Misplaced Pages

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79-534: The Mark 6 was in production from 1951 to 1955 and saw service until 1962. Seven variants and versions were produced, with a total production run of all models of 1100 bombs. The basic Mark 6 design was 61 inches (150 cm) in diameter and 128 inches (330 cm) long, the same basic dimensions as the Mark 4. Various models of the Mark 6 were roughly 25% lighter than either the Mark 4 or Fat Man, and weighed 7,600 to 8,500 pounds (3,400–3,900 kg). Early models of

158-407: A 5 kilogram mass produces 9.68 watts of thermal power. Such a piece would feel warm to the touch, which is no problem if that heat is dissipated promptly and not allowed to build up the temperature. But this is a problem inside a nuclear bomb. For this reason bombs using Pu fuel use aluminum parts to wick away the excess heat, and this complicates bomb design because Al plays no active role in

237-414: A free neutron hits the nucleus of a fissile atom like uranium-235 ( U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons (for U three about as often as two; an average of just under 2.5 per fission). The fission chain reaction in a supercritical mass of fuel can be self-sustaining because it produces enough surplus neutrons to offset losses of neutrons escaping

316-442: A large deflection. The cumulative effect of the many small angle collisions, however, is often larger than the effect of the few large angle collisions that occur, so it is instructive to consider the collision dynamics in the limit of small deflections. We can consider an electron of charge − e {\displaystyle -e} and mass m e {\displaystyle m_{\text{e}}} passing

395-561: A million times more energy than comparable chemical reactions, making nuclear bombs a million times more powerful than non-nuclear bombs, which a French patent claimed in May 1939. In some ways, fission and fusion are opposite and complementary reactions, but the particulars are unique for each. To understand how nuclear weapons are designed, it is useful to know the important similarities and differences between fission and fusion. The following explanation uses rounded numbers and approximations. When

474-437: A necessity for gun-assembled bombs, with their much greater insertion time and much greater mass of fuel required (because of the lack of fuel compression). There is another source of free neutrons that can spoil a fission explosion. All uranium and plutonium nuclei have a decay mode that results in energetic alpha particles . If the fuel mass contains impurity elements of low atomic number (Z), these charged alphas can penetrate

553-428: A protected location outside the physics package, from which they penetrate the pit. This method allows better timing of the first fission events in the chain reaction, which optimally should occur at the point of maximum compression/supercriticality. Timing of the neutron injection is a more important parameter than the number of neutrons injected: the first generations of the chain reaction are vastly more effective due to

632-405: A small neutron absorption cross section and helps protect the plutonium against corrosion . A drawback is that gallium compounds are corrosive and so if the plutonium is recovered from dismantled weapons for conversion to plutonium dioxide for power reactors , there is the difficulty of removing the gallium. Because plutonium is chemically reactive it is common to plate the completed pit with

711-431: A stationary ion of charge + Z e {\displaystyle +Ze} and much larger mass at a distance b {\displaystyle b} with a speed v {\displaystyle v} . The perpendicular force is Z e 2 / ( 4 π ϵ 0 b 2 ) {\displaystyle Ze^{2}/(4\pi \epsilon _{0}b^{2})} at

790-479: A thin layer of inert metal, which also reduces the toxic hazard. The gadget used galvanic silver plating; afterward, nickel deposited from nickel tetracarbonyl vapors was used, but thereafter and since, gold became the preferred material. Recent designs improve safety by plating pits with vanadium to make the pits more fire-resistant. The first improvement on the Fat Man design was to put an air space between

869-400: A true implosion. Coulomb collision A Coulomb collision is a binary elastic collision between two charged particles interacting through their own electric field . As with any inverse-square law , the resulting trajectories of the colliding particles is a hyperbolic Keplerian orbit . This type of collision is common in plasmas where the typical kinetic energy of the particles

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948-413: Is n v ( 2 π b d b ) {\displaystyle nv(2\pi b\mathrm {d} b)} , so the diffusion constant is given by Obviously the integral diverges toward both small and large impact parameters. The divergence at small impact parameters is clearly unphysical since under the assumptions used here, the final perpendicular momentum cannot take on a value higher than

1027-512: Is shielded by the tendency of electrons to cluster in the neighborhood of the ion and other ions to avoid it. The upper cut-off to the impact parameter should thus be approximately equal to the Debye length : The integral of 1 / b {\displaystyle 1/b} thus yields the logarithm of the ratio of the upper and lower cut-offs. This number is known as the Coulomb logarithm and

1106-637: Is about 180 million electron volts (MeV); i.e., 74 TJ/kg. Only 7% of this is gamma radiation and kinetic energy of fission neutrons. The remaining 93% is kinetic energy (or energy of motion) of the charged fission fragments, flying away from each other mutually repelled by the positive charge of their protons (38 for strontium, 54 for xenon). This initial kinetic energy is 67 TJ/kg, imparting an initial speed of about 12,000 kilometers per second (i.e. 1.2 cm per nanosecond). The charged fragments' high electric charge causes many inelastic coulomb collisions with nearby nuclei, and these fragments remain trapped inside

1185-431: Is also the reason that fusion products tend to heat the electrons rather than (as would be desirable) the ions. If an electric field is present, the faster electrons feel less drag and become even faster in a "run-away" process. In passing through a field of ions with density n {\displaystyle n} , an electron will have many such encounters simultaneously, with various impact parameters (distance to

1264-471: Is called the D-T reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium ( D), fuses with hydrogen-3, or tritium ( T), to form helium-4 ( He) plus one neutron (n) and energy: The total energy output, 17.6 MeV, is one tenth of that with fission, but the ingredients are only one-fiftieth as massive, so the energy output per unit mass is approximately five times as great. In this fusion reaction, 14 of

1343-592: Is designated by either ln ⁡ Λ {\displaystyle \ln \Lambda } or λ {\displaystyle \lambda } . It is the factor by which small-angle collisions are more effective than large-angle collisions. The Coulomb logarithm was introduced independently by Lev Landau in 1936 and Subrahmanyan Chandrasekhar in 1943. For many plasmas of interest it takes on values between 5 {\displaystyle 5} and 15 {\displaystyle 15} . (For convenient formulas, see pages 34 and 35 of

1422-410: Is estimated that only about 20% of the plutonium underwent fission; the rest, about 5 kg (11 lb), was scattered. An implosion shock wave might be of such short duration that only part of the pit is compressed at any instant as the wave passes through it. To prevent this, a pusher shell may be needed. The pusher is located between the explosive lens and the tamper. It works by reflecting some of

1501-633: Is hot enough to emit black-body radiation in the X-ray spectrum. These X-rays are absorbed by the surrounding air, producing the fireball and blast of a nuclear explosion. Most fission products have too many neutrons to be stable so they are radioactive by beta decay , converting neutrons into protons by throwing off beta particles (electrons), neutrinos and gamma rays. Their half-lives range from milliseconds to about 200,000 years. Many decay into isotopes that are themselves radioactive, so from 1 to 6 (average 3) decays may be required to reach stability. In reactors,

1580-473: Is known as the pit . 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. Casting and then machining plutonium is difficult not only because of its toxicity, but also because plutonium has many different metallic phases . As plutonium cools, changes in phase result in distortion and cracking. This distortion

1659-404: Is normally overcome by alloying it with 30–35 mMol (0.9–1.0% by weight) gallium , forming a plutonium-gallium alloy , which causes it to take up its delta phase over a wide temperature range. When cooling from molten it then has only a single phase change, from epsilon to delta, instead of the four changes it would otherwise pass through. Other trivalent metals would also work, but gallium has

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1738-406: Is of central importance. The plenitude and cheapness of both bulk dry fusion fuel (lithium deuteride) and U (a byproduct of uranium enrichment) permit the economical production of very large nuclear arsenals, in comparison to pure fission weapons requiring the expensive U or Pu fuels. Fusion produces neutrons which dissipate energy from the reaction. In weapons, the most important fusion reaction

1817-417: Is one in which the percentage of fission-produced neutrons captured by other neighboring fissile nuclei is large enough that each fission event, on average, causes more than one follow-on fission event. Neutrons released by the first fission events induce subsequent fission events at an exponentially accelerating rate. Each follow-on fissioning continues a sequence of these reactions that works its way throughout

1896-474: Is present, one also has some amounts of the following two net reactions: Most lithium is Li, and this gave Castle Bravo a yield 2.5 times larger than expected. The neutrons are supplied by the nuclear reactor in a way similar to production of plutonium Pu from U feedstock: target rods of the Li feedstock are arranged around a uranium-fueled core, and are removed for processing once it has been calculated that most of

1975-581: Is to incorporate material with a large cross-section for neutron capture, such as boron (specifically B comprising 20% of natural boron). Naturally this neutron absorber must be removed before the weapon is detonated. This is easy for a gun-assembled bomb: the projectile mass simply shoves the absorber out of the void between the two subcritical masses by the force of its motion. The use of plutonium affects weapon design due to its high rate of alpha emission. This results in Pu metal spontaneously producing significant heat;

2054-460: Is too large to produce a significant deviation from the initial trajectories of the colliding particles, and the cumulative effect of many collisions is considered instead. The importance of Coulomb collisions was first pointed out by Lev Landau in 1936, who also derived the corresponding kinetic equation which is known as the Landau kinetic equation . In a plasma, a Coulomb collision rarely results in

2133-499: The NRL Plasma formulary .) The limits of the impact parameter integral are not sharp, but are uncertain by factors on the order of unity, leading to theoretical uncertainties on the order of 1 / λ {\displaystyle 1/\lambda } . For this reason it is often justified to simply take the convenient choice λ = 10 {\displaystyle \lambda =10} . The analysis here yields

2212-666: The Los Alamos Laboratory and a remote site 14.3 km (8.9 mi) east of it in Bayo Canyon, proved the practicality of the implosion design for a fission device, with the February 1945 tests positively determining its usability for the final Trinity/Fat Man plutonium implosion design. The key to Fat Man's greater efficiency was the inward momentum of the massive U-238 tamper. (The natural uranium tamper did not undergo fission from thermal neutrons, but did contribute perhaps 20% of

2291-503: The Trinity device and the Fat Man (Nagasaki) bomb, nearly identical plutonium fission through implosion designs were used. The Fat Man device specifically used 6.2 kg (14 lb), about 350 ml or 12 US fl oz in volume, of Pu-239 , which is only 41% of bare-sphere critical mass (see Fat Man article for a detailed drawing) . Surrounded by a U-238 reflector/tamper,

2370-407: The 17.6 MeV (80% of the energy released in the reaction) shows up as the kinetic energy of the neutron, which, having no electric charge and being almost as massive as the hydrogen nuclei that created it, can escape the scene without leaving its energy behind to help sustain the reaction – or to generate x-rays for blast and fire. The only practical way to capture most of the fusion energy is to trap

2449-421: The 1950s, transport due to collisions in non-magnetized plasmas was simultaneously studied by two groups at University of California, Berkeley 's Radiation Laboratory. They quoted each other’s results in their respective papers. The first reference deals with the mean-field part of the interaction by using perturbation theory in electric field amplitude. Within the same approximations, a more elegant derivation of

Mark 6 nuclear bomb - Misplaced Pages Continue

2528-495: The Fat Man's pit was brought close to critical mass by the neutron-reflecting properties of the U-238. During detonation, criticality was achieved by implosion. The plutonium pit was squeezed to increase its density by simultaneous detonation, as with the "Trinity" test detonation three weeks earlier, of the conventional explosives placed uniformly around the pit. The explosives were detonated by multiple exploding-bridgewire detonators . It

2607-489: The Mark 6 used the same 32-point implosion system design concept as the earlier Mark 4 and Mark 3; the Mark 6 Mod 2 and later used a different, 60-point implosion system. Various models and pit options gave nuclear yields of 18, 26, 80, 154, and 160 kilotons for Mark 6 models. There are several Mark 6 casings on display: The Mark 13 nuclear bomb and W13 missile warhead were developed as higher-efficiency Mark 6 successors,

2686-563: The United States, though some were later developed independently by other states. In early news accounts, pure fission weapons were called atomic bombs or A-bombs and weapons involving fusion were called hydrogen bombs or H-bombs . Practitioners of nuclear policy, however, favor the terms nuclear and thermonuclear, respectively. Nuclear fission separates or splits heavier atoms to form lighter atoms. Nuclear fusion combines lighter atoms to form heavier atoms. Both reactions generate roughly

2765-428: The bare-metal critical mass (see Little Boy article for a detailed drawing) . When assembled inside its tamper/reflector of tungsten carbide , the 64 kg (141 lb) was more than twice critical mass. Before the detonation, the uranium-235 was formed into two sub-critical pieces, one of which was later fired down a gun barrel to join the other, starting the nuclear explosion. Analysis shows that less than 2% of

2844-411: The barrel of a much larger gun). Such warheads were deployed by the United States until 1992, accounting for a significant fraction of the U in the arsenal , and were some of the first weapons dismantled to comply with treaties limiting warhead numbers. The rationale for this decision was undoubtedly a combination of the lower yield and grave safety issues associated with the gun-type design. For both

2923-420: The best weapon-grade uranium contains a significant number of U nuclei. These are susceptible to spontaneous fission events, which occur randomly (it is a quantum mechanical phenomenon). Because the fissile material in a gun-assembled critical mass is not compressed, the design need only ensure the two sub-critical masses remain close enough to each other long enough that a U spontaneous fission will occur while

3002-437: The bomb of the number of fission events needed to attain the full design yield. Additionally, heat resulting from the fissions that do occur would work against the continued assembly of the supercritical mass, from thermal expansion of the fuel. This failure is called predetonation . The resulting explosion would be called a "fizzle" by bomb engineers and weapon users. Plutonium's high rate of spontaneous fission makes uranium fuel

3081-456: The bomb's fissile pit and tamper until their kinetic energy is converted into heat . Given the speed of the fragments and the mean free path between nuclei in the compressed fuel assembly (for the implosion design), this takes about a millionth of a second (a microsecond), by which time the core and tamper of the bomb have expanded to a ball of plasma several meters in diameter with a temperature of tens of millions of degrees Celsius. This

3160-404: The bomb's power is the initiation of subsequent fissions. Over half of the neutrons escape the bomb core, but the rest strike U nuclei causing them to fission in an exponentially growing chain reaction (1, 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

3239-416: The calculation correct up to the smallest impact parameters where this full deflection must be used. (ii) The effect of Debye shielding for large impact parameters can be accommodated by using a Debye-shielded Coulomb potential ( Screening effect Debye length ). This cancels the above divergence at large impact parameters. The above Coulomb logarithm turns out to be modified by a constant of order unity. In

Mark 6 nuclear bomb - Misplaced Pages Continue

3318-474: The closest approach and the duration of the encounter is about b / v {\displaystyle b/v} . The product of these expressions divided by the mass is the change in perpendicular velocity: Note that the deflection angle is proportional to 1 / v 2 {\displaystyle 1/v^{2}} . Fast particles are "slippery" and thus dominate many transport processes. The efficiency of velocity-matched interactions

3397-529: The collisional transport coefficients was provided, by using the Balescu–Lenard equation (see Sec. 8.4 of and Secs. 7.3 and 7.4 of ). The second reference uses the Rutherford picture of two-body collisions. The calculation of the first reference is correct for impact parameters much larger than the interparticle distance, while those of the second one work in the opposite case. Both calculations are extended to

3476-404: The cores of boosted fission devices in order to increase their energy yields. This is especially so for the fission primaries of thermonuclear weapons. The second way is indirect, and takes advantage of the fact that the neutrons emitted by a supercritical fission "spark plug" in the secondary assembly of a two-stage thermonuclear bomb will produce tritium in situ when these neutrons collide with

3555-462: The coulomb barrier of these impurity nuclei and undergo a reaction that yields a free neutron. The rate of alpha emission of fissile nuclei is one to two million times that of spontaneous fission, so weapon engineers are careful to use fuel of high purity. Fission weapons used in the vicinity of other nuclear explosions must be protected from the intrusion of free neutrons from outside. Such shielding material will almost always be penetrated, however, if

3634-453: The cross section for large-angle collisions. Under some conditions there is a more stringent lower limit due to quantum mechanics, namely the de Broglie wavelength of the electron, h / m e v {\displaystyle h/m_{\text{e}}v} where h {\displaystyle h} is the Planck constant . At large impact parameters, the charge of the ion

3713-401: The edges of the shaper where it is diffracted around the edges into the main mass of explosive. This causes the detonation to form into a ring that proceeds inward from the shaper. Due to the lack of a tamper or lenses to shape the progression, the detonation does not reach the pit in a spherical shape. To produce the desired spherical implosion, the fissile material itself is shaped to produce

3792-421: The energy carried by the fusion neutrons. In the case of a neutron bomb (see below), the last-mentioned factor does not apply, since the objective is to facilitate the escape of neutrons, rather than to use them to increase the weapon's raw power. An essential nuclear reaction is the one that creates tritium , or hydrogen-3. Tritium is employed in two ways. First, pure tritium gas is produced for placement inside

3871-447: The escape or capture of neutrons. To avoid a premature chain reaction during handling, the fissile material in the weapon must be kept subcritical. 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. Another method of reducing criticality risk

3950-523: The explosion processes. A tamper is an optional layer of dense material surrounding the fissile material. Due to its inertia it delays the thermal expansion of the fissioning fuel mass, keeping it supercritical for longer. Often the same layer serves both as tamper and as neutron reflector. Little Boy , the Hiroshima bomb, used 64 kg (141 lb) of uranium with an average enrichment of around 80%, or 51 kg (112 lb) of uranium-235, just about

4029-414: The exponential function by which neutron multiplication evolves. The critical mass of an uncompressed sphere of bare metal is 50 kg (110 lb) for uranium-235 and 16 kg (35 lb) for delta-phase plutonium-239. In practical applications, the amount of material required for criticality is modified by shape, purity, density, and the proximity to neutron-reflecting material , all of which affect

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4108-564: The hundredth link in the chain. Typically in a modern weapon, the weapon's pit contains 3.5 to 4.5 kilograms (7.7 to 9.9 lb) of plutonium and at detonation produces approximately 5 to 10 kilotonnes of TNT (21 to 42 TJ) yield, representing the fissioning of approximately 0.5 kilograms (1.1 lb) of plutonium. Materials which can sustain a chain reaction are called fissile . The two fissile materials used in nuclear weapons are: U, also known as highly enriched uranium (HEU), "oralloy" meaning "Oak Ridge alloy", or "25" (a combination of

4187-431: The initial momentum. Setting the above estimate for Δ m e v ⊥ {\displaystyle \Delta m_{\text{e}}v_{\perp }} equal to m v {\displaystyle mv} , we find the lower cut-off to the impact parameter to be about We can also use π b 0 2 {\displaystyle \pi b_{0}^{2}} as an estimate of

4266-407: The ion) and directions. The cumulative effect can be described as a diffusion of the perpendicular momentum. The corresponding diffusion constant is found by integrating the squares of the individual changes in momentum. The rate of collisions with impact parameter between b {\displaystyle b} and ( b + d b ) {\displaystyle (b+\mathrm {d} b)}

4345-649: The largest pure-fission (non-thermonuclear) bomb design ever developed by the US. Mark 18 bombs were eventually recycled into Mark 6 Mod 6 bombs after thermonuclear weapons were deployed in quantity. The Mark 18 was tested once in Operation Ivy King . An Atomic Demolition Munition, the XM1 was developed. Few details on the system exist. Nuclear weapon design#Implosion-type weapon Nuclear Weapons Design are physical, chemical, and engineering arrangements that cause

4424-424: The last digit of the atomic number of uranium-235, which is 92, and the last digit of its mass number, which is 235); and Pu, also known as plutonium-239, or "49" (from "94" and "239"). Uranium's most common isotope , U, is fissionable but not fissile, meaning that it cannot sustain a chain reaction because its daughter fission neutrons are not (on average) energetic enough to cause follow-on U fissions. However,

4503-407: The less-dense fuel mass. Each following fission event in the chain approximately doubles the neutron population (net, after losses due to some neutrons escaping the fuel mass, and others that collide with any non-fuel impurity nuclei present). For the gun assembly method (see below) of supercritical mass formation, the fuel itself can be relied upon to initiate the chain reaction. This is because even

4582-441: The lithium nuclei have been transmuted to tritium. Of the four basic types of nuclear weapon, the first, pure fission, uses the first of the three nuclear reactions above. The second, fusion-boosted fission, uses the first two. The third, two-stage thermonuclear, uses all three. The first task of a nuclear weapon design is to rapidly assemble a supercritical mass of fissile (weapon grade) uranium or plutonium. A supercritical mass

4661-454: The lithium nuclei in the bomb's lithium deuteride fuel supply. Elemental gaseous tritium for fission primaries is also made by bombarding lithium-6 ( Li) with neutrons (n), only in a nuclear reactor. This neutron bombardment will cause the lithium-6 nucleus to split, producing an alpha particle, or helium -4 ( He), plus a triton ( T) and energy: But as was discovered in the first test of this type of device, Castle Bravo , when lithium-7

4740-463: The necessity to assemble the supercritical mass of fuel very rapidly. The time required to accomplish this is called the weapon's critical insertion time . If spontaneous fission were to occur when the supercritical mass was only partially assembled, the chain reaction would begin prematurely. Neutron losses through the void between the two subcritical masses (gun assembly) or the voids between not-fully-compressed fuel nuclei (implosion assembly) would sap

4819-436: The neutrons inside a massive bottle of heavy material such as lead, uranium, or plutonium. If the 14 MeV neutron is captured by uranium (of either isotope; 14 MeV is high enough to fission both U and U) or plutonium, the result is fission and the release of 180 MeV of fission energy, multiplying the energy output tenfold. For weapon use, fission is necessary to start fusion, helps to sustain fusion, and captures and multiplies

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4898-454: The neutrons released by fusion of the heavy hydrogen isotopes deuterium and tritium will fission U. This U fission reaction in the outer jacket of the secondary assembly of a two-stage thermonuclear bomb produces by far the greatest fraction of the bomb's energy yield, as well as most of its radioactive debris. For national powers engaged in a nuclear arms race, this fact of U's ability to fast-fission from thermonuclear neutron bombardment

4977-402: The nuclear fuel is cast into a solid shape and placed within the center of a cylinder of high explosive. Detonators are placed at either end of the explosive cylinder, and a plate-like insert, or shaper , is placed in the explosive just inside the detonators. When the detonators are fired, the initial detonation is trapped between the shaper and the end of the cylinder, causing it to travel out to

5056-403: The outside neutron flux is intense enough. When a weapon misfires or fizzles because of the effects of other nuclear detonations, it is called nuclear fratricide . For the implosion-assembled design, once the critical mass is assembled to maximum density, a burst of neutrons must be supplied to start the chain reaction. Early weapons used a modulated neutron generator code named " Urchin " inside

5135-486: The physics package of a nuclear weapon to detonate. There are three existing basic design types: Pure fission weapons have been the first type to be built by new nuclear powers. Large industrial states with well-developed nuclear arsenals have two-stage thermonuclear weapons, which are the most compact, scalable, and cost effective option, once the necessary technical base and industrial infrastructure are built. Most known innovations in nuclear weapon design originated in

5214-511: The pit containing polonium -210 and beryllium separated by a thin barrier. Implosion of the pit crushes the neutron generator, mixing the two metals, thereby allowing alpha particles from the polonium to interact with beryllium to produce free neutrons. In modern weapons, the neutron generator is a high-voltage vacuum tube containing a particle accelerator which bombards a deuterium/tritium-metal hydride target with deuterium and tritium ions . The resulting small-scale fusion produces neutrons at

5293-538: The radioactive products are the nuclear waste in spent fuel . In bombs, they become radioactive fallout, both local and global. Meanwhile, inside the exploding bomb, the free neutrons released by fission carry away about 3% of the initial fission energy. Neutron kinetic energy adds to the blast energy of a bomb, but not as effectively as the energy from charged fragments, since neutrons do not give up their kinetic energy as quickly in collisions with charged nuclei or electrons. The dominant contribution of fission neutrons to

5372-412: The same effect. Due to the physics of the shock wave propagation within the explosive mass, this requires the pit to be a prolate spheroid , that is, roughly egg shaped. The shock wave first reaches the pit at its tips, driving them inward and causing the mass to become spherical. The shock may also change plutonium from delta to alpha phase, increasing its density by 23%, but without the inward momentum of

5451-513: The same size and basic configuration as the Mark 6 but utilizing an improved 92-point implosion system. The Mark 13 was cancelled in August 1954 and the W13 cancelled September 1954, in both cases without ever seeing production service. The Mark 18 nuclear bomb was a follow-on to the Mark 6 and Mark 13, utilizing a fissile pit assembly with around 60 kilograms of HEU and delivering a yield of 500 kilotons ,

5530-403: The scalings and orders of magnitude. An N-body treatment accounting for all impact parameters can be performed by taking into account a few simple facts. The main two ones are: (i) The above change in perpendicular velocity is the lowest order approximation in 1/ b of a full Rutherford deflection. Therefore, the above perturbative theory can also be done by using this full deflection. This makes

5609-416: The severing of the strong nuclear force holding the mutually-repulsive protons together), plus two or three free neutrons. These race away and collide with neighboring fuel nuclei. This process repeats over and over until the fuel assembly goes sub-critical (from thermal expansion), after which the chain reaction shuts down because the daughter neutrons can no longer find new fuel nuclei to hit before escaping

5688-497: The shock wave backward, thereby having the effect of lengthening its duration. It is made out of a low density metal – such as aluminium , beryllium , or an alloy of the two metals (aluminium is easier and safer to shape, and is two orders of magnitude cheaper; beryllium has high neutron-reflective capability). Fat Man used an aluminium pusher. The series of RaLa Experiment tests of implosion-type fission weapon design concepts, carried out from July 1944 through February 1945 at

5767-494: The supercritical assembly. Most of these have the speed (kinetic energy) required to cause new fissions in neighboring uranium nuclei. The uranium-235 nucleus can split in many ways, provided the atomic numbers add up to 92 and the mass numbers add up to 236 (uranium-235 plus the neutron that caused the split). The following equation shows one possible split, namely into strontium-95 ( Sr), xenon-139 ( Xe), and two neutrons (n), plus energy: The immediate energy release per atom

5846-422: The supercritical mass of fuel nuclei. This process is conceived and described colloquially as the nuclear chain reaction . To start the chain reaction in a supercritical assembly, at least one free neutron must be injected and collide with a fissile fuel nucleus. The neutron joins with the nucleus (technically a fusion event) and destabilizes the nucleus, which explodes into two middleweight nuclear fragments (from

5925-468: The tamper and the pit to create a hammer-on-nail impact. The pit, supported on a hollow cone inside the tamper cavity, was said to be "levitated". The three tests of Operation Sandstone , in 1948, used Fat Man designs with levitated pits. The largest yield was 49 kilotons, more than twice the yield of the unlevitated Fat Man. It was immediately clear that implosion was the best design for a fission weapon. Its only drawback seemed to be its diameter. Fat Man

6004-422: The total yield from fission by fast neutrons). After the chain reaction started in the plutonium, it continued until the explosion reversed the momentum of the implosion and expanded enough to stop the chain reaction. By holding everything together for a few hundred nanoseconds more, the tamper increased the efficiency. The core of an implosion weapon – the fissile material and any reflector or tamper bonded to it –

6083-481: The uranium mass underwent fission; the remainder, representing most of the entire wartime output of the giant Y-12 factories at Oak Ridge, scattered uselessly. The inefficiency was caused by the speed with which the uncompressed fissioning uranium expanded and became sub-critical by virtue of decreased density. Despite its inefficiency, this design, because of its shape, was adapted for use in small-diameter, cylindrical artillery shells (a gun-type warhead fired from

6162-438: The weapon is in the vicinity of the target. This is not difficult to arrange as it takes but a second or two in a typical-size fuel mass for this to occur. (Still, many such bombs meant for delivery by air (gravity bomb, artillery shell or rocket) use injected neutrons to gain finer control over the exact detonation altitude, important for the destructive effectiveness of airbursts.) This condition of spontaneous fission highlights

6241-410: Was 1.5 metres (5 ft) wide vs 61 centimetres (2 ft) for Little Boy. The Pu-239 pit of Fat Man was only 9.1 centimetres (3.6 in) in diameter, the size of a softball. The bulk of Fat Man's girth was the implosion mechanism, namely concentric layers of U-238, aluminium, and high explosives. The key to reducing that girth was the two-point implosion design. In the two-point linear implosion,

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