22.6: Nuclear Energy

What is Nuclear Energy? How Does it Work?

Nuclear energy is electricity generated from the heat produced by nuclear fission, the process by which the nucleus of a heavy atom is split into smaller fragments, releasing a large amount of energy. In a nuclear power plant, uranium fuel rods are used to sustain a controlled chain reaction in which neutrons strike uranium-235 nuclei, causing them to split and release heat, additional neutrons, and energy. That heat is used to boil water, produce steam, and drive turbines that generate electricity. Unlike fossil fuels, nuclear fission does not produce carbon dioxide during electricity generation, which has made it an important part of discussions about low-carbon energy sources. However, nuclear energy does produce radioactive waste that must be carefully isolated and stored for thousands of years, and uranium is a nonrenewable resource. The United States currently generates approximately 18 to 19 percent of its electricity from nuclear power, making it the country's largest source of low-carbon electricity.

Uranium Mining and Processing

Uranium is a naturally occurring radioactive element found in trace amounts in most rocks, soils, and even seawater. Although uranium is approximately 100 times more common than silver, the fissile isotope uranium-235, which is the form capable of sustaining a chain reaction, makes up only about 0.7 percent of naturally occurring uranium. The remaining 99.3 percent is uranium-238, which does not directly contribute to fission. Uranium tends to concentrate in the continental crust through geological processes, particularly in granitic rocks and in sedimentary deposits where hydrothermal fluids or groundwater have mobilized and redeposited it. In the United States, the most significant uranium deposits are found across the Colorado Plateau of Utah, Colorado, New Mexico, and Arizona, where groundwater leached uranium from volcanic ash and basement rocks and carried it through permeable sandstone formations. When this uranium-bearing water encountered organic-rich reducing zones within the rock, the uranium precipitated as uraninite, forming the large sedimentary uranium deposits that supplied most of the country's uranium during the Cold War era and remain an important domestic resource today.

Once uranium ore is mined, it is processed at a mill where it is crushed and chemically treated to dissolve the uranium and separate it from the surrounding rock. This produces a solid uranium oxide concentrate commonly called yellowcake, which must be further refined and enriched before it can be used as reactor fuel. Enrichment is necessary because natural uranium contains too little U-235 to sustain the chain reaction required in most commercial reactors. Before enrichment can take place, uranium oxide is converted to uranium hexafluoride gas, a chemical form that can be processed in an enrichment facility. The UF6 gas is then fed into gas centrifuge cylinders and rotated at high speed. Because U-238 is slightly heavier than U-235, the heavier isotope moves to the outer edge of the spinning cylinder while the lighter U-235 concentrates toward the center. This process is repeated through many stages until the concentration of U-235 is raised from its natural level of 0.7 percent to the 3 to 5 percent required for commercial reactor fuel. The enriched uranium is then converted into ceramic fuel pellets and loaded into fuel rods for use in a reactor.