A nuclear reactor known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid; these either turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating; some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research; as of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world. Just as conventional power-stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms; when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, so on; this is known as a nuclear chain reaction. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions. Used moderators include regular water, solid graphite and heavy water; some experimental types of reactor have used beryllium, hydrocarbons have been suggested as another possibility. The reactor core generates heat in a number of ways: The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms; the reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases three million times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates; the heat is carried away from the reactor and is used to generate steam. Most reactor systems employ a cooling system, physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; the rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events.
Nuclear reactors employ several methods of neutron control to adjust the reactor's power output. Some of these methods arising from the physics of radioactive decay and are accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose; the fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods therefore tend to absorb neutrons; when a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces—often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power; the physics of radioactive decay affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes; these delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder released upon fission.
The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, so considerable time is required to determine when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; this last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, other points in the process interpolated in cents. In some reactors, the coolant acts as a neutron moderator. A moderator increases the power of the reactor by causin
A tokamak is a device which uses a powerful magnetic field to confine a hot plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power; as of 2016, it is the leading candidate for a practical fusion reactor. Tokamaks were conceptualized in the 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by a letter by Oleg Lavrentiev. Meanwhile, the first working tokamak was attributed to the work of Natan Yavlinskii on the T-1, it had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix. Devices like the z-pinch and stellarator demonstrated serious instabilities, it was the development of the concept now known as the safety factor that guided tokamak development. The first tokamak, the T-1, began operation in 1958. By the mid-1960s, the tokamak designs began to show improved performance. Initial results were ignored. A second set of results was published in 1968, this time claiming performance far in advance of any other machine, was considered unreliable.
This led to the invitation of a delegation from the United Kingdom to make their own measurements. These confirmed the Soviet results, their 1969 publication resulted in a stampede of tokamak construction. By the mid-1970s, dozens of tokamaks were in use around the world. By the late 1970s, these machines had reached all of the conditions needed for practical fusion, although not at the same time nor in a single reactor. With the goal of breakeven now in sight, a new series of machines were designed that would run on a fusion fuel of deuterium and tritium; these machines, notably the Joint European Torus, Tokamak Fusion Test Reactor and JT-60, had the explicit goal of reaching breakeven. Instead, these machines demonstrated new problems. Solving these would require a much larger and more expensive machine, beyond the abilities of any one country. After an initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985, the International Thermonuclear Experimental Reactor effort emerged and remains the primary international effort to develop practical fusion power.
Many smaller designs, offshoots like the spherical tokamak, continue to be used to investigate performance parameters and other issues. The word tokamak is a transliteration of the Russian word токамак, an acronym of either: "тороидальная камера с магнитными катушками" — toroidal chamber with magnetic coils; the term was created in 1957 by Igor Golovin, the vice-director of the Laboratory of Measuring Apparatus of Academy of Science, today's Kurchatov Institute. A similar term, "tokamag", was proposed for a time. In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to achieve fusion on Earth, using a particle accelerator to shoot deuterium nuclei into a metal foil containing deuterium or other atoms; this allowed them to measure the nuclear cross section of various fusion reactions, determined that the deuterium-deuterium reaction occurred at a lower energy than other reactions, peaking at about 100,000 electronvolts. Accelerator-based fusion is not practical. To maintain fusion, the bulk of the fuel must be raised to high temperatures so its atoms are colliding at high speed.
In 1944, Enrico Fermi calculated the reaction would be self-sustaining at about 50,000,000 K. During the Manhattan Project, the first practical way to reach these temperatures was created, using an atomic bomb. In 1944, Fermi gave a talk on the physics of fusion in the context of a then-hypothetical hydrogen bomb. However, some thought had been given to a controlled fusion device, Jim Tuck and Stanislaw Ulam had attempted such using shaped charges driving a metal foil infused with deuterium, although without success; the first attempts to build a practical fusion machine took place in the United Kingdom, where George Paget Thomson had selected the pinch effect as a promising technique in 1945. After several failed attempts to gain funding, he gave up and asked two graduate students, Stan Cousins and Alan Ware, to build a device out of surplus radar equipment; this was operated in 1948, but showed no clear evidence of fusion and failed to gain the interest of the Atomic Energy Research Establishment.
In 1950, Oleg Lavrentiev a Red Army sergeant stationed on Sakhalin with little to do, wrote a letter to the Central Committee of the Communist Party of the Soviet Union. The letter outlined the idea of using an atomic bomb to ignite a fusion fuel, went on to describe a system that used electrostatic fields to contain a hot plasma in a steady state for energy production; the letter was sent to Andrei Sakharov for comment, who noted
The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave within the nucleus, each has a mass of one atomic mass unit, they are both referred to as nucleons, their properties and interactions are described by nuclear physics. The chemical and nuclear properties of the nucleus are determined by the number of protons, called the atomic number, the number of neutrons, called the neutron number; the atomic mass number is the total number of nucleons. For example, carbon has atomic number 6, its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons; some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes. Within the nucleus and neutrons are bound together through the nuclear force. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen atom.
Neutrons are produced copiously in nuclear fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission and neutron capture processes; the neutron is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations. With the discovery of nuclear fission in 1938, it was realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc. in a cascade known as a nuclear chain reaction. These events and findings led to the first self-sustaining nuclear reactor and the first nuclear weapon. Free neutrons, while not directly ionizing atoms, cause ionizing radiation; as such they can be a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.
Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. An atomic nucleus is formed by a number of protons, Z, a number of neutrons, N, bound together by the nuclear force; the atomic number defines the chemical properties of the atom, the neutron number determines the isotope or nuclide. The terms isotope and nuclide are used synonymously, but they refer to chemical and nuclear properties, respectively. Speaking, isotopes are two or more nuclides with the same number of protons; the atomic mass number, symbol A, equals Z+N. Nuclides with the same atomic mass number are called isobars; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The most common nuclide of the common chemical element lead, 208Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Though it is not a chemical element, the neutron is included in this table; the free neutron has 1.674927471 × 10 − 27 kg, or 1.00866491588 u. The neutron has a mean square radius of about 0.8×10−15 m, or 0.8 fm, it is a spin-½ fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields; the neutron has a magnetic moment, however. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. A free neutron is unstable, decaying to a proton and antineutrino with a mean lifetime of just under 15 minutes; this radioactive decay, known as beta decay, is possible because the mass of the neutron is greater than the proton. The free proton is stable. Neutrons or protons bound in a nucleus can be stable or unstable, depending on the nuclide.
Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, it requires the emission or absorption of electrons and neutrinos, or their antiparticles. Protons and neutrons behave identically under the influence of the nuclear force within the nucleus; the concept of isospin, in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions therefore have an energy density, more than ten million times that of chemical reactions; because of the mass–energy equivalence, nuclear binding energies reduce the mass of nuclei. The ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible.
In nuclear fission, the absorption of a neutron by a heavy nuclide causes the nuclide to become unstable and break into light nuclides and additional neu
The polywell is a type of nuclear fusion reactor that uses an electric field to heat ions to fusion conditions. It is related to the fusor, the high beta fusion reactor, the magnetic mirror, the biconic cusp. A set of electromagnets generates a magnetic field; this creates a negative voltage. As the ions accelerate towards the negative center, their kinetic energy rises. Ions that collide at high enough energies can fuse; the polywell is one of many devices. This branch of fusion research is known as inertial electrostatic confinement; the polywell was developed as an improvement over the fusor. His company, EMC2, Inc. developed prototypical devices for the U. S. Navy. A Farnsworth-Hirsch fusor consists of two wire cages, one inside the other referred to as grids, that are placed inside a vacuum chamber; the outer cage has a positive voltage versus the inner cage. Deuterium gas is injected into this chamber, it is heated past its ionization temperature. The ions move towards the negative inner cage; those that miss the wires of the inner cage fly through the center of the device at high speeds and can fly out the other side of the inner cage.
As the ions move outward, they feel a Coulomb force. Over time, a core of ionized gas can form inside the inner cage. Ions pass forth through the core until they strike either the grid or another nucleus. Most nucleus strikes do not result in fusion. Grid strikes can raise the temperature of the grid as well as eroding it; these strikes conduct energy away from the plasma. In fusors, the potential well is made with a wire cage; because most of the ions and electrons fall into the cage, fusors suffer from high conduction losses. Hence, no fusor has come close to energy break-even; the main problem with the fusor is that the inner cage conducts away too much mass. The solution, suggested by Robert Bussard and Oleg Lavrentiev, was to replace the negative cage with a "virtual cathode" made of a cloud of electrons. A polywell consists of several parts; these are put inside a vacuum chamber A set of positively charged electromagnet coils arranged in a polyhedron. The most common arrangement is a six sided cube.
The six magnetic poles are pointing in the same direction toward the center. The magnetic field vanishes at the center by symmetry. Electron guns facing ring axis; these shoot electrons into the center of the ring structure. Once inside, the electrons are confined by the magnetic fields; this has been measured in polywells using Langmuir probes. Electrons that have enough energy to escape through the magnetic cusps can be re-attracted to the positive rings, they can return to the inside of the rings along the cusps. This reduces conduction losses, improves the overall performance of the machine; the electrons act as a negative voltage drop attracting positive ions. This is a virtual cathode. Gas puffers at corner. Gas is puffed inside the rings; as ions fall down the potential well, the electric field works on them, heating it to fusion conditions. The ions build up speed, they can slam together in the fuse. Ions are electrostatically confined increasing the fusion rate; the magnetic energy density required to confine electrons is far smaller than that required to directly confine ions, as is done in other fusion projects such as ITER.
Magnetic fields exert a pressure on the plasma. Beta is the ratio of plasma pressure to the magnetic field strength, it can be defined separately for ions. The polywell concerns itself only for the electron beta, whereas the ion beta is of greater interest within Tokamak and other neutral-plasma machines; the two vary by a large ratio, because of the enormous difference in mass between an electron and any ion. In other devices the electron beta is neglected, as the ion beta determines more important plasma parameters; this is a significant point of confusion for scientists more familiar with more'conventional' fusion plasma physics. Note that for the electron beta, only the electron number density and temperature are used, as both of these, but the latter, can vary from the ion parameters at the same location. Β e = p p m a g = n e k B T e Most experiments on polywells involve low-beta plasma regimes, where the plasma pressure is weak compared to the magnetic pressure. Several models describe magnetic trapping in polywells.
Tests indicated that plasma confinement is enhanced in a magnetic cusp configuration when β is of order unity. This enhancement is required for a fusion power reactor based on cusp confinement to be feasible. Magnetic mirror dominates in low beta designs. Both ions and electrons are reflected from high to low density fields; this is known as the magnetic mirror effect. The polywell's rings are arranged so the densest fields are on the outside, trapping electrons in the center; this can trap particles at low beta values. In high beta conditions, the machine may operate with cusp confinement; this is an improvement over the simpler magnetic mirror. The MaGr
In fusion power research, the Z-pinch known as zeta pinch, is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it. These systems were referred to as pinch or Bennett pinch, but the introduction of the θ-pinch concept led to the need for increased clarity; the name refers to the direction of the current in the devices, the Z-axis on a normal three-dimensional graph. Any machine that causes a pinch effect due to current running in that direction is referred to as a Z-pinch system, this encompasses a wide variety of devices used for an wide variety of purposes. Early uses focused on fusion research in donut-shaped tubes with the Z-axis running down the inside the tube, while modern devices are cylindrical and used to generate high-intensity x-ray sources for the study of nuclear weapons and other roles; the Z-pinch is an application of the Lorentz force, in which a current-carrying conductor in a magnetic field experiences a force.
One example of the Lorentz force is that, if two parallel wires are carrying current in the same direction, the wires will be pulled toward each other. In a Z-pinch machine the wires are replaced by a plasma, which can be thought of as many current-carrying wires; when a current is run through the plasma, the particles in plasma are pulled toward each other by the Lorentz force, thus the plasma contracts. The contraction is counteracted by the increasing gas pressure of the plasma; as the plasma is electrically conductive, a magnetic field nearby will induce a current in it. This provides a way to run a current into the plasma without physical contact, important as a plasma can erode mechanical electrodes. In practical devices this was arranged by placing the plasma vessel inside the core of a transformer, arranged so the plasma itself would be the secondary; when current was sent into the primary side of the transformer, the magnetic field induced a current into the plasma. As induction requires a changing magnetic field, the induced current is supposed to run in a single direction in most reactor designs, the current in the transformer has to be increased over time to produce the varying magnetic field.
This places a limit on the product of confinement time and magnetic field, for any given source of power. In Z-pinch machines the current is provided from a large bank of capacitors and triggered by a spark gap, known as a Marx Bank or Marx generator; as the conductivity of plasma is good, about that of copper, the energy stored in the power source is depleted by running through the plasma. Z-pinch devices are inherently pulsed in nature. Pinch devices were among the earliest efforts in fusion power. Research began in the UK in the immediate post-war era, but a lack of interest led to little development until the 1950s; the announcement of the Huemul Project in early 1951 led to fusion efforts around the world, notably in the UK and in the US. Small experiments were built at labs as various practical issues were addressed, but all of these machines demonstrated unexpected instabilities of the plasma that would cause it to hit the walls of the container vessel; the problem became known as the "kink instability".
By 1953 the "stabilized pinch" seemed to solve the problems encountered on earlier devices. Stabilized pinch machines added external magnets that created a toroidal magnetic field inside the chamber; when the device was fired, this field added to the one created by the current in the plasma. The result was that the straight magnetic field was twisted into a helix, which the particles followed as they traveled around the tube driven by the current. A particle near the outside of the tube that wanted to kink outward would travel along these lines until it returned to the inside of the tube, where its outward-directed motion would bring it back into the centre of the plasma. Researchers in the UK started construction of ZETA in 1954. ZETA was by far the largest fusion device of its era. At the time all fusion research was classified, so progress on ZETA was unknown outside the labs working on it; however US researchers realized that they were about to be outpaced. Teams on both sides of the Atlantic rushed to be the first to complete stabilized pinch machines.
ZETA won the race, by the summer of 1957 it was producing bursts of neutrons on every run. Despite the researchers' reservations, their results were released with great fanfare as the first successful step on the path to commercial fusion energy. However, further study soon demonstrated that the measurements were misleading, none of the machines were near fusion levels. Interest in pinch devices faded, although ZETA and its cousin Sceptre served for many years as experimental devices. A concept of Z-pinch fusion propulsion system was developed through collaboration between NASA and private companies; the energy released by the Z-pinch effect accelerates the lithium propellant to a high speed, resulting in a specific impulse value of 19400 s and thrust of 38 kN. A magnetic nozzle is required to convert the released energy into a useful impulse; this propulsion method could reduce interplanetary travel times. For example, a mission to Mars would take about 35 days one-way with a total burn time of 20 days and a burned propellant mass of 350 tonnes.
Although it remained unknown for years, Soviet scientists used the pinch concept to develop the tokamak device. Unlike the stabilized pinch devices in the US and UK, the tokamak used more energy in the stabilizing magnets, much less in the plasma current; this reduced the instabilities due to the large currents in the plasma, led to g
A stellarator is a plasma device that relies on external magnets to confine a plasma. In the future, scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions; the name refers to the possibility of harnessing the power source of the stars, including the sun.stellar object. It is one of the earliest fusion power devices, along with the magnetic mirror; the stellarator was invented by Lyman Spitzer of Princeton University in 1951, much of its early development was carried out by his team at what became the Princeton Plasma Physics Laboratory. Lyman's Model A demonstrated that stellarators could confine plasmas. Larger models followed, but these demonstrated poor performance, suffering from a problem known as pump-out that caused them to lose plasma at rates far worse than theoretical predictions. By the early 1960s, any hope of producing a commercial machine faded, attention turned to studying the fundamental theory of high-energy plasmas.
By the mid-1960s, Spitzer was convinced that the stellarator was matching the Bohm diffusion rate, which suggested it would never be a practical fusion device. The release of information on the USSR's tokamak design in 1968 indicated a leap in performance; this led to the Model C stellarator being converted to the Symmetrical Tokamak as a way to confirm or deny these results. ST confirmed them, large-scale work on the stellarator concept ended as the tokamak got most of the attention; the tokamak proved to have similar problems to the stellarators, but for different reasons. Since the 1990s, this has led to renewed interest in the stellarator design. New methods of construction have increased the quality and power of the magnetic fields, improving performance. A number of new devices have been built to test these concepts. Major examples include Wendelstein 7-X in Germany, the Helically Symmetric Experiment in the USA, the Large Helical Device in Japan. In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to achieve fusion on Earth, using a particle accelerator to shoot deuterium nuclei into a metal foil containing deuterium, lithium or other elements.
This system allowed them to measure the nuclear cross section of various fusion reactions, determined that the tritium-deuterium reaction occurred at a lower energy than any other fuel, peaking at about 100,000 electronvolts.100 keV corresponds to a temperature of about a billion kelvins. Due to the Maxwell–Boltzmann statistics, a bulk gas at a much lower temperature will still contain some particles at these much higher energies; because the fusion reactions release so much energy a small number of these reactions can release enough energy to keep the gas at the required temperature. In 1944, Enrico Fermi demonstrated that this would occur at a bulk temperature of about 50 million Celsius, still hot but within the range of existing experimental systems; the key problem was confining such a plasma. But because plasmas are electrically conductive, they are subject to electric and magnetic fields which provide a number of solutions. In a magnetic field, the electrons and nuclei of the plasma circle the magnetic lines of force.
One way to provide some confinement would be to place a tube of fuel inside the open core of a solenoid. A solenoid creates magnetic lines running down its center, fuel would be held away from the walls by orbiting these lines of force, but such an arrangement does not confine the plasma along the length of the tube. The obvious solution is to bend the tube around into a torus shape, so that any one line forms a circle, the particles can circle forever. However, this solution does not work. For purely geometric reasons, the magnets ringing the torus are closer together on the inside curve, inside the "donut hole". Fermi noted this would cause the electrons to drift away from the nuclei causing them to separate and cause large voltages to develop; the resulting electric field would cause the plasma ring inside the torus to expand until it hit the walls of the reactor. In the post-war era, a number of researchers began considering different ways to confine a plasma. George Paget Thomson of Imperial College London proposed a system now known as z-pinch, which runs a current through the plasma.
Due to the Lorentz force, this current creates a magnetic field that pulls the plasma in on itself, keeping it away from the walls of the reactor. This eliminates the need for magnets on the outside. Various teams in the UK had built a number of small experimental devices using this technique by the late 1940s. Another person working on controlled fusion reactors was Ronald Richter, a former German scientist who moved to Argentina after the war, his thermotron used a system of electrical arcs and mechanical compression for heating and confinement. He convinced Juan Perón to fund development of an experimental reactor on an isolated island near the Chilean border. Known as the Huemul Project, this was completed in 1951. Richter soon convinced himself fusion had been achieved in spite of other people working on the project disagreeing; the "success" was announced by Perón on 24 March 1951, becoming the topic of newspaper stories around the world. While preparing for a ski trip to Aspen, Lyman Spitzer received a telephone call from his father, who mentioned an article on Huemul in the New York Times.
Looking over the description in the article, Spitzer concluded it could not work. But the idea stuck with him, he began considering systems that would wor