Centrifuges are used to enrich the uranium that is used in nuclear power plants. The two main isotopes of uranium are U–235 and U–238, with U–235 being the isotope used to generate nuclear power via fission processes. Unfortunately, over 99% of naturally occurring uranium is U–238, so a lot of effort has to go into enriching the fraction of U–235 present in a sample. Centrifuges are often used to isotopically enrich a sample of uranium in the U–235 isotope. As described in an earlier question, this is accomplished by spinning a centrifuge tube, and in this case the heavier U–238 isotope is weighed down more, allowing a greater fraction of U–235 to be collected (in the gas phase) from the top of the centrifuge. This process can also be facilitated by heating the bottom of the centrifuge tube, which also helps the U–235 to move toward the top of the tube where it is collected. The process is typically repeated many times before the desired fraction of U–235 is reached. Highly enriched uranium often contains >85% Uranium-235 though, so clearly people have gotten pretty good at carrying out the isotopic enrichment process.