Nuclear fuel cycle: Encyclopedia II - Nuclear fuel cycle - Front end

Nuclear fuel cycle - Front end

1 Uranium ore - the principal raw material of nuclear fuel

2 Yellowcake - the form in which uranium is transported to an enrichement plant

3 UF6 - used in enrichement

4 Nuclear fuel - a compact, inert, insoluble solid

Nuclear fuel cycle - Exploration

A deposit of uranium, 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. Uranium in nature consists primarily of two isotopes, U²³⁸ and U²³⁵. The numbers refer to the atomic mass number for each isotope, or the number of protons and neutrons in the atomic nucleus. Naturally occurring uranium consists of approximately 99.28 percent U²³⁸ and 0.71 percent U²³⁵. The atomic nucleus of U²³⁵ 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 U²³⁸ 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 U²³⁹. This isotope then undergoes natural radioactive decay to yield Pu²³⁹, which, like U²³⁵, is a fissile isotope. The atoms of U²³⁸ are said to be fertile, because, through neutron irradiation in the core, some eventually yield atoms of fissile Pu²³⁹.

Nuclear fuel cycle - 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 percent uranium oxide (U₃O₈). 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 (50 to 200 parts per million) 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.

Nuclear fuel cycle - Milling

Mined uranium ores normally are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake," which is sold on the uranium market as U₃O₈.

Nuclear fuel cycle - Uranium conversion

Milled uranium oxide, U₃O₈, must be converted to uranium hexafluoride, UF₆, which is the form required by most commercial uranium enrichment facilities currently in use. A solid at room temperature, UF₆ can be changed to a gaseous form at moderately higher temperature of 134°F (57°C). The UF₆ conversion product contains only natural, not enriched, uranium.

U₃O₈ is also converted to ceramic grade UO₂ for use in reactors not requiring enriched fuel, such as CANDU. The volumes of material converted directly to UO₂ are typically quite small compared to the amounts converted to UF₆.

Nuclear fuel cycle - Enrichment

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The concentration of the fissionable isotope, U²³⁵ (0.71 percent in natural uranium) is less than that required to sustain a nuclear chain reaction in light water reactor cores. Natural UF₆ thus must be enriched in the fissionable isotope for it to be used as nuclear fuel. The different levels of enrichment required for a particular nuclear fuel application are specified by the customer: light-water reactor fuel normally is enriched up to about 5 percent U²³⁵, but uranium enriched to lower concentrations also is required. Enrichment is accomplished using some one or more methods of isotope separation.

Gaseous diffusion and gas centrifuge are the commonly used uranium enrichment technologies. The gaseous diffusion process consists of passing the natural UF₆ gas feed under high pressure through a series of diffusion barriers (semiporous membranes) that permit passage of the lighter U²³⁵F₆ atoms at a faster rate than the heavier U²³⁸F₆ atoms. This differential treatment, applied across a large number of diffusion "stages," progressively raises the product stream concentration of U²³⁵ relative to U²³⁸. In the gaseous diffusion technology, the separation achieved per diffusion stage is relatively low, and a large number of stages is required to achieve the desired level of isotope enrichment. Because this technology requires a large capital outlay for facilities and it consumes large amounts of electrical energy, it is relatively cost intensive. In the gas centrifuge process, the natural UF₆ gas is spun at high speed in a series of cylinders. This acts to separate the U²³⁵F₆ and U²³⁸F₆ atoms based on their slightly different atomic masses. Gas centrifuge technology involves relatively high capital costs for the specialized equipment required, but its power costs are below those for the gaseous diffusion technology.

New enrichment technologies currently being developed are the atomic vapor laser isotope separation (AVLIS) and the molecular laser isotope separation (MLIS). Each laser-based enrichment process can achieve higher initial enrichment (isotope separation) factors than the diffusion or centrifuge processes can achieve. Both AVLIS and MLIS will be capable of operating at high material throughput rates.

The bulk (96%) of the byproduct from enrichment is depleted uranium (DU), for which there are few applications; the U.S. Department of Energy alone has 470,000 tonnes in store [1].

Nuclear fuel cycle - Fabrication

For use as nuclear fuel, enriched UF₆ is converted into uranium dioxide (UO₂) powder which is then processed into pellet form. The pellets are then fired in a high temperature sintering furnace to create hard, ceramic pellets of enriched uranium. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The pellets are stacked, according to each nuclear core's design specifications, into tubes of corrosion-resistant metal alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.

The metal used for the tubes depends on the design of the reactor - stainless steel was used in the past, but most reactors now use Zirconium. For the most common types of reactors (BWRs and PWRs) the tubes are assembled into bundles (see picture in [2]) with the tubes spaced precise distances apart. These bundles are then given a unique identification number, which enables them to be tracked from manufacture through use and into disposal

Adapted from the Wikipedia article "Front end", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki