Depleted uranium is the dense leftover from enriching nuclear fuel
Making one kilogram of reactor fuel enriched to 5 per cent uranium-235 takes 11.8 kilograms of natural uranium and leaves around 10.8 kilograms behind. That residue, depleted uranium, is about 40 per cent less radioactive than the natural metal, yet almost as heavy, and it has piled up in enormous quantities.
Natural uranium holds roughly 0.71 per cent of the fissile isotope uranium-235, with nearly all the rest being uranium-238. Enrichment concentrates the lighter isotope for reactors and weapons, leaving a by-product containing only 0.2 to 0.4 per cent. US regulators define depleted uranium as anything below 0.711 per cent. Material recovered from recycled spent fuel carries tiny traces of neptunium and plutonium and can be identified by the isotope uranium-236.
Density is its defining trait. At about 19,050 kilograms per cubic metre it is 1.67 times heavier than lead and only slightly lighter than tungsten or gold. Civilian users value that mass without much added radiation, fitting it as aircraft counterweights, as shielding in radiotherapy and radiography equipment, and in containers for moving radioactive cargo. Armies adopted it for armour plating and armour-piercing rounds, a use that remains contentious because uranium is a toxic heavy metal.
Most of it simply sits in storage. Around 95 per cent of all depleted uranium made so far is kept as uranium hexafluoride in steel cylinders in open yards beside enrichment plants, each holding up to 12.7 tonnes. By 2008, 57,122 such cylinders containing 686,500 tonnes stood near Portsmouth in Ohio, Oak Ridge in Tennessee and Paducah in Kentucky. Contact with moist air turns the compound into toxic uranyl fluoride and hydrogen fluoride, so the US has been converting stocks into solid oxides.
Nature offers only one exception to the usual isotope ratio: the ancient natural fission reactor at Oklo in Gabon. Its uranium can be told apart from the manufactured kind by uranium-234, which sits at 55 parts per million in every natural source.
Source: Depleted uranium
