactive decay of U-238.
Nuclear power is now the only substantial use for uranium. But before uranium can be used in a nuclear reactor, it must undergo several processes. After uranium is mined from geological mineral deposits, it is purified and converted into uranium hexafluoride (UF6). The UF6 is next enriched, increasing the concentration of U-235 by separating out UF6 made with U-238 atoms. The enriched UF6 is then converted into uranium dioxide (UO2), and pressed into fuel pellets for use in the nuclear reactor.
Uranium Mining and Milling
Uranium is found in most rock, in a concentration of two to four parts per million (ppm). Substantially greater average concentrations can be found in mineral deposits, as high as 10,000 ppm, or 10 percent. Most uranium deposits suitable for mining, however, contain an average of less than 1 percent uranium. Uranium is a metal, and thus its acquisition is not unlike the mining of any other metallic ore. Although uranium is found nearly everywhere on the earth, Canada leads the world in uranium production, mostly due to its heavy financial investment in uranium exploration, and to a few sizable deposits in the Saskatchewan territory. Table 1 depicts the total world uranium production in 1997.
Although the nucleus of the uranium atom is relatively stable, it is radioactive, and will remain that way for many years. The half-life of U-238 is over 4.5 billion years; the half-life of U-235 is over 700 million years. (Half-life refers to the amount of time it takes for one half of the radioactive material to undergo radioactive decay, turning into a more stable atom.) Because of uranium radiation, and to a lesser extent other radioactive elements such as radium and radon, uranium mineral deposits emit a finite quantity of radiation that require precautions to protect workers at the mining site. Gamma radiation is the
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most prevalent, followed by beta and alpha radiation. Because airborne particulates can be ingested or inhaled, thus subjecting workers to internal doses of radiation, the most important step taken to protect mine workers is to minimize the amount of airborne particulate matter. In areas where airborne particulate is of great concern, the uranium ore can be wetted to prevent releasing dust to the air. In areas of even greater risk, workers can be outfitted with respiratory equipment. Radon gas is also emitted from uranium, but proper ventilation provides sufficient dilution of the gas so that workers are not affected.
In general, there are three methods available to mine uranium deposits: open-pit, underground, and in-situ leaching. Open-pit mining is used when the uranium deposit is relatively shallow. Underground mining is used when the deposit is relatively deep. But for underground mining to be cost-effective, the deposit must be relatively high-grade and abundant with uranium. For both of these mining methods, the uranium ore is crushed and ground into a fine powder, releasing the uranium from the material surrounding it in the mineral deposit. Next, the crushed uranium is leached from the ore with a solution designed to dissolve only the uranium. After the uranium is dissolved, the solution is separated. The uranium-rich liquid is run off into a collection facility, and the solid waste product is left behind. Note that this waste product, referred to as tailings, can be highly radioactive, and can accumulate in substantial quantity. Since the uranium is often found in deposits with other elements having substantially shorter half-lives, these short-lived isotopes are left behind in a concentrated waste mass. The waste water is evaporated from the sludge, and the remaining metals and minerals are stored, often in an open pit at the mill site, for the life of the facility.
A less environmentally invasive method of obtaining the uranium is through a process called in-situ leaching (ISL). Rather than crush the uranium mineral deposit in order to expose the uranium and then dissolve it, the dissolving solution is pumped directly into the uranium deposit. This way, the dissolved uranium solution is obtained without disturbing the minerals surrounding the uranium in the ore, and all of the waste solids are left behind. This assumes the mineral deposit is able to facilitate the leaching process. Uranium deposits must be permeable, like those found in sand and sandstone, to utilize ISL in order for the dissolving solution to pass easily into the deposit to extract the ore. Between 10 and 15 percent of the world uranium supply is mined via in-situ leaching.
Regardless of the method used to obtain the uranium—open pit, underground, or in-situ leaching—the result is a uranium-rich solution. The uranium is recovered from the solution through ion exchange or solvent extraction, and concentrated into a fine precipitate. The uranium-free solution can then be returned to the mine to extract more uranium. The uranium in the precipitate is a blend of uranium dioxide (UO2) and uranium trioxide (UO3), combined in a general uranium oxide (U3O8). The precipitate is usually dried, forming a yellow cake of uranium, simply called "yellow cake." This concentrated uranium is typically 70 to 90 percent uranium. In order to use the yellow cake as a nuclear fuel, the uranium must go through a process of conversion.
Since the uranium from the milling process is still in an unusable form, the yellow cake is broken down once again. The uranium trioxide is reduced to uranium dioxide at very high temperatures. Refining of the product also takes place. Now the uranium product consists almost entirely of UO2.
During the conversion process, the object is to create uranium hexafluoride (UF6), a highly corrosive substance that is gaseous at high temperatures, but is a white crystalline solid at lower temperatures. Uranium hexafluoride is easily transported in its crystalline form to an enrichment facility (the step taken after conversion), but the gaseous form is well suited for the enrichment process, itself. First, the UO2 is combined with hydrogen fluoride (HF) to form uranium tetrafluoride (UF4). This can be accomplished by reacting the UO2 with either gaseous or liquid HF. The UF4 is combined with gaseous fluorine to produce UF6, and is shipped to a facility for enrichment.
Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split.
Nuclear fuel always contains a mixture of these uranium isotopes. The U-235, being naturally fissile, will sustain a controlled chain reaction in the nuclear reactor early in the fuel cycle. The U-238, being fertile, will eventually turn into plutonium, continuing to sustain the nuclear chain reaction late in the fuel cycle. The amount of U-235 initially blended into the nuclear fuel will dictate certain performance characteristics. Since naturally-occurring uranium is only approximately 0.7 percent U-235, any nuclear fuel containing more than 0.7 percent U-235 is said to be enriched. Light-water reactors, like those in the United States, use a fuel that has been enriched up to 5 percent. Some reactors are designed to use natural (unenriched) uranium, but they must rely on a medium other than light water to moderate the chain reaction. (These types of reactors typically use heavy water or graphite as a moderator for the nuclear reaction.) For these reactors, the U3O8 need only be refined and converted directly to UO2 for use in fuel pellets, and enrichment is not necessary.
Uranium-235 is less massive than U-238, because each atom of U-235 contains three fewer neutrons than U-238. Although this difference appears insignificant, this 1 percent difference is precisely what allows the UF6 to be divided into those molecules containing U-235 and those containing U-238. By separating the two quantities, enrichment plants can control the level of enrichment of the end product. Two processes for dividing the uranium isotopes are in large-scale commercial use. These are gaseous diffusion and centrifuge. A third enrichment process, laser enrichment, is being developed.
Gaseous Diffusion. Gaseous diffusion is responsible for producing over 90 percent of the world's enriched uranium. Gaseous diffusion has the ability to process large quantities of gaseous UF6 at a time. Gaseous UF6 is fed through a series of porous membranes that tend to filter out the heavier UF6 molecules containing U-238 atoms. Gas that passes through the membrane will have a slightly higher percentage of U-235 molecules in the UF6 than when it started, since some of the U-238 molecules did not pass through the membrane. The slightly enriched UF6 is drawn off and forced through another series of membranes. Each iteration produces a slightly higher enrichment of U-235. After more than 1,500 iterations, the UF6 is finally enriched to a usable form, at a U-235 concentration of 3 to 4 percent.
Centrifuge Enrichment. In a centrifuge, the UF6 molecules are spun in a rotating cylinder. Because of the difference in mass between the UF6 molecules containing U-235 and U-238, the lighter molecules remain at the center of the cylinder while the heavier molecules migrate toward the outside. The enriched UF6 is drawn off the center of the centrifuge, and then centrifuged again. Similar to the diffusion process, each iteration produces a slightly higher enrichment of U-235. The centrifuge process is much more efficient than the diffusion process, producing a suitable enrichment after only ten to twenty iterations, and requiring significantly less energy to operate the enrichment plant than does a comparable diffusion plant. The capacity of a centrifuge facility is substantially less than the capacity of a diffusion plant, however, severely limiting its commercial application.
Laser Enrichment. Laser enrichment attempts to ionize gaseous U-235 atoms. By using a laser of a specific frequency, valence electrons of U-235 atoms can be removed without ionizing U-238 atoms. The positively charged U-235 atoms are attracted to a negatively charged metal plate, and liquid U-235 is collected. This process does not yet work with the UF6 created from the existing uranium mining process, so its application is currently limited. Future laser enrichment processes will focus on using UF6 in its enrichment technique.
Fuel Pellet Fabrication
Finally, the enriched uranium of converted back into UO2. The UO2 is pressed into small fuel pellets and packaged in a metal tube (made of a zirconium alloy) for use in a nuclear reactor.
Nuclear fuel can also be fabricated from existing stockpiles of fuel sources. Spent fuel can be reprocessed to extract the remaining U-238 for use in the next fuel cycle. Spent fuel also contains Pu-239, a direct replacement for the fissile U-235 in the enrichment process. Even military grade plutonium and highly enriched uranium can be reprocessed and diluted into a form usable in most nuclear reactors. The reprocessed isotopes are generally manufactured into mixed oxide (MOX) fuels and loaded into conventional nuclear reactors as fuel. The cost of increasing the fissile isotopes in MOX fuel represents a great cost savings over uranium enrichment, and thus reprocessing is increasingly becoming economically viable.
Plutonium (symbol: Pu; atomic number: 93) is not a naturally occurring element. Plutonium is formed in a nuclear reaction from a fertile U-238 atom. Since U-238 is not fissile, it has a tendency to absorb a neutron in a reactor, rather than split apart into smaller fragments. By absorbing the extra neutron, U-238 becomes U-239. Uranium-239 is not very stable, and undergoes spontaneous radioactive decay to produce Pu-239.
Plutonium-239 is a fissile element, and will split into fragments when struck by a neutron in the nuclear reactor. This makes Pu-239 similar to U-235, able to produce heat and sustain a controlled nuclear reaction inside the nuclear reactor. Nuclear power plants derive over one-third of their power output from the fission of Pu-239. Most of the uranium inside nuclear fuel is U-238. Only a small fraction is the fissile U-235. Over the life cycle of the nuclear fuel, the U-238 changes into Pu-239, which continues to provide nuclear energy to generate electricity.
Not all of the Pu-239 will fission during the fuel cycle in a nuclear reactor. Some of the plutonium will not experience neutron bombardment sufficient to cause fission. Other plutonium atoms will absorb one or more neutrons and become higher numbered isotopes of plutonium, such as Pu-240, Pu-241, etc. Plutonium comprises just over 1 percent of nuclear reactor spent fuel—the fuel removed from the nuclear reactor after it has produced all the nuclear fission it can economically provide. Of the plutonium remaining in the spent fuel, slightly less than 60 percent will be fissionable Pu-239.
Because of the high concentration of Pu-240, the plutonium in spent fuel is unsuitable for military application (nuclear warheads). Plutonium-240 is a constant source of neutron decay and is highly unstable. Moreover, the Pu-240 cannot sustain a chain reaction. Military-grade plutonium consists of over 90 percent fissionable plutonium. Because it is impractical to separate the two isotopes of plutonium, special reactors are used to create Pu-239 with extremely small quantities of Pu-240. Even where nations are desperate for electric-generating capacity, the fear of rogue nations teaming to build and operate these special facilities to create military-grade plutonium is a main reason the International Atomic Energy Agency closely safeguards the transfer and use of nuclear energy technology.
Now that much of the world has agreed to nuclear disarmament, scientists and world leaders are searching for uses for the surplus weapons-grade plutonium. Instead of pursuing disposal options, one option is the use of mixed-oxide (MOX) nuclear fuel.
MIXED OXIDE FUEL
In order to deplete stockpiles of weapons-grade plutonium, it is possible to "burn" the fuel in a nuclear reactor. This option has two distinct advantages. First, the fissionable plutonium is a direct replacement for fissionable uranium in a nuclear reactor. Therefore, the plutonium will sustain a controlled chain reaction inside the reactor and provide the heat necessary to create steam for a power plant. Second, the plutonium not undergoing fission will become increasingly contaminated with other isotopes of plutonium (e.g., Pu-240) so that it will no longer be suitable for weapons applications. The longer the plutonium remains inside the nuclear reactor, the more Pu-240 is created as Pu-239 absorbs neutrons. The spent fuel from a reactor using MOX fuel will contain diminished amounts of Pu-239 and increased amounts of Pu-240, effectively removing weapons-grade plutonium from circulation.
Mixed oxide fuel is not appropriate for all nuclear reactors. Plutonium requires faster neutrons in order to operate in a sustained chain reaction. Light-water reactors operate in a highly moderated environment, and thus U-238 and Pu-239 do not make excellent fuel sources. Light-water reactors can handle up to approximately 30 percent MOX fuel. Heavy-water reactors, such as those in Canada, and other fast neutron reactors, can operate with 100 percent MOX fuel.
Other options for eliminating weapons-grade plutonium are to seal it permanently in solid radioactive waste and dispose of it in waste repositories, and to use the plutonium to fuel fast neutron reactors (without reprocessing the plutonium into a MOX fuel).
Plutonium has a much shorter half-life than uranium (24,000 years for Pu-239; 6,500 years for Pu-240). Plutonium is most toxic if it is inhaled. The radioactive decay that plutonium undergoes (alpha decay) is of little external consequence, since the alpha particles are blocked by human skin and travel only a few inches. If inhaled, however, the soft tissue of the lungs will suffer an internal dose of radiation. Particles may also enter the blood stream and irradiate other parts of the body. The safest way to handle plutonium is in its plutonium dioxide (PuO2) form because PuO2 is virtually insoluble inside the human body, greatly reducing the risk of internal contamination.
The main drawback to nuclear power is the production of radioactive waste. Spent fuel from a nuclear reactor is considered a high-level radioactive waste, and remains radioactive for a very long time. Spent fuel consists of fission products from the U-235 and Pu-239 fission process, and also from unspent U-238, Pu-240, and other heavy metals produced during the fuel cycle. That is why special programs exist for the handling and disposal of nuclear waste.
Brian F. Thumm
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