Uranium Background, History, and Properties

¶ … Uranium

Background, History, and Properties

Uranium is a naturally-occurring element on earth that was apparently used in its oxide form in paints or as a coloring additive in pottery and glass as early as the First Century AD (Krauss, 2001; WNA, 2012). There is evidence that uranium was mined in the 1500s, from the mountainous region in between Bohemia and Saxony. For several hundred years, the substance was known as pitchblende (or "pechblende" originally), derived from the German words for "bad luck mineral," presumably owing to the radiation poisoning that affected those who mined and worked with it, mainly as the result of inadvertently having either inhaled its powder residue or ingesting small amounts of it from eating without carefully removing remnants of the element on their hands (Lamarsh, 1975).

The German Chemist credited with having identified uranium oxide as the source of "pitchblende" was Martin Klaproth, who described it as a "strange kind of half metal" in his presentation to the Berlin Academy of Science in 1798 (Krauss, 2001). Before settling on the name uranium, Klaproth had initially suggested that the new element be called uran, after the most recently-discovered planet, Uranus. However, uranium was not actually isolated from uranium oxide until 1841, when Eugene-Melchoir Peligot used potassium to reduce anhydrous chloride to elemental uranium. Finally, the most important property of uranium escaped detection for more than another half century, until Henri Becquerel recognized that uranium was also radioactive (Krauss, 2001).

Molecularly, uranium is composed of 92 protons and 146 neutrons, making it one of the heaviest of the natural elements, at an atomic weight of 238 (Atkins, 1995). Its U-238 isotope is the most common form n which it occurs most abundantly on earth (WNA, 2012). Uranium is hardly a rare element, being found within most rocks of the earth's crust in approximately the same proportion as tin and tungsten and even occurring naturally in common seawater (WNA, 2012). Uranium is a mildly radioactive metal that undergoes alpha-particle decay, according to the principles governed by the weak nuclear force (Liverhant, 1960). It is a very dense, heavy substance that is more than one and one-half times as dense as lead and nearly twenty times as dense as water, but it is also extremely malleable and much softer than steel (Cirincione, 2007). Uranium occurs naturally on earth in three specific radioisotopes in the following proportions: U-238 (99.283%), U-235 (0.711%), and U-234 (0.005%). It melts at 1132.50 C. And boils at 37450 C. Its electronic configuration is [Rn]5f36d17s2 (Rennie, 2003).

Uranium -- Use in Everyday Life

The most well-known and notorious use of Uranium is in its capacity as the fissionable core of a nuclear weapon (Cirincione, 2007). However, it is only the extremely rare U-235 isotope that is fissile, meaning that it has the capacity to undergo the self-sustaining release of massive amounts of energy first demonstrated by Enricho Fermi in 1939 at the University of Chicago (Cirincione, 2007). This use of uranium is limited to highly-enriched uranium that has undergone a complex process of isolating the fissile U-235 isotope from the non-fissile U-238 isotope. In principle, the use of U-235 as the core of a nuclear weapon utilizes the property of fission, or the splitting of a single atomic nucleus, releasing two neutrons in the process. More importantly, the tremendous amount of energy released by nuclear fission is attributable, less to the exponential increase in the number of neutrons released as every split atomic nuclei then triggers the splitting of two more nuclei, but more to the fact that, in the process, a small amount of the original mass of each atom is converted into energy according to Einstein's infamous equation E = mc2 (Cirincione, 2007; Liverhant, 1960). Contrary to popular belief, the deadliness of an atomic bomb is in this energy released as heat and in the shock wave and not in the amount of radiation released, which is not particularly significant (Cirincione, 2007; Feynman, 1997).

Uranium is also used as a weapon in an entirely different form that has nothing to do with its radioactive or fissionable properties, but only on its tremendous relative density. Specifically, in the process of uranium enrichment for the production of fissile uranium for nuclear weapons and low-enriched uranium for nuclear reactor cores, one of the resulting byproducts is uranium with even less U-235 content than the 0.711% that occurs in natural uranium (Rennie, 2003). This material is less radioactive than the U-238 form of uranium found in nature but still retains the other properties of the naturally occurring isotope, in particular, its malleability and its very high density and weight (Cirincione, 2007). This byproduct is ideal for use as projectiles (i.e. bullets) and has been used by the U.S. military in the ammunition fired by combat aircraft in particular (Cirincione, 2007).

Another well-known use of uranium is as a source of energy relied upon within nuclear reactors. Unlike the use of uranium in weapons, nuclear reactors produce only thermal energy from the gradual decay of concentrated but sub-critical masses of approximately 97% U-238 and only 3% U-235 (Cirincione, 2007). Nuclear reactors use a nuclear core composed of minimally-enriched uranium although reactors using light-water as a moderator instead of ordinary water can use un-enriched U-238 (Cirincione, 2007; Liverhant, 1960). In principle, nuclear reactors use their nuclear core to produce heat and steam that powers turbines to produce electric energy; they control the rate of fission carefully by increasing or decreasing the mass of their nuclear cores and by controlling their immersion in the moderator (Cirincione, 2007; Liverhant, 1960).

Other common uses of uranium include as components of home smoke detectors and of nuclear imaging equipment and radioactive dyes in medicine, and for the purpose of irradiating food to increase its safety (Cirincione, 2007).

Ten Uranium Compounds and the Uses of Four Uranium Compounds

Uranium Oxides -- U3O8 and UO2

Both of these forms of uranium oxides occur in nature. They are relatively stable compounds that exist as solids with low water solubility. Most of the nuclear cores within reactors are composed of Uranium dioxide (UO2). It readily converts into triuranium octaoxide or uranium oxide (U3O8) at room temperatures (USDOE, n.d.). Uranium oxide is extracted from crushing natural uranium ore and accounts for approximately 80% of uranium yellowcake (Cirincione, 2007).

Uranium Fluorides -- (UF6), (UF5), and (UO2F2)

The most commonly used form of uranium used in the uranium enrichment process is uranium hexafluoride (UF6). It exists as a solid, liquid, or gas within a relatively narrow range of temperatures. As a solid UF6 is similar in consistency to rock salt. While it does not react with oxygen, nitrogen, carbon dioxide, or dry air, it is reactive to moisture in any form (i.e. liquid water or water vapor), forming corrosive hydrogen fluoride (HF) and uranyl fluoride (UO2F2). Because of its instability in the presence of moisture, it must be handled and stored very carefully (USDOE, n.d.).