Nuclear reactors use black ceramic material called uranium dioxide (UO2) as their fuel. The fuel is loaded into a reactor core, which, for a 1,000 Megawatt power station, is about 14 feet high and 12 feet in diameter. Nuclear power plant operators can generate enormous amounts of heat and electricity from the reactor core. In 2007, a little over 20 ,000 metric tons of uranium was loaded into the 104 commercial U.S. reactors. These reactors generated a record of over 800 billion kilowatthours of electricity, about 20% of all U.S. electricity in 2007.
The nuclear fuel cycle for typical light-water reactors is shown in the figure. The cycle consists of "front end" steps that lead to the preparation of uranium for use in nuclear reactors and "back end" steps to safely manage, prepare, and dispose of the highly radioactive spent nuclear fuel. Chemical processing of the spent fuel material to recover the remaining portion of fissionable products for use in fresh fuel assemblies is technically feasible, although it is not permitted in the United States.
Source: Pennsylvania State University Radiation Science and Engineering Center
The Front End of the Nuclear Fuel Cycle
Exploration
The nuclear fuel cycle starts with the exploration for uranium resources and the development of mines to extract the discovered ore. A variety of techniques are used to find uranium including airborne radiometric surveys, chemical sampling of groundwater and soils, and exploratory drilling to understand the underlying geology.
It is sometimes difficult to locate economic uranium resources because the ore occurrences are not usually continuous like coal seams, but rather they form discrete, concentrated deposits much like the specks in blue cheese. A mining company may drill many holes around a large deposit without finding it. Likewise, one drill hole may hit a single deposit, and yet not be able to confirm the existence of a larger deposit. Once such deposits are located, the mine developer usually follows up with more closely spaced "in fill" or development drilling to further characterize the deposit.
Uranium Mining
Once economic resources have been discovered, the next step in the fuel cycle is to mine the ore using either conventional (underground or open pit) mining techniques or unconventional techniques such as in-place solution mining or heap leaching, which use liquid solvents to dissolve and extract the ore.
Prior to 1980, most U.S. uranium was produced using open pit and underground mining techniques. Today, the majority of uranium is produced using solution mining techniques commonly called in-situ-leach (ISL) or in-situ-recovery (ISR).
Uranium Milling
Once the ore is extracted from the mine, it is then further refined into uranium concentrate at the mill. For vein-type deposits, a typical mill facility at an open pit or underground mine would crush, pulverize, and grind the ore into fine powder that would then be reacted with chemicals to separate the uranium from other minerals. The concentrated uranium product is typically a bright yellow or orange powder called "yellowcake" (U3O8) (see photo), and the waste stream from these operations is called "mill tailings."
In solution mines, the uranium is typically found as a coating on underground sand particles called conglomerates. For these deposits, the uranium is extracted by exposing the sand to a groundwater solution whose pH has been elevated slightly using natural chemicals such as oxygen, carbon dioxide, or caustic soda. The uranium dissolves into the water, which is retrieved and circulated through a resin bed at a facility (also called a mill) in order to extract and further concentrate the uranium into yellowcake. The clean water is then returned to the ground where the mining process is repeated.
Uranium Conversion
The next step in the nuclear fuel cycle involves the conversion of yellowcake into uranium hexafluoride (UF6) gas. This step is required because there are three forms (isotopes) of uranium that occur in nature: U-234, U-235, and U-238. The current U.S. nuclear reactor designs require a stronger concentration (enrichment) of the U-235 isotope in order to operate efficiently. To perform this atomic segregation, the uranium in yellowcake is first converted into a gaseous compound (UF6) from which the individual atoms can be sorted (see photo).
Uranium Enrichment
The uranium hexafluoride gas coming from the converter facility is called "natural UF6" because the original concentrations of uranium isotopes are still unchanged. This gas is then sent to an enrichment plant where the isotope separation takes place. The United States currently has one operating enrichment plant, which uses a process called gaseous diffusion to separate the uranium isotopes. Because the smaller U-235 atoms travel slightly faster than the U-238 atoms, they tend to leak (diffuse) faster through the porous membrane walls, where they are collected and concentrated. The final product has about a 4% to 5% concentration of U-235 and is called "enriched UF6". It is sealed in canisters and allowed to cool and solidify before it is transported to the fuel assembly plant by train, truck, or barge.
Another enrichment technique is the gas centrifuge process, where UF6 gas is spun at high speed in a series of cylinders to separate 235UF6 and 238UF6 atoms based on their different atomic masses. New enrichment technologies currently being developed are atomic vapor laser isotope separation (AVLIS) and molecular laser isotope separation (MLIS). These laser-based enrichment processes can achieve higher initial enrichment (isotope separation) factors than the diffusion or centrifuge processes and are capable of operating at high material throughput rates.
Uranium Re-Conversion and Nuclear Fuel Fabrication
The next step in the production of nuclear fuel takes place at one of the five U.S. fuel fabrication facilities. Here, the enriched UF6 gas is reacted to form a black uranium dioxide powder. The powder is then compressed and formed into the shape of small ceramic fuel pellets. The pellets are stacked and sealed into long metal tubes that are about 1 centimeter in diameter to form fuel rods. The fuel rods are then bundled together to make up a fuel assembly (see photo). Depending on the reactor type, there are about 179 to 264 fuel rods in each fuel assembly; and a typical reactor core holds 121 to 193 fuel assemblies.
At the Reactor
Source: Los Alamos National Laboratory
Following fabrication, the fuel assemblies are shipped by truck to the reactor sites where they are stored onsite in “fresh fuel” storage bins until needed by the reactor operators. At this stage, the uranium is only mildly radioactive and essentially all radiation is contained within the metal tubes. Consequently, the fresh fuel can be handled safely with bare hands and with no special precautions. Typically, about one third of the reactor core (40 to 90 fuel assemblies) is changed out every 12 to 24 months.
The reactor core itself is a cylindrical arrangement of the fuel bundles, about 12 feet in diameter and 14 feet high. It is encased in a several-inch-thick steel pressure vessel. The core has essentially no moving parts except for a small number of control rods that can be inserted to regulate the reaction. Merely placing the fuel assemblies next to each other and adding water is sufficient to initiate the nuclear reaction.
The Back End of the Nuclear Fuel Cycle
Interim Storage
Following use in the reactor, the fuel assembly becomes highly radioactive and must be removed and stored under water in a spent fuel pool at the reactor for several years. Even though the fission reaction has stopped, the spent fuel continues to give off heat from the decay of radioactive elements that were created when the uranium atoms were split apart. The water in the pool serves to both cool the fuel and shield the operators from any radiation. As of 2002, there were over 165,000 spent fuel assemblies stored in about 70 interim storage pools throughout the United States.
After cooling a few years in the pool, the spent fuel element may be moved to a dry cask storage container for further on-site storage. An increasing number of reactor operators now store their older spent fuel in these special outdoor concrete or steel containers with air cooling.
Reprocessing
Less than 4% of the uranium loaded into the reactor is consumed in nuclear reactions. The rest of the uranium remains unchanged. Chemical processing of the spent fuel material to recover the remaining portion of fissionable products for use in fresh fuel assemblies is technically feasible. Some countries, such as France, reprocess spent nuclear fuel, but it is not permitted in the United States.
Final Disposal
The final step in the nuclear fuel cycle is the collection of spent fuel assemblies from the interim storage sites or future reprocessing facilities, and the disposal of any remaining high-level nuclear waste in a permanent underground repository. The United States currently has no such repository.