query: the minimum mass of a sample of fissionable material necessary to sustain a nuclear chain reaction

pos: Nuclear chain reaction and intense it cannot be controlled after it has started. When properly designed, this uncontrolled reaction can lead to an explosive energy release. Nuclear weapons employ high quality, highly enriched fuel exceeding the critical size and geometry (critical mass) necessary in order to obtain an explosive chain reaction. The fuel for energy purposes, such as in a nuclear fission reactor, is very different, usually consisting of a low-enriched oxide material (e.g. UO). When a heavy atom undergoes nuclear fission, it breaks into two or more fission fragments. Also, several free neutrons, gamma rays, and neutrinos are emitted, and a large

pos: Nuclear fission if a critical mass or more is present, then the amount of neutrons is controlled instead by the physics of the chain reaction. The actual mass of a "critical mass" of nuclear fuel depends strongly on the geometry and surrounding materials. Not all fissionable isotopes can sustain a chain reaction. For example, U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. However, too few of the neutrons produced by U fission are energetic enough to induce further fissions in U,

pos: Critical mass the high energy neutrons generated by the primary explosion. This can greatly increase yield, especially if even more neutrons are generated by fusing hydrogen isotopes, in a so-called boosted configuration. The critical size is the minimum size of a nuclear reactor core or nuclear weapon that can be made for a specific geometrical arrangement and material composition. The critical size must at least include enough fissionable material to reach critical mass. If the size of the reactor core is less than a certain minimum, too many fission neutrons escape through its surface and the chain reaction is not sustained. The

pos: Critical mass Critical mass A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. The concept is important in nuclear weapon design. When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a "critical" state in which there is no increase or decrease in power, temperature, or neutron population. A numerical measure of a critical

pos: Critical mass measurement of the reactivity is that suggested by Louis Slotin: that of the dollar and cents. Critical mass A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. The concept is important in nuclear weapon design. When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a "critical" state in which there is

pos: History of nuclear weapons proposed, and in 1934 patented, the idea of a nuclear chain reaction via neutrons. The patent also introduced the term critical mass to describe the minimum amount of material required to sustain the chain reaction and its potential to cause an explosion. (British patent 630,726.) He subsequently assigned the patent to the British Admiralty so that it could be covered by the Official Secrets Act. In a very real sense, Szilárd was the father of the atomic bomb academically. Also in 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with

pos: Tritium the fission reaction in the fissionable core (pit) of the bomb, after it is compressed to a critical mass by explosives. Actuated by an ultrafast switch like a krytron, a small particle accelerator drives ions of tritium and deuterium to energies above the 15 keV or so needed for deuterium-tritium fusion and directs them into a metal target where the tritium and deuterium are adsorbed as hydrides. High-energy fusion neutrons from the resulting fusion radiate in all directions. Some of these strike plutonium or uranium nuclei in the primary's pit, initiating nuclear chain reaction. The quantity of neutrons produced is

pos: Uranium-235 Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235. Highly enriched uranium (HEU), which contains an even greater proportion of uranium-235, is sometimes used in nuclear weapon design. If at least one neutron from uranium-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from

pos: Berkelium fuel in a nuclear reactor. Specifically, berkelium-249 has a moderately large neutron capture cross section of 710 barns for thermal neutrons, 1200 barns resonance integral, but very low fission cross section for thermal neutrons. In a thermal reactor, much of it will therefore be converted to berkelium-250 which quickly decays to californium-250. In principle, berkelium-249 can sustain a nuclear chain reaction in a fast breeder reactor. Its critical mass is relatively high at 192 kg; it can be reduced with a water or steel reflector but would still exceed the world production of this isotope. Berkelium-247 can maintain chain reaction

pos: Tube Alloys of the size of the critical mass that would allow a chain reaction to take place and its practical significance. In the interior of a critical mass sphere, neutrons are spontaneously produced by the fissionable material. A very small portion of these neutrons are colliding with other nuclei, while a larger portion of the neutrons are escaping through the surface of the sphere. Peierls calculated the equilibrium of the system, where the number of neutrons being produced equalled the number escaping. Niels Bohr had theorised that the rare uranium-235 isotope, which makes up only about 0.7% of natural uranium, was

pos: Nuclear reactor which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly 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"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short

pos: Isotopes of americium has an odd number of neutrons) and has a low critical mass, comparable to that of Pu. It has a very high cross section for fission, and if in a nuclear reactor is destroyed relatively quickly. Another report claims that Am has a much lower critical mass, can sustain a chain reaction even as a thin film, and could be used for a novel type of nuclear rocket. Americium-243 has a mass of 243.06138 g/mol and a half-life of 7,370 years, the longest lasting of all americium isotopes. It is formed in the nuclear fuel cycle by neutron capture on

pos: Nuclear technology neutrons are captured by another unstable nucleus, they can fission as well, leading to a chain reaction. The average number of neutrons released per nucleus that go on to fission another nucleus is referred to as "k". Values of "k" larger than 1 mean that the fission reaction is releasing more neutrons than it absorbs, and therefore is referred to as a self-sustaining chain reaction. A mass of fissile material large enough (and in a suitable configuration) to induce a self-sustaining chain reaction is called a critical mass. When a neutron is captured by a suitable nucleus, fission may occur

pos: Tube Alloys a self-sustaining nuclear chain reaction might be possible. The term "atomic bomb" was already familiar to the British public through the writings of H. G. Wells, in his 1913 novel "The World Set Free". It was immediately apparent to many scientists that, in theory at least, an extremely powerful explosive could be created, although most still considered an atomic bomb was an impossibility. Perrin defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction. The neutrons used to cause fission in uranium are considered slow neutrons, but when neutrons are released during a

pos: Critical mass (sociodynamics) Critical mass (sociodynamics) In social dynamics, critical mass is a sufficient number of adopters of an innovation in a social system so that the rate of adoption becomes self-sustaining and creates further growth. The term is borrowed from nuclear physics and in that field it refers to the amount of a substance needed to sustain a chain reaction. Social factors influencing critical mass may involve the size, interrelatedness and level of communication in a society or one of its subcultures. Another is social stigma, or the possibility of public advocacy due to such a factor. Critical mass may be closer

pos: Criticality accident Criticality accident A criticality accident is an uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion or a critical power excursion or a divergent chain reaction. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses, if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self-sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core

pos: Nuclear pulse propulsion reactions can start the chain reaction that would otherwise require a much larger volume of fuel to sustain. Whereas the "normal" critical mass for plutonium is about 11.8 kilograms (for a sphere at standard density), with antimatter catalyzed reactions this could be well under one gram. Several rocket designs using this reaction were proposed, some which would use all-fission reactions for interplanetary missions, and others using fission-fusion (effectively a very small version of Orion's bombs) for interstellar missions. NASA funded MSNW LLC and the University of Washington in 2011 to study and develop a fusion rocket through the NASA Innovative

pos: Otto Robert Frisch and Rudolf Peierls worked together in the Physics Department at the University of Birmingham 1939–40. He went to America in 1943 having been hurriedly made a British citizen. In 1944 at Los Alamos, one of Frisch's tasks as the leader of the Critical Assemblies group was to accurately determine the exact amount of enriched uranium which would be required to create the critical mass, the mass of uranium which would sustain a nuclear chain reaction. He did this by stacking several dozen 3 cm bars of enriched uranium hydride at a time and measuring rising neutron activity as the critical

pos: Nuclear weapon design criticallity is modified by shape, purity, density, and the proximity to neutron-reflecting material, all of which affect the escape or capture of neutrons. To avoid a chain reaction during handling, the fissile material in the weapon must be sub-critical before detonation. It may consist of one or more components containing less than one uncompressed critical mass each. A thin hollow shell can have more than the bare-sphere critical mass, as can a cylinder, which can be arbitrarily long without ever reaching criticality. A "tamper" is an optional layer of dense material surrounding the fissile material. Due to its inertia it

pos: Isotopes of neptunium This particular isotope of neptunium has a mass of 236.04657 u. It is a fissile material with a critical mass of 6.79 kg. decays via the neptunium series, which terminates with thallium-205, which is stable, unlike most other actinides, which decay to stable isotopes of lead. In 2002, was shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon, with a critical mass of around 60 kg. However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for conventional nuclear power plants (as

pos: MAUD Committee critical mass was still only about 8 kilograms. Meanwhile, Pryce investigated how long a runaway nuclear chain reaction in an atomic bomb would continue before it blew itself apart. He calculated that since the neutrons produced by fission have an energy of about this corresponded to a speed of . The major part of the chain reaction would be completed in the order of (ten "shakes"). From 1 to 10 per cent of the fissile material would fission in this time; but even an atomic bomb with 1 per cent efficiency would release as much energy as 180,000 times its

pos: Nuclear fission escaping neutrons are sufficiently contained, then these freshly emitted neutrons outnumber the neutrons that escape from the assembly, and a "sustained nuclear chain reaction" will take place. An assembly that supports a sustained nuclear chain reaction is called a critical assembly or, if the assembly is almost entirely made of a nuclear fuel, a critical mass. The word "critical" refers to a cusp in the behavior of the differential equation that governs the number of free neutrons present in the fuel: if less than a critical mass is present, then the amount of neutrons is determined by radioactive decay, but

pos: History of nuclear weapons on Earth, but is fissile like uranium-235. After Fermi achieved the world's first sustained and controlled nuclear chain reaction with the creation of the first atomic pile, massive reactors were secretly constructed at what is now known as Hanford Site to transform uranium-238 into plutonium for a bomb. The simplest form of nuclear weapon is a gun-type fission weapon, where a sub-critical mass would be shot at another sub-critical mass. The result would be a super-critical mass and an uncontrolled chain reaction that would create the desired explosion. The weapons envisaged in 1942 were the two gun-type weapons, Little Boy

pos: Nuclear reactor physics to withdraw more control rods to achieve this. As a result, the overdriven reaction grew rapidly and uncontrollably, leading to steam explosion in the core, and violent destruction of the facility. While many fissionable isotopes exist in nature, the only usefully fissile isotope found in any quantity is U. About 0.7% of the uranium in most ores is the 235 isotope, and about 99.3% is the non-fissile 238 isotope. For most uses as a nuclear fuel, uranium must be "enriched" - purified so that it contains a higher percentage of U. Because U absorbs fast neutrons, the critical mass needed

pos: Critical mass than the mean free path, such an approximation is only marginally applicable. Finally, note that for some idealized geometries, the critical mass might formally be infinite, and other parameters are used to describe criticality. For example, consider an infinite sheet of fissionable material. For any finite thickness, this corresponds to an infinite mass. However, criticality is only achieved once the thickness of this slab exceeds a critical value. Until detonation is desired, a nuclear weapon must be kept subcritical. In the case of a uranium bomb, this can be achieved by keeping the fuel in a number of separate pieces,

pos: MAUD Committee reached a different conclusion. Oliphant had delegated the task to two German refugee scientists, Rudolf Peierls and Otto Frisch, who could not work on Oliphant's radar project because they were enemy aliens and therefore lacked the necessary security clearance. Francis Perrin had defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction, and had calculated it to be about . He reckoned that if a neutron reflector were placed around it, this might be reduced to . In a theoretical paper written in 1939, Peierls attempted to simplify the problem by using the

pos: Criticality accident has caused significant mechanical damage or even small explosions. Criticality occurs when sufficient fissile material (a "critical mass") is in one place such that each fission of an atom of the material, on average, produces a neutron that in turn strikes another atom causing another fission; this causes the chain reaction to become self-sustaining within the mass of material. Criticality can be achieved by using metallic uranium or plutonium or by mixing compounds or liquid solutions of these elements. The chain reaction is influenced by range of parameters noted by the acronyms MAGIC MERV (for Mass, Absorption, Geometry, Interaction, Concentration,

pos: Neutron moderator of using a moderator in a nuclear explosive is that the amount of fissile material needed to reach criticality may be greatly reduced. Slowing of fast neutrons will increase the cross section for neutron absorption, reducing the critical mass. A side effect is however that as the chain reaction progresses, the moderator will be heated, thus losing its ability to cool the neutrons. Another effect of moderation is that the time between subsequent neutron generations is increased, slowing down the reaction. This makes the containment of the explosion a problem; the inertia that is used to confine implosion type bombs

pos: Enriched uranium stage, but the jacket or tamper secondary stage, which is compressed by the primary nuclear explosion often uses HEU with enrichment between 40% and 80% along with the fusion fuel lithium deuteride. For the secondary of a large nuclear weapon, the higher critical mass of less-enriched uranium can be an advantage as it allows the core at explosion time to contain a larger amount of fuel. The U is not fissile but still fissionable by fusion neutrons. HEU is also used in fast neutron reactors, whose cores require about 20% or more of fissile material, as well as in naval

pos: Stanislaw Ulam and to make slow separation of uranium isotopes the only way to prepare fissile material suitable for use in bombs. To respond, Oppenheimer implemented, in August, a sweeping reorganization of the laboratory to focus on development of an implosion-type weapon and appointed George Kistiakowsky head of the implosion department. He was a professor at Harvard and an expert on precise use of explosives. The basic concept of implosion is to use chemical explosives to crush a chunk of fissile material into a critical mass, where neutron multiplication leads to a nuclear chain reaction, releasing a large amount of energy. Cylindrical

neg: Fissile material in the correct setting. Under this definition, the only nuclides that are fissionable but not fissile are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction. As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control context, particularly in proposals for a Fissile Material Cutoff Treaty, the term "fissile" is often used to describe materials that can be used in the fission primary of a nuclear weapon. These are materials that sustain an explosive fast neutron

neg: Fissile material nuclear fission chain reaction. Under all definitions above, uranium-238 () is fissionable, but because it cannot sustain a neutron chain reaction, it is not fissile. Neutrons produced by fission of have lower energies than the original neutron (they behave as in an inelastic scattering), usually below 1 MeV (i.e., a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of , so fission of does not sustain a nuclear chain reaction. Fast fission of in the secondary stage of a nuclear weapon contributes greatly to yield and to fallout. The fast fission of also makes a significant

neg: Nuclear fission a form of radioactive decay and "induced fission", a form of nuclear reaction. Elemental isotopes that undergo induced fission when struck by a free neutron are called fissionable; isotopes that undergo fission when struck by a slow-moving thermal neutron are also called fissile. A few particularly fissile and readily obtainable isotopes (notably U, U and Pu) are called nuclear fuels because they can sustain a chain reaction and can be obtained in large enough quantities to be useful. All fissionable and fissile isotopes undergo a small amount of spontaneous fission which releases a few free neutrons into any sample of

neg: Fissile material the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is a fissile material. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is a fissionable material but not a fissile material. An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable) and also produce neutrons from such fission that can sustain a nuclear chain reaction

neg: Nuclear fuel cycle fuel is reprocessed, it is referred to as a "closed fuel cycle". Nuclear power relies on fissionable material that can sustain a chain reaction with neutrons. Examples of such materials include uranium and plutonium. Most nuclear reactors use a moderator to lower the kinetic energy of the neutrons and increase the probability that fission will occur. This allows reactors to use material with far lower concentration of fissile isotopes than are needed for nuclear weapons. Graphite and heavy water are the most effective moderators, because they slow the neutrons through collisions without absorbing them. Reactors using heavy water or graphite

neg: Chain reaction Enrico Fermi, Szilárd proved this neutron-multiplying reaction in uranium. In this reaction, a neutron plus a fissionable atom causes a fission resulting in a larger number of neutrons than the single one that was consumed in the initial reaction. Thus was born the practical nuclear chain reaction by the mechanism of neutron-induced nuclear fission. Specifically, if one or more of the produced neutrons themselves interact with other fissionable nuclei, and these also undergo fission, then there is a possibility that the macroscopic overall fission reaction will not stop, but continue throughout the reaction material. This is then a self-propagating and

neg: Nuclear weapon design and 239). Uranium's most common isotope, U-238, is fissionable but not fissile (meaning that it cannot sustain a chain reaction by itself but can be made to fission with fast neutrons). Its aliases include natural or unenriched uranium, depleted uranium (DU), tubealloy (Tu), and 28. It cannot sustain a chain reaction, because its own fission neutrons are not powerful enough to cause more U-238 fission. The neutrons released by fusion will fission U-238. This U-238 fission reaction produces most of the energy in a typical two-stage thermonuclear weapon. Fusion produces neutrons which dissipate energy from the reaction. In weapons, the

neg: Nuclear fuel cycle typically quite small compared to that converted to UF. The natural concentration (0.71%) of the fissionable isotope U-235 is less than that required to sustain a nuclear chain reaction in light water reactor cores. Accordingly UF produced from natural uranium sources must be enriched to a higher concentration of the fissionable isotope before being used as nuclear fuel in such reactors. The level of enrichment for a particular nuclear fuel order is specified by the customer according to the application they will use it for: light-water reactor fuel normally is enriched to 3.5% U-235, but uranium enriched to lower concentrations

neg: Operation Castle "Castle" series. Even though it used lithium fuel for fission boosting, the principal reaction material in the second stage was uranium and plutonium. Similar to the Teller-Ulam configuration, a nuclear fission explosion was used to create high temperatures and pressures to compress a second fissionable mass. This would have otherwise been too large to sustain an efficient reaction if it were triggered with conventional explosives. This experiment was intended to develop intermediate yield weapons for expanding the inventory (around 1-2 Mt vs. 4-8). Many fusion or thermonuclear weapons generate much, or even most, of their yields from fission. Although the

neg: Nuclear fusion–fission hybrid fission, both of these elements release fast neutrons with an energy distribution peaking around 1 to 2 MeV. This energy is too low to cause fission in U-238, which means it cannot sustain a chain reaction. U-235 will undergo fission when struck by neutrons of this energy, so it is possible for U-235 to sustain a chain reaction, as is the case in a nuclear bomb. However, the probability of one neutron causing fission in another U-235 atom before it escapes the fuel is too low to maintain criticality in a mass of natural uranium, so the chain reaction can

neg: Fissile material Fissile material In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives. According to the Ronen fissile rule, for a heavy element with 90 ≤ "Z" ≤ 100, its isotopes with 2 × "Z" − "N" = 43 ± 2, with few exceptions, are fissile (where "N" = number

neg: Fissile material for fission (σ), the cross section for neutron capture with emission of a gamma ray (σ), and the percentage of non-fissions are in the table at right. Fissile material In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives. According to the Ronen fissile rule, for a heavy element

neg: Nuclear fission and in weapons). As noted above, the subgroup of fissionable elements that may be fissioned efficiently with their own fission neutrons (thus potentially causing a nuclear chain reaction in relatively small amounts of the pure material) are termed "fissile." Examples of fissile isotopes are uranium-235 and plutonium-239. Typical fission events release about two hundred million eV (200 MeV) of energy, the equivalent of roughly >2 trillion Kelvin, for each fission event. The exact isotope which is fissioned, and whether or not it is fissionable or fissile, has only a small impact on the amount of energy released. This can be

neg: Nuclear reaction a spontaneous process). Natural nuclear reactions occur in the interaction between cosmic rays and matter, and nuclear reactions can be employed artificially to obtain nuclear energy, at an adjustable rate, on demand. Perhaps the most notable nuclear reactions are the nuclear chain reactions in fissionable materials that produce induced nuclear fission, and the various nuclear fusion reactions of light elements that power the energy production of the Sun and stars. Nuclear reactions may be shown in a form similar to chemical equations, for which invariant mass must balance for each side of the equation, and in which transformations of particles

neg: Critical mass mass is dependent on the effective neutron multiplication factor , the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. When "k" = 1, the mass is critical, and the chain reaction is self-sustaining. A "subcritical" mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, "k" < 1. A steady rate of spontaneous fissions causes a proportionally steady level of

neg: Organic nuclear reactor undergo fission, both of these isotopes release fast neutrons with an energy distribution peaking around 1 to 2 MeV. This energy is too low to cause fission in U-238, which means it cannot sustain a chain reaction. U-235 will undergo fission when struck by neutrons of this energy, so it is possible for U-235 to sustain a chain reaction, as is the case in a nuclear bomb. However, there is too little U-235 in a mass of natural uranium, and the chance any given neutron will cause fission in these isolated atoms is not high enough to reach criticality. Criticality

neg: Nuclear reactor physics place, buildup of long-lived neutron absorbing fission products damps out the chain reaction. This is the reason that nuclear reprocessing is a useful activity: spent nuclear fuel contains about 96% of the original fissionable material present in newly manufactured nuclear fuel. Chemical separation of the fission products restores the nuclear fuel so that it can be used again. Nuclear reprocessing is useful economically because chemical separation is much simpler to accomplish than the difficult isotope separation required to prepare nuclear fuel from natural uranium ore, so that in principle chemical separation yields more generated energy for less effort than mining,

neg: Neutron poison fuel eventually leads to loss of efficiency, and in some cases to instability. In practice, buildup of reactor poisons in nuclear fuel is what determines the lifetime of nuclear fuel in a reactor: long before all possible fissions have taken place, buildup of long-lived neutron-absorbing fission products damps out the chain reaction. This is the reason that nuclear reprocessing is a useful activity: solid spent nuclear fuel contains about 97% of the original fissionable material present in newly manufactured nuclear fuel. Chemical separation of the fission products restores the fuel so that it can be used again. Other potential approaches

neg: Xenon nuclear spin value of and a nonzero quadrupole moment, and has t relaxation times in the millisecond and second ranges. Some radioactive isotopes of xenon (for example, Xe and Xe) are produced by neutron irradiation of fissionable material within nuclear reactors. Xe is of considerable significance in the operation of nuclear fission reactors. Xe has a huge cross section for thermal neutrons, 2.6×10 barns, and operates as a neutron absorber or "poison" that can slow or stop the chain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by the American Manhattan Project for

neg: Nuclear fission neutrons and cause more fission. The chemical element isotopes that can sustain a fission chain reaction are called nuclear fuels, and are said to be "fissile". The most common nuclear fuels are U (the isotope of uranium with an atomic mass of 235 and of use in nuclear reactors) and Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 u (fission products). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha-beta decay chain

neg: Uranium isotopes are fissionable, but not fissile. On bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy and more neutrons. If too many of these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs that results in a burst of heat or (in special circumstances) an explosion. In a nuclear reactor, such a chain reaction is slowed and controlled by a neutron poison, absorbing some of the free neutrons. Such neutron absorbent materials are often part of reactor control rods (see nuclear reactor physics for a

neg: Critical mass than a microsecond, far too fast for man, or even machine, to react. Physicists recognize two points in the gradual increase of neutron flux which are significant: critical, where the chain reaction becomes self-sustaining thanks to the contributions of both kinds of neutron generation, and prompt critical, where the immediate "prompt" neutrons alone will sustain the reaction without need for the decay neutrons. Nuclear power plants operate between these two points of reactivity, while above the prompt critical point is the domain of nuclear weapons and some nuclear power accidents, such as the Chernobyl disaster. A convenient unit for the

neg: Uranium-238 most famous for the determination of the age of the Earth. The Voyager spacecrafts carry small amounts of initially pure U on the covers of their golden records to facilitate dating in the same manner. Uranium-238 Uranium-238 (U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction. However, it is fissionable by fast neutrons, and is "fertile", meaning it can be transmuted to fissile plutonium-239. U cannot support a chain reaction because inelastic scattering reduces neutron energy below

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

neg: Nuclear reactor physics CANDU power reactors used in Canadian power plants are an example of this type. Uranium enrichment is difficult because the chemical properties of U and U are identical, so physical processes such as gaseous diffusion, gas centrifuge or mass spectrometry must be used for isotopic separation based on small differences in mass. Because enrichment is the main technical hurdle to production of nuclear fuel and simple nuclear weapons, enrichment technology is politically sensitive. Modern deposits of uranium contain only up to ~0.7% U (and ~99.3% U), which is not enough to sustain a chain reaction moderated by ordinary water. But

neg: Arthur Jeffrey Dempster the first modern mass spectrometer, a scientific apparatus allowing physicists to identify compounds by the mass of elements in a sample, and determine the isotopic composition of elements in a sample. Dempster's mass spectrometer was over 100 times more accurate than previous versions, and established the basic theory and design of mass spectrometers that is still used to this day. Dempster's research over his career centered on the mass spectrometer and its applications, leading in 1935 to his discovery of the uranium isotope U. This isotope's ability to cause a rapidly expanding fission nuclear chain reaction allowed the development of

neg: Neutron radiation energy distribution (the Maxwell–Boltzmann distribution) to a gas in thermodynamic equilibrium but are easily captured by atomic nuclei and are the primary means by which elements undergo nuclear transmutation. To achieve an effective fission chain reaction, neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the nuclear fuel is not sufficiently refined to absorb enough fast neutrons to carry on the chain reaction, due to the lower cross section for higher-energy neutrons, so a neutron moderator must be introduced to slow the fast neutrons down to thermal velocities

neg: Critical mass free neutron of the desired energy level in order to sustain a chain reaction, and each must find other nuclei and cause them to fission. Most of the neutrons released from a fission event come immediately from that event, but a fraction of them come later, when the fission products decay, which may be on the average from microseconds to minutes later. This is fortunate for atomic power generation, for without this delay "going critical" would always be an immediately catastrophic event, as it is in a nuclear bomb where upwards of 80 generations of chain reaction occur in less

neg: Plutonium element with the highest atomic number to occur in nature. Trace quantities arise in natural uranium-238 deposits when U-238 captures neutrons emitted by decay of other U-238 atoms. Plutonium is much more common on Earth since 1945 as a product of neutron capture and beta decay, where some of the neutrons released by the fission process convert uranium-238 nuclei into plutonium-239. Both plutonium-239 and plutonium-241 are fissile, meaning that they can sustain a nuclear chain reaction, leading to applications in nuclear weapons and nuclear reactors. Plutonium-240 exhibits a high rate of spontaneous fission, raising the neutron flux of any sample

neg: Fast-neutron reactor Fast-neutron reactor A fast-neutron reactor (FNR) or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies of 5 MeV or greater), as opposed to thermal neutrons used in thermal-neutron reactors. Such a reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. In order to sustain a fission chain reaction, the neutrons released in fission events have to react with other atoms in the fuel. The chance of this occurring depends on

neg: Spent fuel pool and overall by the fact that the spent fuel has too low an enrichment level to self-sustain a fission reaction. They also state that if the water covering the spent fuel evaporates, there is no element to enable a chain reaction by moderating neutrons. According to Dr. Kevin Crowley of the Nuclear and Radiation Studies Board, "successful terrorist attacks on spent fuel pools, though difficult, are possible. If an attack leads to a propagating zirconium cladding fire, it could result in the release of large amounts of radioactive material." After the September 11, 2001 attacks the Nuclear Regulatory Commission required

neg: W48 slightly reshape the nuclear material into a supercritical mass which then begins a chain reaction, goes critical, and explodes in a small nuclear blast. Three methods are known to compress and reshape the nuclear material: collapsing hollow spaces inside the nuclear material, using plutonium-gallium alloy which is stabilized in the low density delta phase at a density of (and which collapses to denser alpha-phase under moderate explosive compression), and shaping an explosive around the nuclear material so that the explosive pressure causes a stretched-out elliptical or rugby ball shape to collapse towards a more spherical end shape, which decreases the

neg: Criticality (status) Criticality (status) Criticality, is the state of a nuclear chain reacting medium when the chain reaction is just self-sustaining (or critical), that is, when the reactivity is zero. More loosely, the term is used for states in which the reactivity is greater than zero. In the context of a nuclear reactor, particularly in a nuclear power plant, criticality refers to the normal operating condition of a reactor, in which nuclear fuel sustains a fission chain reaction. A reactor achieves criticality, (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing

neg: W79 to firing the weapon and then, small to moderate amounts of explosive collapse and slightly reshape the nuclear-material into a supercritical-mass which then undergoes chain-reaction and explodes. Three methods are known to compress and reshape the nuclear-material: 1) collapsing hollow spaces inside the nuclear material; 2) using plutonium-gallium alloy, which is stabilized in the low-density delta-phase at a density of 16.4 (and which collapses to denser alpha-phase under moderate explosive-compression); and 3) shaping an explosive and nuclear material so that the explosive pressure changes a stretched-out, elliptical or rugby-football shape to collapse towards a spherical or more spherical end-shape. A

neg: Thorium fuel cycle Thorium fuel cycle The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as ), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, absorbs neutrons to produce . This parallels the process in uranium breeder reactors whereby fertile absorbs neutrons to form

neg: Nuclear fission the most common event is not fission to equal mass nuclei of about mass 120; the most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 u and the other the remaining 130 to 140 u. Unequal fissions are energetically more favorable because this allows one product to be closer to the energetic minimum near mass 60 u (only a quarter of the average fissionable mass), while the other nucleus with mass 135 u is still not far out of the range of the

neg: Nuclear chain reaction = 1/(1-β) is known as delayed supercriticality (or delayed criticality). It is in this region that all nuclear power reactors operate. The region of supercriticality for k > 1/(1-β) is known as prompt supercriticality (or prompt criticality), which is the region in which nuclear weapons operate. The change in k needed to go from critical to prompt critical is defined as a dollar. Nuclear fission weapons require a mass of fissile fuel that is prompt supercritical. For a given mass of fissile material the value of k can be increased by increasing the density. Since the probability per distance traveled

neg: Startup neutron source neutrons produced by the source. The chain reaction in the first critical reactor, CP-1, was initiated by a radium-beryllium neutron source. Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times. In comparison, a bomb is based on immediate neutrons and grows exponentially in nanoseconds. Startup neutron source Startup neutron source is a neutron source used for stable and reliable initiation of nuclear chain reaction in nuclear reactors, when they are loaded with fresh nuclear fuel, whose neutron flux from spontaneous fission is insufficient for a reliable

neg: Nuclear chain reaction orientation. The value of k can also be increased by using a neutron reflector surrounding the fissile material Once the mass of fuel is prompt supercritical, the power increases exponentially. However, the exponential power increase cannot continue for long since k decreases when the amount of fission material that is left decreases (i.e. it is consumed by fissions). Also, the geometry and density are expected to change during detonation since the remaining fission material is torn apart from the explosion. Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly. During part of this

neg: Small modular reactor which can then be absorbed by other unstable nuclei, causing a chain reaction. A sustained fission chain is necessary to generate nuclear power. SMR designs include thermal-neutron reactors and fast-neutron reactors. Thermal-neutron reactors rely on a moderator to slow neutrons and generally use as fissile material. Most currently operating nuclear reactors are of this type. Fast reactors don’t use moderators to slow down the neutrons, therefore they rely on the nuclear fuel being able to absorb neutrons travelling at higher speeds. This usually means changing the fuel arrangement within the core, or using different fuel types. is more likely to

neg: Sustainable energy is the reaction that exists in stars, including the sun, and remains impractical for use on Earth, as fusion reactors are not yet available. However nuclear power is controversial politically and scientifically due to concerns about radioactive waste disposal, safety, the risks of a severe accident, and technical and economical problems in dismantling of old power plants. Thorium is a fissionable material used in thorium-based nuclear power. The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including greater abundance, superior physical and nuclear properties, better resistance to nuclear weapons proliferation and reduced plutonium and actinide production.

neg: Uranium radioactive isotope uranium-239. U decays by beta emission to neptunium-239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239. Pu was used as fissile material in the first atomic bomb detonated in the "Trinity test" on 15 July 1945 in New Mexico. In nature, uranium is found as uranium-238 (99.2742%) and uranium-235 (0.7204%). Isotope separation concentrates (enriches) the fissionable uranium-235 for nuclear weapons and most nuclear power plants, except for gas cooled reactors and pressurised heavy water reactors. Most neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the

neg: Nuclear reaction analysis distance traveled) in the sample has to be known. To contribute to the nuclear reaction the projectile nuclei have to slow down in the sample to reach the resonance energy. Thus each initial kinetic energy corresponds to a depth in the sample where the reaction occurs (the higher the energy, the deeper the reaction). For example, a commonly used reaction to profile hydrogen with an energetic N ion beam is with a resonance when the incident N ion has an energy of 6.385 MeV. Since the ion loses energy in the material the ion beam must have an energy higher

neg: Burnup per fission event. A typical value is 193.7 MeV () of thermal energy per fission (see Nuclear fission). With this value, the maximum burnup of 100%, which includes fissioning not just fissile content but also the other fissionable nuclides, is equivalent to about 909 GWd/t. Nuclear engineers often use this to roughly approximate 10% burnup as just less than 100 GWd/t. The actual fuel may be any actinide that can support a chain reaction, including uranium, plutonium, and more exotic transuranic fuels. This fuel content is often referred to as the "heavy metal" to distinguish it from other metals present

neg: Nuclear reactor a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas. Such reactors can no longer form on Earth in its present geologic period. Radioactive decay of formerly abundant uranium-235 over the time span of hundreds of millions of years has reduced the proportion of this naturally occurring fissile isotope to below the amount required to sustain a chain reaction. The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away

neg: Iran and weapons of mass destruction material is required to achieve a sustained nuclear chain reaction. HEU enriched to 90% and above is most typically used in a weapons development program. In an interview in October 2011, President Ahmadinejad of Iran said: "We have already expressed our views about nuclear bombs. We said those who are seeking to build nuclear bombs or those who stockpile, they are politically and mentally retarded. We think they are stupid because the era of nuclear bombs is over. [Why] for example, should Iran continue its efforts and tolerate all international treasures only to build a nuclear bomb, or a few

neg: Nuclear chain reaction for a neutron to collide with a nucleus is proportional to the material density, increasing the density of a fissile material can increase k. This concept is utilized in the implosion method for nuclear weapons. In these devices, the nuclear chain reaction begins after increasing the density of the fissile material with a conventional explosive. In the gun-type fission weapon two subcritical pieces of fuel are rapidly brought together. The value of k for a combination of two masses is always greater than that of its components. The magnitude of the difference depends on distance, as well as the physical

neg: Nuclear fission the fission of plutonium-239 produced in place, over the total life-cycle of a fuel load. Fissionable, non-fissile isotopes can be used as fission energy source even without a chain reaction. Bombarding U with fast neutrons induces fissions, releasing energy as long as the external neutron source is present. This is an important effect in all reactors where fast neutrons from the fissile isotope can cause the fission of nearby U nuclei, which means that some small part of the U is "burned-up" in all nuclear fuels, especially in fast breeder reactors that operate with higher-energy neutrons. That same fast-fission effect

neg: Chain reaction use neutrons theoretically produced from certain nuclear reactions in lighter isotopes, to induce further reactions in light isotopes that produced more neutrons. This would in theory produce a chain reaction at the level of the nucleus. He did not envision fission as one of these neutron-producing reactions, since this reaction was not known at the time. Experiments he proposed using beryllium and indium failed. Later, after fission was discovered in 1938, Szilárd immediately realized the possibility of using neutron-induced fission as the particular nuclear reaction necessary to create a chain-reaction, so long as fission also produced neutrons. In 1939, with

neg: Nunn–Lugar Cooperative Threat Reduction United States sent storage containers to Russia to store fissionable material under their control. The United States provided "10,000 fissile material storage containers by the end of 1995 and a total of nearly 33,000 containers by early 1997". These containers aided in Russia’s ability to store nuclear material from dismantled warheads. Another contribution from the United States to Russia was "75 million dollars to help Russia build a new fissile material storage facility at Chelyabinsk for plutonium "pits" from dismantled warheads". The Nuclear Threat Reduction program was not just used to remove everything fissionable from Russia; it also included ideas

neg: Nuclear fusion or nickel-62 will generally yield a net energy release. These elements have the smallest mass per nucleon and the largest binding energy per nucleon, respectively. Fusion of light elements toward these releases energy (an exothermic process), while a fusion producing nuclei heavier than these elements will result in energy retained by the resulting nucleons, and the resulting reaction is endothermic. The opposite is true for the reverse process, nuclear fission. This means that the lighter elements, such as hydrogen and helium, are in general more fusible; while the heavier elements, such as uranium, thorium and plutonium, are more fissionable. The

neg: Operation Sandstone retrieved by the backup drone tank. Both tanks were subsequently dumped in the ocean. The successful testing of the new cores in the Sandstone tests had a profound effect. Practically every component of the old weapons was rendered obsolete. Even before the third test had been carried out, Bradbury had halted production of the old cores, and ordered that all effort was to be concentrated on the Mark 4 nuclear bomb, which would become the first mass-produced nuclear weapon. The more efficient use of fissionable material would increase the nuclear stockpile from 56 bombs in June 1948 to 169 in

neg: Operation Sandstone of nuclear weapons. Three tests were carried out in April and May 1948 by Joint Task Force 7, with a work force of 10,366 personnel, of whom 9,890 were military. The successful testing of the new cores in the "Operation Sandstone" tests rendered every component of the old weapons obsolete. Even before the third test had been carried out, production of the old cores was halted, and all effort concentrated on the new Mark 4 nuclear bomb, which would become the first mass-produced nuclear weapon. More efficient use of fissionable material as a result of Operation Sandstone would increase the

neg: Nuclear weapon design the greatest challenge of fission weapon design. The heat of fission rapidly expands the fission core, spreading apart the target nuclei and making space for the neutrons to escape without being captured. The chain reaction stops. Materials which can sustain a chain reaction are called fissile. The two fissile materials used in nuclear weapons are: U-235, also known as highly enriched uranium (HEU), oralloy (Oy) meaning Oak Ridge Alloy, or 25 (the last digits of the atomic number, which is 92 for uranium, and the atomic weight, here 235, respectively); and Pu-239, also known as plutonium, or 49 (from 94

neg: Bomb the right circumstances, rapid consolidation can provoke a chain reaction that can proliferate and intensify by many orders of magnitude within microseconds. The energy released by a nuclear fission bomb may be tens of thousands of times greater than a chemical bomb of the same mass. A thermonuclear weapon is a type of nuclear bomb that releases energy through the combination of fission and fusion of the light atomic nuclei of deuterium and tritium. With this type of bomb, a thermonuclear detonation is triggered by the detonation of a fission type nuclear bomb contained within a material containing high concentrations

neg: Condensed aerosol fire suppression = KOH KOH + H• = K• + H2O The potassium radicals are propagated since they are both consumed and produced by reaction with the fire radicals. Disrupting the reactions necessary to sustain the flame's combustion, the cycle continues until the combustion's chain reactions are terminated and the flame is extinguished. Condensed aerosol agents also have secondary extinguishing mechanisms implicating the other three elements of the fire tetrahedron described above. The aerosol cools the flame by engulfing it with a cloud with large concentrations of microparticles which have mass median aerodynamic diameter sizes (MMAD) as small as 1 to 2

neg: Nuclear fusion–fission hybrid neutrons which in turn converts and fissions the outer fissionable blanket. In parallel with the ICF approach, the University of Texas at Austin is developing a system based on the tokamak fusion reactor, optimising for nuclear waste disposal versus power generation. The principles behind using either ICF or tokamak reactors as a neutron source are essentially the same (the primary difference being that ICF is essentially a point-source of neutrons while Tokamaks are more diffuse toroidal sources). The surrounding blanket can be a fissile material (enriched uranium or plutonium) or a fertile material (capable of conversion to a fissionable material

neg: Nuclear reactor physics Nuclear reactor physics Nuclear reactor physics is the branch of science that deals with the study and application of chain reaction to induce a controlled rate of fission in a nuclear reactor for the production of energy. Most nuclear reactors use a chain reaction to induce a controlled rate of nuclear fission in fissile material, releasing both energy and free neutrons. A reactor consists of an assembly of nuclear fuel (a reactor core), usually surrounded by a neutron moderator such as regular water, heavy water, graphite, or zirconium hydride, and fitted with mechanisms such as control rods that control the

neg: Neutron reflector great. The reason for this is that the process of reflection is time-consuming. By the time reflected neutrons make it back into the core, several generations of the chain reaction have passed, meaning the contribution from the older generation is a tiny fraction of the neutron population. Neutron reflector A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, tungsten carbide, or other materials. A neutron reflector can make an otherwise subcritical mass of fissile material critical, or increase the amount of nuclear

neg: Nuclear reactor physics traces of stable daughters of long-decayed fission products. Fermi age theory Notes on nuclear diffusion by Dr. Abdelhamid Dokhane Nuclear reactor physics Nuclear reactor physics is the branch of science that deals with the study and application of chain reaction to induce a controlled rate of fission in a nuclear reactor for the production of energy. Most nuclear reactors use a chain reaction to induce a controlled rate of nuclear fission in fissile material, releasing both energy and free neutrons. A reactor consists of an assembly of nuclear fuel (a reactor core), usually surrounded by a neutron moderator such as

neg: Corium (nuclear reactor) Corium (nuclear reactor) Corium (also called fuel containing material (FCM) or lava-like fuel containing material (LFCM)) is the lava-like mixture of fissile material created in a nuclear reactor's core during a nuclear meltdown. It consists of nuclear fuel, fission products, control rods, structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam, and, in the event that the reactor vessel is breached, molten concrete from the floor of the reactor room. The heat causing the melting of a reactor may originate from the nuclear chain reaction, but more commonly decay heat

neg: Partial Nuclear Test Ban Treaty establishment of bilateral committees on nuclear matters. In early 1958, Eisenhower publicly stated that amendments to the McMahon Act were a necessary condition of a test ban, framing the policy shift in the context of US commitment to its NATO allies. In August 1957, the US assented to a two-year testing moratorium proposed by the Soviet Union, but required that it be linked to restrictions on the production of fissionable material with military uses, a condition that the Soviet Union rejected. While Eisenhower insisted on linking a test ban to a broader disarmament effort (e.g., the production cut-off), Moscow insisted

neg: Nuclear chain reaction due to a positive void coefficient). However, nuclear reactors are still capable of causing smaller explosions even after complete shutdown, such as was the case of the Fukushima Daiichi nuclear disaster. In such cases, residual decay heat from the core may cause high temperatures if there is loss of coolant flow, even a day after the chain reaction has been shut down (see SCRAM). This may cause a chemical reaction between water and fuel that produces hydrogen gas which can explode after mixing with air, with severe contamination consequences, since fuel rod material may still be exposed to the atmosphere

neg: Uranium-235 fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments which are highly radioactive and produce further energy by their radioactive decay. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates

neg: Nuclear chain reaction from this process. However, such explosions do not happen during a chain reaction, but rather as a result of energy from radioactive beta decay, after the fission chain reaction has been stopped. Nuclear chain reaction A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, this leading to the possibility of a self-propagating series of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g., uranium-235, U). The nuclear chain reaction releases several million times more energy per reaction than any chemical reaction. Chemical chain reactions

neg: Nuclear material Nuclear material Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium (U-235), uranium-233, and plutonium-239. Uranium ore concentrates are considered to be a "source material", although these are not subject to safeguards under the Nuclear Non-Proliferation Treaty. Different countries may use different terminology: in the United States of America, "nuclear material" most commonly refers to "special nuclear materials" (SNM), with the potential to be made into nuclear weapons as defined in

neg: Nuclear material the Atomic Energy Act of 1954. The "special nuclear materials" are also plutonium-239, uranium-233, and enriched uranium (U-235). Note that the 1980 Convention on the Physical Protection of Nuclear Material definition of nuclear material does not include thorium. Nuclear material Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium (U-235), uranium-233, and plutonium-239. Uranium ore concentrates are considered to be a "source material", although these are not subject to safeguards under

neg: Nuclear chain reaction Nuclear chain reaction A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, this leading to the possibility of a self-propagating series of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g., uranium-235, U). The nuclear chain reaction releases several million times more energy per reaction than any chemical reaction. Chemical chain reactions were first proposed by German chemist Max Bodenstein in 1913, and were reasonably well understood before nuclear chain reactions were proposed. It was understood that chemical chain reactions were responsible for exponentially

neg: Real-time polymerase chain reaction amount of an expressed gene in a cell can be measured by the number of copies of an RNA transcript of that gene present in a sample. In order to robustly detect and quantify gene expression from small amounts of RNA, amplification of the gene transcript is necessary. The polymerase chain reaction (PCR) is a common method for amplifying DNA; for RNA-based PCR the RNA sample is first reverse-transcribed to complementary DNA (cDNA) with reverse transcriptase. In order to amplify small amounts of DNA, the same methodology is used as in conventional PCR using a DNA template, at least one

neg: Alan H. Goldstein monitoring the coming transformation from natural to artificial biology: Let us define a life form as an entity that reduces entropy by self-executing the minimum set of physical and chemical operations necessary to sustain the ability to execute functionally equivalent negentropic operations indefinitely across time. Given that, a life form will be considered an animat (living material) if all the information necessary to execute that minimum set of physical and chemical operations cannot be stored in DNA or RNA. The corollary: If all the information necessary to execute that minimum set of physical and chemical operations can be stored in