PrintTechnology Overview (nuclear fuel, power plants, radioactive waste)
The series of steps involved in supplying fuel for nuclear power reactors include the following:
Uranium recovery to extract (or mine) uranium ore, and concentrate (or mill) the ore to produce "yellowcake"
As the precursor to the nuclear fuel cycle, uranium recovery focuses on extracting (or mining) natural uranium ore from the Earth and concentrating (or milling) that ore. These recovery operations produce a product, called "yellowcake," which is then transported to a fuel cycle facility. There, the yellowcake is transformed into fuel for nuclear power reactors. In addition to yellowcake, uranium recovery operations generate waste products, called byproduct materials, that contain low levels of radioactivity.
Conversion of yellowcake into uranium hexafluoride (UF6)
After the yellowcake is produced at the mill, the next step is conversion into pure uranium hexafluoride (UF6) gas suitable for use in enrichment operations. During this conversion, impurities are removed and the uranium is combined with fluorine to create the UF6 gas. The UF6 is then pressurized and cooled to a liquid. In its liquid state it is drained into 14-ton cylinders where it solidifies after cooling for approximately five days. The UF6 cyclinder, in the solid form, is then shipped to an enrichment plant. UF6 is the only uranium compound that exists as a gas at a suitable temperature.
One conversion plant is operating in the United States: Honeywell International Inc. in Metropolis, Illinois. Canada, France, United Kingdom, China, and Russia also have conversion plants.
As with mining and milling, the primary risks associated with conversion are chemical and radiological. Strong acids and alkalis are used in the conversion process, which involves converting the yellowcake (uranium oxide) powder to very soluble forms, leading to possible inhalation of uranium. In addition, conversion produces extremely corrosive chemicals that could cause fire and explosion hazards.
Enrichment to increase the concentration of uranium-235 (U-235) in UF6
Enriching uranium increases the proportion of uranium atoms that can be "split" by fission to release energy (usually in the form of heat) that can be used to produce electricity. Not all uranium atoms are the same. When uranium is mined, it consists of about 99.3% uranium-238 or U-238 (U238), 0.7% uranium-235 or U-235 (U235), and < 0.01% uranium-234 or U-234 (U234). These are the different isotopes of uranium, which means that while they all contain 92 protons in the atom’s center, or nucleus (which is what makes it uranium), the U238 atoms contain 146 neutrons, the U235 atoms contain 143 neutrons, and the U234 atoms contain only 142 neutrons. (The total number of protons plus neutrons gives the atomic mass of each isotope — that is, 238, 235, or 234, respectively.)
Caption: Natural uranium contains 99% U238 and only about 0.7% U235 by weight. (Source: http://www.nrc.gov/materials/fuel-cycle-fac/uranium-enrichment.pdf)
The fuel for nuclear reactors has to have a higher concentration of U235 than exists in natural uranium ore. This is because U235 is "fissionable," meaning that it starts a nuclear reaction and keeps it going. Normally, the amount of the U235 isotope is enriched from 0.7% of the uranium mass to about 5%, as illustrated in this diagram PDF Icon of the enrichment process.
Caption: The uranium enrichment process increases the concentration of U235 to the amount needed for use in reactor fuel. (Source: http://www.nrc.gov/materials/fuel-cycle-fac/uranium-enrichment.pdf)
Gaseous diffusion is the only process currently being used in the United States to commercially enrich uranium. Gas centrifuges and laser separation can also be used to enrich uranium, as described below.
Deconversion to reduce the hazards associated with the depleted uranium hexafluoride (DUF6), or “tailings,” produced in earlier stages of the fuel cycle
As uranium-235 (U235) is extracted, converted, and enriched in the uranium recovery, conversion, and enrichment processes for use in fabricating fuel for nuclear reactors, large quantities of depleted uranium hexafluoride (DUF6), or “tailings,” are produced. These tailings are transferred into 14-ton cylinders which are stored in large yards near the enrichment facilities. A process called "deconversion" is then used to chemically extract the fluoride from the DUF6 stored in the cylinders. This deconversion process produces stable compounds, known as uranium oxides, which are generally suitable for disposal as low-level radioactive waste.
Enriching 1,000 kilograms (kg) of natural uranium to 5% U235 produces 85 kg of enriched uranium hexafluoride (UF6) and about 915 kg of DUF6 (0.3 percent U235). As a result, enrichment processes in the United States produce approximately 12,000 – 15,000 tons of DUF6 tailings per year, which are then transferred to storage cylinders. The uranium in these cylinders consists of high purity U238 with less than 0.7% other uranium isotopes (e.g., U234 and U235). In addition the cylinders contain small quantities of impurities resulting from the natural radioactive decay of the uranium. The high purity of the U238 and self-shielding of the bulk material limit the radiological hazard from the full cylinders. Nonetheless, DUF6 represents a chemical hazard if it is released to the environment.
The deconversion process significantly reduces the chemical hazards associated with DUF6 by extracting the fluoride atoms and replacing them with oxygen. This deconversion process results in depleted uranium dioxide (DUO2) and depleted triuranium octoxide (DU3O8) compounds. These compounds are chemically stable, compared to DUF6, and are generally suitable for disposal as low-level radioactive waste. These oxides are similar to the chemical form of uranium in nature. Depleted uranium has a lower specific radioactivity per mass than natural uranium because the enrichment process reduces the percentage of other isotopes, e.g. U234 and U235. The specific radioactivity of the storage containers increases over time as the daughter products, removed during uranium recovery and conversion processes, return to natural levels due to radioactive decay. Most of the daughter products return to natural levels over the course of several million years.
Deconversion also enables the recovery of high purity fluoride compounds which have commercial value. These fluoride compounds are used in the production of refrigerants, herbicides, pharmaceuticals, high-octane gasoline, aluminum, plastics, electrical components, and fluorescent light bulbs.
Chemical exposure is the dominant hazard at deconversion facilities because uranium and fluoride compounds (such as hydrogen fluoride) are hazardous at low levels of exposure. In particular, these compounds have the following characteristics:
- When DUF6 comes in contact with moisture in the air, it reacts to form hydrogen fluoride and uranyl fluoride.
- Uranium is a heavy metal, which can be toxic to the kidneys when ingested.
- Hydrogen fluoride is a corrosive acid, which can be very dangerous if inhaled.
Deconversion facilities are designed to reduce the likelihood and consequences of accidental releases of hazardous radiological and chemical compounds through safety systems, onsite and offsite monitoring, and emergency planning.
Fuel fabrication to convert enriched UF6 into fuel for nuclear reactors
Fuel fabrication facilities convert enriched UF6 into fuel for nuclear reactors. Fabrication also can involve mixed oxide (MOX) fuel, which is a combination of uranium and plutonium components.
Fuel fabrication for light (regular) water power reactors (LWR) typically begins with receipt of low-enriched uranium (LEU) hexafluoride (UF6) from an enrichment plant. The UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form LEU uranium dioxide (UO2) powder. This powder is then pressed into pellets, sintered into ceramic form, loaded into Zircaloy tubes, and constructed into fuel assemblies. Depending on the type of light water reactor, a fuel assembly may contain up to 264 fuel rods and have dimensions of 5 to 9 inches square by about 12 feet long.
Caption: Typical Light Water Reactor Fuel Fabrication Facility (Source: http://www.nrc.gov/materials/fuel-cycle-fac/fuel-fab.html)
MOX fuel differs from LEU fuel in that the dioxide powder from which the fuel pellets are pressed is a combination of UO2 and plutonium oxide (PuO2). The NRC was directed by Congress to regulate the Department of Energy's (DOE's) fabrication of MOX fuel used for disposal of plutonium from international nuclear disarmament agreements. For more information about this fuel, see Mixed Oxide Fuel Fabrication Facility Licensing.
Use of the fuel in reactors (nuclear power, research, or naval propulsion)
There are several types of commercial nuclear power plants that generate electricity. Of these, only the Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs) are in commercial operation in the United States.
In a typical commercial pressurized light-water reactor, the core inside the reactor vessel creates heat, pressurized water in the primary coolant loop carries the heat to the steam generator, and a steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted in to the condenser where it condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the reactor vessel. The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need power.
Caption: Pressurized Water Reactors (PWRs) keep water under pressure so that it heats, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area. (Source and to see animation, visit http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html)
n a typical commercial boiling-water reactor (BWR), the core inside the reactor vessel creates heat, a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat, the steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line, and the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted in to the condenser where it it condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated and pumped back to the reactor vessel. The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need electric power.
aption: Boiling Water Reactors actually boil the water, producing the steam that turns the turbine. (Source and to see animation, visit http://www.nrc.gov/reading-rm/basic-ref/students/animated-bwr.html)
Interim storage of spent nuclear fuel
There are two acceptable storage methods for spent fuel after it is removed from the reactor core:
- Spent Fuel Pools - Currently, most spent nuclear fuel is safely stored in specially designed pools at individual reactor sites around the country.
- Dry Cask Storage - If pool capacity is reached, licensees may move toward use of above-ground dry storage casks.
The water-pool option involves storing spent fuel rods under at least 20 feet of water, which provides adequate shielding from the radiation for anyone near the pool. The rods are moved into the water pools from the reactor along the bottom of water canals, so that the spent fuel is always shielded to protect workers.
In the late 1970s and early 1980s, the need for alternative storage began to grow when pools at many nuclear reactors began to fill up with stored spent fuel. Utilities began looking at options such as dry cask storage for increasing spent fuel storage capacity.
Dry cask storage allows spent fuel that has already been cooled in the spent fuel pool for at least one year to be surrounded by inert gas inside a container called a cask. The casks are typically steel cylinders that are either welded or bolted closed. The steel cylinder provides a leak-tight containment of the spent fuel. Each cylinder is surrounded by additional steel, concrete, or other material to provide radiation shielding to workers and members of the public. Some of the cask designs can be used for both storage and transportation.
Reprocessing of high-level waste to recover the fissionable material remaining in the spent fuel (currently not done in the United States)
High-level radioactive wastes are the highly radioactive materials produced as a byproduct of the reactions that occur inside nuclear reactors. High-level wastes take one of two forms:
- Spent (used) reactor fuel when it is accepted for disposal
- Waste materials remaining after spent fuel is reprocessed
Spent nuclear fuel is used fuel from a reactor that is no longer efficient in creating electricity, because its fission process has slowed. However, it is still thermally hot, highly radioactive, and potentially harmful. Until a permanent disposal repository for spent nuclear fuel is built, licensees must safely store this fuel at their reactors.
Reprocessing extracts isotopes from spent fuel that can be used again as reactor fuel. Commercial reprocessing is currently not practiced in the United States, although it has been allowed in the past. However, significant quantities of high-level radioactive waste are produced by the defense reprocessing programs at Department of Energy (DOE) exit icon facilities, such as Hanford, Washington, and Savannah River, South Carolina, and by commercial reprocessing operations at West Valley, New York. These wastes, which are generally managed by DOE, are not regulated by NRC. However they must be included in any high-level radioactive waste disposal plans, along with all high-level waste from spent reactor fuel.
Because of their highly radioactive fission products, high-level waste and spent fuel must be handled and stored with care. Since the only way radioactive waste finally becomes harmless is through decay, which for high-level wastes can take hundreds of thousands of years, the wastes must be stored and finally disposed of in a way that provides adequate protection of the public for a very long time.
Transportation of Spent Nuclear Fuel
Spent nuclear fuel refers to uranium-bearing fuel elements that have been used at commercial nuclear reactors and that are no longer producing enough energy to sustain a nuclear reaction. Once the spent fuel is removed from the reactor the fission process has stopped, but the spent fuel assemblies still generate significant amounts of radiation and heat. Because of the residual hazard, spent fuel must be shipped in containers or casks that shield and contain the radioactivity and dissipate the heat.
Over the last 30 years, thousands of shipments of commercially generated spent nuclear fuel have been made throughout the United States without causing any radiological releases to the environment or harm to the public.
Most of these shipments occur between different reactors owned by the same utility to share storage space for spent fuel, or they may be shipped to a research facility to perform tests on the spent fuel itself. In the near future, because of a potential high-level waste repository being built, the number of these shipments by road and rail is expected to increase.
Final disposition (disposal) of high-level waste
On June 3, 2008, the U.S. Department of Energy (DOE) submitted a license application to the U.S. Nuclear Regulatory Commission (NRC), seeking authorization to construct a deep geologic repository for disposal of high-level radioactive waste at Yucca Mountain, Nevada. The NRC's review of that application will require evaluation of a wide range of technical and scientific issues. The NRC will issue a construction authorization only if DOE can demonstrate that it can safely construct and operate the repository in compliance with the NRC's regulations.
United States policies governing the permanent disposal of HLW are defined by the Nuclear Waste Policy Act of 1982, as amended (NWPA). This Act specifies that HLW will be disposed of underground, in a deep geologic repository, and that Yucca Mountain, Nevada, will be the single candidate site for characterization as a potential geologic repository. Under the Act, the NRC is one of three Federal agencies with a role in the disposal of spent nuclear fuel, as well as the HLW from the Nation's nuclear weapons production activities:
The U.S. Department of Energy (DOE) is responsible for designing, constructing, operating, and decommissioning a permanent disposal facility for HLW, under NRC licensing and regulation.
The U.S. Environmental Protection Agency (EPA) is responsible for developing site-specific environmental standards for use in evaluating the safety of a geologic repository.
The NRC is responsible for developing regulations to implement the EPA's safety standards, and for licensing and overseeing the construction and operation of the repository. In addition, the NRC will consider any future DOE applications for license amendments to permanently close the repository, dismantle surface facilities, remove controls to restrict access to the site, or undertake any other activities involving an unreviewed safety question.
Final disposition of low-level waste
Low-level waste includes items that have become contaminated with radioactive material or have become radioactive through exposure to neutron radiation. This waste typically consists of contaminated protective shoe covers and clothing, wiping rags, mops, filters, reactor water treatment residues, equipments and tools, luminous dials, medical tubes, swabs, injection needles, syringes, and laboratory animal carcasses and tissues. The radioactivity can range from just above background levels found in nature to very highly radioactive in certain cases such as parts from inside the reactor vessel in a nuclear power plant. Low-level waste is typically stored on-site by licensees, either until it has decayed away and can be disposed of as ordinary trash, or until amounts are large enough for shipment to a low-level waste disposal site in containers approved by the Department of Transportation.
Low-level waste disposal occurs at commercially operated low-level waste disposal facilities that must be licensed by either NRC or Agreement States. The facilities must be designed, constructed, and operated to meet safety standards. The operator of the facility must also extensively characterize the site on which the facility is located and analyze how the facility will perform for thousands of years into the future.
There are three existing low-level waste disposal facilities in the United States that accept various types of low-level waste. All are in Agreement States.
The Low-level Radioactive Waste Policy Amendments Act of 1985 gave the states responsibility for the disposal of their low-level radioactive waste. The Act encouraged the states to enter into compacts that would allow them to dispose of waste at a common disposal facility. Most states have entered into compacts; however, no new disposal facilities have been built since the Act was passed.
NRC backgrounder on radioactive waste (high level & low level) http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/radwaste.html