Uranium Milling and Processing

Uranium oxide concentrate (often known as “yellowcake”) is produced from naturally occurring uranium minerals through milling uranium ore extracted through conventional mining or processing  uranium-bearing solution from ISR operations. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. Milling produces a uranium oxide concentrate, which is shipped from the mill. In the milling process, uranium is extracted from the crushed and ground-up ore by leaching, in which either a strong acid or a strong alkaline solution is used to dissolve the uranium oxide. The uranium oxide is then precipitated and removed from the solution. After drying and usually heating, it is packed in drums as a concentrate.

The remainder of the ore, nearly all the rock material, becomes tailings, which are emplaced in engineered facilities near the mine (often in a mined-out pit). Tailings are isolated from the environment because they contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals. The tailings are placed into a pond in the ground on top of a plastic liner to prevent leakage. The waste is then covered with a layer of soil and then water. In ISR facilities, uranium is concentrated and extracted from solutions into uranium oxide concentrate at a processing plant. As in conventional mining, one processing facility may serve a number of ISR operations. For more information on uranium production, go to the U.S. Energy Information Administration website: http://www.eia.gov/nuclear/

Conversion

For most types of reactors, the concentration of the fissile ²³⁵U isotope in natural uranium must be enriched typically to between 3 percent and 5 percent. Natural uranium oxide from mines and processing plants is chemically converted into uranium hexafluoride (UF₆), a compound that when heated forms a gas that can be fed into enrichment plants. Honeywell International Inc. operates the only uranium conversion facility in the U.S., in Metropolis, Illinois.

Enrichment

The enrichment process separates gaseous uranium hexafluoride into two streams, one being enriched to the required level known as low-enriched uranium (LEU); the other stream is progressively depleted in ²³⁵U and is called “tails," or simply depleted uranium.

There are two types of enrichment technologies in large-scale commercial use, each of which uses uranium hexafluoride gas as feed: gaseous diffusion and gas centrifuge. These processes both use the physical properties of molecules, specifically the 1 percent mass difference between the two uranium isotopes, to separate them. A third technology that can be used to enrich uranium is called laser enrichment. This technology has not been utilized at the commercial level as of today.

Gaseous Diffusion

The gas diffusion process involves forcing uranium hexafluoride gas under pressure through a series of porous membranes or diaphragms. As ²³⁵U molecules are lighter than the ²³⁸U molecules they move faster and have a slightly better chance of passing through the pores in the membrane. The UF₆ that diffuses through the membrane is thus slightly enriched, while the gas that did not pass through is depleted in ²³⁵U.

This process is repeated many times in a series of diffusion stages called a cascade. Each stage consists of a compressor, a diffuser and a heat exchanger to remove the heat of compression. The enriched UF₆ product is withdrawn from one end of the cascade and the depleted UF₆ is removed at the other end. The gas must be processed through some 1,400 stages to obtain a product with a concentration of 3 to 5 percent ²³⁵U.

The gaseous diffusion process was first developed in 1943 on a large scale at the U.S. Department of Energy (DOE) plant in Oak Ridge, Tennessee. Two additional uranium enrichment plants were subsequently constructed in Paducah, Kentucky, and Portsmouth, Ohio. The Ohio plant ceased operation in 2001. As of today, USEC Inc. operates the only gaseous diffusion plant in the U.S., located in Paducah, Kentucky.

Gas Centrifuge 

The gas centrifuge like the diffusion process uses UF₆ gas as its feed and makes use of the slight difference in mass between ²³⁵U and ²³⁸U. The gas is fed into a series of vacuum tubes rotated at very high speeds to obtain efficient separation of the two isotopes. The slightly heavier ²³⁸U isotope is concentrated closer to the cylinder wall with the lighter ²³⁵U increasing toward the center of the cylinder where it can be drawn off. Although the capacity of a single centrifuge is much smaller than that of a single diffusion stage, its separative capability is significantly greater. In the centrifuge process, the number of stages may only be 10 to 20, instead of a thousand or more for diffusion. Centrifuge stages are arranged in parallel into cascades. The gas centrifuge technology consumes only about 5 percent as much electricity as the gaseous diffusion technology to produce a given amount of product.

Three companies, Areva Enrichment Services (AES), a wholly owned subsidiary of AREVA; Louisiana Enrichment Services (LES), a wholly owned subsidiary of URENCO, Ltd.; and USEC have received licenses from the NRC to build and operate uranium enrichment facilities in the United States using centrifuge technology. The NRC issued a license in 2004 to USEC to construct a test and demonstration facility known as the Lead Cascade at the Piketon, Ohio, site, and a separate license in 2007 to construct and operate the full-scale American Centrifuge Plant. In June 2006, the NRC issued a license to LES to construct and operate the National Enrichment Facility in Lea County, New Mexico. The National Enrichment Facility is currently in operating status. A third gas centrifuge plant is being planned by AES as the Eagle Rock Enrichment Facility near Idaho Falls, Idaho.  

Laser Separation

Laser separation uses laser technology to selectively excite ²³⁵U, the fissile isotope, from the much more abundant ²³⁸U isotope. This technology promises to provide improved enrichment method as compared to first generation gaseous diffusion and second generation gaseous centrifugation methods. Development work began in Australia in 1990s by Silex Systems Ltd. USEC began collaboration with Australian researchers in 1996. In 2000, USEC acquired rights to the Separation of Isotopes by Laser Excitation (SILEX) technology, but due to technical and market concerns, USEC relinquished the license in 2003. In 2006, Silex Systems Ltd. signed an exclusive commercialization and license agreement for the SILEX uranium enrichment technology with GE Hitachi Nuclear Energy (GEH) through its subsidiary, Global Laser Enrichment (GLE). In October 2006, GEH received the required U.S. government authorizations to proceed with the technology exchange. Since that time, GLE has relocated equipment and key personnel from Australia to its Wilmington, NC facility. No laser separation uranium enrichment plants are currently operating in the United States. However, in July 2007, GEH submitted a license request to the NRC, seeking approval for research and development associated with laser enrichment to be conducted at its Global Nuclear Fuels-Americas, LLC, facility in Wilmington, NC. The NRC approved the amendment on May 12, 2008, and GEH is currently operating a test loop with the intention of beginning operations in the near future. In September 2012, GEH was granted a license by the NRC to construct a commercial laser enrichment plant in Wilmington, North Carolina.

Fuel Fabrication

Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide (UO₂), which is sintered (baked) at a high temperature (over 2550°F). The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of the fuel. Nuclear fuel assemblies are specifically designed for particular types of reactors and are made to quality assurance specifications. The most common reactor, the pressurized-water reactor (PWR), contains 150-200 fuel assemblies, whereas the boiling-water reactor, the second most common reactor, contains 370-800 fuel assemblies.

In a fuel fabrication plant great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation). With low-enriched fuel criticality is most unlikely, but in plants handling special fuels for research reactors this is a vital consideration.

There are currently three fuel fabrication plants in the U.S.: 1) AREVA Inc. in Richland, Washington, 2) Global Nuclear Fuel-Americas, LLC in Wilmington, North Carolina, and 3) Westinghouse Electric Co., LLC in Columbia, South Carolina.

Power Reactor

Generation of electricity in a nuclear reactor is similar to a coal-fired steam station. The difference is the source of heat. Fissioning, or splitting, of uranium atoms produces energy in the same way burning coal, gas, or oil is used as a source of heat in fossil fuel power plants. The fuel used in nuclear generation is ²³⁵U and/or ²³⁹Pu. The process of producing electricity begins when uranium atoms are split (i.e., fission) by particles known as neutrons. ²³⁵U has a unique quality that causes it to break apart when it collides with a neutron. Once an atom of ²³⁵U is split, neutrons from the uranium atom collide with other atoms of the ²³⁵U. A chain reaction begins that produces heat. This heat is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator that produces electricity. Some of the ²³⁸U in the nuclear fuel is turned into plutonium in the reactor core during the fission process. The plutonium isotope is also fissile and yields about one third of the energy in a typical nuclear reactor. Typically, some 44 million kilowatt-hours of electricity are produced from one ton of natural uranium.