Uranium-235 in the context of "Fast neutron"

Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

Nuclear fission was discovered by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Hahn and Strassmann proved that a fission reaction had taken place on 19 December 1938, and Meitner and her nephew Frisch explained it theoretically in January 1939. Frisch named the process "fission" by analogy with biological fission of living cells. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction.

Uranium is a chemical element; it has symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium radioactively decays, usually by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 (which has 146 neutrons and accounts for over 99% of uranium on Earth) and uranium-235 (which has 143 neutrons). Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.

Many contemporary uses of uranium exploit its unique nuclear properties. Uranium is used in nuclear power plants and nuclear weapons because it is the only naturally occurring element with a fissile isotope – uranium-235 – present in non-trace amounts. However, because of the low abundance of uranium-235 in natural uranium (which is overwhelmingly uranium-238), uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is studied for future industrial use in nuclear technology. Uranium-238 has a small probability for spontaneous fission or even induced fission with fast neutrons; uranium-235, and to a lesser degree uranium-233, have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction. This generates the heat in nuclear power reactors and produces the fissile material for nuclear weapons. The primary civilian use for uranium harnesses the heat energy to produce electricity. Depleted uranium (U) is used in kinetic energy penetrators and armor plating.

A nuclear reactor is a device used to sustain a controlled fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei (primarily uranium-235 or plutonium-239) absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further fission. Reactors stabilize this, regulating neutron absorbers and moderators in the core. Fuel efficiency is exceptionally high; low-enriched uranium is 120,000 times more energy-dense than coal.

Heat from nuclear fission is passed to a working fluid coolant. In commercial reactors, this drives turbines and electrical generator shafts. Some reactors are used for district heating, and isotope production for medical and industrial use.

Whether nuclear power should be considered a form of renewable energy is an ongoing subject of debate. Statutory definitions of renewable energy usually exclude many present nuclear energy technologies, with the notable exception of the U.S. state of Utah. Dictionary-sourced definitions of renewable energy technologies often omit or explicitly exclude mention of nuclear energy sources, with an exception made for the natural nuclear decay heat generated within the Earth.

The most common fuel used in conventional nuclear fission power stations, uranium-235 is "non-renewable" according to the United States' Energy Information Administration, the organization, however, is silent on the recycled MOX fuel. The National Renewable Energy Laboratory does not mention nuclear power in its "energy basics" definition.

Uranium-238 (
U or U-238) is the most common isotope of uranium found in nature, with a relative abundance above 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

The isotope has a half-life of 4.463 billion years (1.408×10 s). Due to its abundance and half-life relative rate of decay to other radioactive elements, U is responsible for about 40% of the radioactive heat produced within the Earth. The U decay chain contributes six electron anti-neutrinos per U nucleus (one per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of U to daughter isotopes is extensively used in radiometric dating, particularly for material older than approximately 1 million years.

Little Boy was a type of atomic bomb created by the Manhattan Project during World War II. The name is also often used to describe the specific bomb (L-11) used in the bombing of the Japanese city of Hiroshima by the Boeing B-29 Superfortress Enola Gay on 6 August 1945, making it the first nuclear weapon used in warfare, and the second nuclear explosion in history, after the Trinity nuclear test. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and had an explosion radius of approximately 1.3 kilometres (0.81 mi) which caused widespread death across the city. It was a gun-type fission weapon which used uranium that had been enriched in the isotope uranium-235 to power its explosive reaction.

Little Boy was developed by Lieutenant Commander Francis Birch's group at the Los Alamos Laboratory. It was the successor to a plutonium-fueled gun-type fission design, Thin Man, which was abandoned in 1944 after technical difficulties were discovered. Little Boy used a charge of cordite to fire a hollow cylinder (the "bullet") of highly enriched uranium through an artillery gun barrel into a solid cylinder (the "target") of the same material. The design was highly inefficient: the weapon used on Hiroshima contained 64 kilograms (141 lb) of uranium, but less than a kilogram underwent nuclear fission. Unlike the implosion design developed for the Trinity test and the Fat Man bomb design that was used against Nagasaki, which required sophisticated coordination of shaped explosive charges, the simpler but inefficient gun-type design was considered almost certain to work, and was never tested prior to its use at Hiroshima.

Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (U with 99.2732–99.2752% natural abundance), uranium-235 (U, 0.7198–0.7210%), and uranium-234 (U, 0.0049–0.0059%). U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons.

Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons. Low-enriched uranium (below 20% U) is necessary to operate light water reactors, which make up almost 90% of nuclear electricity generation. Highly enriched uranium (above 20% U) is used for the cores of many nuclear weapons, as well as compact reactors for naval propulsion and research, as well as breeder reactors. There are about 2,000 tonnes of highly enriched uranium in the world.

High-level waste (HLW) is a type of nuclear waste created by the irradiation of nuclear fuel in a reactor. Irradiation causes a build-up of fission products and transuranic elements (generated by capture of neutrons) in the fuel. Fission products typically have a much shorter half-life than uranium, which means the irradiated fuel is more radioactive and thus hotter than fresh fuel – high-level waste has heat output of >2 kW/m. At the same time, the fissile material (usually uranium-235) is used up, so that the fuel is no longer able to sustain the operation of the reactor and must be recycled or disposed of as waste.

High-level waste includes spent nuclear fuel itself as well as the byproducts of nuclear reprocessing, which results in liquid raffinates and other waste streams. Liquid wastes are not suitable for disposal, so these are vitrified to convert them into a solid, glass form which is suitable for disposal.

In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g., uranium-235, U). A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.