Palliser, Janna, Science Scope
Concerns about global warming and climate change have prompted the analysis of our energy sources and their environmental impact. Greenhouse gas emissions come from the combustion of nonrenewable resources such as coal, petroleum, and natural gas; and from landfills, agriculture, and certain industrial and waste management processes (U.S. EIA 2011). To address energy and climate concerns, the use of nonrenewable and renewable energy sources (hydropower, solar, wind, geothermal, biomass) is being analyzed. Interest in nuclear power has increased, with proposed government-backed loans to support the building of new plants (Wallsten and Yang 2011). In this month's column, nuclear energy will be examined.
The United States has 104 commercial nuclear power plants in 31 states, which produce 20% of the country's electricity. No nuclear plants have been built in the United States since 1996 (EPA 2010). Worldwide, more than 400 nuclear power plants produce 16% of the world's electricity (CASEnergy Coalition 2009).
Nuclear plants use uranium (uranium-238 and uranium-235) in the form of solid ceramic pellets packaged into long, vertical tubes (NEI 2011a). The pellets are bombarded with neutrons, causing the uranium atoms to split (fission) and release heat and neutrons. These neutrons collide with other uranium atoms and release additional heat and neutrons in a chain reaction. The heat is used to generate steam, which is used by a turbine to generate electricity (EPA 2010).
In 2003, U.S. uranium ore reserves were estimated at about 890 million pounds. The reserves are located primarily in New Mexico and Wyoming (EPA 2010). Canada and Australia account for 40% of global uranium production and the United States accounts for 3% (NEI 2011b).
All isotopes of uranium are radioactive; most have extremely long half-lives. A half-life is a measure of the time it takes for one half of the atoms of a particular radionuclide to disintegrate or decay into another nuclear form. Because radioactivity is a measure of the rate at which a radionuclide decays, the longer the half-life of a radionuclide, the less radioactive it is for a given mass. The half-life of uranium-238 is about 4.5 billion years (Argonne National Laboratory and U.S. DOE).
Before it is used in a reactor, uranium must be converted from an ore to solid ceramic fuel pellets. The steps involved are mining and milling, conversion, enrichment, and fabrication.
Mining and milling: Miners use several techniques: surface (open pit) mining, underground mining, and in-situ recovery (an extraction method used to recover uranium from low-grade ores where other methods may be too disruptive or expensive) (NEI 2011b; U.S. NRC 2011a). Uranium is also a byproduct of other mineral processing operations. Solvents remove uranium from mined ore or in-situ leaching and the resulting uranium oxide, or yellowcake, undergoes filtering and drying.
Conversion: Yellowcake then goes to a conversion plant, where chemical processes convert it to uranium hexafluoride. The uranium hexafluoride is heated to become a gas and loaded into cylinders, where it cools and condenses into a solid.
Enrichment: Uranium hexafluoride consists of the isotopes uranium-235 (less than 1%) and uranimum-238. The amount of uranium-235 is increased to 3%-5% so that the uranium is usable as a fuel.
Fuel fabrication: After enrichment, a fuel fabricator converts the uranium hexafluoride into uranium dioxide powder and presses the powder into fuel pellets. The pellets are loaded into long tubes made of a noncorrosive metal. Once grouped together in a bundle, these tubes form a fuel assembly. The tubes are sealed in a reactor, which is sealed inside a containment structure (NEI 2011b).
FIGURE 1 Nuclear energy time line * 1934: Physicist Enrico Fermi conducts experiments in Rome showing that neutrons can split many kinds of atoms. …