I'll try to answer my own question here.
When uranium decays,another nuclide or isotope is created,which in turn is also radioactive,leading to a long chain of radionuclides (uranium daughter products)being produced (see Appendix O 'Data on Uranium').DU is roughly 40 per cent less radioactive than natural uranium and,consequently, less radiotoxic.This is because during the industrial process by which uranium ore is converted to uranium metal,uranium is chemically separated from all its daughter products beyond U-234,i.e. radium,radon and others.
In the enrichment process used for the production of nuclear fuel,the uranium concentration of the isotope U-235 is enriched from 0.7 per cent in natural uranium to roughly 4 per cent in the uranium destined for fuel in nuclear reactors.
The by-product is uranium with a lower concentration of U-235,i.e.depleted uranium (DU). The U-235 concentration in the DU produced is usually 0.2-0.3 per cent.In enrichment plants,U-235,which is slightly lighter in mass than U-238,is used to separate the two iso- topes,allowing the enrichment process of U-235.
Since U-234 is an even lighter isotope,its concentration is correspondingly higher in fuel uranium and lower in DU when compared with natural uranium.The fact that DU has lower concentrations of U-235 and U-234 than natural uranium also explains why DU is less radioactive than natural uranium.Data on the specific activity of DU are given in Table 2.1.
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The overwhelming part of the radiation emitted from the nuclides in the U-238 series is emitted from the isotopes that follow after U-234.Compared with the sum of the energy of alpha radiation emitted per transformation from all isotopes in the U-238 series,the isotopes that follow after U-234 emit about 89 per cent of the alpha energy,roughly 58 per cent of the beta radiation energy and about 98.6 per cent of the gamma radiation energy (Appendix O,Table O.4).
If reprocessed uranium from a nuclear reactor is used (fully or partially)as feed material in the enrichment process of uranium,or if this was the case during earlier runs of the technical facilities of the enrichment plant,the DU may contain tiny traces of fission products,uranium isotopes and transuranic elements that are specific to reprocessed reactor fuel.In DU penetrator material found during earlier UNEP missions to the Balkans region (UNEP,2001; UNEP,2002),traces of U-236 and Pu-239/240 could be identified.U-236 was analysed around 0.003 per cent (mass per cent),and Pu-239/240 contamination of the DU was around 20 Bq/kg (10 -2 micrograms per kilogram),which is equivalent to the very low content of one plutonium atom per 100 billion uranium atoms.This indicates that the DU found in the Balkans came into contact with reprocessed uranium at some point during its fabrication process.The concentration of contaminating nuclides is indeed so low that their contribution to the total radiation dose of DU is insignificant and can be neglected in assessing risk to humans or the environment.