Editing Nuclear weapon design (section)

====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}}