Editing Nuclear weapon design (section)

==Pure fission weapons==
{{Unreferenced section|date=October 2022}}
[[File:TrinityDetonation1945GIF.gif|thumb|Trinity-''Gadget'' was the first ever pure-fission nuclear device to be detonated, with an estimated yield of 25&nbsp;kilotons.]]

The first task of a nuclear weapon design is to rapidly assemble a [[critical mass|supercritical mass]] of fissile (weapon grade) uranium or plutonium. A supercritical mass 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 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 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 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 the best weapon-grade uranium contains a significant number of <sup>238</sup>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 <sup>238</sup>U spontaneous fission will occur while 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 the necessity to assemble the supercritical mass of fuel very rapidly. The time required to accomplish this is called the weapon's [[insertion time|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 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 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 particle]]s. If the fuel mass contains impurity elements of low atomic number (Z), these charged alphas can penetrate 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 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 (detonator)|Urchin]]" inside 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 [[ion]]s. The resulting small-scale fusion produces neutrons at 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 the exponential function by which neutron multiplication evolves.

The critical mass of an uncompressed sphere of bare metal is {{convert|50|kg|lb|abbr=on}} for uranium-235 and {{convert|16|kg|lb|abbr=on}} 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 reflector|neutron-reflecting material]], all of which affect 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 is to incorporate material with a large cross-section for neutron capture, such as boron (specifically <sup>10</sup>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; a 5&nbsp;kilogram mass produces 9.68&nbsp;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 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{{when|date=October 2023}} the same layer serves both as tamper and as neutron reflector.

===Gun-type assembly===
[[File:Gun-type fission weapon en-labels thin lines.svg|thumb|350px|Diagram of a gun-type fission weapon]]
{{Main|Gun-type fission weapon}}
[[Little Boy]], the Hiroshima bomb, used {{convert|64|kg|lb|abbr=on}} of uranium with an average enrichment of around 80%, or {{convert|51|kg|lb|abbr=on}} of uranium-235, just about the bare-metal critical mass {{xref|(see [[Little Boy#Assembly details|Little Boy]] article for a detailed drawing)}}. When assembled inside its tamper/reflector of [[tungsten carbide]], the {{convert|64|kg|lb|abbr=on}} 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 the uranium mass underwent fission;<ref>Glasstone and Dolan, ''Effects'', pp. 12–13. When 454 g (one pound) of <sup>235</sup>U undergoes complete fission, the yield is 8 kilotons. The 13 to 16-kiloton yield of the Little Boy bomb was therefore produced by the fission of no more than {{convert|2|lb|g}} of <sup>235</sup>U, out of the {{convert|141|lb|g}} in the pit. Thus, the remaining {{convert|139|lb|kg}}, 98.5% of the total, contributed nothing to the energy yield.</ref> the remainder, representing most of the entire wartime output of the [[Y-12 National Security Complex|giant Y-12 factories]] at Oak Ridge, scattered uselessly.<ref>Compere, A.L., and Griffith, W.L. 1991. "The U.S. Calutron Program for Uranium Enrichment: History,. Technology, Operations, and Production. Report", ORNL-5928, as cited in John Coster-Mullen, "Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man", 2003, footnote 28, p. 18. The total wartime output of Oralloy produced at Oak Ridge by July 28, 1945, was {{convert|165|lb|kg}}. Of this amount, 84% was scattered over Hiroshima (see previous footnote).</ref>

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 fission weapon#US nuclear artillery|gun-type warhead]] fired from the barrel of a much larger gun).{{Citation needed|date=October 2023}} Such warheads were deployed by the United States until 1992, accounting for a significant fraction of the <sup>235</sup>U in the arsenal{{Citation needed|date=June 2021}}, and were some of the first weapons dismantled to comply with treaties limiting warhead numbers.{{Citation needed|date=June 2021|reason=Very doubtful given the only treaty dealing with tactical weapons was the intermediate-ranged nuclear forces treaty}} The rationale for this decision was undoubtedly a combination of the lower yield and grave safety issues associated with the gun-type design.{{Citation needed|date=June 2021|reason=W33s were stored disassembled}}

===Implosion-type{{anchor|Implosion-type_weapon}}===
[[File:Implosion Nuclear weapon.svg|right|350px]]
For both the [[Trinity (nuclear test)|Trinity device]] and the [[Fat Man]] (Nagasaki) bomb, nearly identical plutonium fission through implosion designs were used. The Fat Man device specifically used {{convert|6.2|kg|lb|abbr=on}}, about {{convert|350|ml|usoz|abbr=on|disp=or}} in volume, of [[Pu-239]], which is only 41% of bare-sphere critical mass {{xref|(see [[Fat Man#Interior|Fat Man]] article for a detailed drawing)}}. Surrounded by a [[uranium-238|U-238]] reflector/tamper, 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 detonator]]s. It is estimated that only about 20% of the plutonium underwent fission; the rest, about {{convert|5|kg|lb|abbr=on}}, was scattered.

[[File:Implosion bomb animated.gif|left|175px]]

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 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 the [[Los Alamos Laboratory]] and a remote site {{convert|14.3|km|mi|abbr=on}} 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.<ref>{{cite book |last=Hoddeson |first=Lillian |display-authors=etal |title=Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945 |date=2004 |publisher=Cambridge University Press |page=271 |isbn=978-0-521-54117-6}}</ref>

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 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.

====Plutonium pit====
{{Main|Pit (nuclear weapon)}}
[[File:X-Ray-Image-HE-Lens-Test-Shot.gif|thumb|right|Flash X-Ray images of the converging shock waves formed during a test of the high explosive lens system.]]

The core of an implosion weapon – the fissile material and any reflector or tamper bonded to it – is known as the ''pit''. Some weapons tested during the 1950s used pits made with [[uranium-235|U-235]] alone, or in [[composite material|composite]] with [[plutonium]],<ref>[https://fas.org/sgp/othergov/doe/rdd-7.html "Restricted Data Declassification Decisions from 1945 until Present"] {{webarchive |url=https://web.archive.org/web/20160423121258/https://fas.org/sgp/othergov/doe/rdd-7.html |date=April 23, 2016}} – "Fact that plutonium and uranium may be bonded to each other in unspecified pits or weapons."</ref> but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.{{Citation needed|date=June 2021}}

Casting and then machining plutonium is difficult not only because of its toxicity, but also because plutonium has many different [[allotropes of plutonium|metallic phases]]. As plutonium cools, changes in phase result in distortion and cracking. This distortion 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.<ref name="RDD-7"/> 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 [[valence (chemistry)|trivalent]] [[metal]]s would also work, but gallium has 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 [[nuclear reactor|power reactors]], there is the difficulty of removing the gallium.{{Citation needed|date=June 2021}}

Because plutonium is chemically reactive it is common to plate the completed pit with a thin layer of inert metal, which also reduces the toxic hazard.<ref name="NWFAQ-6.2"/> [[The gadget]] used galvanic silver plating; afterward, [[nickel]] deposited from [[nickel tetracarbonyl]] vapors was used,<ref name="NWFAQ-6.2"/> but thereafter and since, [[gold]] became the preferred material.{{Citation needed|date=May 2009|reason=not found in nuclearweaponarchive.org cite}} Recent designs improve safety by plating pits with [[vanadium]] to make the pits more fire-resistant.{{Citation needed|date=June 2021|reason=Modern pits are sealed in a fire resistant shell, vanadium was an innovation in the never produced W89}}

===Levitated-pit implosion===
[[File:SandstoneYoke.gif|thumb|The ''Sandstone'' series of nuclear-weapons tests in 1948 proved the feasibility of increased yield efficiency via the levitated-pit design method.]]
The first improvement on the Fat Man design was to put an air space between 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.<ref>All information on nuclear weapon tests comes from Chuck Hansen, ''The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945'', October 1995, Chucklea Productions, Volume VIII, p. 154, Table A-1, "U.S. Nuclear Detonations and Tests, 1945–1962".</ref>

It was immediately clear{{according to whom|date=October 2023}} that implosion was the best design for a fission weapon. Its only drawback seemed to be its diameter. Fat Man was {{convert|1.5|m|ft|0}} wide vs {{convert|61|cm|ft|0}} for Little Boy.

The Pu-239 pit of Fat Man was only {{convert|9.1|cm|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.{{Citation needed|date=June 2021|reason=Doubtful given Swan would be challenging to harden for laydown and ground penetration delivery.}}

===Two-point linear implosion===
[[File:Linear implosion schematic.svg|right|350 px]]
In the two-point linear implosion, 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 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.<ref>[https://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.6.3 Nuclear Weapons FAQ: 4.1.6.3 Hybrid Assembly Techniques] {{webarchive |url=https://web.archive.org/web/20160419071500/https://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.6.3 |date=April 19, 2016}}, accessed December 1, 2007. Drawing adapted from the same source.</ref>

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 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 a true implosion.{{Citation needed|date=June 2021}}

The lack of compression makes such designs inefficient, but the simplicity and small diameter make it suitable for use in artillery shells and atomic demolition munitions – ADMs – also known as backpack or [[suitcase nuke]]s; an example is the [[W48]] artillery shell, the smallest nuclear weapon ever built or deployed. All such low-yield battlefield weapons, whether gun-type U-235 designs or linear implosion Pu-239 designs, pay a high price in fissile material in order to achieve diameters between six and ten inches (15 and 25&nbsp;cm).{{Citation needed|date=June 2021}}

===Hollow-pit implosion===
{{Unreferenced section|date=October 2022}}
A more efficient implosion system uses a hollow pit.{{Citation needed|date=June 2021}}

A hollow plutonium pit was the original plan for the 1945 Fat Man bomb, but there was not enough time to develop and test the implosion system for it. A simpler solid-pit design was considered more reliable, given the time constraints, but it required a heavy U-238 tamper, a thick aluminium pusher, and three tons of high explosives.{{Citation needed|date=June 2021}}

After the war, interest in the hollow pit design was revived. Its obvious advantage is that a hollow shell of plutonium, shock-deformed and driven inward toward its empty center, would carry momentum into its violent assembly as a solid sphere. It would be self-tamping, requiring a smaller U-238 tamper, no aluminium pusher, and less high explosive.{{Citation needed|date=June 2021}}