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. * 1938: Scientists Otto Hahn, Fritz Strassman, and Lise Meitner prove the occurrence of fission using Einstein's theory of the relationship between mass and energy, E = [mc.sup.2] (energy equals mass times the speed of light squared) (U.S. DOE 1994). * 1942: A group of scientists led by Fermi produce the first controlled, self-sustaining nuclear chain reaction in Chicago. * 1945: An atomic bomb is dropped on Hiroshima, Japan; three days later, a smaller bomb is dropped on Nagasaki, Japan. * 1946: The Atomic Energy Commission is created to control nuclear energy development and explore peaceful uses of nuclear energy. * 1951: In Arco, Idaho, Experimental Breeder Reactor I produces the first electric power from nuclear energy. * 1957: The first large-scale nuclear power plant begins operation in Shippingport, Pennsylvania (NYT 2011; PBS 2009). * 1971: Twenty-two commercial nuclear power plants are in operation in the United States. * 1979: On March 28, at Three Mile Island nuclear power station near Harrisburg, Pennsylanvia, an accident occurs due to mechanical malfunction and human error. * 1979: Seventy-two licensed reactors generate 12% of U.S electricity. * 1986: On April 26, an operator error causes two explosions in reactor number 4 at Chernobyl nuclear power plant in the former Soviet Union. * 1992: One hundred and ten nuclear power plants provide nearly 22% of all electricity used in the United States (U.S. DOE 1994). * 2011: Fukushima power plant in Japan is damaged by 9.0 earthquake resulting in meltdown of its three active nuclear cores.
Nuclear power plants
Commercial nuclear power plants in the United States are either boiling water reactors (BWRs) or pressurized water reactors (PWRs). Both are cooled by regular water. BWRs heat the water surrounding the nuclear fuel directly into steam in the reactor vessel. Pipes carry steam to the turbine, which drives the electric generator to produce electricity. PWRs heat the water surrounding the nuclear fuel, but keep the water under pressure to prevent boiling. The hot water is pumped from the reactor vessel to a steam generator. There, the heat from the water is transferred to a second, separate supply of water. This water boils to make steam. The steam spins the turbine, which drives the electric generator to produce electricity. About two-thirds of the nuclear reactors in the United States are PWRs, and one-third are BWRs (NEI 2011a).
Advocates of nuclear power cite its clean energy--nuclear power plants do not emit carbon dioxide, sulfur dioxide, or nitrogen oxides (EPA 2010). Large amounts of electricity can be generated in a relatively small space with less land than other energy sources. One uranium fuel pellet contains the same amount of energy as 17,000 cubic feet of natural gas, 1,780 pounds of coal, or 149 gallons of oil. Nuclear waste is small in physical size compared to waste produced by other forms of energy. Electricity from a nuclear power plant is available when needed and not dependent on the wind or the Sun (INL 2011). The water that is discharged from power plants is relatively clean and not radioactive (EPA 2010). Some sources also cite benefits such as state and tax revenues and the potential to develop wetlands around a power plant (Westinghouse 2011).
Uranium-238 emits alpha particles, which are less penetrating than other forms of radiation, and weak gamma rays. If uranium remains outside the body, it poses few health hazards. However, if ingested or inhaled, uranium's radioactivity increases the risk of lung and bone cancers. At high concentrations, uranium is chemically toxic and can cause damage to internal organs, especially the kidneys. Animal studies suggest that uranium may affect reproduction and a developing fetus and increase the risk of soft-tissue cancer and leukemia (IEEE 2005). The decay process of uranium-238 produces radon-222, a radioactive gas. Uranium miners exposed to radon-222 and uranium-238 have developed lung cancer and other lung diseases (EPA 2011b).
The cost of building a new power plant is about $4 billion, though costs often increase during the construction process, making the actual cost much higher (Schoen 2007). When a plant's license expires (usually after 40 years), it can cost $300 million or more to shut down and decommission the plant (U.S. NRC 2011b).
Uranium mining and enrichment
Uranium is a nonrenewable resource that cannot be replenished on a human timescale. The uranium enrichment process (converting uranium ore into uranium oxide pellets) generates radioactive waste. Equipment used in nuclear power plants also becomes contaminated with radiation and will become radioactive waste once the plant is closed (EPA 2010). Fossil fuel emissions are associated with the uranium mining and enrichment process and the transport of uranium fuel to the nuclear power plant (EPA 2010). Conventional mining techniques (extraction from open pits and underground mines) generate a large amount of mill tailings (radioactive, sandlike materials that remain after uranium is extracted) during the milling (refining) process (EPA 2011c; IEEE 2005). The hazard per gram of mill tailings is low relative to most other radioactive wastes, but the large volume of tailings and lack of regulations since 1980 have resulted in widespread environmental contamination. When uranium is enriched and converted, uranium hexafluoride is used, which is chemically toxic and radioactive (IEEE 2005).
Nuclear power plants use large amounts of water for steam production and cooling. Removing water from lakes and rivers for this purpose can negatively affect aquatic life. Heavy metals and salts that build up in the water inside power plants, as well as the high temperatures of the water discharge, can also negatively affect aquatic life (EPA 2010). Waste from the uranium enrichment process can contaminate groundwater and surface-water resources with heavy metals and traces of radioactive uranium (EPA 2010). The radioactive materials in mill tailings can leach into the groundwater (IEEE 2005).
Land resource use
Construction of nuclear power plants can eliminate natural habitat for animals and plants and contaminate the land with toxic by-products. Also, the storage of radioactive waste at a plant can prevent any future use of the land (EPA 2010).
Radioactive waste and storage
Every 18 to 24 months, nuclear power plants must shut down to remove and replace spent uranium fuel. U.S. nuclear power plants produce 2,000 metric tons of radioactive waste per year, which is stored at the plant where it is generated. Waste is stored in steel-lined, concrete vaults filled with water or in aboveground steel or steel-reinforced concrete containers with steel inner canisters (EPA 2010). Nuclear waste is being stored at more than 70 locations in two-thirds of the states, including nuclear power plants and five U.S. Department of Energy facilities (EPA 2011a). The half-life of uranium-238 is 4.5 billion years.
Since 1978, Yucca Mountain in Nevada has been studied and considered as a final disposal site for spent nuclear fuel and high-level nuclear waste (EPA 2011a). Yucca Mountain would be a geologic repository; packaged waste would be stored deep below the Earth's surface in an underground tunnel (EPA 2011a). About $10 billion has been spent on the Yucca Mountain project so far (EPA 2010). Recent concerns have emerged, including the realization that water flows through the mountain faster than originally thought. Nuclear waste could leach over time, polluting the water table (NYT 2010). As of now, Yucca Mountain is not being pursued and no clear alternatives have been presented (EPA 2010).
Three Mile Island
On March 28, 1979, an accident occurred at Three Mile Island nuclear power station near Harrisburg, Pennsylvania. The accident was caused by a loss of coolant from the reactor core due to a combination of mechanical and human error. No overexposure to radiation occurred and no one was injured (U.S. DOE 1994).
On April 26, 1986, an operator error caused two explosions in reactor number 4 at Chernobyl nuclear power plant in the former Soviet Union. The reactor had an inadequate containment building; large amounts of radiation escaped (U.S. DOE 1994). According to the World Health Organization, 28 people died in 1986 due to acute radiation sickness (2006). There has been a large increase in thyroid cancers in people who were children or adolescents at the time (and lived in the most contaminated areas) due to exposure to radiated iodine. Cardiovascular disease, cataracts, and negative impacts on emotional well-being were additional effects of Chernobyl (WHO 2006).
On March 11, 2011, a magnitude-9.0 earthquake occurred off the coast of Honshu, Japan. The earthquake generated a tsunami that devastated the area and was felt across the Pacific region. More than 15,000 people were killed (ITIC 2011). At the Fukushima Daiichi Nuclear Power Plant Station, 170 miles north of Tokyo, power was knocked out by the earthquake and the tsunami disabled the backup generators that keep cooling systems working. Three active reactors at the plant melted down and a series of explosions and fires led to the release of radioactive gases (NYT 2011). As of August 27, 2011, the Fukushima plant was still leaking low levels of radiation and readings were above the 20 millisieverts per year deemed safe. People are banned from coming within 20 km (12 miles) of the plant, and the area will be inhospitable for at least 20 years (Reuters 2011).
To address energy concerns, the comprehensive analysis of our energy sources, both renewable and nonrenewable, must continue. As with all forms of energy, every aspect (economic, environmental, function, long-term impact) of nuclear energy should be closely examined and considered; only then can the future of nuclear energy be determined.
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Wallsten, P., and J.L. Yang. 2011. Obama's support for nuclear power faces a test. Washington Post. March 18. www.washingtonpost.com/politics/obamas support-for-nuclear-power-faces-a-test/2011/03/18/ ABQLu8r_story.html.
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Energy for educators--www.energyforeducators.org/ lessonplanstopic/nuclear.shtml
Energy kids: Uranium--www.eia.gov/kids/energy. cfm?page=nuclear_home-basics
Interactive graphic: Nuclear power and you--www.dom. com/about/stations/nuclear/nuctour.html
Thinkfinity: Nuclear energy lesson plans and primary sources--www.thinkfinity.org/nuclear-energy
United States Department of Energy: Nuclear energy student zone--www.ne.doe.gov/students/Track_ura. html
United States Nuclear Regulatory Commission: Student's corner--www.nrc.gov/reading-rm/basic-ref/students. html
Janna Palliser (email@example.com) is consulting editor for Science Scope.