The Nuclear Fuel Cycle
Uranium is the primary fuel for nuclear power plants. To be used as fuel, uranium must be extracted from its occurrence in the earth, milled, refined, and processed into a form from which its energy can be harnessed. As uranium decays, it releases energy in the form of heat. This heat is then used to create steam to turn turbine generators in the same way coal-fired or hydroelectric plants do. Developing countries around the world are demanding more and more electrical generating capacity. Presently this growth is exponential. Nuclear power’s great advantage today is that it is emission-free (no CO2 produced). With increasing efforts to halt anthropogenic climate change and reduce emissions of other pollutants such as sulfur and soot, more nations are turning to nuclear power.
Mining and milling
After locating and characterizing uranium deposits, the next step in the nuclear fuel cycle is extracting uranium from the ground. Depth and groundwater conditions dictate the mining method used. Conventional mining requires the removal of ore (uranium minerals and their host rock) from the ground to a mill, where it is crushed and leached to separate the uranium from the host rock. In the case of ISR, the produced water is chemically treated in a series of steps to recover the uranium. A solution containing oxidized uranium is stirred, causing the uranium oxide to crystallize, precipitate out of solution, and settle to the bottom of a large funnel-shaped tank. This paste-like material is then filtered to remove any remaining chloride solution and dried The dried yellowcake is dispensed into 55-gallon barrels and shipped to a conversion facility.
To serve as fuel for nuclear power plants, yellowcake must be converted into uranium hexafluoride (UF6). Conversion facilities cleanse and refine the uranium to remove impurities, then heat the purified uranium and combine it with fluorine (F) gas to create uranium hexafluoride (UF6). It is then cooled and condensed to form a solid.
Five yellowcake conversion facilities exist worldwide: one here in the U.S., and one each in Canada, France, the United Kingdom, and Russia.
Naturally, uranium can occur as three different isotopes: 238U, 235U, and 234U. All three isotopes are the same element (uranium) and have the same number of protons (92) in the nucleus, but differ in the number of neutrons in the nucleus. Their chemical properties are largely the same but their radioactive properties vary somewhat. For example, 235U decays slightly faster than 238U and is more fissionable, which makes it more desirable for nuclear reactions.
The uranium in yellowcake is 99.27% 238U and 0.73% 235U. That ratio remains the same after conversion to UF6. The 235U isotope is fissionable (or fissile), meaning that it can split into two or more other elements. For power generation, the 235U content of UF6 must increase to about 3–5%. This is accomplished by either gaseous diffusion or centrifugation. Gaseous diffusion involves passing UF6 through barriers which can separate the isotopes by weight (238U is more dense, or heavier per unit volume). Centrifugation spins the gases at high speed and causes atoms of the denser 238U isotope to separate from atoms of the lighter 235U isotope. The difference in density between the two isotopes is so small that it takes thousands of centrifuges to accomplish the enrichment.
Fuel pellet fabrication
Once the uranium is enriched to the required level, fuel fabricators convert the material back into uranium oxide and press it into small pellets, about the size of your little fingertip. The pellets are then inserted into long tubular fuel rods, which are grouped together to boil water at nuclear power plants.
Spent nuclear fuel pellets look the same as they did before use in the reactor, but are now more radioactive due to the various daughter isotopes spawned by fission of 235U. These pellets are quite small and storage space is not an issue. The volume of all the nuclear waste generated in the U.S. since the dawn of nuclear power would cover a football field to a height of less than twenty feet. The challenge of spent fuel disposal lies in finding suitable sites for long-term storage (centuries) that minimize risk to the environment and the public. In the United States, the most likely disposal site is Yucca Mountain, Nevada.