In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction or radioactive decay process in which the atomic nucleus is divided into smaller parts (lighter nuclei). The fission process often produces free neutrons and gamma photons, and releases enormous amounts of energy even by energetic standards of radioactive decay.
The heavy nuclear fission was discovered on December 17, 1938 by German Otto Hahn and his assistant Fritz Strassmann, and was explained theoretically in January 1939 by Lise Meitner and his nephew, Otto Robert Frisch. Frisch named the process by analogy with the biological physiology of living cells. This is an exothermic reaction that can release large amounts of energy both as electromagnetic radiation and as the kinetic energy of fragments (heats up bulk material where fission occurs). In order for the fission to generate energy, the total binding energy of the resulting element must be more negative (greater binding energy) than the starting element.
Fission is a form of nuclear transmutation because the resulting fragment is not the same as the original atom. The two most commonly generated cores have comparable but slightly different sizes, usually with a product mass ratio of about 3 to 2, for common fissile isotopes. Most fissions are binary fissions (resulting in two charged fragments), but sometimes (2 to 4 times per 1000 events), three positively charged fragments are generated, in a ternary fission. The smallest of these fragments in the ternary process range in size from proton to argon nucleus.
In addition to fission induced by neutrons, exploited and exploited by humans, the natural form of spontaneous radioactivity (not requiring neutrons) is also called fission, and occurs mainly in isotopes with very high mass. Spontaneous cleavage was discovered in 1940 by Flyorov, Petrzhak and Kurchatov in Moscow, when they decided to confirm that, without a bombardment by neutrons, the uranium fission rate was neglected, as Niels Bohr predicted; It is not that.
Unpredictable product composition (which varies in a probabilistic and somewhat chaotic manner) distinguishes the fission of pure quantum filtration processes such as proton emissions, alpha decay, and clump decay, which gives the same product every time. Nuclear fission generates energy for nuclear power and encourages nuclear weapons explosions. Both uses are possible because certain substances called nuclear fuel undergo fission when attacked by fission neutrons, and in turn emit neutrons when they are ruptured. This allows for an independent nuclear chain reaction, releasing energy at a controlled rate in a nuclear reactor or at a very rapid and uncontrollable rate in a nuclear weapon.
The amount of free energy contained in nuclear fuel millions of times the amount of free energy contained in the mass of a kind of chemical fuel such as gasoline makes nuclear fission a very dense source of energy. However, the average nuclear fission product is much more radioactive than the heavy element that is usually diffusioned as fuel, and remains so for quite a long time, causing problems of nuclear waste. Concerns over the accumulation of nuclear waste and the destructive potential of nuclear weapons are a counterweight to the peaceful desire to use fission as a source of energy.
Video Nuclear fission
Physical description
Mechanism
Radioactive Decay
Nuclear fission can occur without the neutron bombing as a type of radioactive decay. This type of fission (called spontaneous fission) is rare except in some of the heavy isotopes.
Nuclear Reaction
In engineered nuclear equipment, basically all nuclear divisions occur as "nuclear reactions" - a process driven by the bombardment resulting from the collision of two subatomic particles. In nuclear reactions, subatomic particles collide with the atomic nucleus and cause changes to it. Nuclear reactions are thus driven by bombing mechanisms, not by the relatively constant exponential and part-time decay of spontaneous radioactive processes.
Many types of nuclear reactions are now known. Nuclear fission differs significantly from other types of nuclear reactions, as it can be strengthened and sometimes controlled through nuclear chain reactions (one common chain reaction type). In such a reaction, the free neutrons released by each fission event can trigger more events, which in turn release more neutrons and cause more fission.
Elements of chemical elements that can sustain a fission chain reaction are called nuclear fuel, and are said to be fissile . The most common nuclear fuel is 235 U (uranium isotope with atomic mass 235 and use in nuclear reactor) and 239 Pu (isotope plutonium with atomic mass 239). This fuel breaks into bimodals of various chemical elements with atomic mass centering near 95 and 135 u (fission products). Much of the nuclear fuel undergoes spontaneous fission only very slowly, especially decaying through the alpha-beta decay chain over a period of millennium up to thousands of years. In nuclear reactors or nuclear weapons, the majority of fission events are triggered by bombing with other particles, neutrons, generated by previous fission events.
The nuclear fission in fossil fuel is the result of nuclear excitation energy generated when the fissile nucleus captures the neutrons. This energy, which results from the capture of neutrons, is the result of an attractive nuclear force that works between neutrons and nuclei. This is enough to change the shape of the nucleus into a double "drop", to the point that the nuclear fragments exceed the distance at which the nuclear forces can hold two groups of charged nuclei together and, when this happens, the two fragments complete their separation. and then pushed further apart by allegations of mutual disgust, in a process that becomes irreversible with greater and greater distances. A similar process occurs on isotopes that can be fissioned (such as uranium-238), but for fission, these isotopes require the additional energy provided by fast neutrons (such as those produced by nuclear fusion in thermonuclear weapons).
The liquid drop model of the atomic nucleus predicts fission products of the same size as a result of nuclear deformation. More sophisticated nuclear shell models are needed to mechanically explain routes to more profitable results, where one fission product is slightly smaller than the other. A theory of fission based on the shell model has been formulated by Maria Goeppert Mayer.
The most common fission process is binary division, and produces the above-mentioned fission products, at 95 ± 15 and 135 Â ± 15 u . However, the binary process only happens because it is the most likely. Anywhere from 2 to 4 missions per 1000 in a nuclear reactor, a process called ternary fission produces three positively charged fragments (plus neutrons) and the smallest of these can range from very small charge and mass as protons (Z = 1) , as large fragments such as argon (Z = 18). The most common small fragments, however, consist of 90% helium-4 nuclei with more energy than the alpha particles of alpha decay (called "long-range alpha" at ~16 MeV), plus helium-6 nuclei, and tritons of tritium). The ternary process is less common, but ultimately produces significant helium-4 and tritium gas accumulation in the fuel rods of modern nuclear reactors.
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Energetika
Input
The heavy core division requires a total input energy of about 7 to 8 million electron volts (MeV) to initially overcome the nuclear force that holds the nucleus into a round or almost spherical shape, and from there, transforms the shape into two lobed ("pea") shapes in which the lobes can be kept separate from each other, driven by their mutual positive charge, in the most common binary process (two fission products positively charged neutrons). Once the nuclear lobe has been pushed to a critical distance, beyond that the strong forces at close range can no longer hold them, their separation process results from energy from electromagnetic (long-term) repulsion between fragments. The result is two fission fragments that move away from each other, with high energy.
Approximately 6 MeV of fission-input energy is provided by simple binding of the extra neutrons to the heavy nuclei through a strong force; However, in many fission isotopes, this amount of energy is not sufficient for fission. Uranium-238, for example, has a nearly zero cross-section of fission for neutrons of less than one MeV energy. If no additional energy is supplied by another mechanism, the nucleus will not divide, but will absorb only the neutrons, as happens when U-238 absorbs slowly and even some rapid neutron fractions, becoming U-239. The energy remaining to initiate fission can be supplied by two other mechanisms: one of which is the kinetic energy of the incoming neutron, which is increasingly capable of dividing diffusing diffusing nuclei by exceeding a single MeV or kinetic energy (so-called fast neutrons). Such high energy neutrons are capable of splitting the U-238 directly (see thermonuclear weapons for applications, where neutrons are rapidly supplied by nuclear fusion). However, this process can not occur in nuclear reactors, because the too small fraction of the fission neutrons produced by all fission types has enough energy to efficiently split the U-238 (fission neutrons have a 2 MeV mode energy, but the median is only 0, 75 MeV, which means half of them have less energy than this is not enough).
Among the heavy actinide elements, however, isotopes that have an odd number of neutrons (such as U-235 with 143 neutrons) bind extra neutrons in addition to 1 to 2 MeV of energy above isotopes of the same element even the number of neutrons (such as U-238 with 146 neutrons). This extra binding energy is available as a result of the mechanism of effect of neutron pairs. This extra energy is generated from the Pauli exclusion principle that allows additional neutrons to occupy the same nuclear orbitals with the last neutrons in the nucleus, so they form a pair. In such isotopes, therefore, no kinetic energy of neutrons is required, since all the energy required is supplied by any neutron absorption, either from slow or fast varieties (first used in a moderated nuclear reactor, and the latter used in a neutron reactor , and in weapons). As mentioned above, a subgroup of fission elements that can be efficiently diffused with their own physical neutrons (thus potentially causing a relatively small nuclear chain reaction of pure material) is called "fissile". Examples of fissile isotopes are uranium-235 and plutonium-239.
Output
A typical fission event releases about two hundred million eV (200 MeV) of energy, equivalent to about & gt; 2 Kelvin quadrillion, for each fission event. The exact isotope that has been fissed, and whether it can be fissionable or fissile, has only a small impact on the amount of energy released. This can be easily seen by examining the binding energy curve (figure below), and noting that the average binding energy of actinide nuclides starting with uranium is about 7.6 MeV per nucleon. Looking further to the left on the binding energy curve, where fission product clusters, it is easy to observe that the fusion energies of fission products tend to center around 8.5 MeV per nucleon. Thus, in any event of isotope fission within the actinide mass range, approximately 0.9 MeV is released per nucleon of the starting element. The cleavage of U235 by a slow neutron generates almost identical energy to U238 fission by rapid neutrons. This energy release profile applies to thorium and various minor actinides as well.
In contrast, most chemical oxidation reactions (such as coal combustion or TNT) emit at most several eV per incidence. Thus, nuclear fuel contains at least ten million times more energy that can be used per unit mass rather than chemical fuel. Nuclear fission energy is released as the kinetic energy of fission products and fragments, and as electromagnetic radiation in the form of gamma rays; in a nuclear reactor, energy is converted to heat because particles and gamma rays collide with the atoms that make up the reactor and its working fluid, usually water or sometimes heavy water or molten salt.
When a uranium nucleus evolves into two female nuclei fragments, about 0.1 percent of the uranium nucleus mass appears as a ~ 200 MeV fission energy. For uranium-235 (total fission energy averaging 202.79 MeV), usually ~ 169Ã, MeV appears as the kinetic energy of the daughter nuclei, which flies apart at about 3% of the speed of light, due to Coulomb repulsion. Also, an average of 2.5 emitted neutrons, with average kinetic energy per neutron ~ 2 MeV (total of 4.8 M MeV). The fission reaction also releases ~ 7 MeV in a fast gamma-ray photon. The latter number means that nuclear fission explosions or critical accidents emit about 3.5% of their energy as gamma rays, less than 2.5% of their energy as fast neutrons (total of both types of radiation ~ 6%), and the remainder as kinetic energy of fission fragments (this appears immediately when the fragments impact material around, such as simple heat). In an atomic bomb, this heat can serve to raise the temperature of the bomb nucleus to 100 million kelvins and cause the secondary emission of soft X-rays, which convert some of this energy into ionizing radiation. However, in a nuclear reactor, the carpide energy of the fission fragments remains as low temperature heat, which in itself causes little or no ionization.
The so-called neutron bombs (radiation-enhanced weapons) have been built that release most of their energy as ionizing radiation (in particular, neutrons), but these are all thermonuclear devices that depend on nuclear fusion stages to produce additional radiation. The energy dynamics of pure fission bombs always remain about 6% of the total radiation results, as a quick result of fission.
The total energy fission prompt is about 181 MeV, or ~ 89% of the total energy ultimately released by fission over time. The remaining ~ 11% is released in beta decay having various beaks, but begins as a process in the fission product immediately; and delayed gamma emissions associated with this beta decay. For example, in uranium-235, this delayed energy is divided into about 6.5Ã, MeV in beta, 8.8 MeV in antineutrino (released at the same time as beta), and finally, additional 6.3Ã, MeV in delayed gamma emission from a happy beta-decay product (for a mean total of ~ 10 gamma-ray emissions per fission, in all). Thus, about 6.5% of the total fission energy is released shortly after the event, as ionizing radiation is not fast or delayed, and the delayed ionizing energy is evenly distributed between gamma and beta-ray energy.
In a reactor that has been in operation for some time, the radioactive fission product will build steady state concentrations so that its decomposition rate is equal to its rate of formation, so the total contribution of its fraction to the reactor heat (via beta decay)) equals the contribution of this radioisotopic fractional to the fission energy. Under these conditions, 6.5% of fission emerging as delayed ionizing radiation (delayed gammas and beta from radioactive fission products) contributes to the production of heat of the steady-state reactor under power. This output fraction is left when the reactor suddenly dies (scram). For this reason, the reactor decay heat output begins at 6.5% of the fission power of the full reactor steady state reactor, after the reactor is turned off. However, within a few hours, because of this isotope decay, the decay power output is much less. See decay heat for details.
The remainder of the delayed energy (8.8 MeV/202.5 â € <â € Some processes involving neutrons are important for absorbing or ultimately producing energy - for example the neutron kinetic energy does not generate heat immediately if neutrons are captured by uranium-238 atoms for plutonium-239 breeding, but this energy is emitted if plutonium-239 is then cleaved. On the other hand, so-called delayed neutrons emitted as radioactive decay products with a half-life up to several minutes, from fission-girls, are essential for reactor control, since they provide typical "reaction" times for total nuclear reactions. to double the size, if the reaction is run in a "delayed-critical zone" that intentionally relies on this neutron for a supercritical chain reaction (one in which each fission cycle produces more neutrons than absorbed). Without their existence, nuclear chain reactions will soon become critical and size increases faster than can be controlled by human intervention. In this case, the first experimental atomic reactor will escape to a dangerous and messy "critical reaction" before its operator can turn it off manually (for this reason, designer Enrico Fermi including radiation-induced control rods, suspended by an electromagnet, can automatically fall into the center of Chicago Pile-1). If these delayed neutrons are captured without producing fission, they produce heat as well. Product core and binding energy
In fission there is a preference for generating fragments with even proton numbers, called odd effects on the distribution of fragment charges. However, no odd effects were observed in the division of fragments mass numbers . This result is attributed to the breakdown of the nucleon pairs.
In nuclear fission events, nuclei may break into some lighter core combinations, but the most common event is no fission to the same mass nucleus of about 120 masses; the most common event (depending on the isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 and u and the remaining remaining 130 to 140 . An energetic unequal fission is more advantageous because it allows one product to be closer to the minimum mass near the 60 p energetic (only a quarter of the average fissionable mass), while another nucleus with a mass of 135 u is still not far from the closest bound nucleus range (another statement about this, is that the atomic binding energy curve is slightly sharper to the left of mass 120 u than on the right). Origin of the active energy and binding energy curve
The heavy nuclear fission generates exploitable energy because the specific binding energy (mass-per-mass energy) of the middle-mass nucleus with atomic number and atomic mass is close to 62 Ni and 56 Fe greater than the nucleon-specific binding energy of very heavy nuclei, so that energy is released when the heavy nucleus is split. The total mass of fission products remaining ( Mp ) from a single reaction is less than the original fuel core mass ( M ). Excess mass ? M Ã, -Ã, Mp is the invariant mass of energy released as photons (gamma rays) and kinetic energy of the fission fragments , corresponding to the mass-energy equivalence formula E Ã, = Ã, mc 2 .
The variation in specific binding energy with the atomic number is due to the interaction of two fundamental forces acting on the component nucleons (protons and neutrons) that form the nucleus. Nuclei are bound by an attractive nuclear force between nucleons, which overcome electrostatic repulsions between protons. However, the nuclear force acts only on a relatively short range (some diameter of the nucleons), as it follows the exponentially deteriorating potential of Yukawa that makes it insignificant at longer distances. Electrostatic demand is a longer range, as it decays with an inverse square rule, so that a nucleus larger than about 12 nucleons in diameter reaches the point that total electrostatic repulsion overcomes the nuclear force and causes them to become spontaneously unstable. For the same reason, larger nuclei (more than about eight nucleons in diameter) are less closely bound per unit mass than smaller nuclei; breaking the large nucleus into two or more middle-sized nuclei releasing energy.
Also because of the short range of strong binding forces, large stable nuclei must contain more neutrons proportionately than the lightest elements, most stable with protons and neutrons 1 to 1 . Nuclei that have more than 20 protons can not be stable unless they have more than the same number of neutrons. Extra neutrons stabilize heavy elements because they add a strong binding force (which works between all nucleons) without adding proton-proton repulsions. On average, fission products have the same neutron and proton ratios as their parent nuclei, and therefore are usually unstable against beta decay (which converts neutrons into protons) because they have too many neutrons compared to stable isotopes of the same mass.
This tendency for the fission product core to beta decay is the fundamental cause of the high-level radioactive waste problem from nuclear reactors. The fission product tends to be a beta transmitter, emitting fast-moving electrons to conserve electrical charges, since excess neutrons are converted to protons in fission product atoms. View Fission products (based on elements) for descriptions of fission products sorted by element.
Chain reactions
Some heavy elements, such as uranium, thorium, and plutonium, undergo spontaneous fission, a form of radioactive decay and fission induction , a form of nuclear reaction. The isotope of an element undergoing induction fission when struck by a free neutron is called fissionable; isotopes that experience fission when being hit by slow-moving thermal neutrons are also called fissile. Some of the most fissile and easily available isotopes (especially 233 U, 235 U and 239 Pu) are called nuclear fuel because they can retain the chain of reactions and can be obtained in large enough quantities to be useful.
All fission and fissile isotopes undergo a small amount of spontaneous fission that releases some free neutrons into nuclear fuel samples. Such neutrons will come out quickly from the fuel and become free neutrons, with an average lifespan of about 15 minutes before decaying into protons and beta particles. However, neutrons are almost always impacted and absorbed by other nuclei around them long before this happens (newly created fission neutrons move about 7% of the speed of light, and even a moderated neutron moves about 8 times the speed of sound). Some neutrons will affect the fuel core and further induce fission, releasing more neutrons. If enough nuclear fuel is assembled in one place, or if the escaping neutron is sufficiently contained, the newly emitted neutrons outnumber the neutrons that have escaped the assembly, and a sustained nuclear chain reaction will occur.
An assembly that supports a sustainable nuclear chain reaction is called a critical assembly or, if the assembly is almost entirely made of nuclear fuel, the mass is critical. The word "critical" refers to the culmination in the behavior of differential equations governing the amount of free neutrons present in the fuel: if less than the critical mass is present, then the number of neutrons is determined by radioactive decay, but if more or more critical mass is present then the number of neutrons is controlled by the physics of chain reaction. The true mass of the critical mass of nuclear fuel depends heavily on the geometry and the surrounding material.
Not all isotopes that can be destroyed can sustain a chain reaction. For example, 238 U, the most abundant form of uranium, is fissile but not fissile: it undergoes an induction fission when influenced by energetic neutrons with more than 1 MeV of kinetic energy. However, too few neutrons produced by the fission 238 U are sufficiently energetic to induce further fission in 238 U, so there is no possible chain reaction with this isotope. Conversely, bombarding 238 U with slow neutrons causes it to absorb it (being 239 U) and decayed by beta emission to 239 Np which then decays again with the process the same for 239 Pu; the process was used to produce 239 Pu in the breeder reactor. In-situ plutonium production also contributes to the neutron chain reaction in other types of reactors after sufficient plutonium-239 has been produced, since plutonium-239 is also a fissile element that serves as a fuel. It is estimated that nearly half of the power generated by a standard "non-breeder" reactor is produced by the fission of plutonium-239 produced in place, above the total fuel load life cycle.
Fisiables, non-fissile isotopes can be used as a fission energy source even without a chain reaction. Bombarding 238 U with fast neutrons induces fission, releasing energy as long as external neutron sources are present. This is an important effect in all reactors in which the rapid neutrons of the fissile isotope can cause a cleavage of nearby nuclei 238 U, which means that some small parts of 238 U are "Burned" in all nuclear fuel, especially in fast-breed reactors that operate with high-energy neutrons. The same fast-fission effect is used to increase the energy released by modern thermonuclear weapons, by coating weapons with 238 U to react with neutrons released by nuclear fusion at the center of the device. But the effect of nuclear fission chain reaction outbursts can be reduced by using substances like moderators that slow down the speed of secondary neutrons.
Fission reactor
Critical fission reactors are the most common type of nuclear reactor. In critical fission reactors, the neutrons generated by the fission of the fuel atom are used to encourage more fission, to maintain the controlled release amount of energy. Devices that produce fission reactions that are engineered but not independently are subcritical fission reactors. The device uses radioactive decay or a particle accelerator to trigger fission.
Critical fission reactors are built for three main purposes, which usually involve different trade-off techniques to harness the heat or neutrons produced by fission chain reactions:
- power plant is intended to generate heat for nuclear power, either as part of a generating station or a local power system such as a nuclear submarine.
- research reactors are intended to produce neutrons and/or activate radioactive sources for scientific, medical, engineering, or other research purposes.
- the breeder reactor is intended to mass-produce nuclear fuel from more isotopes. The better known breeder reactor makes 239 Pu (nuclear fuel) from 238 U very abundant (not nuclear fuel). The thermal batch reactor previously tested using 232 Th to breed the fissile isotope 233 U (thorium fuel cycle) continues to be studied and developed.
While, in principle, all fission reactors can act in all three capacities, in practice tasks lead to conflicting engineering objectives and most reactors have been built with only one of the above tasks in mind. (There are some early counter-examples, such as the Hanford N reactor, now disabled). The power plant generally converts the kinetic energy of the fission product into heat, which is used to heat the working fluid and drive a heat engine that produces mechanical or electrical power. The working fluids are usually water with steam turbines, but some designs use other materials such as helium gas. Research reactors produce neutrons that are used in various ways, with fission heat treated as an unavoidable waste product. The breeding reactor is a special form of the research reactor, with the caveat that the irradiated sample is usually the fuel itself, a mixture of 238 U and 235 U. For a more detailed description of physics and operating principles of a critical fission reactor, see the physics of a nuclear reactor. For a description of their social, political, and environmental aspects, see nuclear power.
Fantasy Bomb
A class of nuclear weapons, fission bombs (not to be confused with fusion bombs), otherwise known as atomic bombs or bomb atoms , is a fission reactor designed to release as much energy as quickly as possible, before the energy released causes the reactor to explode (and the chain reaction stops). The development of nuclear weapons was the motivation behind initial research into nuclear fission where the Manhattan Project during World War II (September 1, 1939 - September 2, 1945) performed most of the early scientific work on fission chain reactions, culminating in three events involving fission bombs that occurred during war. The first fission bomb, codenamed "The Gadget", was detonated during the Trinity Test in the New Mexico desert on July 16, 1945. Two other fission bombs, codenamed "Little Boy" and "Fat Man", were used in battles against cities, the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August 1945 respectively.
Even the first fission bombs were thousands of times more explosive than a comparable mass of chemical explosives. For example, Little Boy weighs a total of about four tons (of which 60 kg is nuclear fuel) and is 11 feet (3.4 m) long; it also produces an explosion equivalent to about 15 kilotons of TNT, destroying much of Hiroshima city. Modern nuclear weapons (which include thermonuclear fusion and one or more fission stages) are hundreds of times more energetic for their weight than the first pure atomic fission atom (see nuclear weapon results), so a modern single warhead bomb weighing less than 1/8 such as Little Boy (see for example W88) has a yield of 475,000 tons of TNT, and can cause damage about 10 times that of the city.
While the basic physics of fission chain reactions in nuclear weapons is similar to the physics of a controlled nuclear reactor, the two types of devices must be engineered quite differently (see physics of a nuclear reactor). A nuclear bomb is designed to release all its energy at once, while a reactor is designed to produce a stable, useful power supply. While excessive heat from the reactor can cause, and has caused, melting and steam explosions, the enrichment of the much lower uranium makes it impossible for nuclear reactors to explode with the same destructive force as nuclear weapons. It is also difficult to extract useful power from nuclear bombs, although at least one rocket propulsion system, Project Orion, is intended to work by detonating a fission bomb behind a large, rubber-lined spacecraft.
The strategic importance of nuclear weapons is the main reason why nuclear fission technology is politically sensitive. The design of a decent fission bomb, arguably, in many people's abilities, is relatively simple from a technical point of view. However, the difficulty of obtaining fissile fissile material for design is key to the relative unavailability of nuclear weapons for all but the modern industrial government with a special program to produce fissile material (see uranium enrichment and nuclear fuel cycle).
Maps Nuclear fission
History
The discovery of nuclear fission
The discovery of nuclear fission occurred in 1938 in the buildings of the Kaiser Wilhelm Society for Chemistry, today part of the Free University of Berlin, after nearly five decades working on the science of radioactivity and the elaboration of new nuclear physics describing the atomic components.
In 1911, Ernest Rutherford proposed an atomic model in which a very small, solid and positively charged nuclear nucleus (neutrons not yet discovered) is surrounded by negatively charged and orbiting electrons (Rutherford's model). Niels Bohr refined this in 1913 by reconciling the quantum behavior of the electron (Bohr model). The works of Henri Becquerel, Marie Curie, Pierre Curie, and Rutherford further elaborate that the nucleus, although firmly attached, can undergo various forms of radioactive decay, and thus transform into other elements. (For example, with alpha decay: emission of alpha particles - two protons and two neutrons bonded together into a particle identical to the helium nucleus.)
Some work in nuclear transmutation has been done. In 1917, Rutherford was able to complete the transmutation of nitrogen into oxygen, using alpha particles directed at nitrogen 14 N? -> 17 O p. This is the first observation of a nuclear reaction, that is, the reaction in which particles of a decay are used to change the nuclei of the other. Finally, in 1932, artificial nuclear reactions and nuclear transmutation were achieved by Rutherford's colleagues, Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium-7, to split this nucleus into two alpha particles. The feat was popularly known as "atom splitting", and would win them the 1951 Nobel Prize in Physics for "Transmutation of atomic nuclei by artificial atomic acceleration particles" , even though it was not a modern nuclear fission. The reaction is then found in the heavy elements, which are discussed below.
Meanwhile, the possibility of incorporating nuclear fusion nuclei has been studied in terms of understanding the processes supported by the stars. The first artificial fusion reaction was achieved by Mark Oliphant in 1932, using two accelerated deuterium nuclei (each consisting of one proton bound to one neutron) to create a helium-3 nucleus.
After British physicist James Chadwick discovered neutrons in 1932, Enrico Fermi and his colleagues in Rome studied the results of bombarding uranium with neutrons in 1934. Fermi concluded that his experiments had created new elements with 93 and 94 protons, called ausonium groups and hesperium. However, not all are convinced by Fermi's analysis of the results, although he will win the 1938 Nobel Prize in Physics for "demonstration of the existence of new radioactive elements produced by neutron irradiation, and for the discovery of nuclear-related reactions caused by slow neutrons".
The German chemist, Ida Noddack, especially suggested printing in 1934 that instead of creating a heavier new element 93, "it is conceivable that the nucleus breaks into several large fragments." However, Noddack's conclusions were not pursued at the time.
After the publication of Fermi, Otto Hahn, Lise Meitner, and Fritz Strassmann began conducting similar experiments in Berlin. Meitner, an Austrian Jew, lost his citizenship with the Austrian "Anschluss", occupation and annexation into Nazi Germany in March 1938, but he escaped in July 1938 to Sweden and initiated correspondence by mail with Hahn in Berlin. Coincidentally, his nephew Otto Robert Frisch, also a refugee, was also in Sweden when Meitner received a letter from Hahn dated December 19 which explains the chemical evidence that some of the products of bombarded uranium with neutrons are barium. Hahn suggests exploding the nucleus, but he is not sure what the physical basis for the outcome is. Bariums have an atomic mass of 40% less than uranium, and no previously known radioactive decay method can explain large differences in nuclear masses. Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie has separated the barium from radium for years, and the technique is very well known. According to Frisch:
Is that a mistake? No, says Lise Meitner; Hahn the chemist is too good for that. But how can barium form from uranium? No larger fragment of proton or helium nuclei (alpha particles) ever peeled from the nuclei, and to break up large amounts of insufficient energy available. Nor is it possible for the uranium nucleus to be split. A nucleus is not like a fragile solid that can be split or broken; George Gamow has suggested from the beginning, and Bohr has given a good argument that the nucleus is more like a liquid droplet. Maybe a drop can divide into two smaller drops in a more gradual way, by first being elongated, then narrowed, and eventually tearing instead of being broken into two? We know that there is a strong force that will resist such a process, just as the surface tension of ordinary liquid tends to deny its division into two smaller ones. But nuclei differ from ordinary drops in an important way: they are electrically charged, and they are known to resist surface tension.
The allegations of the uranium nucleus, we found, are indeed large enough to overcome the effects of surface tension almost completely; So the uranium nucleus may indeed resemble a very unstable decline, ready to divide in the slightest provocation, such as the impact of a neutron. But there is another problem. After separation, two drops will be pushed apart by their mutual electrical repulsion and will gain high speed and hence enormous energy, around 200 MeV in all; where does that energy come from?... Lise Meitner... succeeds that two nuclei formed by the division of the uranium nucleus together will be lighter than the original uranium nucleus of about one-fifth the mass of the proton. Now every time the mass disappears the energy is created, according to the formula Einstein E = mc 2 , and one fifth of the mass of the proton is only equivalent to 200 MeV. So here is the source of that energy; everything fits!
In short, Meitner and Frisch have correctly interpreted Hahn's results meaning that the uranium nucleus has been split into two. Frisch suggested the process was called "nuclear fission," by analogy of the process of dividing living cells into two cells, which were then called binary divisions. As the term "chain reaction" of nuclear would later be borrowed from chemistry, the term "fission" was borrowed from biology.
On December 22, 1938, Hahn and Strassmann sent a script to Naturwissenschaften reporting that they had discovered the element of barium after bombarding uranium with neutrons. At the same time, they communicate these results to Meitner in Sweden. He and Frisch correctly interpret the results as evidence of nuclear division. Frisch confirmed this experiment on January 13, 1939. To prove that the barium produced from his uranium bombardment with neutrons was a product of nuclear fission, Hahn was awarded the Nobel Prize for Chemistry in 1944 (the only recipient) "for his discovery of heavy nuclear fission". (The award was actually given to Hahn in 1945, because "the Nobel Committee of Chemistry decided that no nomination this year fulfilled the criteria outlined in the will of Alfred Nobel." In such a case, the Nobel Foundation law permits the prize of the year to be kept up to next year.)
The news spread rapidly from new discoveries, which are properly seen as entirely new physical effects with a potentially scientific - and potentially practical - opportunity. Meitner and Frisch's interpretation of Hahn and Strassmann's invention crossed the Atlantic Ocean with Niels Bohr, who was to attend Princeton University. I.I. Rabi and Willis Lamb, two Columbia University physicists working at Princeton, heard the news and took him back to Columbia. The rabbi says he told Enrico Fermi; Fermi rewarded Lamb. Bohr soon after that went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr descended into the cyclotron area and found Herbert L. Anderson. Bohr pulled his shoulder and said, "Young man, let me explain to you about something new and exciting in physics." It is clear to some scientists at Columbia that they should try to detect the energy released in nuclear uranium splits from neutron bombing. On January 25, 1939, a team of Columbia University conducted the first nuclear fission experiment in the United States, conducted in the basement of Pupin Hall; team members are Herbert L. Anderson, Eugene T. Booth, John R. Dunning, Enrico Fermi, G. Norris Glasoe, and Francis G. Slack. The experiment involves placing the uranium oxide inside an ionization chamber and irradiating it with neutrons, and measuring the energy released. The result confirms that fission has occurred and strongly suggests that the uranium isotope 235 is particularly the fission. The next day, the 5th Washington Conference on Theoretical Physics began in Washington, D.C. under the joint assistance of George Washington University and the Carnegie Institution of Washington. There, news of nuclear fission spreads further, leading to more experimental demonstrations.
During this period the Hungarian physicist LeÃÆ'³ SzilÃÆ'¡rd, who lived in the United States at the time, realized that neutron-driven fission of heavy atoms could be used to create nuclear chain reactions. Such a reaction using neutrons was the idea he first formulated in 1933, after reading Rutherford's disparaging comments about generating power from his 1932 team experiment using protons to break down lithium. However, SzilÃÆ'¡rd has not been able to achieve a neutron-driven chain reaction with light-rich neutron atoms. Theoretically, if in the neutron driven chain reaction the resulting number of secondary neutrons is greater than one, then each of these reactions can trigger some additional reactions, resulting in an increasing number of exponentially reactions. Thus it is likely that uranium fission can produce large amounts of energy for civil or military purposes (ie, power plants or atomic bombs).
Szilard now urges Fermi (in New York) and Father Joliot-Curie (in Paris) not to publicize a possible chain reaction, lest the Nazi government become aware of the possibilities of what night will come. known as World War II. With little hesitation, Fermi agreed to censor himself. But Joliot-Curie did not, and in April 1939 his team in Paris, including Hans von Halban and Lew Kowarski, reported in the journal Nature that the number of neutrons emitted with the 235 U is then reported at 3.5 per fission. (They then corrected this to 2.6 per fission.) Simultaneous work by Szilard and Walter Zinn confirm these results. The results indicate the possibility of building a nuclear reactor (first called "neutronic reactors" by Szilard and Fermi) and even nuclear bombs. However, much remains unknown about fission reactions and chain reactions.
Reaction chain reaction realized
The "chain reaction" at the time was a phenomenon known in chemistry, but the analogy process in nuclear physics, using neutrons, was predicted as early as 1933 by SzilÃÆ'¡rd, although SzilÃÆ'¡rd at the time did not know with what material the process might start. SzilÃÆ'¡rd considers that neutrons would be ideal for such situations, since they have no electrostatic charge.
With news of fission neutrons from fission uranium, SzilÃÆ'¡rd immediately understands the possibility of nuclear chain reactions using uranium. In the summer, Fermi and Szilard proposed the idea of ​​a nuclear reactor (pole) to mediate this process. The pile will use natural uranium as fuel. Fermi have shown earlier that neutrons are much more effectively captured by atoms if they have low energy (so-called "slow" or "thermal" neutrons), because for quantum reasons it makes atoms look like much larger targets to neutrons.. Thus to slow down the secondary neutrons released by fission uranium nuclei, Fermi and Szilard proposed graphite "moderators", which resisted rapid, high-energy secondary neutrons to collide, effectively slowing them down. With uranium sufficient, and with fairly pure graphite, their "piles" can theoretically support slow neutron chain reactions. This will produce heat production, as well as the creation of radioactive fission products.
In August 1939, Szilard and fellow Hungarian refugee physicists Teller and Wigner thought that Germany might be exploiting a fissional chain reaction and spurred to try to draw the attention of the United States government to this issue. Against this, they persuaded German-Jewish Albert Einstein refugees to lend his name to a letter addressed to President Franklin Roosevelt. The Einstein-SzilÃÆ'¡rd letter suggested the possibility of sending uranium bombs by ship, which would destroy "the entire port and many surrounding areas." The President received the letter on October 11, 1939 - shortly after World War II began in Europe, but two years before entering the US. Roosevelt ordered that the scientific committee be authorized to oversee uranium work and allocate small sums of money for stack research.
In England, James Chadwick proposed an atomic bomb utilizing natural uranium, based on a paper by Rudolf Peierls with the required mass for critical states being 30-40 tonnes. In America, J. Robert Oppenheimer thinks that a 10 cm uranium deuteride cube next to (about 11 kg of uranium) may "blow himself up to hell." In this design it is still thought that a moderator needs to be used for nuclear bomb division (this will not happen if the fissile isotope is separated). In December, Werner Heisenberg submitted a report to the German War Ministry about possible uranium bombs. Most of these models are still under the assumption that bombs will be supported by slow neutron reactions - and thus resemble crumbling reactors.
In Birmingham, England, Frisch worked with Peierls, a German-Jewish refugee. They have the idea of ​​using the purified mass of the uranium isotope 235 U, which has a specified cross section, and which is much larger than 238 U or naturally uranium (which is 99.3 % of the last isotope). Assuming that the cross-section for fast neutron splitting 235 U equals slow fission of neutrons, they determined that a pure <232 U bomb would have a critical mass of just 6 kg instead of tons, and the resulting explosion will be amazing. (In fact it was actually 15kg, although several times this amount was used in real uranium bombs.In February 1940 they delivered the Frisch-Peierls memorandum, ironically, they were still officially considered "foreign enemies" at the time Glenn Seaborg, Joseph W. Kennedy, Arthur Wahl, and Italian Jewish refugee Emilio SegrÃÆ'¨ soon found 239 Pu in decay products 239 U produced by bombarding 238 U with a neutron, and assign it to a fissile material, such as 235 U.
The possibility of isolating uranium-235 is technically frightening, since uranium-235 and uranium-238 are chemically identical, and vary in their mass with only the weight of three neutrons. However, if sufficient amounts of uranium-235 can be isolated, this will allow rapid neutron fission chain reactions. This will be very explosive, true "atomic bomb". The discovery that plutonium-239 can be produced in a nuclear reactor points toward another approach to a rapid neutron fission bomb. Both approaches are very new and not well understood, and there is considerable scientific skepticism on the idea that they can be developed in a short time.
On June 28, 1941, the Office of Scientific Research and Development was formed in the US to mobilize scientific resources and apply research results to national defense. In September, Fermi collected his first "reactor" or nuclear reactor, in an attempt to create an induced neutron chain reaction in uranium, but the experiment failed to achieve criticality, due to lack of appropriate material, or insufficient accuracy. ingredients available.
Generating a fissional chain reaction in natural uranium fuel is found far from trivial. Early nuclear reactors did not use isotopic enriched uranium, and therefore they were required to use large quantities of highly purified graphite as a neutron moderating material. The use of plain water (compared to heavy water) in nuclear reactors requires enriched fuel - partial separation and relative enrichment of the rare <23> U isotope from the more common <238> U isotope supply. Typically, the reactor also requires the entry of chemically pure neutron moderator materials such as deuterium (in heavy water), helium, beryllium, or carbon, the latter usually as graphite. (High purity for carbon is required because many chemical impurities such as boron-10 natural boron components, are powerful neutron dampers and thus poison the chain reaction and terminate prematurely.)
Production of such materials on an industrial scale must be solved for nuclear power and weapons production to be completed. Until 1940, the total amount of uranium metal produced in the US was no more than a few grams, and even this was a doubtful purity; of metallic beryllium not more than a few kilograms; and concentrated deuterium oxide (heavy water) is not more than a few kilograms. Finally, carbon is never produced in quantity with something like the purity that a moderator needs.
The problem of producing large quantities of high purity uranium is solved by Frank Spedding using the thermite or "Ames" process. The Ames Laboratory was established in 1942 to produce the large quantities of uranium (non-enriched) metal needed for future research. The success of chain-critical nuclear reactions of Chicago Pile-1 (December 2, 1942) using unenriched uranium (natural), like all "piles" of atoms that produce plutonium for atomic bombs, also due specifically to Szilard's realization that very pure graphite used to moderate "piles" of natural uranium. In German wartime, the failure to appreciate the very pure graphite quality caused the design of the reactor to depend on heavy water, which in turn was rejected by the German Allied attack in Norway, where heavy water was produced. These difficulties - among many others - prevented the Nazis from building nuclear reactors capable of being critical during the war, although they never attempted as much as the United States in nuclear research, focusing on other technologies (see German nuclear energy project for more details).
Manhattan Project and beyond
In the United States, an all-out effort to make atomic weapons began in late 1942. This work was taken over by the US Army Engineer Corps in 1943, and is known as the Manhattan Engineer District. The secret Manhattan project, as it is known everyday, is headed by General Leslie R. Groves. Among the dozens of project sites are: Hanford Site in Washington state, which has the first industrial-scale nuclear reactor; Oak Ridge, Tennessee, which deals primarily with uranium enrichment; and Los Alamos, in New Mexico, which is a scientific center for research on the development and design of bombs. Other sites, notably the Berkeley Radiation Laboratory and the Metallurgical Laboratory at the University of Chicago, play a significant role in contributing. The overall scientific direction of the project is managed by physicist J. Robert Oppenheimer.
In July 1945, the first atomic explosive device, dubbed "Trinity", was detonated in the New Mexico desert. Fueled by plutonium made at Hanford. In August 1945, two more atomic devices - "Little Boy", uranium-235 bombs, and "Fat Man", plutonium bombs - were used against Japanese cities, Hiroshima and Nagasaki.
In the years following World War II, many countries were involved in the further development of nuclear fission for the purposes of nuclear and nuclear reactors. Britain opened its first commercial nuclear power plant in 1956. By 2013, there are 437 reactors in 31 countries.
Natural fission chain reactor on Earth
Criticality in nature is rare. At three ore deposits in Oklo in Gabon, sixteen sites (called Oklo Fossil Reactor) have been found where independent nuclear fission took place about 2 billion years ago. Unknown until 1972 (but postulated by Paul Kuroda in 1956), when the French physicist Francis Perrin discovered the Oklo Fossil Reactor, it was realized that nature had beaten humans. The reaction of large-scale natural uranium fission chains, moderated by normal water, has occurred deep in the past and will not be possible now. This ancient process was able to use ordinary water as a moderator simply because 2 billion years before this time, natural uranium is richer in short-lived phisical isotopes 235 U (about 3%), from natural uranium available today only 0.7%, and should be enriched to 3% for use in light water reactors).
See also
- Cold fission
- Hybrid fusion/fission
- nuclear fusion
- The nuclear drive
- Photophots
References
Further reading
- DOE Fundamentals Handbook: Nuclear Physics and Volume 1 Reactor Theory (PDF) . US Department of Energy. January 1993. Archived from the original (PDF) in 2014-03-19 . Retrieved 2012-01-03 .
- DOE Fundamentals Handbook: Nuclear Physics and Volume 2 Reactor Theory (PDF) . US Department of Energy. January 1993. Archived from the original (PDF) in 2013-12-03 . Retrieved 2012-01-03 .
External links
- Nuclear Weapon Effects
- Bibliography annotated for nuclear fission from Alsos Digital Library
- The Discovery of Nuclear Fission Historical account complete with audio guides and teachers from the American Institute of Physics History Center
- atomicarchive.com Explained Nuclear Fuel
- Nuclear Files.org What is Nuclear Fission?
- Nuclear Fission Animation
Source of the article : Wikipedia
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