Illustration of the concept of "critical mass" in respects to nuclear weapons design. The circles represent spheres of critical material (enriched uranium or plutonium), arrows represent neutron paths, and stars represent fission reactions.

In the top frame, there is too little active material and so the fission reaction quickly ends. In the middle frame the addition of more material allows for more fission reactions and more neutrons, thus opening up the possibility of a self-perpetuating reaction (criticality and super-criticality). In the bottom frame, the addition of a neutron reflector to the original material has increased the efficiency of the reaction by preventing as many neutrons from escaping, also opening up the possibility of criticality. The diagram is meant to illustrate that there is not necessarily one set "critical mass" — the amount of mass needed to go critical can depend on the exact arrangement used in assembling the material.

Created by User:Fastfission in Inkscape.

 

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