Air Force Materials Report: Depleted Uranium
C3C Alex Forbes
CHEM 200, US Air Force Academy, USAFA, CO 80841
German scientist Martin Klaproth discovers uranium and since the 1940’s till 1970’s, virtually all uranium was used for nuclear weapons (CIAAW). Naturally occurring uranium contains three major isotopes, U-238, 235, and 234 (Bleise). Depleted Uranium does not naturally occur; rather it is a byproduct of enriched uranium which fuels nuclear reactors for the manufacture of nuclear products. The production of nuclear energy is dependent of U-235, which natural uranium contains about .7% (WNA.). Enrichment is simply purifying the product until the critical mass requirements are met. There are several methods of enrichment that are used around the world that range from centrifuge, laser separation, gaseous diffusion, electromagnetic and aerodynamic enrichment. During the enrichment process reactors attempt to separate the U-235 from other isotopes and harvest that particular isotope for nuclear energy. In order to capture the U-235 isotope from natural uranium it needs to be converted into UF_6. After the enrichment process, what is left over is U-235 in the form of UF_6 and it’s by product, depleted uranium. Depleted uranium or DU can only be created by process of enrichment; to be labeled as DU – according to the Department of Energy- DU has to contain less than .2% of U-235 isotope (Argonne National Laboratory). In other words, DU is uranium that has had a portion of U-235 isotope removed. Every tone of natural uranium produces about 3.5% DU material (WNA). This byproduct, DU is something that is favored by the military and private sector for its extremely high density of 19.1 g/cm^3(RSC). Its high density and pyrophoric property results in several uses of DU, to briefly note, radiation therapy, transportation, aircraft, munitions and armor. If DU isn’t being used for manufacturing then it is commonly deposited as UF_6 for long term storage.
DU has several names; its IUPAC nomenclature however is the same as uranium, simply being uranium (PubChem). The structure of DU mirrors that of uranium, differing only in the amount of U-235 isotope present after enrichment. The difference between DU and uranium is the radioactive properties, due to the differing levels of isotopes present, the mass difference it negligible (Idaho University). However DU’s structure plays a significant and advantageous role in military applications. DU when in its metallic form is able to resist ignition due to the surface layer oxidation into UO_2.This thin layer of UO_2that forms protects the inner metal from oxidation and ultimately ignition (Bleise). However when DU is in its powder form it’s highly susceptible to ignition by compounds found naturally in air, such as CO_2and N_2 (Center for Disease Control). When DU is formed as a kinetic missile-in its metallic state- and launch at a target 10-35% of the DU missile may become an aerosol or powder due to impact speeds and kinetic force and will ignite, the 65 to 90% of DU missile maintains it structure (Bleise). Gaseous Diffusion is an enrichment process that makes up the majority of U.S.’s entire DU supply, approximately 50 percent quoted in 2000 (WNA). When UF_6 enters the diffusion system, it forces the gas through extremely high pressures and a series of porous membranes (WNA).¬ The high pressures forces the lighter UF_6 molecules, U-234 and 235, through the membranes faster thus pushing the heavier U-238 molecules (USNRC); this is due to differing isotopic masses. This process is sometimes done 1400 times to ensure the concentration of isotope U-235 is around 3 to 4% (WNA). This isotopic separation done via gaseous diffusion,
Dr. Totemeier of Argonne National Laboratory provides the reaction rate of DU when being oxidized and pyrophoricity during impact and collision. Reaction rate as represented by =C_exp [(-E)/RT] , where C is the pre-exponential term (referenced only when dealing with chemical kinetics), E is the activation energy (the negative sign to represent the exothermic energy), R is the universal gas constant, T is absolute temperature measured in Kelvin’s, and k being the reaction rate, unit-less (Totemeier). An average reaction rate, according to Dr. Totemeier for DU undergoing oxidation and combustion, is 633,330. Signifying an extremely fast reaction which makes sense, due to uranium’s pyrophoric’ s properties, it burns around 1373.15 K (Peacock).
DU goes through a multitude of steps during its decay, starting at U-238 or DU, it takes approximately 4.5 billon years for 1 gram of U-238 to turn into half a gram of U-238 and the other half being thorium, from thorium to protactinium and so forth (CCNR). Each of the radioactive elements listed in appendix A give off alpha or beta particles.
Depleted Uranium is a fascinating topic as it has emerged from being a waste product to being utilized in our everyday work. Initially depleted uranium had tough competition when being researched as a new metal to be used by the Air Force. When it came down to it, according to U.S. Air Force Technical, it was depleted uranium or tungsten (Alexandra). When analyzing tungsten we see that it’s a stronger and denser material, measuring at 19.25 g/cm^3. In addition tungsten has the highest melting point of any metal (ITIA). However when compared to depleted uranium, its pyrophoric property chemically very reactive and self-sharpening upon impact. Another unique fact when utilizing depleted uranium as a kinetic penetrator is the smaller impact but deeper penetration when comparing munitions of the same caliber. Depleted uranium’s density causes deeper penetration and when impacting the target and coming in contact with the air, has a violent reaction-an inadvertent incendiary round (Alexandra). In the military’s munitions, these DU penetrators come in two types. One for armor penetration and the other for air to ground; land warfare requiring to penetrate armor necessitate 105 to 120 mm guns to fire approximately 5kgs of DU. Air-to-ground warfare call for a special weapon that fires up to 400 grams of depleted uranium. Depleted uranium has influenced and revolutionized the way the Air Force handles it business and carries out missions. Main applications of DU in the military have been armor penetrating rounds and armor. As stated above DU has unique properties, extremely high density and pyrophoricity that appeals to munition’s side. In the early 1970’s the Air Force designs a gun, an air to surface gun – GAU-8/A– that fires DU rounds (Force Health Protection). DU when applied to other composites makes armor seriously strong. M1 tank armor, although kept secret, is made up of steel, a DU core, and other elastic materials (Davitt). In addition, DU has also revolutionized the private sector by using a waste product and transforming it to a useable material. On the civilian side, there are many uses for DU, a popular one being ballast for aircraft or radiation shielding for medical purpose such as x-rays. DU makes for great radiation shielding, especially through the battlefield. When combat medics or when transporting nuclear weaponry, the density of DU suffices in that category (Journal of Environmental Radioactivity).
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I received help from Capt Hare about the chemistry portion; advising on pyrophoric properties and research and cadet Holden for reviewing and making small grammatical corrections.
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