Editing Critical mass (section)

==Criticality in nuclear weapon design==
[[File:Nuclear predetonation.svg|right|thumb|250px|If two pieces of subcritical material are not brought together fast enough, nuclear predetonation ([[Fizzle (nuclear test)|fizzle]]) can occur, whereby a very small explosion will blow the bulk of the material apart.]]
Until detonation is desired, a [[nuclear weapon]] must be kept subcritical. In the case of a uranium gun-type bomb, this can be achieved by keeping the fuel in a number of separate pieces, each below the [[critical size]] either because they are too small or unfavorably shaped. To produce detonation, the pieces of uranium are brought together rapidly. In [[Little Boy]], this was achieved by firing a piece of uranium (a 'doughnut') down a [[gun barrel]] onto another piece (a 'spike'). This design is referred to as a ''[[gun-type fission weapon]]''.

A theoretical 100% pure <sup>239</sup>Pu weapon could also be constructed as a gun-type weapon, like the Manhattan Project's proposed [[Thin Man (nuclear bomb)|Thin Man]] design.  In reality, this is impractical because even "weapons grade" <sup>239</sup>Pu is contaminated with a small amount of <sup>240</sup>Pu, which has a strong propensity toward spontaneous fission.  Because of this, a reasonably sized gun-type weapon would suffer nuclear reaction ([[predetonation]]) before the masses of plutonium would be in a position for a full-fledged explosion to occur.
   
Instead, the plutonium is present as a subcritical sphere (or other shape), which may or may not be hollow. Detonation is produced by exploding a [[shaped charge]] surrounding the sphere, increasing the density (and collapsing the cavity, if present) to produce a [[prompt critical]] configuration. This is known as an [[Nuclear weapon design#Implosion-type weapon|implosion type weapon]].