Nuclear Fuel
From Thermal-FluidsPedia
Most nuclear reactors use uranium as fuel. Uranium is a common element on earth, formed during the earlier stages of its formation. At the time our planet was formed 4.5 billion years ago, about 75% of all uranium was in the form of U-238 and the remaining 25% was in the form of isotope U-235. As uranium decayed (T1/2 = 700 million years), the ratio of U-235 to U-238 dropped considerably. Today only 0.7 % of uranium found in nature is U-235. The non-fissile isotope U-238 makes up the rest.
Uranium is scattered throughout the earth’s crust in most rocks and soils, as well as in many rivers and in seawater. Uranium is not an unlimited resource and, like fossil fuel, has only a limited lifetime. With 24% of the world’s total uranium resources, Australia is the leading producer. Other countries with vast uranium reserves are Kazakhstan (16%), Canada (10%), and South Africa (7%). The United States, with 340,000 tons, has about 5% of the earth’s uranium reserves. Eighty-five percent (85%) of all US uranium reserves are in New Mexico and Wyoming.
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Processing
From extraction to disposal, uranium fuel undergoes many changes. Various stages of the nuclear fuel cycle are shown in Figures 11-8 and 11-9 and consist of:
1. Mining and milling - Like coal and other minerals, uranium must be mined and hauled away to a mill where uranium ore is crushed and ground to a fine slurry that is poured into an acid, which dissolves the uranium, but not the rest of the crushed rock. The acid solution is dried into a yellow powder called yellow cake. The leftover rock is known as tailing.
2. Conversion - The yellow cake is then shipped to a conversion plant where it is purified and chemically converted to uranium hexafluoride (UF6) gas.
3. Enrichment - The flow stream has very little uranium-235, the fuel for nuclear fission. During the enrichment process, about 85% of uranium-238 is removed from the flue gas to raise the concentration of U-235 isotopes from the 0.7% which is naturally present in the ore to the 3-5% required for use as nuclear fuel (a). The enrichment is carried by gaseous diffusion or gaseous centrifuge. In gaseous diffusion, the gas is pumped through filters with holes large enough to allow uranium-235 atoms to pass through but not the slightly bigger atoms of uranium-238. In a gaseous centrifuge plant, gas is sent through hundreds of centrifuges that spin uranium hexafluoride gas at very high speeds, separating the lighter uranium-235 hexafluoride molecules from the heavier uranium-238 ones.
4. Fabrication – Enriched UF6 gas is sent to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small ceramic pellets that are then stacked inside long zirconium or stainless steel tubes to form fuel rods. Finally, fuel rods are bundled together to make the fuel assemblies used in nuclear reactors. Some U-235 remains in the tail and is called depleted uranium. Because of its high density, depleted uranium has been used in yacht keels, antitank missiles, and artillery shells.
5. Burn-up – A reactor core contains several hundred fuel assemblies where the fission process takes place and fuel is “burned,” producing heat in a process called a chain reaction. Fission products are plutonium and various fission-fragments.
6. Spent Fuel Storage – Over time, the uranium fuel becomes depleted and the concentration of fission fragments begins to build up in the reactor. As more and more neutrons are absorbed, a point is reached where old fuel is no longer efficient and must be replaced with new fuel. The spent fuel and other fission products remain highly radioactive and continue to release heat and radiation long after they are removed from the reactor core. Typically, spent fuel is unloaded into a storage water pond immediately adjacent to the reactor. This allows the fuel to cool and the short-lived isotopes to decay and radiation levels to decrease by as much as 90%(Figure 3). Spent fuel is held in such pools from several months to many years before it is sent out to be reprocessed or dried (vitrified) and deposited in storage facilities. Each year, over 10,000 tons of spent fuel is generated by the world’s current 441 operating nuclear plants, of which less than one-third is reprocessed for recycling as mixed-oxide (MOX) fuel (b). The remainder is placed into interim storage facilities.
7. Reprocessing - The fissionable part of the fuel rod assembly is only a few percentages. About 95-97% of the uranium is still intact after fuel is burned. Plutonium makes up 1% of its mass and the rest are other radioactive elements. During reprocessing, uranium and plutonium are separated out from the rest, returned to the conversion plant, and blended with additional enriched uranium to build new fuel rods. During WWII, nuclear reactors were primarily designed for manufacturing large quantities of plutonium. The existing stockpile of plutonium is sufficient to fuel many breeder reactors. All US reprocessing plants were shut down at the end of the twentieth century and no new reprocessing plants are believed to be necessary in the foreseeable future.
8. Vitrification – High-level liquid waste is dried and stored in special containment vessels called casks. Casks are currently made of stainless steel alloy, but ceramic might be a more favorable material. Ceramics do not rust and have good resistance to radiation and heat.
9. Disposal – The canisters are stored in an underground permanent repository.
Example 11-4: How much U-238 must be removed from the uranium oxide (UO2) ore to enrich the U-235 concentration to 3.5%? Solution: Initially, only 0.7% of the uranium oxide is U-235, while the remaining 99.3% is U-238. If X is the fraction of U-238 to be removed from the mixture, then its concentration has reduced to 99.3(1-X) percent and we have:
[U-235]/[U-238]= 0.7/(99.3(1-X)) = 3.5% or X = 0.8
That means 80% of the U-238 must be removed and discarded. This is a major source of radioactive waste.
Example 11-5: How much energy is released from the fission of 1 kg of natural uranium enriched to 3.5% U-235? Assume that each atom of uranium releases 200 MeV when undergoing the fission process.
Solution: Assuming natural uranium is mainly U-238, then the number of nuclei in one kg of uranium is N = (1000 g)(6.02x1023 atoms/mole)/(238 g/mole) = 2.5x1024 Assuming that only 3% of these nuclei participate in fission reaction, the total energy release is E = (0.03x2.5x1024)(200x106)(1.6x10-19) = 2.4x1012 J = 667 MWh
Plutonium
Plutonium is only slightly heavier than uranium, but unlike uranium that is abundant in mines, there is no plutonium left in nature. This is because, compared to uranium, plutonium has a very short life and has therefore decayed to lighter, more stable elements. Plutonium, however, can be manufactured in nuclear reactors. When uranium-238 – which comprises the bulk of nuclear fuel – is collided with neutrons it turns to highly radioactive uranium-239 with a half-life of only 23 minutes. The resulting isotope is called neptunium-239, which is also radioactive and decays further to the isotope plutonium-239. Plutonium is not only the fuel for breeder reactors but also the ideal fuel for making nuclear bombs. Plutonium is also highly toxic and causes death within a few hours or days if ingested. In bulk quantities, plutonium is not very dangerous, but if vaporized or aerosolized it can cause great damage and must therefore be carefully safeguarded from falling into the hands of terrorist organizations.
References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) Enrichment is a physical process which relies on the small mass difference between atoms of two isotopes. No chemical methods can separate uranium-isotopes, as all isotopes are chemically identical. The two main enrichment processes are diffusion and centrifuge. In the diffusion technique, the mass differences between different isotopes result in different rates of diffusion through a membrane. In centrifuge, a similar separation occurs as the uranium substrate is spun at a very high speed. Today, most uranium enrichment plants use centrifuges to separate lighter uranium-235 hexafluoride from heavier uranium-238 hexafluoride. Successive operations allow production of more and more highly concentrated uranium-235.
(b) In the last few years many European countries have been using mixed uranium with plutonium extracted from their surplus stockpiles of nuclear weapons as a nuclear fuel (called MOX for mixed oxide fuel). The United States has not been using MOX to fuel its nuclear reactors yet, but is planning to build a MOX facility at a DoE site at Savannah River, South Carolina. The project is a part of the disarmament process and is aimed at reducing the nuclear threat by minimizing the number of nuclear weapons and disposing of nuclear-grade plutonium which has accumulated over many years of weapons manufacturing and reactor operation. Critics of the plan warn that opening the large stockpiles of plutonium to commercial sites make power plants more attractive to thieves and terrorists, because the plutonium can be separated from the MOX fuel rather easily and used for constructing an atomic bomb. By 2010 it is expected that MOX fuels will power some 15-20% of the world’s power reactors.
Further Reading
Bodansky, Nuclear Energy Principles, Practices, and Prospects, Second Ed., Springer, 2004.
Seaborg, G., T., Peaceful Uses of Nuclear Energy, University Press of the Pacific, 2005.
International Journal of Nuclear Engineering and Design, Direct Science Elsevier Publishing Company, devoted to the Thermal, Mechanical, Material and Structural Aspects of Nuclear Fission.
Journal of Fusion Energy, Springer Netherlands. It features articles pertinent to development of thermonuclear fusion.
External Links
Federation of American Scientists (http://www.fas.org/nuke/intro/nuke/index.html).
International Atomic Energy Agency (http://www.iaea.org).
DoE Office of Nuclear Energy, Science & Technology (http://www.ne.doe.gov).
American Nuclear Society, (http://www.ans.org).
World Association of Nuclear Operator (WANO) (http://www.wano.org.uk).