Depleted Uranium

Uranium is a naturally occurring radioactive element. It is a very heavy metal and a small 10-centimetre metal cube weighs 20 kilograms! It’s specific activity (radioactivity per unit gram) is low. In its natural state, uranium consists of three isotopes (U-234 (0.0057%), U-235 (0.72%) and U-238 (99.28%). Out of the three isotopes, U-238 has the longest half life of 4.51 billion years. Isotopes of natural uranium decay by emitting mainly alpha particles. The emission of beta particles and gamma radiations are low. Uranium is found in trace amounts in all rocks and soil, in water and air, and in materials made from natural substances.

Natural uranium is used as nuclear fuel in Indian PHWR-type power reactors. On use in the reactors, the percentage of U-235 isotope gets reduced due to nuclear fission (burning) and the uranium recovered chemically from the “burnt” fuel after the reactor operation is called Depleted Uranium (DU). The depleted uranium produced in this method contains some fission-product contamination.

The depleted uranium is also the by-product of enrichment process used to produce fuel for certain other types of nuclear reactors and nuclear weapons. During the enrichment process the fraction of U-235 is increased from its natural level (0.72% by mass) to between 2% and 94% by mass. The by-product uranium mixture (after the enriched uranium is removed) has reduced concentrations of U-235. In depleted uranium, the percentage fraction of U-235 is less than 0.72% and can go down up to 0.2%. The DU is considerably less radioactive than natural uranium because it has lesser percentage of U-234 and U-235 per unit mass than natural uranium. The specific activity of uranium alone in DU is 14.8 Bq per mg compared with 25.4 Bq per mg for natural uranium. Becquerel (Bq) is the unit quantifying radioactivity.

DU is less harmful than natural uranium. However, according to the Indian Atomic Energy Act 1962, it is a Prescribed Substance and hence possession of the material in unauthorized persons is illegal.

The DU has more of industrial application (as heavy shield material against penetrating radiation like gamma rays) than military applications such as in the manufacturing of ammunitions used to pierce armour plating on tanks and in missile nose cones.