Editing Nuclear weapon design (section)

===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}}