How uranium ore is made into nuclear fuel
Uranium is a naturally-occurring element in the Earth's crust. Traces of it occur almost everywhere, although mining takes place in locations where it is naturally concentrated. To make nuclear fuel from the uranium ore requires first for the uranium to be extracted from the rock in which it is found, then enriched in the uranium-235 isotope, before being made into pellets that are loaded into assemblies of nuclear fuel rods.
Uranium mines operate in some twenty countries, though about half of world production comes from just ten mines in six countries, in Canada, Australia, Niger, Kazakhstan, Russia and Namibia.
At conventional mines, the ore goes through a mill where it is first crushed. It is then ground in water to produce a slurry of fine ore particles suspended in the water. The slurry is leached with sulphuric acid to dissolve the uranium oxides, leaving the remaining rock and other minerals undissolved, as mine tailings.
However, nearly half the world's mines now use a mining method called in situ leaching . This means that the mining is accomplished without any major ground disturbance. Groundwater with a lot of oxygen injected into it is circulated through the uranium ore, extracting the uranium. The solution with dissolved uranium is pumped to the surface.
Both mining methods produce a liquid with uranium dissolved in it. This is filtered and the uranium then separated by ion exchange, precipitated from the solution, filtered and dried to produce a uranium oxide concentrate, which is then sealed in drums. This concentrate may be a bright yellow colour, hence known as 'yellowcake', or if dried at high temperatures it is khaki.
The uranium oxide is only mildly radioactive. (The radiation level one metre from a drum of freshly-processed uranium oxide about half that - experienced from cosmic rays - on a commercial jet flight.)
Yellowcake in a drum for storage or transport (Cameco)
The vast majority of all nuclear power reactors require 'enriched' uranium fuel in which the proportion of the uranium-235 isotope has been raised from the natural level of 0.7% to about 3.5% to 5%. The enrichment process needs to have the uranium in gaseous form, so on the way from the mine it goes through a conversion plant which turns the uranium oxide into uranium hexafluoride.
The enrichment plant concentrates the useful uranium-235, leaving about 85% of the uranium by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level of uranium-235 and then passes to the next stage of the fuel cycle. The other stream is depleted in uranium-235 and is called 'tails' or depleted uranium. It is mostly uranium-238 and has little immediate use.
Today's enrichment plants use the centrifuge process, with thousands of rapidly-spinning vertical tubes. Research is being conducted into laser enrichment, which appears to be a promising new technology.
A small number of reactors, notably the Canadian CANDU reactors, do not require uranium to be enriched.
About 27 tonnes of fresh fuel is required each year by a 1000 MWe nuclear reactor. In contrast, a coal power station requires more than two and a half million tonnes of coal to produce as much electricity. (1)Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide powder. This powder is then pressed to form small fuel pellets, which are then heated to make a hard ceramic material. The pellets are then inserted into thin tubes to form fuel rods. These fuel rods are then grouped together to form fuel assemblies, which are several meters long.
Uranium dioxide in powder and pellet form (Candu)
The number of fuel rods used to make each fuel assembly depends on the type of reactor. A pressurized water reactor may use between 121-193 fuel assemblies, each consisting of between 179-264 fuel rods. A boiling water reactor has between 91-96 fuel rods per assembly, with between 350-800 fuel assemblies per reactor.
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