For many years, uranium was primarily used as a colorant in ceramic glazes, yielding hues ranging from orange-red to lemon yellow. It was also used in early photography for tinting. Its radioactive properties were not discovered until 1896 and its potential as an energy source was not realized until the mid-twentieth century. Its primary application is as a fuel in nuclear power reactors, which generate electricity. It is also used in weapons and small nuclear reactors around the world to produce isotopes for medical and industrial purposes. In this article, we will learn about the properties, sources and isotopes of uranium element.
Sources of uranium
Small amounts of uranium can be found in almost any soil, rock or even water. Concentrated deposits of uranium ores, on the other hand, are only found in a few places, usually in hard rock or sandstone. Normally, these deposits are covered with earth and vegetation. Uranium has been mined in Canada, the United States’ southwest, Australia, parts of Europe, the former Soviet Union, Namibia, South Africa, Niger and other places.
What is uranium?
Uranium is a metallic, silver-gray element in the actinide series. It serves as the primary fuel for nuclear reactors, but it is also used in the production of nuclear weapons. Symbol of uranium is U and the atomic number of uranium is 92, which means it contains 92 protons and 92 electrons. The nucleus of U-238 contains 146 neutrons, but the number of neutrons can range from 141 to 146. Because uranium is radioactive, it emits particles and changes into other elements all the time. The series of radioactive decay of uranium is well established. The most common isotopes of uranium are U-238 and U-235 (which has 143 neutrons). Without our intervention, uranium naturally contains all three isotopes (U-238, U-235, and U-234), and it rarely varies more than 0.01 percent from the average composition.
What is depleted uranium?
Depleted uranium is a dense metal produced as a byproduct of natural uranium enrichment for nuclear fuel. It is still radioactive, but much less so than the starting material. It is used to increase the penetrating power of armour-piercing shells and bombs. Such weapons were used in both Gulf Wars, as well as in Serbia and Kosovo. Their use has raised concerns about the health risks associated with uranium exposure.
Properties of uranium
Uranium is a radioactive element found in varying but minute amounts in soil, rocks, water, plants, animals and all humans. The majority of the weight of a typical natural uranium sample consists of uranium-238 atoms. Atoms of uranium-235 account for approximately 0.72 percent of the weight, with uranium-234 accounting for a very small amount (0.0055 percent by weight). Pure uranium is a silvery-white, radioactive metal that is harder than most other elements. It is malleable and ductile, slightly paramagnetic, highly electropositive and a poor electrical conductor. Uranium metal has a very high density, about 70% denser than lead but slightly less dense than gold. Uranium metal has three crystallographic variations: alpha (688°C) > beta (776°C) > gamma.
Uranium is slightly softer than steel and, in a finely divided state, is attacked by cold water. Acids dissolve the metal, forming the +3 oxidation state, which is quickly oxidized by water and air to form higher oxidation states. Alkalis have no effect on uranium metal. Uranium metal reacts with almost all nonmetallic elements and their compounds, with temperature increasing reactivity. In the presence of oxygen, uranium metal oxidizes and becomes coated with a dark layer of uranium oxide. Uranium can be found in a wide range of alloys and compounds, with the most common oxidation states being uranium (IV) and uranium (VI), and their two corresponding oxides being uranium dioxide, UO2, and uranium trioxide, UO3.
Fluorides, chlorides, bromides, iodides, carbonates, hydrides, carbides, nitrides, phosphates are all uranium compounds. Uranium is radioactive because all naturally occurring isotopes of uranium are unstable, with half-lives ranging from 159,200 to 4.5 billion years. There are 27 known isotopes of uranium, with atomic weights ranging from 217–219, 222–240, and 242, and half-lives ranging from billions of years to a few nanoseconds. There are three major isotopes of naturally occurring uranium. All three isotopes are radioactive, with a low probability of spontaneous fission but a preference for alpha emission decay.
Isotopes of uranium
Natural uranium is primarily composed of uranium-238, with 0.7 percent uranium-235 and a trace of isotope 234. Reactors from uranium-235 and thorium also produce three isotopes, uranium 236, 233, and 232. Thus, there are three naturally occurring isotopes of uranium and three artificial isotopes of uranium.
Natural occurring isotopes of uranium
- Uranium 238: It accounts for 99.3 percent of natural uranium, has the longest lifetime: 4.5 billion years, or roughly the age of the Earth. It is not particularly radioactive. It’s extremely long period indicates that it is still present in the Earth’s crust. This nucleus’ capture of neutrons results in the formation of fissile plutonium-239 in a reactor. U-238, which is not fissile, contributes to the operation of reactors and the generation of electricity. This impressive potential of fission energy is largely untapped. The goal of fourth-generation breeder reactors is to recapture this incredible potential.
- Uranium 235: The only fissile nucleus found in natural uranium, is used as a nuclear fuel in reactors and as a nuclear weapon explosive. This extremely rare isotope, which occurs at a concentration of 0.7 percent in natural uranium, is thus a highly strategic and sought-after material. It has extremely long period, 700 million years, which is 6.5 times shorter than that of isotope U-238. U-235 was 85 times more abundant during Earth’s formation. The 0.7 percent observed today is a pale relic of this previous abundance. Humans would not have needed to enrich uranium to make atomic bombs or run their reactors if they had been present at the beginning of Earth!
- Uranium 234: It is the uranium-238’s first long-lived descendant. These nuclei are present in a natural uranium sample in the unalterable proportions of the radioactive equilibrium of the uranium-238 filiation at a ratio of one atom of uranium-234 for 18 800 atoms of uranium-238, so that the two isotopes contribute equally to the radiations emitted by uranium.
Artificial isotopes of uranium
- Uranium 236: It is formed in nuclear fuel from uranium 235 by neutron capture that does not result in fission. The presence of this isotope in a uranium sample indicates that the sample was processed in a reactor.
- Uranium 233: It is a fissile nucleus that does not exist naturally, unlike plutonium 239, which it is similar due to its manufacturing method. It is produced by neutron capture in thorium reactors. This nucleus, under fission reaction, by both fast and slow neutrons, has some intriguing properties for energy production. Thorium and uranium-233 reactors are one of the options being considered for future fourth-generation reactors.
- Uranium 232: It is a byproduct of reactors that use thorium and uranium 233 fuel. This isotope is formed as a result of specific neutron capture by uranium 233, which results in the ejection of two neutrons. Uranium 232 has a relatively short period of 68.9 years, but its radioactive filiation produces a descendant, thallium 208, which emits gamma rays of 2.6 MeV energy, which are highly energetic and penetrating. Because of these intense radiations, handling fissile uranium-233 contaminated with uranium 232 is far riskier than handling conventional uranium 235 or plutonium 239 fuels. They act as a barrier to the spread of bombs made of this fissile uranium.
Radioactive isotopes of uranium
From the above mentioned isotopes, each of the three naturally occurring isotopes of uranium is radioactive, which means the nuclei decay spontaneously. The radioactivity emitted by uranium isotopes consists of alpha particles and gamma rays. The rate at which the nuclei in an isotope sample decays, is known as activity, which is defined as the number of disintegrations per second. As the atoms disintegrate, the activity of an isotope sample decreases over time. Each isotope has its own half-life, which is the time it takes for half of the atoms in an isotope sample to decay and the sample’s activity to be proportionately reduced.
In addition to being naturally radioactive, the uranium-235 isotope is capable of fission, which is the splitting of the nucleus into two parts caused by neutron absorption. When uranium-235 splits, a significant amount of energy is released, making it valuable as a fuel in nuclear reactors used to generate electricity and for use.