The nuclear fuel cycle

A 1,000 MWe reactor consumes about 200 tonnes of natural uranium a year. That uranium leaves the mine as a concentrate, yellowcake, with over 80% U₃O₈ in 200-liter drums. Conversion turns it into uranium hexafluoride, UF₆, the only uranium compound that is a gas at moderate temperature and can be used to separate isotopes. In enrichment, cascades of centrifuges concentrate the U-235 from the natural 0.7% to the 3.5 to 5% that light-water reactors need, leaving depleted tails at 0.22 to 0.25%.

Separative work is measured in separative work units, SWU: a year of fuel for that 1,000 MWe reactor takes about 182,000 SWU at 4.5% enrichment, and 198,000 at 5%. That is why 27.6 tonnes of enriched uranium dioxide are enough, pressed into pellets, stacked in zirconium cladding and assembled into fuel elements. Heavy-water reactors skip enrichment and use natural uranium directly.

Inside the reactor the fuel burns up. Burnup is measured in thermal gigawatt-days per tonne of uranium, GWd/t: the energy extracted from each tonne. Light-water reactors were designed for about 40 GWd/t and now exceed that comfortably: current fuel goes beyond 60 GWd/t and the industry is aiming at 70. More burnup means less uranium and less spent fuel per kilowatt-hour. Every 12 to 24 months a quarter to a third of the core is replaced, so each element spends three to six years inside. On the way out it still contains 96% of the original uranium, now with under 1% U-235, 1% plutonium and 3% fission products and minor actinides, which hold almost all the radioactivity.

There the cycle forks. In the open cycle, that of the United States, Sweden or Finland, spent fuel is treated as waste and goes to pools, dry casks and finally a geological repository. In the closed cycle, that of France, Russia and Japan, reprocessing separates uranium and plutonium from the fission products by chemical extraction, the PUREX process of La Hague, and recycles them: plutonium mixed with depleted uranium forms mixed-oxide fuel, MOX, and about eight reprocessed assemblies yield one of MOX. Reprocessing extracts 25 to 30% more energy and cuts the volume of high-level waste by 85%, to about 750 kg per reactor per year, which vitrified fits in twelve glass cylinders 1.3 meters tall.