Critical mass in physics - an explanation

06-21  Source: Network gathering  Views:247 


Critical mass in physics - an explanation

The chain reaction needs a critical mass.

Nuclear fission - physics knowledge briefly

  • The two chemists Otto Hahn and Friedrich Strassmann discovered together with the physicist Liese Meitner in 1939 that a core of a particular uranium isotope (namely uranium-235) can be split by bombardment with slow neutrons in two moderate rubble cores. The cleavage products are (due to their neutron excess) usually radioactive.
  • Closer investigation some time later showed that this division is not only energy that was stored in the core, free will, but also 2-3 neutrons.
  • The released during nuclear fission neutrons can further uranium nuclei split, where more neutrons are released.
  • Are there enough uranium nuclei present, the process is growing like an avalanche - a chain reaction occurs in the vast amounts of energy are released (keyword: nuclear bomb).

Critical Mass - what is it?

  • Requirement for physical chain reaction in the cleavage of course is that the released neutrons in turn split uranium nuclei.
  • Here are a few conditions, however, must be met: The neutron afford to lose either by other reactions (eg trapping by other atomic nuclei or Wandeinfang.) Nor to the outside, ie can from the uranium-bearing material out, escape.
  • The simplest option is therefore to take as much fissile material (uranium-235) that formed neutron meet again only on fissile material, so make their way through the material to a new reactant before they leave it, or otherwise react.
  • Of course, you can achieve this by having a minimum mass of fissile material.
  • If this mass spherical, it is called the critical mass of the amount of fissile material that is at least necessary for the end of the chain reaction.
  • The critical mass of pure (!) Uranium-235 is about 50 kg Enriched Fissile materials correspondingly more.
  • Special Cases of neutron reflective material to reduce this critical mass, of course, so already about 15 kg U-235 and about 4.4 kg of the likewise fissile plutonium as a critical mass (military technical, so for a nuclear explosion) are sufficient.
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