Editing Nuclear weapon design (section)

==Fusion-boosted fission==
{{Main|Boosted fission weapon}}
[[File:Greenhouse - Item Device 3.png|thumb|''Item'' of the Greenhouse-series of tests was the first nuclear weapon device to achieve yield utilizing boosting-principles.]]
The next step in miniaturization was to speed up the fissioning of the pit to reduce the minimum inertial confinement time. This would allow the efficient fission of the fuel with less mass in the form of tamper or the fuel itself. The key to achieving faster fission would be to introduce more neutrons, and among the many ways to do this, adding a fusion reaction was relatively easy in the case of a hollow pit.{{Citation needed|date=June 2021}}

The easiest fusion reaction to achieve is found in a 50–50 mixture of tritium and deuterium.<ref name="Fission-Fusion Hybrid Weapons">{{cite web |last1=Sublette |first1=Carey |title=Fission-Fusion Hybrid Weapons |website=nuclearweaponarchive |url=https://nuclearweaponarchive.org/}}</ref> For [[fusion power]] experiments this mixture must be held at high temperatures for relatively lengthy times in order to have an efficient reaction. For explosive use, however, the goal is not to produce efficient fusion, but simply provide extra neutrons early in the process.{{Citation needed|date=June 2021|reason=extremely doubtful. The fusion fuel needs to fuse rapidly to provide said neutrons.}} Since a nuclear explosion is supercritical, any extra neutrons will be multiplied by the chain reaction, so even tiny quantities introduced early can have a large effect on the outcome. For this reason, even the relatively low compression pressures and times (in fusion terms) found in the center of a hollow pit warhead are enough to create the desired effect.{{Citation needed|date=June 2021|reason=Even a modest ~0.1kt provides enormous pressures and temperatures in a pit suitable for fusion.}}

In the boosted design, the fusion fuel in gas form is pumped into the pit during arming. This will fuse into helium and release free neutrons soon after fission begins.{{citation needed|date=August 2023}} The neutrons will start a large number of new chain reactions while the pit is still critical or nearly critical. Once the hollow pit is perfected, there is little reason not to boost; deuterium and tritium are easily produced in the small quantities needed, and the technical aspects are trivial.<ref name="Fission-Fusion Hybrid Weapons"/>

The concept of fusion-boosted fission was first tested on May 25, 1951, in the [[Greenhouse Item|Item]] shot of [[Operation Greenhouse]], [[Enewetak Atoll|Eniwetok]], yield 45.5 kilotons.{{Citation needed|date=June 2021}}

Boosting reduces diameter in three ways, all the result of faster fission:
* Since the compressed pit does not need to be held together as long, the massive U-238 tamper can be replaced by a light-weight beryllium shell (to reflect escaping neutrons back into the pit). The diameter is reduced.{{Citation needed|date=June 2021}}
* The mass of the pit can be reduced by half, without reducing yield. Diameter is reduced again.{{Citation needed|date=June 2021}}
* Since the mass of the metal being imploded (tamper plus pit) is reduced, a smaller charge of high explosive is needed, reducing diameter even further.{{Citation needed|date=June 2021}}

[[File:U.S. Swan Device.svg|center]]{{Citation needed|date=June 2021|reason=image is also uncited}}

The first device whose dimensions suggest employment of all these features (two-point, hollow-pit, fusion-boosted implosion) was the [[Swan (nuclear primary)|Swan]] device. It had a cylindrical shape with a diameter of {{convert|11.6|in|cm|abbr=on|order=flip}} and a length of {{convert|22.8|in|cm|abbr=on|order=flip}}.{{Citation needed|date=June 2021|reason=The B28 predates Swan and was a compact (sub 20") boosted weapon}}

It was first tested standalone and then as the primary of a two-stage thermonuclear device during [[Operation Redwing]]. It was weaponized as the [[Robin primary]] and became the first off-the-shelf, multi-use primary, and the prototype for all that followed.{{Citation needed|date=June 2021}}

[[File:Nuclear Weapon Miniaturization.png|400px|right]]

After the success of Swan, {{convert|11|or|12|in|cm|order=flip}} seemed to become the standard diameter of boosted single-stage devices tested during the 1950s.{{Citation needed|date=June 2021|reason=The W81 suggests the B61 has a spherical primary}} Length was usually twice the diameter, but one such device, which became the [[W54]] warhead, was closer to a sphere, only {{convert|15|in|cm|order=flip}} long.

One of the applications of the W54 was the [[Davy Crockett (nuclear device)|Davy Crockett XM-388 recoilless rifle projectile]]. It had a dimension of just {{convert|11|in|cm|order=flip}}, and is shown here in comparison to its Fat Man predecessor ({{convert|60|in|cm|order=flip|disp=or}}).

Another benefit of boosting, in addition to making weapons smaller, lighter, and with less fissile material for a given yield, is that it renders weapons immune to predetonation.{{Citation needed|date=June 2021|reason=hardens the weapon to predetonation, does not make it immune. terminology also seems to be made up, it was "predetonation", not "radiation interference"}} It was discovered in the mid-1950s that plutonium pits would be particularly susceptible to partial [[predetonation]] if exposed to the intense radiation of a nearby nuclear explosion (electronics might also be damaged, but this was a separate problem).{{Citation needed|date=June 2021|reason=doubtful, delayed neutrons are a thing in U235 too}} RI was a particular problem before effective [[early warning radar]] systems because a first strike attack might make retaliatory weapons useless. Boosting reduces the amount of plutonium needed in a weapon to below the quantity which would be vulnerable to this effect.{{Citation needed|date=June 2021|reason=The concern was with fraticide or defensive warhead detonations, not weapons in ICBMs or on the ground. Any launcher close enough to a weapon burst to worry about neutrons is shattered by the blast wave}}