Depleted Uranium [DU]
Depleted uranium [DU] results from the enriching of natural uranium for use in nuclear reactors. Natural uranium is a slightly radioactive metal that is present in most rocks and soils as well as in many rivers and sea water. Natural uranium consists primarily of a mixture of two isotopes (forms) of uranium, Uranium-235 (U235) and Uranium-238 (U238), in the proportion of about 0.7 and 99.3 percent, respectively. Nuclear reactors require U235 to produce energy, therefore, the natural uranium has to be enriched to obtain the isotope U235 by removing a large part of the U238. Uranium-238 becomes DU, which is 0.7 times as radioactive as natural uranium. Since DU has a half-life of 4.5 billion years, there is very little decay of those DU materials.
Staballoy are metal alloys made from high-density depleted uranium mixed with other metals for use in kinetic energy penetrators for armor-piercing munitions. Several different metals, such as titanium or molybdenum, can be used for the purpose. The various staballoy metals have low radioactivity that is not considered to be a significant health hazard.
The Agency for Toxic Substances and Disease Registry (ATSDR) for the Department of Health and Human Services estimates there are an average of 4 tons of uranium in the top foot of soil in every square mile of land. A heavy metal similar to tungsten and lead, uranium occurs in soils in typical concentrations of a few parts per million (equivalent to about half a teaspoon of uranium in a typical 8-cubic yard dump truck-load of dirt).
The Department of Energy (DOE) has reported that the DU it provided to DoD for manufacturing armor plates and munitions may contain trace levels (a few parts per billion ) of contaminants including neptunium, plutonium, americium, technitium-99 and uranium-236. From a radiological perspective, these contaminants in DU add less than one percent to the radioactivity of DU itself.
In military applications, when alloyed, Depleted Uranium is ideal for use in armor penetrators. These solid metal projectiles have the speed, mass and physical properties to perform exceptionally well against armored targets. DU provides a substantial performance advantage, well above other competing materials. This allows DU penetrators to defeat an armored target at a significantly greater distance. Also, DU's density and physical properties make it ideal for use as armor plate. DU has been used in weapon systems for many years in both applications.
DU can be used to engage the enemy at greater distances than tungsten penetrators or high explosive anti-tank (HEAT) rounds because of improved ballistic properties. When they strike a target, tungsten penetrators blunt while DU has a self-sharpening property. DU ammunition routinely provides a 25 percent increase in effective range over traditional kinetic energy rounds.
On impact with a hard target (such as a tank) the penetrator may generate a cloud of DU dust within the struck vehicle that ignites spontaneously creating a fire that increases the damage to the target. Due to the pyrophoric nature of DU, many of the DU particles and fragments that are formed during and following impact and perforation will spontaneously ignite, resulting in a shift of the particle size probability distribution function to a smaller mean diameter. As a result of physical differences between DU and its oxides, the oxide particles tend to crumble under relatively weak mechanical forces, further shifting the particle size to an even smaller mean diameter.
The amount of depleted uranium which is transformed into dust will depend upon the type of munition, the nature of the impact, and the type of target. The number of penetrators hitting a target depends upon many factors, including the type and size of the target. On average, not more than 10% of the penetrators fired by planes equipped with large machine guns hit the target (20 - 30 mm rounds). DU munitions which do not hit hard targets will penetrate into the soft ground or remain more or less intact on the surface. These will corrode over time, as metalic DU is not stable under environmental conditions.
US forces also use DU to enhance their tanks' armor protection. In one noteworthy incident, an M1A1 Abrams Main Battle Tank, its thick steel armor reinforced by a layer of DU sandwiched between two layers of steel, rebuffed a close-in attack by three of Iraq's T-72 tanks. After deflecting three hits from Iraq's tanks, the Abrams' crew dispatched the T-72s with a single DU round to each of the three Iraqi tanks.
Depleted uranium is also used in numerous commercial applications requiring a very dense material. These include: ballast and counterweights; balancing control devices on aircraft; balancing and vibration damping on aircraft; machinery ballast and counterweights; gyrorotors and other electromechanical counterweights; shielding for medicine and industry; shipping container shielding for radiopharmaceuticals; chemical catalyst; pigments; and, x-ray tubes.
Artist depiction shows why a DU penetrator, which
sharpens itself as it moves through armor, is much
more effective than tungsten, which becomes blunt.
DU's self-sharpening properties are evident in this
x-ray. Note how the tungsten penetrator's tip deforms
into a mushroom shape.
The Department of Defense has a need for gun-launched kinetic-energy penetrators with length-to-diameter ra- tios sufficiently high that the rods will penetrate modern armor steel configurations. However, such rods must have high stiffness (that is, high elastic mod- ulus) to resist bending during launch and flight because slight bending may lead to yaw during flight and a glanc- ing blow off the target. The uranium- titanium alloy described above is a marginal candidate for use in the pro- posed penetrator rods because its elastic modulus is not high enough. Design analysis shows that composites of de- pleted uranium and of tungsten (whose elastic modulus for bending is three times that of uranium) improve the stiff- ness of the rod and thus, potentially, its performance. The stiffness of the composite rod is directly related to the ge- ometric placement of the high-modulus material in the rod. It is possible to arrange the composite so that maximum stiffening is achieved with the least change in penetrator density.
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