Critical mass (nuclear reaction)

See also: Critical mass

The critical mass of a fissile material is the quantity of this material necessary to the release of a Chain nuclear reaction of Nuclear fission.

Characterization

It depends on the nuclear properties of material considered (cross Section of fission, and many neutrons produced by fission), but also of its physical properties (in particular of its Densité), of its Forme and its Pureté.

By surrounding fissile material of a reflecting of neutrons, one supports fission, which decreases the quantity necessary to the release of the reaction; on the other hand, the presence of a Neutron absorber within the fissile material with the contrary effect.

Examples

  • In a Nuclear reactor, where the forming die of fissile materials (Plutonium or Uranium) is fixed, but not its composition, since the fissile material is consumed slowly, control is obtained by the insertion or the withdrawal of control rods (containing a neutron absorber, Bore or Cadmium). When the bars are inserted, the critical mass is not reached.

  • In the atomic bombs (see diagrams in this article), two methods exist to obtain this critical mass.

    • Gun device - the Method simplest: in a kind of tube of gun, two subcritical masses are projected one inside the other, the assembly being critical. Example: bend Hiroshima with uranium.
    • Implosion - more powerful Method, because more rapid: A hollow fissile material sphere is compressed by explosive lenses to form a very dense, supercritical ball. Example: bend Nagasaki with plutonium.

See too

Internal bonds

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