Natural uranium contains 0.7% of the uranium-235 isotope and 99.3% of the uranium-238 isotope (with traces of other uranium isotopes). Enrichment increases the percentage of uranium-235 to 3-5% which makes it suitable for use as fuel in most of the world’s nuclear power reactors.
The enrichment technologies commercially available at present are the gaseous diffusion process and the centrifuge process. Both of them require the prior conversion of the uranium to gaseous uranium hexafluoride (UF6).
Depleted uranium (DU) is a radioactive by-product of the uranium enrichment process. It gets its name from the fact that much of the uranium-235 has been extracted from it. When natural uranium is enriched, one-seventh of the original amount becomes enriched uranium fuel; the other six-sevenths becomes DU waste.
Thus very large stockpiles of DU waste have been created, estimated at 1.5 million tonnes at the start of 2005 (IAEA, 2006).
Peter Diehl (2010) from the World Information Service on Energy summarises storage and disposal issues: “Most of the depleted uranium produced to date is being stored as UF6 in steel cylinders in the open air in so-called cylinder yards located adjacent to the enrichment plants. … Chemically, UF6 is very reactive: with water it forms the extremely corrosive hydrofluoric acid and the highly toxic uranyl fluoride (UO2F2). The hydrofluoric acid causes skin burns, and, after inhalation, damages the lungs. Further health hazards result from the chemical toxicity of the uranium to the kidneys, and from the radiation of the uranium (an alpha emitter). In the storage yards, the cylinders are subject to corrosion. The integrity of the cylinders must therefore be monitored and the painting must be refreshed from time to time. This maintenance work requires moving of the cylinders, causing further hazards from breaching of corroded cylinders, and from handling errors. … For long-term storage or disposal, the depleted UF6 must be converted to a less reactive chemical form: candidates are UF4, U3O8, and UO2.”
DU has military uses:
* It is used in munitions (e.g. missile nose cones) used to pierce armour plating. It has been used in munitions used by the US and NATO in Iraq, the Balkans and Afghanistan. This has generated controversy because of the long-term public health and environmental risks associated with DU.
* Because DU is rich in uranium-238 it is ideal for producing fissile plutonium-239 for use in nuclear weapons. This can be done by inserting a ‘blanket’ or target into a reactor.
* Enrichment plants are of military significance since they can produce not only low enriched uranium for use as reactor fuel, but also highly enriched uranium which can be used as the fissile (explosive) material in nuclear weapons.
There are also civil uses of DU. It can be re-enriched to provide more fuel for reactors. It is sometimes used as a radiation shield or as ballast (because of its weight). It can also be used to provide fuel for fast neutron reactors although very few of these reactors exist.
(For more information on DU and enrichment plants see Makhijani and Smith, 2005.)