Hello!
Critical mass is an amount of fissionable material capable of sustaining a constant rate of fission.
Uranium-235 undergoes fission to produce two neutrons per fission event. If a neturon-absorbing material is mixed in with this sample so that the rate of neutron production drops down to 0.5 neutrons per fission event. It surround the sample with a deflective material to prevent. neutrons from escaping to the surroundings. And it compress the uranium-235 sample.
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