What two word phrase means the smallest amount of fissile material needed for a sustained nuclear chain reaction?
"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"
"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"
"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"
"size and mass to sustain a prompt criticality but the duration without tamper or neutron reflection would be short. The slope of exponential growth, estimated number of fissions, and specific fissile material are not recorded. Neptunium-236 is fissile and possesses the smallest and lightest critical mass, but isolation of the specific radionuclide makes it an impractical choice. Several other novel fissile materials are known, but U-235 and Pu-239 are the only practical options although two US tests using U-233 (critical mass some 32% less than U235) have taken place. Carey Sublette stated that neither the United States nor the Soviet"
"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"
"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"
"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"
"minimal neutron energy needed being about 5.9 MeV. Considerable amounts of neptunium-239 and uranium-237 are indicators of a fission-fusion-fission explosion. Minor amounts of uranium-240 are also formed, and capture of large numbers of neutrons by individual nuclei leads to formation of small but detectable amounts of higher transuranium elements, e.g. einsteinium-255 and fermium-255. One of the important fission products is krypton-90, a radioactive noble gas. It diffuses easily in the cloud, and undergoes two decays to rubidium-90 and then strontium-90, with half-lives of 33 seconds and 3 minutes. The noble gas nonreactivity and rapid diffusion is responsible for depletion of"
"In fission weapons, a mass of fissile material (enriched uranium or plutonium) is forced into supercriticality—allowing an exponential growth of nuclear chain reactions—either by shooting one piece of sub-critical material into another (the ""gun"" method) or by compressing using explosive lenses a sub-critical sphere of material using chemical explosives to many times its original density (the ""implosion"" method). The latter approach is considered more sophisticated than the former, and only the latter approach can be used if the fissile material is plutonium. A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel"
"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"
"thermal neutrons). In fact, U is the only naturally occurring fissile nucleus. Because natural uranium is only about 0.72% U by mass, it must be enriched to a concentration of 2–5% to be able to support a continuous nuclear chain reaction when normal water is used as the moderator. The product of this enrichment process is called enriched uranium. Gaseous diffusion is based on Graham's law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. For example, in a box with a semi-permeable membrane containing a mixture of"
"about 1 x 10 fissions. In the design of nuclear weapons, on the other hand, achieving prompt criticality is essential. Indeed, one of the design problems to overcome in constructing a bomb is to compress the fissile materials enough to achieve prompt criticality before the chain reaction has a chance to produce enough energy to cause the core to expand. A good bomb design must therefore win the race to a dense, prompt critical core before a less-powerful chain reaction disassembles the core without allowing a significant amount of fuel to fission (known as a fizzle). This generally means that"
"is consumed before the weapon destroys itself. The amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons (500 kilotons) of TNT (). All fission reactions generate fission products, the remains of the split atomic nuclei. Many fission products are either highly radioactive (but short-lived) or moderately radioactive (but long-lived), and as such, they are a serious form of radioactive contamination if not fully contained. Fission products are the principal radioactive component of nuclear fallout. The most commonly used fissile materials for nuclear weapons applications have been uranium-235"
"for use in artillery shells and atomic demolition munitions – ADMs – also known as backpack or suitcase nukes; an example is the W48 artillery shell, the smallest nuclear weapon ever built or deployed. All such low-yield battlefield weapons, whether gun-type U-235 designs or linear implosion Pu-239 designs, pay a high price in fissile material in order to achieve diameters between six and ten inches (15 and 25 cm). A more efficient two-point implosion system uses two high explosive lenses and a hollow pit. A hollow plutonium pit was the original plan for the 1945 Fat Man bomb, but there"
"confinement must be 500 times better than that required for the D-T reaction, and the power density will be 2500 times lower than for D-T. Nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing and sustaining nuclear fission. The three most relevant fissile isotopes are Uranium-233, Uranium-235 and Plutonium-239. When the unstable nuclei of these atoms are hit by a slow-moving neutron, they split, creating two daughter nuclei and two"
"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,"
"a nuclear explosion. The Little Boy gun type atomic bomb dropped on Hiroshima on August 6, 1945 was made of highly enriched uranium with a large tamper. The nominal spherical critical mass for an untampered U nuclear weapon is , a sphere 17.32 cm (6.8"") in diameter. The material must be 85% or more of U and is known as weapons grade uranium, though for a crude, inefficient weapon 20% is sufficient (called ""weapon(s)-usable""). Even lower enrichment can be used, but then the required critical mass rapidly increases. Use of a large tamper, implosion geometries, trigger tubes, polonium triggers, tritium"
"Pit (nuclear weapon) The pit, named after the hard core found in fruits such as peaches and apricots, is the core of an implosion nuclear weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s. The pits of the first nuclear weapons were solid, with an ""urchin"" neutron initiator in their center. The Gadget and Fat Man used pits made of 6.2"
"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"
"sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes. Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), ""three"" positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus. Apart from fission induced by a neutron, harnessed and exploited by humans, a natural form of spontaneous radioactive decay (not requiring a neutron) is also referred to as fission, and occurs especially in very high-mass-number isotopes. Spontaneous"
"been sufficiently short, this spent fuel could be reprocessed to produce weapons grade plutonium. Weapons-grade nuclear material Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples. (These nuclear materials have other categorizations based on their purity.) Only fissile isotopes of certain elements have the potential for use in nuclear weapons. For such use, the concentration of fissile isotopes uranium-235 and plutonium-239 in the element used"
"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"
"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"
"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"
"the U.S.'s withdrawal, the EU enacted an updated blocking statute on 7 August 2018 to nullify US sanctions on countries trading with Iran. In November 2018, U.S. sanctions came back into effect intended to force Iran to dramatically alter its policies, including its support for militant groups in the region and its development of ballistic missiles. A nuclear weapon uses a fissile material to cause a nuclear chain reaction. The most commonly used materials have been uranium 235 (U-235) and plutonium 239 (Pu-239). Both uranium 233 (U-233) and reactor-grade plutonium have also been used. The amount of uranium or plutonium"
"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"
"Nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing and sustaining nuclear fission. The three most relevant fissile isotopes are Uranium-233, Uranium-235 and Plutonium-239. When the unstable nuclei of these atoms are hit by a slow-moving neutron, they split, creating two daughter nuclei and two or three more neutrons. These neutrons then go on to split more nuclei. This creates a self-sustaining chain reaction that is controlled in a nuclear"
"amount of shielding material. For instance, a spent nuclear fuel cask requires shielding calculations to determine how much concrete and steel is needed to safely protect the truck driver who is shipping it. Fission is the process through which a nucleus splits into (typically two) smaller atoms. If fission is occurring, it is often of interest to know the asymptotic behavior of the system. A reactor is called “critical” if the chain reaction is self-sustaining and time-independent. If the system is not in equilibrium the asymptotic neutron distribution, or the fundamental mode, will grow or decay exponentially over time. Criticality"
"of a nuclear weapon design is to rapidly assemble a supercritical mass of fissile uranium or plutonium. A supercritical mass is one in which the percentage of fission-produced neutrons captured by another fissile nucleus is large enough that each fission event, on average, causes more than one additional fission event. Once the critical mass is assembled, at maximum density, a burst of neutrons is supplied to start as many chain reactions as possible. Early weapons used an ""urchin"" inside the pit containing polonium-210 and beryllium separated by a thin barrier. Implosion of the pit crushed the urchin, mixing the two"
"a large explosive yield, a linear implosion weapon needs somewhat more material, about . The mass, known as the ""pit"", is configured in a lower density non-spherical configuration. Tampers or reflectors could be used to reduce the mass but the overall diameter of the fissile material plus tamper/reflector increases compared to the volume required for an untamped, unreflected pit. To fit weapons into small artillery shells ( or are known and has been alleged to be possible by nuclear weapon designer Ted Taylor) bare pits are required. On firing the weapon, small to moderate amounts of high explosive collapse and"
"critical mass establishes a nuclear chain reaction, resulting in an exponential rate of change in the neutron population over space and time leading to neutron radiation and a neutron flux. This radiation contains both a neutron and gamma ray component and is extremely dangerous to any unprotected nearby life-form. The rate of change of neutron population depends on the neutron generation time, which is characteristic of the neutron population, the state of ""criticality"", and the fissile medium. A nuclear fission creates approximately 2.5 neutrons per fission event on average. Hence, to maintain a stable, exactly critical chain reaction, 1.5 neutrons"
"2, 4, 8, 16, etc.). Starting from one atom, the number of fissions can theoretically double a hundred times in a microsecond, which could consume all uranium or plutonium up to hundreds of tons by the hundredth link in the chain. In practice, bombs do not contain hundreds of tons of uranium or plutonium. Instead, typically (in a modern weapon) the core of a weapon contains only about 5 kilograms of plutonium, of which only 2 to 2.5 kilograms, representing 40 to 50 kilotons of energy, undergoes fission before the core blows itself apart. Holding an exploding bomb together is"
"and ended when the ever-decreasing fissile materials no longer could sustain a chain reaction. Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down to complete a 3-hour cycle. A key factor that made the reaction possible was that, at"