Editing Nuclear weapon design (section)

==Warhead design safety==
Because even low-yield nuclear warheads have astounding destructive power, weapon designers have always recognised the need to incorporate mechanisms and associated procedures intended to prevent accidental detonation.{{Citation needed|date=June 2021}}

[[File:Steel balls.png|thumb|right|300px|A diagram of the ''[[Green Grass (nuclear warhead)|Green Grass]]'' warhead's steel ball safety device, shown left, filled (safe) and right, empty (live). The steel balls were emptied into a hopper underneath the aircraft before flight, and could be re-inserted using a funnel by rotating the bomb on its trolley and raising the hopper.]]

===Gun-type===
It is inherently dangerous to have a weapon containing a quantity and shape of fissile material which can form a critical mass through a relatively simple accident. Because of this danger, the propellant in Little Boy (four bags of [[cordite]]) was inserted into the bomb in flight, shortly after takeoff on August 6, 1945. This was the first time a gun-type nuclear weapon had ever been fully assembled.{{Citation needed|date=June 2021}}

If the weapon falls into water, the [[neutron moderator|moderating]] effect of the [[light-water reactor|water]] can also cause a [[criticality accident]], even without the weapon being physically damaged. Similarly, a fire caused by an aircraft crashing could easily ignite the propellant, with catastrophic results. Gun-type weapons have always been inherently unsafe.{{Citation needed|date=June 2021|reason=Safing schemes for the reliable earth penetrator warhead describes safing schemes for gun-type weapons}}

===In-flight pit insertion===
Neither of these effects is likely with implosion weapons since there is normally insufficient fissile material to form a critical mass without the correct detonation of the lenses. However, the earliest implosion weapons had pits so close to criticality that accidental detonation with some nuclear yield was a concern.{{Citation needed|date=June 2021|reason=modern weapons were still one-point tested}}

On August 9, 1945, Fat Man was loaded onto its airplane fully assembled, but later, when levitated pits made a space between the pit and the tamper, it was feasible to use in-flight pit insertion. The bomber would take off with no fissile material in the bomb. Some older implosion-type weapons, such as the US [[Mark 4 nuclear bomb|Mark 4]] and [[Mark 5 nuclear bomb|Mark 5]], used this system.{{Citation needed|date=June 2021|reason=images of IFI systems show a cylinder with HE one end and the pit on the other being inserted}}

In-flight pit insertion will not work with a hollow pit in contact with its tamper.{{Citation needed|date=June 2021|reason=utter nonsense. Whoever wrote that has not even done basic research about IFI}}

===Steel ball safety method===
As shown in the diagram above, one method used to decrease the likelihood of accidental detonation employed [[ball (bearing)|metal balls]]. The balls were emptied into the pit: this prevented detonation by increasing the density of the hollow pit, thereby preventing symmetrical implosion in the event of an accident. This design was used in the Green Grass weapon, also known as the Interim Megaton Weapon, which was used in the [[Violet Club]] and [[Yellow Sun (nuclear weapon)|Yellow Sun Mk.1]] bombs.{{Citation needed|date=June 2021}}

[[File:One-Point Safety Test.svg|right]]

===Chain safety method===
Alternatively, the pit can be "safed" by having its normally hollow core filled with an inert material such as a fine metal chain, possibly made of [[cadmium]] to absorb neutrons. While the chain is in the center of the pit, the pit cannot be compressed into an appropriate shape to fission; when the weapon is to be armed, the chain is removed. Similarly, although a serious fire could detonate the explosives, destroying the pit and spreading plutonium to contaminate the surroundings as has happened in [[list of military nuclear accidents|several weapons accidents]], it could not cause a nuclear explosion.{{Citation needed|date=June 2021}}

===One-point safety===
While the firing of one detonator out of many will not cause a hollow pit to go critical, especially a low-mass hollow pit that requires boosting, the introduction of two-point implosion systems made that possibility a real concern.{{Citation needed|date=June 2021|reason=utter nonsense. Big citation needed for the claim hollow pits are one-point safe. Also boosting requires significant yield to function. A weapon making 0.1kt of yield from a one point detonation is not safe}}

In a two-point system, if one detonator fires, one entire hemisphere of the pit will implode as designed. The high-explosive charge surrounding the other hemisphere will explode progressively, from the equator toward the opposite pole. Ideally, this will pinch the equator and squeeze the second hemisphere away from the first, like toothpaste in a tube. By the time the explosion envelops it, its implosion will be separated both in time and space from the implosion of the first hemisphere. The resulting dumbbell shape, with each end reaching maximum density at a different time, may not become critical.{{Citation needed|date=June 2021|reason=}}

It is not possible to tell on the drawing board how this will play out. Nor is it possible using a dummy pit of U-238 and high-speed x-ray cameras, although such tests are helpful. For final determination, a test needs to be made with real fissile material. Consequently, starting in 1957, a year after Swan, both labs began one-point safety tests.{{Citation needed|date=June 2021|reason=}}

Out of 25 one-point safety tests conducted in 1957 and 1958, seven had zero or slight nuclear yield (success), three had high yields of 300 t to 500 t (severe failure), and the rest had unacceptable yields between those extremes.{{Citation needed|date=June 2021|reason=}}

Of particular concern was Livermore's [[W47]], which generated unacceptably high yields in one-point testing. To prevent an accidental detonation, Livermore decided to use mechanical safing on the W47. The wire safety scheme described below was the result.{{Citation needed|date=June 2021|reason=wan device, failed massively, would suggest above claims are very wrong}}

When testing resumed in 1961, and continued for three decades, there was sufficient time to make all warhead designs inherently one-point safe, without need for mechanical safing.{{Citation needed|date=June 2021|reason=}}

===Wire safety method===
In the last test before the 1958 moratorium the W47 warhead for the Polaris SLBM was found to not be one-point safe, producing an unacceptably high nuclear yield of {{convert|200|kg|lb|abbr=on}} of TNT equivalent (Hardtack II Titania). With the test moratorium in force, there was no way to refine the design and make it inherently one-point safe. A solution was devised consisting of a [[boron]]-coated wire inserted into the weapon's hollow pit at manufacture. The warhead was armed by withdrawing the wire onto a spool driven by an electric motor. Once withdrawn, the wire could not be re-inserted.<ref>Chuck Hansen, ''The Swords of Armageddon'', Volume VII, pp. 396–397.</ref> The wire had a tendency to become brittle during storage, and break or get stuck during arming, preventing complete removal and rendering the warhead a dud.<ref name="dud"/> It was estimated that 50–75% of warheads would fail. This required a complete rebuild of all W47 primaries.<ref>{{cite journal |last1=Harvey |first1=John R. |last2=Michalowski |first2=Stefan |title=Nuclear Weapons Safety:The Case of Trident |journal=Science & Global Security |volume=4 |issue=3 |pages=261–337 |bibcode=1994S&GS....4..261H |doi=10.1080/08929889408426405 |date=1994 |url=https://scienceandglobalsecurity.org/archive/sgs04harvey.pdf |url-status=live |archive-url=https://web.archive.org/web/20121016101827/http://www.princeton.edu/sgs/publications/sgs/pdf/4_3harvey.pdf |archive-date=2012-10-16}}</ref> The oil used for lubricating the wire also promoted corrosion of the pit.<ref>{{cite book |isbn=978-0521054010 |title=From Polaris to Trident: The Development of the U.S. Fleet Ballistic Missile Technology |url=https://books.google.com/books?id=95eoQSNDp6gC&q=warhead+corrosion&pg=PA214}}.{{dead link|date=November 2016|bot=InternetArchiveBot |fix-attempted=yes}}</ref>

===Strong link/weak link===
{{See also|Strong link/weak link}}
Under the strong link/weak link system, "weak links" are constructed between critical nuclear weapon components (the "hard links"). In the event of an accident the weak links are designed to fail first in a manner that precludes energy transfer between them. Then, if a hard link fails in a manner that transfers or releases energy, energy can't be transferred into other weapon systems, potentially starting a nuclear detonation. Hard links are usually critical weapon components that have been hardened to survive extreme environments, while weak links can be both components deliberately inserted into the system to act as a weak link and critical nuclear components that can fail predictably.{{Citation needed|date=June 2021|reason=}}

An example of a weak link would be an electrical connector that contains electrical wires made from a low melting point alloy. During a fire, those wires would melt, breaking any electrical connection.{{Citation needed|date=June 2021|reason=}}

===Permissive action link===
{{See also|Permissive action link}}
A ''permissive action link'' is an [[access control]] device designed to prevent unauthorised use of nuclear weapons. Early PALs were simple electromechanical switches and have evolved into complex arming systems that include integrated yield control options, lockout devices and anti-tamper devices.{{Citation needed|date=April 2024}}