Catherine Philp reports that uranium is mined and then processed as nuclear fuel for military or civilian purposes. If uranium ore is near the surface, it is drilled,blasted, excavated from an open-pit mine, transported to a processing plant where it is ground up and chemically treated to yield “yellowcake”, a fine powder of uranium oxide.. If the ore is deeper, and occurs in sandstone, it is drilled and recovered by wells by solution mining methods where fluids charged with CO2 dissolve the uranium within the rock and is piped over some distance to a central processing plant where it is chemically treated to yield "yellowcake." The uranium oxide is then typically converted into uranium hexafluoride gas so that it can be enriched. After enrichment, it is converted into purified fuel rods, where it can be used in a variety of reactors where heat is generated under controlled conditions to boil water. One type is a pressurised heavy water reactor. The enrichment process increases the percentage of a particular isotope, uranium-235, which makes up 0.7 per cent of natural uranium. The rest is uranium-238.
Any method of enrichment is to create fuel through isotope separation, the most common is by gas centrifuge. Centrifuges rotate at high speed, separating the isotopes by weight and sending the heavier uranium-238 to the outside of the cylinder while the lighter 235 collects at the center. The slightly enriched stream is extracted and fed into the next centrifuge, where the process is repeated, enriching it further. Most nuclear power reactors use uranium that has been enriched to a composition of between 3 and 5 per cent uranium-235. Anything up to 20 per cent uranium-235 is called low-enriched uranium. Uranium enriched to between 12 and 19.75 per cent is used in the production of medical isotopes in research reactors. Uranium enriched above 20 per cent is called highly enriched uranium, while 20 per cent is the lowest theoretical threshold for weapons-grade uranium. Most weapons use uranium that is 90 per cent enriched.
The first stages require more centrifuges due to the volume of uranium. The process gets easier as purity increases, making the leap from low to high-enrichment easier than the leap from natural uranium to low-enriched. Once the 20 per cent threshold is breached, weapons grade is within reach.
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