Uranium and nuclear power

Summary: Uranium (U) is a metallic element atomic number 92, that occurs in six main isotopes. U-238 is by far the most common in nature, followed by U-235, which is the key to generating the intense heat used to power nuclear reactors and to fuel nuclear bomb explosions.

The only naturally occurring fissile (capable of sustaining a chain reaction) isotope of any element is U-235. In practice, uranium ore must be enriched to increase the concentration of U-235 to at least 3 percent for power plants, and about 85 percent for weapons grade material.

Occurrence and Mining

Uranium occurs naturally, in low concentrations of a few parts per million, in rocks, soil, and seawater. Uranium ore is so defined by the estimated cost of mining of a given deposit, calculated as tons recoverable up to a certain cost. Global production was more than 49,000 tonnes U in 2022; major producers included Australia, Canada, Kazakhstan, Namibia, Niger, and Russia. Kazakhstan produced 43 percent of the world supply in 2022, while Canada and Namibia followed at 15 percent and 11 percent, respectively.

The full range of mining techniques is employed, including open-pit, underground, and borehole. Ore is treated to dissolve out the uranium content, using either an acid or alkali agent. Yellowcake (U3O8) is the vernacular name of the mining and milling end product; it contains 75 percent uranium or higher.

By 2022, about 56 percent of uranium produced was acquired using a method called in situ leach (ISL) mining or in situ recovery (ISR) mining, while 38 percent was produced using underground and open pit mining. ISL mining involves leaving the ore in the ground, and dissolving them using a solution that is then pumped to the surface, where the minerals are separated out. ISL is seen as a more economical and less environmentally damaging method. The majority of uranium mined in the United States, Kazakhstan, and Uzbekistan is produced using ISL. The method is also used in Australia, China, and Russia.

Purchases of uranium are sanctioned by the Nuclear Non-Proliferation Treaty; those nations that have signed it must allow international inspection teams to verify that its use is strictly intended for peaceful (e.g., power generation, medical diagnostics, scientific research) purposes. Countries purchasing uranium from Australia or Canada must sign and meet additional bilateral safeguards.

History

The use of natural uranium oxide to impart color to ceramic glazes dates to 79 CE; artifacts showing this application were first found in Italy in 1912. The scientific discovery of it in the mineral pitchblende is ascribed to the German chemist Martin Heinrich Klaproth in 1789. He named the new element after the planet Uranus, which had been discovered eight years earlier. In 1841, Eugène-Melchior Péligot isolated uranium as a metal. His method involved application of heating to uranium tetrachloride in the presence of potassium.

Its radioactive properties were reported in 1896 by French physicistHenri Becquerel. He determined that invisible rays had fogged a photographic plate with a sample of the uranium salt K2UO2(SO4)2 on top of it. In the nineteenth century, when uranium was not yet understood to be dangerous, it was commonly used for tinting in early photography. Uranium salts are mordants of silk or wool.

Research led by Enrico Fermi, starting in 1934, involved bombarding uranium with neutrons; this was observed to produce the emission of beta particles and resulted later, after the process was explained by Otto Hahn (1938) and Lise Meitner (1939), in its use as a fuel in the nuclear power industry and in nuclear weapons. In 1942, as part of the Manhattan Project, another team led by Fermi initiated the first artificial self-sustained nuclear chain reaction, Chicago-Pile 1. They ultimately built Little Boy, the first nuclear bomb, detonated over Hiroshima, Japan, in 1945. Exploding with a yield equivalent to 12,500 tons of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people outright.

The arms race during the cold war between the United States and the Soviet Union produced tens of thousands of nuclear weapons. After the Cold War, from 1993 to 2005, the U.S. Material Protection, Control, and Accounting Program spent approximately $550 million to help safeguard uranium and plutonium stockpiles in Russia and some former Soviet republics.

The first commercial nuclear power station, Obninsk in the Soviet Union, began operating in 1954. The same year, the USS Nautilus was the first submarine to use nuclear power for propulsion.

Chemistry

Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.5 billion years. U-235 decays with a half-life of about 700 million years, and uranium-234 has a half-life of about 245,000 years. U-238 forms the beginning of a long decay series of elements, passing through a number of other uranium isotopes (some referred to as thorium), as well as radium and radon, continuing until a stable, nonradioactive decay product is formed—in this case lead-206. The constant, precisely known rates of decay of these isotopes make them useful in radiometric dating of soils, organic matter, and artifacts.

The fissile nature of Uranium-235 means that when hit by a relatively slow-moving neutron, the nucleus of U-235 will split into two smaller parts, releasing great amounts of nuclear binding energy as heat and radiation, as well as two or three additional neutrons. If these freed neutrons in turn hit other U-235 nuclei, the process repeats itself in an uncontrolled chain reaction. Fuel elements in a nuclear reactor are surrounded by a moderator, such as water or graphite, to slow the speed of the emitted neutrons and thus enable the chain reaction to continue at a controlled rate.

Uranium-238 is fertile. It can capture a flying neutron and become plutonium-239 (Pu-239), which is also fissile. Pu-239 was used in the atomic bomb detonated in the Trinity test in 1945 in New Mexico. Spent plutonium can be recycled as fuel.

Enrichment

To increase the concentration of one isotope relative to another, they must become separated. Therefore, solid uranium oxide is converted into gaseous uranium hexafluoride (UF6). This is further processed in gas centrifuges or through gaseous diffusion into a lighter (U-235) and a heavier (U-238) fraction. This process is called enrichment.

For use in commercial nuclear power plants as fuel, the isotope U-235 needs to be enriched from its natural level of 0.7 percent to around 3 to 5 percent. Then, the enriched UF6 is converted to UO2, which is formed into fuel pellets. They are placed inside thin metal tubes, which are assembled in bundles to become the fuel elements for the core of the reactor. Each pellet, about the size of a fingertip, contains the energy of approximately 150 gallons of oil.

Nuclear weapons require a degree of enrichment of U-235 of at least 85 percent. From as little as 15 pounds (7 kilograms) of U-235, an atomic bomb can be built.

Civilian and Military Uses

The main use of uranium in the civilian sector is to fuel nuclear power plants. Assuming complete fission, 1 kilogram of uranium-235 produces about 80 terajoules of energy (8 × 10¹³ joules), as much energy as 3,000 tons of coal. Today, some nuclear fuel comes from nuclear weapons being dismantled and reprocessed. The heat created by splitting U-235 atoms is used to make steam, which spins a turbine, driving a generator and producing electricity. Afterward, the steam is cooled back into water in a separate cooling tower, to be reused. (The heat produced by nuclear reactors can also be used directly to heat buildings or to provide heat for water desalination rather than for generating electricity.) In 2008, of the 20.3 terawatt-hours of electricity produced worldwide, approximately 13 percent, or about 2.6 terawatt-hours, was generated from uranium in more than 430 nuclear reactors. The United States, France, and Japan together comprised about half of this total. One year after the disaster at Fukushima, Japan had shut down all 52 of its commercial nuclear power reactors, and several other countries had announced plans to phase out their nuclear power generation, Germany being the largest of these. The proportion of global electricity generation from nuclear thus dropped to below 10 percent in 2012.

Depleted uranium is, because of its very high density of 19.1 grams per cubic meter, used in helicopters and airplanes as counterweights on certain wing parts, and in the keels of yachts as ballast, as well as for radiation shielding in medical radiation therapy and containers used to transport radioactive materials. Uranium is also used in gyroscopic compasses. Smaller nuclear reactors power ships ranging from icebreakers to aircraft carriers. These can stay at sea for long periods without having to make refueling stops.

Other military uses include defensive armor plating as well as weapons tipped with depleted uranium to boost density and enhance armor-piercing ability. Enriched uranium is also used in nuclear weapon warheads.

Health and the Environment

Uranium or released alpha particles do not commonly possess enough energy to penetrate the skin. However, they are extremely toxic when ingested, with no known upper limit on cumulative cell damage they can cause. Soluble uranium salts, relatively low in radioactivity, are toxic upon inhalation or swallowing, but not demonstrably more so than equivalent amounts of lead, mercury, or other heavy metals. The greatest health risk is damage to the kidneys, because 99 percent of absorbed uranium salts are excreted in the urine within a few days. The brain, liver, heart, DNA, gastrointestinal, immune, and other systems can be affected. Uranium can cause birth defects. Exposure to its radioactivity increases the risk of getting cancer. The World Health Organization has established a daily “tolerated intake,” or nonhazardous exposure, of soluble uranium salts for the general public of 0.5 microgram (µg) per kilogram of body weight.

Radioactive wastes such as uranium mill tailings and spent reactor fuel can remain radioactive and dangerous to human health for thousands of years. The U.S. Environmental Protection Agency (EPA) has issued special criteria for cleaning up contaminated sites under the Uranium Mill Tailings Radiation Control Act. Additional EPA standards under the Clean Air Act limit uranium in the air, and the Safe Drinking Water Act covers water supplies, limiting uranium to 30 micrograms per liter.

Radioactive wastes are classified in three levels. Reactor operators store their spent fuel in spent fuel pools or in dry storage concrete or steel containers with air cooling. There is currently no permanent disposal facility in the United States for high-level nuclear waste. High-level waste is being stored on site at nuclear plants, monitored by the Nuclear Regulatory Commission. The uranium oxides U3O8 and UO2 are both solids having low solubility in water. Due to their stability, they are the preferred chemical form for storage or disposal.

Unlike fossil–fuel-fired power plants, the operation of nuclear reactors does not produce direct carbon dioxide emissions. However, the processes for mining and refining uranium ore, producing reactor fuel, and related activities require large amounts of energy, contributing to emissions.

Some bacteria, such as Shewanella putrefaciens and Geobacter metallireducens, can reduce uranium(VI) to uranium(IV) and change uranium from a soluble to insoluble form, respectively. Organisms such as the lichen Trapelia involuta or the bacterium Citrobacter can absorb concentrations of uranium that are up to 300 times higher than in their environment. Research on these organisms suggests they could be used in bioremediation to decontaminate uranium-polluted water.

Bibliography

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International Atomic Energy Agency. “Uranium Mine Productions to Meet Growths Needs.” Staff Report 2010. www.iaea.org/newscenter/news/2010/uraniummine.html.

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