Nuclear reactor in the context of "Primary containment"

The neutron is a subatomic particle, symbol n or n
, that has no electric charge, and a mass slightly greater than that of a proton. The neutron was discovered by James Chadwick in 1932, leading to the discovery of nuclear fission in 1938, the first self-sustaining nuclear reactor (Chicago Pile-1, 1942), and the first nuclear weapon (Trinity, 1945).

Neutrons are found, together with a similar number of protons in the nuclei of atoms. Atoms of a chemical element that differ only in neutron number are called isotopes. Free neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes. Neutron stars, formed from massive collapsing stars, consist of neutrons at the density of atomic nuclei but a total mass more than the Sun.

Arabelle Solutions, formerly GE Alstom Nuclear Systems, or GEAST, for ‘GE Alstom’, most of which was spun off from GE Steam Power, is a French multinational specialising in nuclear activities related to steam turbines (Arabelle) for the turbine islands. It is present in nearly 16 countries including China, Finland, India, Romania and the United Kingdom, and headquartered in Nanterre, France. At Belfort, it is developing the Arabelle nuclear turbine, the most powerful in the world.

Historically based in France, notably at its Belfort site, it has been a subsidiary of EDF since 31 May 2024. Originally a joint subsidiary named GE Alstom Nuclear Systems (GEAST) between General Electric and Alstom, it became an 80%-owned subsidiary of General Electric, then of GE Vernova, in October 2018. The French state held a 20% stake in GEAST. Headed by Frédéric Wiscart, GE Alstom Nuclear Systems also brought together GE Steam Power's nuclear activities, resulting from the acquisition of Alstom Power in 2015, through its two subsidiaries GE Steam Power Systems (formerly Alstom Power Systems) and GE Steam Power Service (formerly Alstom Power Services).

A nuclear power plant (NPP), also known as a nuclear power station (NPS), nuclear generating station (NGS) or atomic power station (APS) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a generator that produces electricity. As of October 2025, the International Atomic Energy Agency reported that there were 416 nuclear power reactors in operation in 31 countries around the world, and 62 nuclear power reactors under construction.

Most nuclear power plants use thermal reactors with enriched uranium in a once-through fuel cycle. Fuel is removed when the percentage of neutron absorbing atoms becomes so large that a chain reaction can no longer be sustained, typically three years. It is then cooled for several years in on-site spent fuel pools before being transferred to long-term storage. The spent fuel, though low in volume, is high-level radioactive waste. While its radioactivity decreases exponentially, it must be isolated from the biosphere for hundreds of thousands of years, though newer technologies (like fast reactors) have the potential to significantly reduce this. Because the spent fuel is still mostly fissionable material, some countries (e.g. France and Russia) reprocess their spent fuel by extracting fissile and fertile elements for fabrication into new fuel, although this process is more expensive than producing new fuel from mined uranium. All reactors breed some plutonium-239, which is found in the spent fuel, and because Pu-239 is the preferred material for nuclear weapons, reprocessing is seen as a weapon proliferation risk.

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.

An atomic battery, nuclear battery, radioisotope battery or radioisotope generator uses energy from the decay of a radioactive isotope to generate electricity. Like a nuclear reactor, it generates electricity from nuclear energy, but it differs by not using a chain reaction. Although commonly called batteries, atomic batteries are technically not electrochemical and cannot be charged or recharged. Although they are very costly, they have extremely long lives and high energy density, so they are typically used as power sources for equipment that must operate unattended for long periods, such as spacecraft, pacemakers, medical devices, underwater systems, and automated scientific stations in remote parts of the world.

Nuclear batteries began in 1913, when Henry Moseley first demonstrated a current generated by charged-particle radiation. Since RCA's initial nuclear research and development in the early 1950s, many types and methods have been designed to extract electrical energy from nuclear sources.

Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam cars (and other motor vehicles), agricultural engines used for ploughing or threshing, marine engines, and the steam turbines used as the mechanism of power generation for most nuclear power plants.

The development of the steam engine was gradual. They were introduced during the 18th century and widely made for the whole of the 19th century and most of the first half of the 20th century, only declining as electricity supply and the internal combustion engine became more widespread. Over time, they would improve in pressure, expansion and speed.

Neutrino astronomy is a branch of astronomy that gathers information about astronomical objects by observing and studying neutrinos emitted by them with the help of neutrino detectors in special Earth observatories. It is an emerging field in astroparticle physics providing insights into the high-energy and non-thermal processes in the universe.

Neutrinos are nearly massless and electrically neutral or chargeless elementary particles. They are created as a result of certain types of radioactive decay, nuclear reactions such as those that take place in the Sun or high energy astrophysical phenomena, in nuclear reactors, or when cosmic rays hit atoms in the atmosphere. Neutrinos rarely interact with matter (only via the weak nuclear force), travel at nearly the speed of light in straight lines, pass through large amounts of matter without any notable absorption or without being deflected by magnetic fields. Unlike photons, neutrinos rarely scatter along their trajectory. But like photons, neutrinos are some of the most common particles in the universe. Because of this, neutrinos offer a unique opportunity to observe processes that are inaccessible to optical telescopes, such as reactions in the Sun's core. Neutrinos that are created in the Sun's core are barely absorbed, so a large quantity of them escape from the Sun and reach the Earth. Neutrinos can also offer a very strong pointing direction compared to charged particle cosmic rays.

A nuclear meltdown (core meltdown, core melt accident, meltdown or partial core melt) is a severe nuclear reactor accident that results in core damage from overheating. The term nuclear meltdown is not officially defined by the International Atomic Energy Agency, however it has been defined to mean the accidental melting of the core or fuel of a nuclear reactor, and is in common usage a reference to the core's either complete or partial collapse.

A core meltdown accident occurs when the heat generated by a nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point. This differs from a fuel element failure, which is not caused by high temperatures. A meltdown may be caused by a loss of coolant, loss of coolant pressure, or low coolant flow rate, or be the result of a criticality excursion in which the reactor's power level exceeds its design limits.

Nazi Germany undertook several research programs relating to nuclear technology, including nuclear weapons and nuclear reactors, before and during World War II. These were variously called Uranverein (Uranium Society) or Uranprojekt (Uranium Project). The first effort started in April 1939, just months after the discovery of nuclear fission in Berlin in December 1938, but ended shortly ahead of the September 1939 German invasion of Poland, for which many German physicists were drafted into the Wehrmacht. A second effort under the administrative purview of the Wehrmacht's Heereswaffenamt began on September 1, 1939, the day of the invasion of Poland. The program eventually expanded into three main efforts: Uranmaschine (nuclear reactor) development, uranium and heavy water production, and uranium isotope separation. Eventually, the German military determined that nuclear fission would not contribute significantly to the war, and in January 1942 the Heereswaffenamt turned the program over to the Reich Research Council (Reichsforschungsrat) while continuing to fund the activity.

The program was split up among nine major institutes where the directors dominated research and set their own objectives. Subsequently, the number of scientists working on applied nuclear fission began to diminish as many researchers applied their talents to more pressing wartime demands. The most influential people in the Uranverein included Kurt Diebner, Abraham Esau, Walther Gerlach, and Erich Schumann. Schumann was one of the most powerful and influential physicists in Germany. Diebner, throughout the life of the nuclear weapon project, had more control over nuclear fission research than did Walther Bothe, Klaus Clusius, Otto Hahn, Paul Harteck, or Werner Heisenberg. Esau was appointed as Reichsmarschall Hermann Göring's plenipotentiary for nuclear physics research in December 1942, and was succeeded by Walther Gerlach after he resigned in December 1943.