Inertial confinement fusion
Inertial confinement fusion is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target in the form of a pellet that most contains a mixture of deuterium and tritium. Typical fuel pellets contain around 10 milligrams of fuel. To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons all ICF devices as of 2015 have used lasers; the heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards, compressing the target. This process is designed to create shock waves. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur. ICF is one of two major branches of fusion energy research, the other being magnetic confinement fusion; when it was first proposed in the early 1970s, ICF appeared to be a practical approach to power production and the field flourished.
Experiments during the 1970s and'80s demonstrated that the efficiency of these devices was much lower than expected, reaching ignition would not be easy. Throughout the 1980s and'90s, many experiments were conducted in order to understand the complex interaction of high-intensity laser light and plasma; these led to the design of newer machines, much larger, that would reach ignition energies. The largest operational ICF experiment is the National Ignition Facility in the US, designed using the decades-long experience of earlier experiments. Like those earlier experiments, however, NIF has failed to reach ignition and is, as of 2015, generating about 1⁄3 of the required energy levels. Fusion reactions combine lighter atoms, such as hydrogen, together to form larger ones; the reactions take place at such high temperatures that the atoms have been ionized, their electrons stripped off by the heat. Nuclei are positively charged, thus repel each other due to the electrostatic force. Overcoming this repulsion costs a considerable amount of energy, known as the Coulomb barrier or fusion barrier energy.
Less energy will be needed to cause lighter nuclei to fuse, as they have less charge and thus a lower barrier energy, when they do fuse, more energy will be released. As the mass of the nuclei increase, there is a point where the reaction no longer gives off net energy—the energy needed to overcome the energy barrier is greater than the energy released in the resulting fusion reaction; the best fuel from an energy perspective is a one-to-one mix of tritium. The D-T mix has a low barrier because of its high ratio of neutrons to protons; the presence of neutral neutrons in the nuclei helps pull them together via the nuclear force, while the presence of positively charged protons pushes the nuclei apart via electrostatic force. Tritium has one of the highest ratios of neutrons to protons of any stable or moderately unstable nuclide—two neutrons and one proton. Adding protons or removing neutrons increases the energy barrier. A mix of D-T at standard conditions does not undergo fusion. In the hot, dense center of the sun, the average proton will exist for billions of years before it fuses.
For practical fusion power systems, the rate must be increased by heating the fuel to tens of millions of degrees, and/or compressing it to immense pressures. The temperature and pressure required for any particular fuel to fuse is known as the Lawson criterion; these conditions have been known since the 1950s. To meet the Lawson Criterion is difficult on Earth, which explains why fusion research has taken many years to reach the current high state of technical prowess. In a hydrogen bomb, the fusion fuel is heated with a separate fission bomb. A variety of mechanisms transfers the energy of the fission "primary" explosion into the fusion fuel. A primary mechanism is that the flash of x-rays given off by the primary is trapped within the engineered case of the bomb, causing the volume between the case and the bomb to fill with an x-ray "gas"; these x-rays evenly illuminate the outside of the fusion section, the "secondary" heating it until it explodes outward. This outward blowoff causes the rest of the secondary to be compressed inward until it reaches the temperature and density where fusion reactions begin.
The requirement of a fission bomb makes the method impractical for power generation. Not only would the triggers be prohibitively expensive to produce, but there is a minimum size that such a bomb can be built, defined by the critical mass of the plutonium fuel used, it seems difficult to build nuclear devices smaller than about 1 kiloton in yield, the fusion secondary would add to this. This makes it a difficult engineering problem to extract power from the resulting explosions. One of the PACER participants, John Nuckolls, began to explore what happened to the size of the primary required to start the fusion reaction as the size of the secondary was scaled down, he discovered that as the secondary reaches the miligram size, the amount of energy needed to spark it fell into the megajoule range. This was far below what was needed for a bomb, where the primary was in the tera
Nuclear power is the use of nuclear reactions that release nuclear energy to generate heat, which most is used in steam turbines to produce electricity in a nuclear power plant. As a nuclear technology, nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced by nuclear fission of uranium and plutonium. Nuclear decay processes are used in niche applications such as radioisotope thermoelectric generators. Generating electricity from fusion power remains at the focus of international research; this article deals with nuclear fission power for electricity generation. Civilian nuclear power supplied 2,488 terawatt hours of electricity in 2017, equivalent to about 10% of global electricity generation; as of April 2018, there are 449 civilian fission reactors in the world, with a combined electrical capacity of 394 gigawatt. As of 2018, there are 58 power reactors under construction and 154 reactors planned, with a combined capacity of 63 GW and 157 GW, respectively.
As of January 2019, 337 more reactors were proposed. Most reactors under construction are generation III reactors in Asia. Nuclear power is classified as a low greenhouse gas energy supply technology, along with renewable energy, by the Intergovernmental Panel on Climate Change. Since its commercialization in the 1970s, nuclear power has prevented about 1.84 million air pollution-related deaths and the emission of about 64 billion tonnes of carbon dioxide equivalent that would have otherwise resulted from the burning of fossil fuels. There is a debate about nuclear power. Proponents, such as the World Nuclear Association and Environmentalists for Nuclear Energy, contend that nuclear power is a safe, sustainable energy source that reduces carbon emissions. Opponents, such as Greenpeace and NIRS, contend that nuclear power poses many threats to people and the environment. Accidents in nuclear power plants include the Chernobyl disaster in the Soviet Union in 1986, the Fukushima Daiichi nuclear disaster in Japan in 2011, the more contained Three Mile Island accident in the United States in 1979.
There have been some nuclear submarine accidents. Nuclear reactors have caused the lowest number of fatalities per unit of energy generated when compared to fossil fuels and hydropower. Coal, natural gas and hydroelectricity each have caused a greater number of fatalities per unit of energy, due to air pollution and accidents. Collaboration on research and development towards greater efficiency and recycling of spent fuel in future generation IV reactors presently includes Euratom and the co-operation of more than 10 permanent member countries globally. In 1932 physicist Ernest Rutherford discovered that when lithium atoms were "split" by protons from a proton accelerator, immense amounts of energy were released in accordance with the principle of mass–energy equivalence. However, he and other nuclear physics pioneers Niels Bohr and Albert Einstein believed harnessing the power of the atom for practical purposes anytime in the near future was unlikely; the same year, his doctoral student James Chadwick discovered the neutron, recognized as a potential tool for nuclear experimentation because of its lack of an electric charge.
Experiments bombarding materials with neutrons led Frédéric and Irène Joliot-Curie to discover induced radioactivity in 1934, which allowed the creation of radium-like elements. Further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Experiments bombarding uranium with neutrons led Fermi to believe he had created a new, transuranic element, dubbed hesperium. In 1938, German chemists Otto Hahn and Fritz Strassmann, along with Austrian physicist Lise Meitner and Meitner's nephew, Otto Robert Frisch, conducted experiments with the products of neutron-bombarded uranium, as a means of further investigating Fermi's claims, they determined that the tiny neutron split the nucleus of the massive uranium atoms into two equal pieces, contradicting Fermi. This was an surprising result: all other forms of nuclear decay involved only small changes to the mass of the nucleus, whereas this process—dubbed "fission" as a reference to biology—involved a complete rupture of the nucleus.
Numerous scientists, including Leó Szilárd, one of the first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. Once this was experimentally confirmed and announced by Frédéric Joliot-Curie in 1939, scientists in many countries petitioned their governments for support of nuclear fission research, just on the cusp of World War II, for the development of a nuclear weapon. In the United States, where Fermi and Szilárd had both emigrated, the discovery of the nuclear chain reaction led to the creation of the first man-made reactor, the research reactor known as Chicago Pile-1, which achieved self-sustaining power/criticality on December 2, 1942; the reactor's development was part of the Manhattan Project, the Allied effort to create atomic bombs during World War II. It led to the building of larger single-purpose production reactors, such as the X-10 Pile, for the production of weapons-grade plutonium for use in the first nuclear weapons.
The United States tested the first nuclear weapon in July 1945, the Trinity test, with the atomic bombings of Hiroshima and Nagasaki taking place one month later. In August 1945, the first distributed account of nuclear energy, in the form of the pocketbook The Atomic Age, discussed the peaceful future uses of nuclear energy and depicted a future where fo
Iodine is a chemical element with symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a lustrous, purple-black non-metallic solid at standard conditions that melts to form a deep violet liquid at 114 degrees Celsius, boils to a violet gas at 184 degrees Celsius; the element was discovered by the French chemist Bernard Courtois in 1811. It was named two years by Joseph Louis Gay-Lussac from this property, after the Greek ἰώδης "violet-coloured". Iodine occurs in many oxidation states, including iodide and the various periodate anions, it is the least abundant of the stable halogens. It is the heaviest essential mineral nutrient. Iodine is essential in the synthesis of thyroid hormones. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities; the dominant producers of iodine today are Japan. Iodine and its compounds are used in nutrition. Due to its high atomic number and ease of attachment to organic compounds, it has found favour as a non-toxic radiocontrast material.
Because of the specificity of its uptake by the human body, radioactive isotopes of iodine can be used to treat thyroid cancer. Iodine is used as a catalyst in the industrial production of acetic acid and some polymers. In 1811, iodine was discovered by French chemist Bernard Courtois, born to a manufacturer of saltpetre. At the time of the Napoleonic Wars, saltpetre was in great demand in France. Saltpetre produced from French nitre beds required sodium carbonate, which could be isolated from seaweed collected on the coasts of Normandy and Brittany. To isolate the sodium carbonate, seaweed was burned and the ash washed with water; the remaining waste was destroyed by adding sulfuric acid. Courtois once added a cloud of purple vapour rose, he noted. Courtois lacked funding to pursue it further. Courtois gave samples to his friends, Charles Bernard Desormes and Nicolas Clément, to continue research, he gave some of the substance to chemist Joseph Louis Gay-Lussac, to physicist André-Marie Ampère. On 29 November 1813, Clément made Courtois' discovery public.
They described the substance to a meeting of the Imperial Institute of France. On 6 December, Gay-Lussac announced that the new substance was either an element or a compound of oxygen, it was Gay-Lussac. Ampère had given some of his sample to English chemist Humphry Davy, who experimented on the substance and noted its similarity to chlorine. Davy sent a letter dated 10 December to the Royal Society of London stating that he had identified a new element. Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists acknowledged Courtois as the first to isolate the element. In early periodic tables, iodine is given the symbol J, for jod, its name in German. Iodine is the fourth halogen, being a member of group 17 in the periodic table, below fluorine and bromine. Iodine has an electron configuration of 4d105s25p5, with the seven electrons in the fifth and outermost shell being its valence electrons. Like the other halogens, it is one electron short of a full octet and is hence a strong oxidising agent, reacting with many elements in order to complete its outer shell, although in keeping with periodic trends, it is the weakest oxidising agent among the stable halogens: it has the lowest electronegativity among them, just 2.66 on the Pauling scale.
Elemental iodine hence forms diatomic molecules with chemical formula I2, where two iodine atoms share a pair of electrons in order to each achieve a stable octet for themselves. The iodide anion, I−, is the strongest reducing agent among the stable halogens, being the most oxidised back to diatomic I2; the halogens darken in colour as the group is descended: fluorine is a pale yellow gas, chlorine is greenish-yellow, bromine is a reddish-brown volatile liquid. Iodine conforms to the prevailing trend, being a shiny black crystalline solid that melts at 114 °C and boils at 183 °C to form a violet gas; this trend occurs because the wavelengths of visible light absorbed by the halogens increase down the group. The violet colour of iodine gas results from the electron transition between the highest occupied antibonding πg molecular orbital and the lowest vacant antibonding σu molecular orbital. Elemental iodine is soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C.
Nonpolar solvents such as hexane and carbon tetrachloride provide a higher solubility. Polar solutions, such as aqueous solution
Reversed field pinch
A reversed-field pinch is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma to study magnetic fusion energy, its magnetic geometry is somewhat different from that of the more common tokamak. As one moves out radially, the portion of the magnetic field pointing toroidally reverses its direction, giving rise to the term "reversed field"; this configuration can be sustained with comparatively lower fields than that of a tokamak of similar power density. One of the disadvantages of this configuration is that it tends to be more susceptible to non-linear effects and turbulence; this makes it a perfect laboratory for non-ideal magnetohydrodynamics. RFPs are used in the study of astrophysical plasmas as they share many features; the largest Reversed Field Pinch device presently in operation is the RFX in Italy. Others include the MST in the United States, EXTRAP T2R in Sweden, TPE-RX in Japan, KTX in China.
Unlike the Tokamak, which has a much larger magnetic field in the toroidal direction than the poloidal direction, an RFP has a comparable field strength in both directions. Moreover, a typical RFP has a field strength one half to one tenth that of a comparable Tokamak; the RFP relies on driving current in the plasma to reinforce the field from the magnets through the dynamo effect. The reversed-field pinch works towards a state of minimum energy; the magnetic field lines coil loosely around a center torus. They coil outwards. Near the plasma edge, the toroidal magnetic field reverses and the field lines coil in the reverse direction. Internal fields are bigger than the fields at the magnets; the RFP has many features. Due to the lower overall fields, an RFP reactor might not need superconducting magnets; this is a large advantage over tokamaks since superconducting magnets are delicate and expensive and so must be shielded from the neutron rich fusion environment. RFPs are susceptible to so require a close fitting shell.
Some experiments use their close fitting shell as a magnetic coil by driving current through the shell itself. This is attractive from a reactor standpoint since a solid copper shell would be robust against high energy neutrons, compared with superconducting magnets. There is no established beta limit for RFPs. There exists a possibility that a reversed field pinch could achieve ignition with ohmic power, which would be much simpler than tokamak designs, though it could not be operated in steady state. RFPs require a large amount of current to be driven, although promising experiments are underway, there is no established method of replacing ohmically driven current, fundamentally limited by the machine parameters. RFPs are prone to tearing modes which lead to overlapping magnetic islands and therefore rapid transport from the core of the plasma to the edge; these problems are areas of active research in the RFP community. The plasma confinement in the best RFP's is only about 1% as good as in the best tokamaks.
One reason for this is that all existing RFP's are small. MST was larger than any previous RFP device, thus it tested this important size issue.. The RFP is believed to require a shell with high electrical conductivity close to the boundary of the plasma; this requirement is an unfortunate complication in a reactor. The Madison Symmetric Torus was designed to test this assumption and to learn how good the conductor must be and how close to the plasma it must be placed. In RFX, the thick shell was replaced with an active system of 192 coils, which cover the entire torus with their saddle shape, response to the magnetic push of the plasma. Active control of plasma modes is possible with this system; the Reversed Field Pinch is interesting from a physics standpoint. RFP dynamics are turbulent. RFPs exhibit a strong plasma dynamo, similar to many astrophysical bodies. Basic plasma science is another important aspect of Reversed Field Pinch research. Self-organized plasmas RFX: Reversed-Field eXperiment Measurement of superthermal electron flow and temperature in a reversed-field pinch experiment by an electrostatic electron energy analyser
A field-reversed configuration is a type of plasma device that confines a plasma on closed magnetic field lines without a central penetration. In an FRC, the plasma has the form of a self-stable torus, similar to a smoke ring. FRCs are related to another self-stable magnetic confinement fusion device, the spheromak. Both are considered part of the compact toroid class of fusion devices. FRCs have a plasma, more elongated than spheromaks, having the overall shape of a hollowed out sausage rather than the spherical spheromak. FRCs were a major area of research in the 1960s and into the 1970s, but had problems scaling up into practical fusion triple products. In the 1990s it saw renewed interest, As of 2019 the FRC remains an active area of research; the FRC was first observed in laboratories in the late 1950s during theta pinch experiments with a reversed background magnetic field. The first studies of the effect started at the United States Naval Research Laboratory in the 1960s. Considerable data has been collected since with over 600 published papers.
All research was conducted during Project Sherwood at Los Alamos National Laboratory from 1975 to 1990, during 18 years at the Redmond Plasma Physics Laboratory of the University of Washington, with the large s experiment. More some research has been done at the Air Force Research Laboratory, the Fusion Technology Institute of the University of Wisconsin-Madison, Princeton Plasma Physics Laboratory, the University of California, Irvine; some private companies now theoretically and experimentally study FRCs in order to use this configuration in future fusion power plants they try to build, like General Fusion, Tri-Alpha Energy, Inc. and Helion Energy. The FRC is considered for deep space exploration, not only as a possible nuclear energy source, but as means of accelerating a propellant to high levels of specific impulse for electrically powered spaceships and fusion rockets, with interest expressed by NASA and the media; the difference between a spheromak and a field-reversed configuration is that a spheromak has an extra toroidal field.
This toroidal field can run counterclockwise to the spinning plasma direction. One approach to producing fusion power is to confine the plasma with magnetic fields; this is most effective if the field lines do not penetrate solid surfaces but close on themselves into circles or toroidal surfaces. The mainline confinement concepts of tokamak and stellarator do this in a toroidal chamber, which allows a great deal of control over the magnetic configuration, but requires a complex construction; the field-reversed configuration offers an alternative in that the field lines are closed, providing good confinement, but the chamber is cylindrical, allowing simpler, easier construction and maintenance. Field-reversed configurations and spheromaks are together known as compact toroids. Unlike the spheromak, where the strength of the toroidal magnetic field is similar to that of the poloidal field, the FRC has little to no toroidal field component and is confined by a poloidal field; the lack of a toroidal field means that the FRC has no magnetic helicity and that it has a high beta.
The high beta makes the FRC attractive as a fusion reactor and uniquely suited to aneutronic fuels because of the low required magnetic field. Spheromaks have β ≈ 0.1 whereas a typical FRC has β ≈ 1. In modern FRC experiments, the plasma current that reverses the magnetic field can be induced in a variety of ways; when a field-reversed configuration is formed using the theta-pinch method, a cylindrical coil first produces an axial magnetic field. The gas is pre-ionized, which "freezes in" the bias field from a magnetohydrodynamic standpoint the axial field is reversed, hence "field-reversed configuration." At the ends, reconnection of the bias field and the main field occurs. The main field is raised further and heating the plasma and providing a vacuum field between the plasma and the wall. Neutral beams are known to drive current in Tokamaks by directly injecting charged particles. FRCs can be formed and heated by application of neutral beams. In such experiments, as above, a cylindrical coil produces a uniform axial magnetic field and gas is introduced and ionized, creating a background plasma.
Neutral particles are injected into the plasma. They ionize and the heavier, positively-charged particles form a current ring which reverses the magnetic field. Spheromaks are FRC-like configurations with finite toroidal magnetic field. FRCs have been formed through the merging of spheromaks of canceling toroidal field. Rotating magnetic fields have been used to drive current. In such experiments, as above, gas is ionized and an axial magnetic field is produced. A rotating magnetic field is produced by external magnetic coils perpendicular to the axis of the machine, the direction of this field is rotated about the axis; when the rotation frequency is between the ion and electron gyro-frequencies, the electrons in the plasma co-rotate with the magnetic field, producing current and reversing the magnetic field. More so-called odd parity rotating magnetic fields have been used to preserve the closed topology of the FRC. FRCs contain an important and uncommon feature: a "magnetic null," or circular line on which the magnetic field is zero.
This is the case, as inside the null the magnetic field points one direction and outside the null the magnetic field points the opposite direction. Particles far from the null trace closed cyclotron orbits as in other magnetic fusion geometries. Particles which cross the null, trace not cyclotron o
In astrophysics, silicon burning is a brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8-11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung-Russell diagram, it follows the previous stages of hydrogen, carbon and oxygen burning processes. Silicon burning begins when gravitational contraction raises the star's core temperature to 2.7–3.5 billion Kelvin. The exact temperature depends on mass; when a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically may explode in what is known as a Type II supernova. After a star completes the oxygen burning process, its core is composed of silicon and sulfur. If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 GK. At these temperatures and other elements can photodisintegrate, emitting a proton or an alpha particle.
Silicon burning proceeds by photodisintegration rearrangement, which creates new elements by adding one of these freed alpha particles per capture step in the following sequence: The silicon-burning sequence lasts about one day before being struck by the shock wave, launched by the core collapse. Burning becomes much more rapid at the elevated temperature and stops only when the rearrangement chain has been converted to nickel-56 or is stopped by supernova ejection and cooling; the star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon of all the elements in the alpha process sequence. Only minutes are available for the nickel-56 to decay within the core of a massive star, only seconds if in the ejecta; the star has run out of nuclear fuel and within minutes its core begins to contract. During this phase of the contraction, the potential energy of gravitational contraction heats the interior to 5 GK and this opposes and delays the contraction.
However, since no additional heat energy can be generated via new fusion reactions, the final unopposed contraction accelerates into a collapse lasting only a few seconds. The central portion of the star is now crushed into either a neutron star or, if the star is massive enough, a black hole; the outer layers of the star are blown off in an explosion known as a Type II supernova that lasts days to months. The supernova explosion releases a large burst of neutrons, which may synthesize in about one second half of the supply of elements in the universe that are heavier than iron, via a rapid neutron-capture sequence known as the r-process; the graph above shows the binding energy per nucleon of various elements. As can be seen, light elements such as hydrogen release large amounts of energy when combined to form heavier elements—the process of fusion. Conversely, heavy elements such as uranium release energy when broken into lighter elements—the process of nuclear fission. In stars, rapid nucleosynthesis proceeds by adding helium nuclei to heavier nuclei.
Although nuclei with 58 and 62 nucleons have the highest binding energy per nucleon, converting nickel-56 to the next element, zinc-60, is a decrease in binding energy per nucleon and consumes energy rather than releasing any. Accordingly, nickel-56 is the last fusion product produced in the core of a high-mass star. Decay of nickel-56 explains the large amount of iron-56 seen in metallic meteorites and the cores of rocky planets. Alpha nuclide Alpha process Stellar evolution Supernova nucleosynthesis Neutron capture: p-process, r-process, s-process Stellar Evolution: The Life and Death of Our Luminous Neighbors, by Arthur Holland and Mark Williams of the University of Michigan The Evolution and Death of Stars, by Ian Short Origin of Heavy Elements, by Tufts University Chapter 21: Stellar Explosions, by G. Hermann Arnett, W. D. Advanced evolution of massive stars. VII – Silicon burning / Astrophysical Journal Supplement Series, vol. 35, Oct. 1977, p. 145–159. Hix, W. Raphael. "Silicon Burning.
I. Neutronization and the Physics of Quasi-Equilibrium"; the Astrophysical Journal. 460: 869. ArXiv:astro-ph/9511088v1. Bibcode:1996ApJ...460..869H. Doi:10.1086/177016. Retrieved 29 July 2015