Yellow Sun was the first British operational high-yield strategic nuclear weapon warhead. The name refers only to the outer casing; the warhead (or physics package) was known as "Green Grass" in Yellow Sun Mk.1 and " Red Snow " (a US design) in Yellow Sun Mk.2.
159-451: Yellow Sun was designed to contain a variety of warheads. The initial plan was that it would carry an alarm-clock-type warhead known as " Green Bamboo ", and then replace it with a true thermonuclear warhead known as "Green Granite". After signing a weapon technology agreement with the US, both concepts were dropped. Green Granite would be replaced by Red Snow at an earlier service date. This meant
318-437: A parachute to retard its fall. Instead, it had a completely flat nose which induced drag, thereby slowing the fall of the weapon sufficiently to permit the bomber to escape the danger zone. Additionally, the blunt nose ensured that Yellow Sun did not encounter the transonic/supersonic shock waves which had caused many difficulties with barometric fuzing gates which had plagued an earlier weapon, Blue Danube . Electrical power
477-594: 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 fission was discovered in 1940 by Flyorov , Petrzhak , and Kurchatov in Moscow, in an experiment intended to confirm that, without bombardment by neutrons,
636-407: 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
795-445: A capacity of 398 GWE , with about 85% being light-water cooled reactors such as pressurized water reactors or boiling water reactors . Energy from fission is transmitted through conduction or convection to the nuclear reactor coolant , then to a heat exchanger , and the resultant generated steam is used to drive a turbine or generator. The objective of an atomic bomb is to produce a device, according to Serber, "...in which energy
954-508: A classically curved nose created, which made it difficult to measure altitude barometrically. Mark 1 with Green Grass began to enter service in 1959, replacing the massive Blue Danube over the next year. Mark 2 with Red Snow began to replace it in 1961. Beginning in 1966, Yellow Sun was replaced by the WE.177 , based on another US design. The casing was around 21 feet (6.4 m) long, and 48 inches (1.2 m) in diameter. The Mark 1 version with
1113-500: A complex 72-point explosive system, and led to a very large weapon overall. The resulting 45-inch (110 cm) diameter of Green Bamboo determined the 48 in (120 cm) diameter of both Yellow Sun and the Blue Steel missile . The launch of Sputnik 1 coincided with ongoing negotiations between the US and UK about nuclear technology, and the sudden shock of an apparent Soviet superiority swept aside lingering US concerns about
1272-401: A deformed nucleus relative to a spherical form for the surface and Coulomb terms. Additional terms can be included such as symmetry, pairing, the finite range of the nuclear force, and charge distribution within the nuclei to improve the estimate. Normally binding energy is referred to and plotted as average binding energy per nucleon. According to Lilley, "The binding energy of a nucleus B
1431-448: A fast neutron. This energy release profile holds for thorium and the various minor actinides as well. When a uranium nucleus fissions into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus appears as the fission energy of ~200 MeV. For uranium-235 (total mean fission energy 202.79 MeV ), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of
1590-480: A fission bomb where growth is at an explosive rate. If k is exactly unity, the reactions proceed at a steady rate and the reactor is said to be critical. It is possible to achieve criticality in a reactor using natural uranium as fuel, provided that the neutrons have been efficiently moderated to thermal energies." Moderators include light water, heavy water , and graphite . According to John C. Lee, "For all nuclear reactors in operation and those under development,
1749-521: A fission reaction had taken place on 19 December 1938, and Meitner and her nephew Frisch explained it theoretically in January 1939. Frisch named the process "fission" by analogy with biological fission of living cells. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up
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#17328014025431908-432: A fission reaction is produced by its fission products , though a large majority of it, about 85 percent, is found in fragment kinetic energy , while about 6 percent each comes from initial neutrons and gamma rays and those emitted after beta decay , plus about 3 percent from neutrinos as the product of such decay. Nuclear fission can occur without neutron bombardment as a type of radioactive decay. This type of fission
2067-413: A limitation associated with the energy of his alpha particle source. Eventually, in 1932, a fully artificial nuclear reaction and nuclear transmutation was achieved by Rutherford's colleagues Ernest Walton and John Cockcroft , who used artificially accelerated protons against lithium-7, to split this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", and would win them
2226-408: A major gamma ray emitter. All actinides are fertile or fissile and fast breeder reactors can fission them all albeit only in certain configurations. Nuclear reprocessing aims to recover usable material from spent nuclear fuel to both enable uranium (and thorium) supplies to last longer and to reduce the amount of "waste". The industry term for a process that fissions all or nearly all actinides
2385-437: 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
2544-469: A neutron-driven chain reaction using beryllium. Szilard stated, "...if we could find an element which is split by neutrons and which would emit two neutrons when it absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction." On 25 January 1939, after learning of Hahn's discovery from Eugene Wigner , Szilard noted, "...if enough neutrons are emitted...then it should be, of course, possible to sustain
2703-667: A new, heavier element 93, that "it is conceivable that the nucleus breaks up into several large fragments." However, the quoted objection comes some distance down, and was but one of several gaps she noted in Fermi's claim. Although Noddack was a renowned analytical chemist, she lacked the background in physics to appreciate the enormity of what she was proposing. After the Fermi publication, Otto Hahn , Lise Meitner , and Fritz Strassmann began performing similar experiments in Berlin . Meitner, an Austrian Jew, lost her Austrian citizenship with
2862-416: A nuclear reaction. Cross sections are a function of incident neutron energy, and those for U and Pu are a million times higher than U at lower neutron energy levels. Absorption of any neutron makes available to the nucleus binding energy of about 5.3 MeV. U needs a fast neutron to supply the additional 1 MeV needed to cross the critical energy barrier for fission. In
3021-404: A nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with another particle, a neutron, which is itself produced by prior fission events. Fissionable isotopes such as uranium-238 require additional energy provided by fast neutrons (such as those produced by nuclear fusion in thermonuclear weapons ). While some of the neutrons released from
3180-622: A nuclear reactor, ternary fission can produce three positively charged fragments (plus neutrons) and the smallest of these may range from so small a charge and mass as a proton ( Z = 1), to as large a fragment as argon ( Z = 18). The most common small fragments, however, are composed of 90% helium-4 nuclei with more energy than alpha particles from alpha decay (so-called "long range alphas" at ~16 megaelectronvolts (MeV)), plus helium-6 nuclei, and tritons (the nuclei of tritium ). Though less common than binary fission, it still produces significant helium-4 and tritium gas buildup in
3339-428: 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
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#17328014025433498-583: A small fraction of fission products. Neutron absorption which does not lead to fission produces plutonium (from U ) and minor actinides (from both U and U ) whose radiotoxicity is far higher than that of the long lived fission products. Concerns over nuclear waste accumulation and the destructive potential of nuclear weapons are a counterbalance to the peaceful desire to use fission as an energy source . The thorium fuel cycle produces virtually no plutonium and much less minor actinides, but U - or rather its decay products - are
3657-405: 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
3816-601: A supercritical chain-reaction (one in which each fission cycle yields more neutrons than it absorbs). Without their existence, the nuclear chain-reaction would be prompt critical and increase in size faster than it could be controlled by human intervention. In this case, the first experimental atomic reactors would have run away to a dangerous and messy "prompt critical reaction" before their operators could have manually shut them down (for this reason, designer Enrico Fermi included radiation-counter-triggered control rods, suspended by electromagnets, which could automatically drop into
3975-414: A supercritical mass of fuel can be self-sustaining because it produces enough surplus neutrons to offset losses of neutrons escaping 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
4134-631: A superior breeding potential for fast reactors." Critical fission reactors are the most common type of nuclear reactor. In a critical fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release. Devices that produce engineered but non-self-sustaining fission reactions are subcritical fission reactors . Such devices use radioactive decay or particle accelerators to trigger fissions. Critical fission reactors are built for three primary purposes, which typically involve different engineering trade-offs to take advantage of either
4293-484: 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
4452-520: A thin-walled spherical shell. To guard against the accidental crushing of the core into a critical condition, the shell was filled with 133,000 steel ball-bearings, weighing 450 kilograms (990 lb). In a conflict, these would have had to be removed before flight. The RAF thought it unsafe (see Violet Club § Controversy ). Red Snow was the US W28 warhead used in the US Mk-28 nuclear bomb . This
4611-411: A third particle is emitted. This third particle is commonly an α particle . Since in nuclear fission, the nucleus emits more neutrons than the one it absorbs, a chain reaction is possible. Binary fission may produce any of the fission products, at 95±15 and 135±15 daltons . However, the binary process happens merely because it is the most probable. In anywhere from two to four fissions per 1000 in
4770-480: A true implosion. Nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons , and releases a very large amount of energy even by the energetic standards of radioactive decay . Nuclear fission was discovered by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch . Hahn and Strassmann proved that
4929-496: Is a " closed fuel cycle ". Younes and Loveland define fission as, "...a collective motion of the protons and neutrons that make up the nucleus, and as such it is distinguishable from other phenomena that break up the nucleus. Nuclear fission is an extreme example of large- amplitude collective motion that results in the division of a parent nucleus into two or more fragment nuclei. The fission process can occur spontaneously, or it can be induced by an incident particle." The energy from
Yellow Sun (nuclear weapon) - Misplaced Pages Continue
5088-506: Is by definition a reactor that produces more fissile material than it consumes and needs a minimum of two neutrons produced for each neutron absorbed in a fissile nucleus. Thus, in general, the conversion ratio (CR) is defined as the ratio of fissile material produced to that destroyed ...when the CR is greater than 1.0, it is called the breeding ratio (BR)... U offers a superior breeding potential for both thermal and fast reactors, while Pu offers
5247-427: Is called spontaneous fission , and was first observed in 1940. During induced fission, a compound system is formed after an incident particle fuses with a target. The resultant excitation energy may be sufficient to emit neutrons, or gamma-rays, and nuclear scission. Fission into two fragments is called binary fission, and is the most common nuclear reaction . Occurring least frequently is ternary fission , in which
5406-474: 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
5565-438: Is called the odd–even effect on the fragments' charge distribution. This can be seen in the empirical fragment yield data for each fission product, as products with even Z have higher yield values. However, no odd–even effect is observed on fragment distribution based on their A . This result is attributed to nucleon pair breaking . In nuclear fission events the nuclei may break into any combination of lighter nuclei, but
5724-413: Is characterized by the neutron multiplication factor k , which is defined as the ratio of the number of neutrons in one generation to the number in the preceding generation. If, in a reactor, k is less than unity, the reactor is subcritical, the number of neutrons decreases and the chain reaction dies out. If k > 1, the reactor is supercritical and the chain reaction diverges. This is the situation in
5883-410: 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
6042-637: 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,
6201-473: 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
6360-407: Is much less than the prompt energy, but it is a significant amount and is why reactors must continue to be cooled after they have been shut down and why the waste products must be handled with great care and stored safely." John Lilley states, "...neutron-induced fission generates extra neutrons which can induce further fissions in the next generation and so on in a chain reaction. The chain reaction
6519-404: 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
Yellow Sun (nuclear weapon) - Misplaced Pages Continue
6678-406: 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
6837-608: Is on display at the Air Force Museum in Cosford, England. Nuclear weapon design#Alarm Clock Nuclear Weapons Design are physical, chemical, and engineering arrangements that cause 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
6996-417: 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
7155-477: 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
7314-417: Is recoverable, Prompt fission fragments amount to 168 MeV, which are easily stopped with a fraction of a millimeter. Prompt neutrons total 5 MeV, and this energy is recovered as heat via scattering in the reactor. However, many fission fragments are neutron-rich and decay via β emissions. According to Lilley, "The radioactive decay energy from the fission chains is the second release of energy due to fission. It
7473-402: Is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission." According to Rhodes, "Untamped, a bomb core even as large as twice the critical mass would completely fission less than 1 percent of its nuclear material before it expanded enough to stop the chain reaction from proceeding. Tamper always increased efficiency: it reflected neutrons back into
7632-423: Is the atomic mass of a hydrogen atom, m n is the mass of a neutron, and c is the speed of light . Thus, the mass of an atom is less than the mass of its constituent protons and neutrons, assuming the average binding energy of its electrons is negligible. The binding energy B is expressed in energy units, using Einstein's mass-energy equivalence relationship. The binding energy also provides an estimate of
7791-418: Is the energy required to separate it into its constituent neutrons and protons." m ( A , Z ) = Z m H + N m n − B / c 2 {\displaystyle m(\mathbf {A} ,\mathbf {Z} )=\mathbf {Z} m_{H}+\mathbf {N} m_{n}-\mathbf {B} /c^{2}} where A is mass number , Z is atomic number , m H
7950-584: 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;
8109-667: The Anschluss , the union of Austria with Germany in March 1938, but she fled in July 1938 to Sweden and started a correspondence by mail with Hahn in Berlin. By coincidence, her nephew Otto Robert Frisch , also a refugee, was also in Sweden when Meitner received a letter from Hahn dated 19 December describing his chemical proof that some of the product of the bombardment of uranium with neutrons
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#17328014025438268-707: The Kaiser Wilhelm Society for Chemistry, today part of the Free University of Berlin , following over four decades of work on the science of radioactivity and the elaboration of new nuclear physics that described the components of atoms. In 1911, Ernest Rutherford proposed a model of the atom in which a very small, dense and positively charged nucleus of protons was surrounded by orbiting, negatively charged electrons (the Rutherford model ). Niels Bohr improved upon this in 1913 by reconciling
8427-630: 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
8586-513: 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,
8745-421: The nuclear fuel cycle is based on one of three fissile materials, U, U, and Pu, and the associated isotopic chains. For the current generation of LWRs , the enriched U contains 2.5~4.5 wt% of U, which is fabricated into UO 2 fuel rods and loaded into fuel assemblies." Lee states, "One important comparison for the three major fissile nuclides, U, U, and Pu, is their breeding potential. A breeder
8904-621: The nuclear shell model for the nucleus. The nuclides that can sustain a fission chain reaction are suitable for use as nuclear fuels . The most common nuclear fuels are U (the isotope of uranium with mass number 235 and of use in nuclear reactors) and Pu (the isotope of plutonium with mass number 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 daltons ( fission products ). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha - beta decay chain over periods of millennia to eons . In
9063-647: The "layer-cake" type thought similar to the Soviet Sloika and the US Alarm Clock concepts. These hybrid designs are not now regarded as truly thermonuclear , but were then thought to be a stepping-stone on the route to a fusion bomb. Stage 2 was to follow when a true thermonuclear warhead based on the Green Granite design became available. To produce the required yield, the implosion of the fissile core had to be extremely uniform. This required
9222-407: 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
9381-548: The 1951 Nobel Prize in Physics for "Transmutation of atomic nuclei by artificially accelerated atomic particles" , although it was not the nuclear fission reaction later discovered in heavy elements. English physicist James Chadwick discovered the neutron in 1932. Chadwick used an ionization chamber to observe protons knocked out of several elements by beryllium radiation, following up on earlier observations made by Joliot-Curies . In Chadwick's words, "...In order to explain
9540-499: 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
9699-501: The Green Grass warhead weighed 7,250 pounds (3,290 kg). The Mk.2 version with the lighter 1,700 pounds (770 kg). The Red Snow warhead had ballast added to maintain overall weight, ballistic and aerodynamic properties, and avoid further lengthy and expensive testing, and changes to the electrical power generating and airburst fuze. Unlike contemporary United States bombs of similar destructive power, Yellow Sun did not deploy
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#17328014025439858-582: The UK after the Suez Crisis . These negotiations would lead to the US–UK Mutual Defence Agreement in 1958. With access to US designs, the original plan to use Green Granite for Stage 2 was abandoned as a new thermonuclear warhead would be available before Granite would be. This meant the need for a powerful interim design to fill this time period was also considerably less important. The decision
10017-699: The US W-28 warhead used in the US Mark 28 nuclear bomb . This was anglicised to adapt it to British engineering practices, and manufactured in Britain using British fissile materials and known as Red Snow. Red Snow was more powerful, lighter and smaller than Green Grass. It was always envisaged that the Yellow Sun bomb casing would be adapted for successor warheads to minimise unessential development time and cost. Yellow Sun Mk. 2 entered service in 1961, and remained
10176-428: 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
10335-474: 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
10494-420: 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
10653-698: The binding energy as the sum of five terms, which are the volume energy, a surface correction, Coulomb energy, a symmetry term, and a pairing term: B = a v A − a s A 2 / 3 − a c Z 2 A 1 / 3 − a a ( N − Z ) 2 A ± Δ {\displaystyle B=a_{v}\mathbf {A} -a_{s}\mathbf {A} ^{2/3}-a_{c}{\frac {\mathbf {Z} ^{2}}{\mathbf {A} ^{1/3}}}-a_{a}{\frac {(\mathbf {N} -\mathbf {Z} )^{2}}{\mathbf {A} }}\pm \Delta } where
10812-437: 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
10971-456: 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
11130-404: 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
11289-456: The case of U however, that extra energy is provided when U adjusts from an odd to an even mass. In the words of Younes and Lovelace, "...the neutron absorption on a U target forms a U nucleus with excitation energy greater than the critical fission energy, whereas in the case of n + U , the resulting U nucleus has an excitation energy below the critical fission energy." About 6 MeV of
11448-446: The center of Chicago Pile-1 ). If these delayed neutrons are captured without producing fissions, they produce heat as well. The binding energy of the nucleus is the difference between the rest-mass energy of the nucleus and the rest-mass energy of the neutron and proton nucleons. The binding energy formula includes volume, surface and Coulomb energy terms that include empirically derived coefficients for all three, plus energy ratios of
11607-440: 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
11766-419: The core and its inertia...slowed the core's expansion and helped keep the core surface from blowing away." Rearrangement of the core material's subcritical components would need to proceed as fast as possible to ensure effective detonation. Additionally, a third basic component was necessary, "...an initiator—a Ra + Be source or, better, a Po + Be source, with the radium or polonium attached perhaps to one piece of
11925-405: The core and the beryllium to the other, to smash together and spray neutrons when the parts mated to start the chain reaction." However, any bomb would "necessitate locating, mining and processing hundreds of tons of uranium ore...", while U-235 separation or the production of Pu-239 would require additional industrial capacity. The discovery of nuclear fission occurred in 1938 in the buildings of
12084-404: 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
12243-462: 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
12402-414: The curve of binding energy, where the fission products cluster, it is easily observed that the binding energy of the fission products tends to center around 8.5 MeV per nucleon. Thus, in any fission event of an isotope in the actinide mass range, roughly 0.9 MeV are released per nucleon of the starting element. The fission of U by a slow neutron yields nearly identical energy to the fission of U by
12561-401: 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
12720-416: The element thorium was slowly and spontaneously transmuting itself into argon gas!" In 1919, following up on an earlier anomaly Ernest Marsden noted in 1915, Rutherford attempted to "break up the atom." Rutherford was able to accomplish the first artificial transmutation of nitrogen into oxygen, using alpha particles directed at nitrogen N + α → O + p. Rutherford stated, "...we must conclude that
12879-421: 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
13038-406: The energy spectrum for fast fission is similar. ) Among the heavy actinide elements, however, those isotopes that have an odd number of neutrons (such as U with 143 neutrons) bind an extra neutron with an additional 1 to 2 MeV of energy over an isotope of the same element with an even number of neutrons (such as U with 146 neutrons). This extra binding energy is made available as a result of
13197-699: The energy thus released. The results confirmed that fission was occurring and hinted strongly that it was the isotope uranium 235 in particular that was fissioning. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C. under the joint auspices of the George Washington University and the Carnegie Institution of Washington . There, the news on nuclear fission
13356-450: 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 easily seen by examining the curve of binding energy (image below), and noting that the average binding energy of the actinide nuclides beginning with uranium is around 7.6 MeV per nucleon. Looking further left on
13515-447: 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
13674-449: The excitation energy is sufficient, the nucleus breaks into fragments. This is called scission, and occurs at about 10 seconds. The fragments can emit prompt neutrons at between 10 and 10 seconds. At about 10 seconds, the fragments can emit gamma rays. At 10 seconds β decay, β- delayed neutrons , and gamma rays are emitted from the decay products . Typical fission events release about two hundred million eV (200 MeV) of energy,
13833-432: The explosion of nuclear weapons . Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes a self-sustaining nuclear chain reaction possible, releasing energy at a controlled rate in a nuclear reactor or at a very rapid, uncontrolled rate in a nuclear weapon. The amount of free energy released in
13992-528: 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
14151-414: 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
14310-424: The fact that effective forces in the nucleus are stronger for unlike neutron-proton pairs, rather than like neutron–neutron or proton–proton pairs. The pairing term arises from the fact that like nucleons form spin-zero pairs in the same spatial state. The pairing is positive if N and Z are both even, adding to the binding energy. In fission there is a preference for fission fragments with even Z , which
14469-426: The fast neutrons are supplied by nuclear fusion). However, this process cannot happen to a great extent in a nuclear reactor, as too small a fraction of the fission neutrons produced by any type of fission have enough energy to efficiently fission U . (For example, neutrons from thermal fission of U have a mean energy of 2 MeV, a median energy of 1.6 MeV, and a mode of 0.75 MeV, and
14628-409: The fission of U are fast enough to induce another fission in U , most are not, meaning it can never achieve criticality. While there is a very small (albeit nonzero) chance of a thermal neutron inducing fission in U , neutron absorption is orders of magnitude more likely. Fission cross sections are a measurable property related to the probability that fission will occur in
14787-450: The fission of an equivalent amount of U is a million times more than that released in the combustion of methane or from hydrogen fuel cells . The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. However, the seven long-lived fission products make up only
14946-443: The fission rate of uranium was negligible, as predicted by Niels Bohr ; it was not negligible. The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum tunneling processes such as proton emission , alpha decay , and cluster decay , which give the same products each time. Nuclear fission produces energy for nuclear power and drives
15105-431: The fission-input energy is supplied by the simple binding of an extra neutron to the heavy nucleus via the strong force; however, in many fissionable isotopes, this amount of energy is not enough for fission. Uranium-238, for example, has a near-zero fission cross section for neutrons of less than 1 MeV energy. If no additional energy is supplied by any other mechanism, the nucleus will not fission, but will merely absorb
15264-410: The fuel rods of modern nuclear reactors. Bohr and Wheeler used their liquid drop model , the packing fraction curve of Arthur Jeffrey Dempster , and Eugene Feenberg's estimates of nucleus radius and surface tension, to estimate the mass differences of parent and daughters in fission. They then equated this mass difference to energy using Einstein's mass-energy equivalence formula. The stimulation of
15423-405: The great penetrating power of the radiation we must further assume that the particle has no net charge..." The existence of the neutron was first postulated by Rutherford in 1920, and in the words of Chadwick, "...how on earth were you going to build up a big nucleus with a large positive charge? And the answer was a neutral particle." Subsequently, he communicated his findings in more detail. In
15582-435: The group dubbed ausenium and hesperium . However, not all were convinced by Fermi's analysis of his results, though he would win the 1938 Nobel Prize in Physics for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". The German chemist Ida Noddack notably suggested in 1934 that instead of creating
15741-492: The heat or the neutrons produced by the fission chain reaction: While, in principle, all fission reactors can act in all three capacities, in practice the tasks lead to conflicting engineering goals and most reactors have been built with only one of the above tasks in mind. (There are several early counter-examples, such as the Hanford N reactor , now decommissioned). As of 2019, the 448 nuclear power plants worldwide provided
15900-564: 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
16059-400: The interim Green Bamboo was less important and it was replaced by the less powerful and simpler " Green Grass ". A unique feature of the Yellow Sun casing was its completely flat nose. This provided two benefits, one was that the drag allowed the bomb to fall behind the bomber a safe distance before detonation, and the other was that it did not generate the complex pattern of shock waves that
16218-424: 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,
16377-412: The latter are used in fast-neutron reactors , and in weapons). According to Younes and Loveland, "Actinides like U that fission easily following the absorption of a thermal (0.25 meV) neutron are called fissile , whereas those like U that do not easily fission when they absorb a thermal neutron are called fissionable ." After an incident particle has fused with a parent nucleus, if
16536-407: 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
16695-427: The line has the slope N = Z , while the heavier nuclei require additional neutrons to remain stable. Nuclei that are neutron- or proton-rich have excessive binding energy for stability, and the excess energy may convert a neutron to a proton or a proton to a neutron via the weak nuclear force, a process known as beta decay . Neutron-induced fission of U-235 emits a total energy of 207 MeV, of which about 200 MeV
16854-441: 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
17013-454: 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
17172-478: The mechanism of neutron pairing effects , which itself is caused by the Pauli exclusion principle , allowing an extra neutron to occupy the same nuclear orbital as the last neutron in the nucleus. In such isotopes, therefore, no neutron kinetic energy is needed, for all the necessary energy is supplied by absorption of any neutron, either of the slow or fast variety (the former are used in moderated nuclear reactors, and
17331-465: 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 daltons and the other the remaining 130 to 140 daltons. Stable nuclei, and unstable nuclei with very long half-lives , follow a trend of stability evident when Z is plotted against N . For lighter nuclei less than N = 20,
17490-482: 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 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
17649-463: 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
17808-410: 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 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
17967-523: The neutron, as happens when U absorbs slow and even some fraction of fast neutrons, to become U . The remaining energy to initiate fission can be supplied by two other mechanisms: one of these is more kinetic energy of the incoming neutron, which is increasingly able to fission a fissionable heavy nucleus as it exceeds a kinetic energy of 1 MeV or more (so-called fast neutrons). Such high energy neutrons are able to fission U directly (see thermonuclear weapon for application, where
18126-436: 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
18285-454: 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
18444-399: The news and carried it back to Columbia. Rabi said he told Enrico Fermi; Fermi gave credit to Lamb. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Herbert L. Anderson . Bohr grabbed him by the shoulder and said: "Young man, let me explain to you about something new and exciting in physics." It
18603-433: The nitrogen atom is disintegrated," while the newspapers stated he had split the atom . This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. It also offered a new way to study the nucleus. Rutherford and James Chadwick then used alpha particles to "disintegrate" boron, fluorine, sodium, aluminum, and phosphorus before reaching
18762-441: The nuclear binding energy is proportional to the nuclear volume, while nucleons near the surface interact with fewer nucleons, reducing the effect of the volume term. According to Lilley, "For all naturally occurring nuclei, the surface-energy term dominates and the nucleus exists in a state of equilibrium." The negative contribution of Coulomb energy arises from the repulsive electric force of the protons. The symmetry term arises from
18921-457: The nuclear force approaches a constant value for large A , while the Coulomb acts over a larger distance so that electrical potential energy per proton grows as Z increases. Fission energy is released when a A is larger than 120 nucleus fragments. Fusion energy is released when lighter nuclei combine. Carl Friedrich von Weizsäcker's semi-empirical mass formula may be used to express
19080-402: 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
19239-470: The nucleus after neutron bombardment was analogous to the vibrations of a liquid drop, with surface tension and the Coulomb force in opposition. Plotting the sum of these two energies as a function of elongated shape, they determined the resultant energy surface had a saddle shape. The saddle provided an energy barrier called the critical energy barrier. Energy of about 6 MeV provided by the incident neutron
19398-403: 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
19557-532: 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 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
19716-511: 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
19875-430: The plutonium-239 is later fissioned. On the other hand, so-called delayed neutrons emitted as radioactive decay products with half-lives up to several minutes, from fission-daughters, are very important to reactor control , because they give a characteristic "reaction" time for the total nuclear reaction to double in size, if the reaction is run in a " delayed-critical " zone which deliberately relies on these neutrons for
20034-413: The possibility of a nuclear chain reaction . For heavy nuclides , it is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments ( heating the bulk material where fission takes place). Like nuclear fusion , for fission to produce energy, the total binding energy of the resulting elements must be greater than that of
20193-410: The possibility of a nuclear chain reaction. The 11 February 1939 paper by Meitner and Frisch compared the process to the division of a liquid drop and estimated the energy released at 200 MeV. The 1 September 1939 paper by Bohr and Wheeler used this liquid drop model to quantify fission details, including the energy released, estimated the cross section for neutron-induced fission, and deduced U
20352-576: The primary air-dropped strategic weapon until replaced with WE.177B in 1966. Although the first British designed thermonuclear weapon to be deployed, Yellow Sun was not the first to be deployed with the RAF. US Mk-28 and Mk-43 thermonuclear bombs and others had been supplied to the RAF for use in V bombers prior to the deployment of Yellow Sun. Some bombers of the V-force only ever used American weapons supplied under dual-key arrangements. An authentic shell
20511-522: The quantum behavior of electrons (the Bohr model ). In 1928, George Gamow proposed the Liquid drop model , which became essential to understanding the physics of fission. In 1896, Henri Becquerel had found, and Marie Curie named, radioactivity. In 1900, Rutherford and Frederick Soddy , investigating the radioactive gas emanating from thorium , "conveyed the tremendous and inevitable conclusion that
20670-538: 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
20829-406: The rest as kinetic energy of fission fragments (this appears almost immediately when the fragments impact surrounding matter, as simple heat). Some processes involving neutrons are notable for absorbing or finally yielding energy — for example neutron kinetic energy does not yield heat immediately if the neutron is captured by a uranium-238 atom to breed plutonium-239, but this energy is emitted if
20988-412: 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
21147-416: 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
21306-497: 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
21465-475: The speed of light, due to Coulomb repulsion . Also, an average of 2.5 neutrons are emitted, with a mean kinetic energy per neutron of ~2 MeV (total of 4.8 MeV). The fission reaction also releases ~7 MeV in prompt gamma ray photons . The latter figure means that a nuclear fission explosion or criticality accident emits about 3.5% of its energy as gamma rays, less than 2.5% of its energy as fast neutrons (total of both types of radiation ~6%), and
21624-533: The starting element. Fission is a form of nuclear transmutation because the resulting fragments (or daughter atoms) are not the same element as the original parent atom. The two (or more) nuclei produced are most often of comparable but slightly different 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
21783-422: 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
21942-468: 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
22101-403: The techniques were well-known. Meitner and Frisch then correctly interpreted Hahn's results to mean that the nucleus of uranium had split roughly in half. Frisch suggested the process be named "nuclear fission", by analogy to the process of living cell division into two cells, which was then called binary fission . Just as the term nuclear "chain reaction" would later be borrowed from chemistry, so
22260-543: The term "fission" was borrowed from biology. News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities. Meitner's and Frisch's interpretation of the discovery of Hahn and Strassmann crossed the Atlantic Ocean with Niels Bohr, who was to lecture at Princeton University . I.I. Rabi and Willis Lamb , two Columbia University physicists working at Princeton, heard
22419-531: 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 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
22578-414: The total energy released from fission. The curve of binding energy is characterized by a broad maximum near mass number 60 at 8.6 MeV, then gradually decreases to 7.6 MeV at the highest mass numbers. Mass numbers higher than 238 are rare. At the lighter end of the scale, peaks are noted for helium-4, and the multiples such as beryllium-8, carbon-12, oxygen-16, neon-20 and magnesium-24. Binding energy due to
22737-422: 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 –
22896-481: 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
23055-588: The vicinity of the nucleus, and that gave it more time to be captured." Fermi's team, studying radiative capture which is the emission of gamma radiation after the nucleus captures a neutron, studied sixty elements, inducing radioactivity in forty. In the process, they discovered the ability of hydrogen to slow down the neutrons. Enrico Fermi and his colleagues in Rome studied the results of bombarding uranium with neutrons in 1934. Fermi concluded that his experiments had created new elements with 93 and 94 protons, which
23214-438: 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
23373-428: The words of Richard Rhodes , referring to the neutron, "It would therefore serve as a new nuclear probe of surpassing power of penetration." Philip Morrison stated, "A beam of thermal neutrons moving at about the speed of sound...produces nuclear reactions in many materials much more easily than a beam of protons...traveling thousands of times faster." According to Rhodes, "Slowing down a neutron gave it more time in
23532-438: Was barium . Hahn suggested a bursting of the nucleus, but he was unsure of what the physical basis for the results were. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus. Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie had been separating barium from radium for many years, and
23691-410: 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,
23850-501: Was always envisaged that a Mk.2 version would be available later fitted with a true thermonuclear warhead derived from the Granite type tested at Grapple, or an American type made available after the 1958 US-UK Mutual Defence Agreement . It was carried only by RAF V bombers . In September 1958 a decision was made to abandon the Granite type warheads intended for Yellow Sun Mk.2 (and Blue Steel, and Blue Streak MRBM ) and instead adopt
24009-457: Was anglicised to adapt it to British engineering practices, and manufactured in Britain using British fissile materials. For further information see the "Deployment" section below. Deployment started in 1959–60. The RAF Service designations were Bomb, Aircraft HE 7000 lb HC Mk.1 or Bomb, Aircraft HE 7000 lb HC Mk.2 . Yellow Sun Mk.1 was intended as an "emergency" weapon, and had not been engineered for reliable long-term stockpiling. It
24168-599: Was based in part on the core of the Orange Herald device tested at Grapple , with some of the implosion and firing features of Green Bamboo. The modulated neutron initiator was Blue Stone . Twelve Green Grass warheads were fitted in the much larger, older casings derived from Blue Danube and known as " Violet Club ". These twelve warheads were later transferred to the Yellow Sun Mk.1 casings and supplemented by further warheads totalling 37. Green Grass yield
24327-500: Was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall . The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring
24486-512: Was made to cancel Green Bamboo and replace it with a simpler concept. After Green Bamboo was abandoned a decision was made to use the Interim Megaton Weapon known as Green Grass in the Yellow Sun casing and designate it as Yellow Sun Mk.1 until better warheads were available for a Mk.2. Green Grass was of similar layout to Green Bamboo, although it was not thermonuclear, being a very large unboosted pure fission warhead that
24645-494: Was necessary to overcome this barrier and cause the nucleus to fission. According to John Lilley, "The energy required to overcome the barrier to fission is called the activation energy or fission barrier and is about 6 MeV for A ≈ 240. It is found that the activation energy decreases as A increases. Eventually, a point is reached where activation energy disappears altogether...it would undergo very rapid spontaneous fission." Maria Goeppert Mayer later proposed
24804-502: Was originally stated to the Royal Air Force (RAF) as 500 kilotons of TNT equivalent (2.1 PJ ), but the designers' estimate was later revised downwards to 400 kt of TNT. The Green Grass warhead was never tested. It used a dangerously large quantity of fissile material – thought to be in excess of 70 kilograms (150 lb), and considerably more than an uncompressed critical mass . It was kept subcritical by being fashioned into
24963-415: Was spread even further, which fostered many more experimental demonstrations. The 6 January 1939 Hahn and Strassman paper announced the discover of fission. In their second publication on nuclear fission in February 1939, Hahn and Strassmann used the term Uranspaltung (uranium fission) for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up
25122-413: Was supplied by duplicated ram-air turbines located behind the twin air intakes in the flat nose. The earlier Blue Danube design had relied on lead–acid batteries which had proven to be both unreliable and to require time-consuming pre-flight warming. Yellow Sun Stage 1 and Stage 2 were the original designations. Stage 1 was intended as an interim design to carry a one megaton Green Bamboo warhead of
25281-516: Was the major contributor to that cross section and slow-neutron fission. During this period the Hungarian physicist Leó Szilárd realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Rutherford's disparaging remarks about generating power from neutron collisions. However, Szilárd had not been able to achieve
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