Nuclear fuel cycle

The nuclear fuel cycle, also known as the nuclear fuel chain, is the series of stages that nuclear fuel undergoes during its production, use, and recycling or disposal. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

Basic concepts

relies on fissionable material that can sustain a chain reaction with neutrons. Examples of such materials include uranium and plutonium. Most nuclear reactors use a moderator to lower the kinetic energy of the neutrons and increase the probability that fission will occur. This allows reactors to use material with far lower concentration of fissile isotopes than are needed for nuclear weapons. Graphite and heavy water are the most effective moderators, because they slow the neutrons through collisions without absorbing them. Reactors using heavy water or graphite as the moderator can operate using natural uranium.
A light water reactor uses water in the form that occurs in nature, and requires fuel enriched to higher concentrations of fissile isotopes. Typically, LWRs use uranium enriched to 3–5% U-235, the only fissile isotope that is found in significant quantity in nature. One alternative to this low-enriched uranium fuel is mixed oxide fuel produced by blending plutonium with natural or depleted uranium, and these fuels provide an avenue to utilize surplus weapons-grade plutonium. Another type of MOX fuel involves mixing LEU with thorium, which generates the fissile isotope U-233. Both plutonium and U-233 are produced from the absorption of neutrons by irradiating fertile materials in a reactor, in particular the common uranium isotope U-238 and thorium, respectively, and can be separated from spent uranium and thorium fuels in reprocessing plants.
Some reactors do not use moderators to slow the neutrons. Like nuclear weapons, which also use unmoderated or "fast" neutrons, these fast-neutron reactors require much higher concentrations of fissile isotopes in order to sustain a chain reaction. They are also capable of breeding fissile isotopes from fertile materials; a breeder reactor is one that generates more fissile material in this way than it consumes.
During the nuclear reaction inside a reactor, the fissile isotopes in nuclear fuel are consumed, producing more and more fission products, most of which are considered radioactive waste. The buildup of fission products and consumption of fissile isotopes eventually stop the nuclear reaction, causing the fuel to become a spent nuclear fuel. When 3% enriched LEU fuel is used, the spent fuel typically consists of roughly 1% U-235, 95% U-238, 1% plutonium and 3% fission products. Spent fuel and other high-level radioactive waste is extremely hazardous, although nuclear reactors produce orders of magnitude smaller volumes of waste compared to other power plants because of the high energy density of nuclear fuel. Safe management of these byproducts of nuclear power, including their storage and disposal, is a difficult problem for any country using nuclear power.

Front end

Exploration

A deposit of uranium, such as uraninite, discovered by geophysical techniques, is evaluated and sampled to determine the amounts of uranium materials that are extractable at specified costs from the deposit. Uranium reserves are the amounts of ore that are estimated to be recoverable at stated costs.
Naturally occurring uranium consists primarily of two isotopes U-238 and U-235, with 99.28% of the metal being U-238 while 0.71% is U-235, and the remaining 0.01% is mostly U-234. The number in such names refers to the isotope's atomic mass number, which is the number of protons plus the number of neutrons in the atomic nucleus.
The atomic nucleus of U-235 will nearly always fission when struck by a free neutron, and the isotope is therefore said to be a "fissile" isotope. The nucleus of a U-238 atom on the other hand, rather than undergoing fission when struck by a free neutron, will nearly always absorb the neutron and yield an atom of the isotope U-239. This isotope then undergoes natural radioactive decay to yield Pu-239, which, like U-235, is a fissile isotope. The atoms of U-238 are said to be fertile, because, through neutron irradiation in the core, some eventually yield atoms of fissile Pu-239.

Mining

Uranium ore can be extracted through conventional mining in open pit and underground methods similar to those used for mining other metals. In-situ leach mining methods also are used to mine uranium in the United States. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. Uranium ores in the United States typically range from about 0.05 to 0.3% uranium oxide. Some uranium deposits developed in other countries are of higher grade and are also larger than deposits mined in the United States. Uranium is also present in very low-grade amounts in some domestic phosphate-bearing deposits of marine origin. Because very large quantities of phosphate-bearing rock are mined for the production of wet-process phosphoric acid used in high analysis fertilizers and other phosphate chemicals, at some phosphate processing plants the uranium, although present in very low concentrations, can be economically recovered from the process stream.

Milling

When Uranium is mined out of the ground it does not contain enough pure uranium per pound to be used. The process of milling is how the cycle extracts the usable uranium from the rest of the materials, also known as tailings. To begin the milling process the ore is either ground into fine dust with water or crushed into dust without water. Once the Materials have been physically treated, they then begin the process of being chemically treated by being doused in acids. Acids used include hydrochloric and nitrous acids but the most common acids are sulfuric acids. Alternatively if the material that the ore is made of is particularly resistant to acids then an alkali is used instead. After being treated chemically the uranium particles are dissolved into the solution used to treat them. This solution is then filtered until what solids remain are separated from the liquids that contain the uranium. The undesirable solids are disposed of as tailings. Once the solution has had the tailings removed the uranium is extracted from the rest of the liquid solution, in one of two ways, solvent exchange or ion exchange. In the first of these a solvent is mixed into the solution. The dissolved uranium binds to the solvent and floats to the top while the other dissolved materials remain in the mixture. During ion exchange a different material is mixed into the solution and the uranium binds to it. Once filtered the material is panned out and washed off. The solution will repeat this process of filtration to pull as much usable uranium out as possible. The filtered uranium is then dried out into U₃O₈ uranium. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake", which is sold on the uranium market as U₃O₈. Note that the material is not always yellow.

Uranium conversion

Usually milled uranium oxide, U₃O₈ is then processed into either of two substances depending on the intended use.
For use in most reactors, U₃O₈ is usually converted to uranium hexafluoride, the input stock for most commercial uranium enrichment facilities. A solid at room temperature, uranium hexafluoride becomes gaseous at 57 °C. At this stage of the cycle, the uranium hexafluoride conversion product still has the natural isotopic mix.
There are two ways to convert uranium oxide into its usable forms uranium dioxide and uranium hexafluoride; the wet option and the dry option. In the wet option the yellowcake is dissolved in nitric acid then extracted using tributyl phosphate. The resulting mixture is then dried and washed resulting in uranium trioxide. The uranium trioxide is then mixed with pure hydrogen resulting in uranium dioxide and dihydrogen monoxide or water. After that the uranium dioxide is mixed with four parts hydrogen fluoride resulting in more water and uranium tetrafluoride. Finally the end product of uranium hexafluoride is created by simply adding more fluoride to the mixture.
For use in reactors such as CANDU which do not require enriched fuel, the U₃O₈ may instead be converted to uranium dioxide which can be included in ceramic fuel elements.
In the current nuclear industry, the volume of material converted directly to UO₂ is typically quite small compared to that converted to UF₆.

Enrichment

The natural concentration of the fissile isotope U-235 is less than that required to sustain a nuclear chain reaction in light water reactor cores. Accordingly, UF₆ produced from natural uranium sources must be enriched to a higher concentration of the fissionable isotope before being used as nuclear fuel in such reactors. The level of enrichment for a particular nuclear fuel order is specified by the customer according to the application they will use it for: light-water reactor fuel normally is enriched to 3.5% U-235, but uranium enriched to lower concentrations is also required. Enrichment is accomplished using any of several methods of isotope separation. Gaseous diffusion and gas centrifuge are the commonly used uranium enrichment methods, but new enrichment technologies are currently being developed.
The bulk of the byproduct from enrichment is depleted uranium, which can be used for armor, kinetic energy penetrators, radiation shielding and ballast. As of 2008 there are vast quantities of depleted uranium in storage. The United States Department of Energy alone has 470,000 tonnes. About 95% of depleted uranium is stored as uranium hexafluoride.