Uranium deposits

Uranium is a radioactive element that naturally occurs on Earth. The element is used in the manufacture of weapons and as a fuel source for power plants. Uranium has proved to be a valuable economic and political resource, and its extraction and use have become a pressing international concern.

Chemical and Physical Properties

Uranium is an element containing ninety-two protons in the nucleus. It is the heaviest element found in any significant quantity on Earth. (Larger elements, such as plutonium, are found only in small quantities. Most of the heavier elements exist only when created in laboratories.) Uranium’s uses and properties can vary based on the number of neutrons present.

Three main isotopes of uranium are found in nature. More than 99 percent of naturally occurring uranium is uranium-238. All isotopes of uranium are radioactive.

Uranium-238, which contains 146 neutrons, takes the longest of the isotopes to decay, with a half-life of roughly 4.5 billion years. In a lengthy chain of decay, uranium-234 is one of the eventual by-products, with a half-life of 240,000 years. Uranium-238 cannot sustain the chain reactions used by nuclear fission to generate energy. With the addition of a neutron to the nucleus, however, which is done in a nuclear reactor, uranium-238 can be converted to plutonium-239, which can be used for nuclear fission. Uranium-235 is the only isotope of uranium that can sustain a chain reaction necessary for nuclear fission. As a general rule, for isotopes to be usable for nuclear fission, they must have an odd number of neutrons in the nucleus.

Radioactive nuclei can decay in different ways. In alpha decay, which is what occurs with all three isotopes of uranium, an alpha particle is emitted from the nucleus. When the nucleus has an odd number of neutrons, a significant amount of energy is released in the form of a gamma ray. This energy allows the breakdown of the nucleus to continue further, particularly when a large number of odd-neutron–numbered nuclei are present. Enrichment of uranium, which involves increasing the ratio of uranium-235 to uranium-238 in a sample, is the focus of the primary applications for the element.

A History of Uranium

Uranium was officially discovered as an element in the late eighteenth century, but some of its features have been known for much longer. For example, the coloring effects of uranium have been found in the stained glass of the ancient Romans.

Uranium was found largely by accident. Originally, the shiny black rocks of uranium ore were considered the waste product of a silver mining operation in the town Joachimsthal, in modern-day Czech Republic. The material was called pitchblende; it is now known as uranium ore. The silver mining operation also led to illness among the workers, illness that was not immediately diagnosed by doctors of the time.

The discovery of uranium itself is credited to German chemist and pharmacist Martin Klaproth, who, in 1789, isolated the metal from uranium ore mined in Joachimsthal that had been sent to him. Klaproth named uranium for the planet Uranus, which was the most recently discovered planet at the time. Uranium originally was intended to be a temporary name.

Even after the discovery of uranium, its most important properties remained unknown. In the nineteenth century it was used in artwork. This began to change in the twentieth century, soon after French physicist Antoine Henri Becquerel discovered radiation with a sample of the uranium. At the time, theories of emission tied radiation to light from the sun. In 1896, Becquerel left a sample of the uranium compound on a sheet of photograph paper on a cloudy day. The image generated from the compound indicated that the radiation came from the uranium rather than the sun.

The discovery of radiation unleashed a number of scientific discoveries, including the work of Pierre and Marie Curie, who discovered the radioactive elements polonium and radium from those same pitchblende rocks from Joachimsthal. In 1903, Becquerel and the Curies were awarded the Nobel Prize in Physics for the discovery of radiation. (Marie Curie received the Nobel Prize in Chemistry in 1911 for the discovery of the two elements.)

Discoveries followed that would cast further light on the radioactive nature of uranium. Working together at McGill University in Montreal, Canada, Ernest Rutherford and Frederick Soddy discovered the alpha particle, the key component in the radioactive decay of uranium. Rutherford received the Nobel Prize in Chemistry in 1908, while Soddy received the award in 1921.

The research that ultimately brought uranium to the forefront of the public consciousness was the U.S. government’s Manhattan Project, directed by physicist J. Robert Oppenheimer. The project involved the development, construction, and detonation of the world’s first atomic bombs, near the end of World War II.

On July 16, 1945, the United States tested the first atomic bomb, in New Mexico. On August 6, an atomic bomb was dropped on the Japanese city of Hiroshima. The immediate death toll was estimated at seventy thousand people, with that number doubling in the next year as a result of radiation exposure. A second bomb was dropped on Nagasaki, Japan, on August 9. Fighting stopped the next day, and Japan surrendered on August 14, ending World War II. Within the next few years, both the United Kingdom and the Soviet Union successfully tested nuclear bombs of their own, changing the landscape of geopolitics for the nuclear age.

Civilian uses for nuclear power soon followed. In 1951, a working nuclear reactor was built in southeastern Idaho. By the end of the decade, full-scale nuclear power plants had been built. In the years to come, people would get a glimpse of the strong potential for nuclear power, and its destructive capabilities. Many of the issues that would arise continue into the twenty-first century and will shape the future of nuclear power use.

In 1963, discussions began regarding guidelines for nuclear weapon testing and usage. The framework was thus begun for the Treaty on the Non-Proliferation of Nuclear Weapons (1970). The treaty called for inspections to prevent development of further nuclear weapons beyond the five countries that had built the bomb (the United States, the Soviet Union [now Russia], the United Kingdom, France, and China). Many other nations signed the treaty; the only major notable holdouts were Israel, India, North Korea, and Pakistan.

In part as a result of this framework, nations began reducing their stockpiles of weapons-grade uranium, both for reasons of utility and for reasons of security. Outside the scope of the treaty, nations have gained access to uranium through existing stockpiles.

Mining Uranium

Sources of uranium have been found throughout the world. Uranium has been mined in the American Southwest. The three nations mining the most uranium are Kazakhstan, Canada, and Australia. In World War II, much of the uranium used for the Manhattan Project came from Shinkolobwe, in the Belgian Congo (now the Democratic Republic of the Congo).

Mined uranium is milled to form yellowcake uranium (U₃O₈), which is the form in which it is shipped. Yellowcake uranium is then enriched for use in power plants or weapons. In uranium enrichment, the percentage of uranium-235 in the sample is increased. Low-enriched uranium (LEU) involves uranium composed of 3 to 5 percent uranium-235, which is useful as a fuel for power plants. Highly enriched uranium (HEU), which is enriched to be 90 percent uranium-235, is used for nuclear weapons.

Enriching uranium involves the use of fluorine gas, which binds to the uranium to form uranium hexafluoride (UF₆). The uranium-235 is then separated using diffusion of a gas or, more recently, using centrifuges. Separating by weight produces uranium oxide (UO₂), which contains a greater proportion of uranium-235. The uranium from which it was taken, now called depleted uranium, is disposed of, while the uranium oxide becomes fuel.

Uranium’s Uses

Uranium has a number of uses. Historically, it was used in artwork for its shine and coloring. After discovery of its atomic properties, it became a fuel source for atomic weapons and nuclear power plants.

HEU also can be used to produce technetium-99. Fission of uranium-235 is used with molybdenum-98 to produce molybdenum-99, which decays to technetium-99. This isotope is used for diagnostic medical imaging. Because of the desire to reduce available stocks of HEU, ongoing efforts have focused on ways to produce technetium-99 from LEU. The first such stocks were produced at the end of 2010, but efforts are ongoing to ensure the transition without a supply shortage.

While other applications involve the processed form of the element, uranium ore (because of its radioactivity) is useful for radiometric dating, a method geologists use to determine age. In time, uranium-238 decays into lead-206. Knowing the half-life of uranium allows scientists to calculate the age of the sample based on the ratio of presence of these two elements. (Because lead has multiple isotopes—uranium-235 becomes lead-207, for example—the presence of other lead in the sample will not necessarily interfere with this calculation.)

Health Risks of Uranium

Uranium ore is relatively safe. The alpha particles released by the decay of uranium-238 can be blocked by a sheet of paper and do not penetrate the skin. While uranium in its natural form may have fewer health risks from radiation, it still can cause significant problems if dust that contains uranium particles is inhaled into the lungs. High rates of lung cancers have been found among people who work in uranium mines; as a result some uranium mining operations have been shut down.

A much greater health risk arises when uranium is enriched. The gamma radiation from uranium-235 can penetrate much deeper, including through human tissue. This can lead to radiation poisoning (which led to the death of Marie Curie in 1934). The energy from an atomic blast will cause death instantly.

Politics of Uranium

Because of its chemical properties and its use as an engine for both power and warfare, uranium has been a source of controversy since the early twentieth century. Mining operations present a health risk, and the most publicized early use of uranium was as history’s most deadly weapon. Furthermore, power plants designed to enhance civilian life hold the potential for large-scale destruction.

While scientists created the atomic bomb, the term “atomic bomb” was not created by a researcher on the Manhattan Project but much earlier by science fiction writer H. G. Wells, who, having read a scientific paper about the potential of uranium, created the term in the book The World Set Free (1914), a novella about an element with the potential to destroy the world. In the story, Wells foreshadowed uranium’s destructive potential more than thirty years before the first atomic bomb was detonated.

In addition to the fears raised by the possibility of the world’s destruction from atomic bombs, particularly during the Cold War, concerns have been raised after accidents at nuclear power plants around the world. A 1979 accident at Three Mile Island in Pennsylvania led to a loss of coolant. The heat of the energy from the plant began to cause the metal containing the uranium to melt. While it was brought under control and no deaths were caused by the accident, the meltdown led to public fear. A 1986 accident at the Chernobyl nuclear plant in Ukraine led to two deaths from the immediate explosion and another twenty-eight deaths from radiation poisoning in the weeks that followed. Others have claimed the disaster has had long-term health effects that continue into the twenty-first century.

In the aftermath of a devastating earthquake and tsunami that impacted the eastern coast of Japan in March 2011, the Fukushima Daiichi nuclear plant suffered a nuclear emergency when a 50-foot (15-meter) wall of water slammed into the plant. The tsunami knocked out the power supply and cooling systems to the plant’s four reactors. Three of the reactors underwent nuclear meltdown, releasing significant amounts of radiation into the environment. Although no one was killed as a result of the radiation, more than 100,000 people were evacuated from their homes. More than a decade later, much of the area remained off limits and many evacuees were still displaced.

Technically, nuclear plants present less of an environmental hazard than, for example, coal, which emits tons of pollutants into the atmosphere. However, while the risks from nuclear power plants remain relatively small from a quantitative perspective, the severity of an adverse outcome, in the minds of many, leads those risks to outweigh the potential benefits. Despite its potential, civilian uses for uranium face an uncertain future.

Principal Terms

alpha particle: essentially a helium nucleus, a particle consisting of two protons and two neutrons, emitted by radioactive atoms during alpha decay

beta particle: an electron emitted from the nucleus of an atom during the process of beta decay

gamma ray: a high-energy photon, with no mass or charge, emitted when atomic nuclei are broken down

half-life: the amount of time it takes for half of a sample of a radioactive element to decay

highly enriched uranium: uranium that has been processed to alter its composition of uranium isotopes, making it more useful as fuel or for nuclear weapons

ionizing radiation: radiation that creates ions by breaking down atomic nuclei

isotopes: atoms of the same element with differing numbers of neutrons in their nuclei

nuclear fission: a reaction in which atomic nuclei are broken down; generates a large amount of energy

plutonium: a radioactive element with ninety-four protons in its nucleus; can be used as an energy source in the same way as uranium

radiation: unstable isotopes of an element decay in various ways; the emission of particles and energy from nuclei is known as radiation

uranium: an element with ninety-two protons in the nucleus and with only radioactive isotopes

uranium ore: uranium as it is typically found in nature, as shiny black rocks that may be found combined with a variety of other elements

yellowcake: uranium bonded with oxygen that has been milled from the uranium ore found in nature; has a yellow, powdery appearance

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