Editing Nuclear chain reaction (section)

==Nuclear weapons==
{{Seealso|Nuclear weapon design}}
Nuclear fission weapons require a mass of fissile fuel that is prompt supercritical. For a given mass of fissile material the value of ''k'' can be increased by increasing the density. Since the probability per distance travelled for a neutron to collide with a nucleus is proportional to the material density, increasing the density of a fissile material can increase ''k''. This concept is utilized in the [[Nuclear weapon design#Implosion-type weapon|implosion method]] for nuclear weapons. In these devices, the nuclear chain reaction begins after increasing the density of the fissile material with a conventional explosive.

In a [[gun-type fission weapon]], two subcritical masses of fuel are rapidly brought together. The value of ''k'' for a combination of two masses is always greater than that of its components. The magnitude of the difference depends on distance, as well as the physical orientation. The value of ''k'' can also be increased by using a [[neutron reflector]] surrounding the fissile material.

Once the mass of fuel is prompt supercritical, the power increases exponentially. However, the exponential power increase cannot continue for long since ''k'' decreases when the amount of fission material that is left decreases (i.e. it is consumed by fissions). Also, the geometry and density are expected to change during detonation since the remaining fission material is torn apart from the explosion.

===Predetonation===
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[[File:Nuclear predetonation.svg|right|thumb|250px|{{resize|90%|If two pieces of subcritical material are not brought together fast enough, nuclear predetonation can occur, whereby a smaller explosion than expected will blow the bulk of the material apart. See [[Fizzle (nuclear test)]].}}]]
Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly (about one [[microsecond]], or one-millionth of a second). During part of this process, the assembly is supercritical, but not yet in an optimal state for a chain reaction. Free neutrons, in particular from [[spontaneous fission]]s, can cause the device to undergo a preliminary chain reaction that destroys the fissile material before it is ready to produce a large explosion, which is known as '''predetonation'''.<ref name=nukearchive-predet>{{cite web |url=http://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html |author=Carey Sublette |website=4. Engineering and Design of Nuclear Weapons: 4.1 Elements of Fission Weapon Design  |title=4.1.5.3 Predetonation |date=February 20, 1999 |access-date=June 29, 2014}}</ref>

To keep the probability of predetonation low, the duration of the non-optimal assembly period is minimized, and fissile and other materials are used that have low spontaneous fission rates. In fact, the combination of materials has to be such that it is unlikely that there is even a single spontaneous fission during the period of supercritical assembly. In particular, the gun method cannot be used with plutonium.