Eli5:How does this critical mass trigger a rapid and uncontrolled explosive chain reaction.?
In: Engineering
When a fissile isotope spotaneously decays, one or more neutrons are released. Two things can happen to a neutron, it can escape the piece of material and do nothing, or it can be absorbed by another atom of fissile material and trigger that one to undergo fission too, producing more neutrons.
Overall, if more neutrons escape than are produced, the number of neutrons in the material decays exponentially. Contrary, if neutrons can’t escape material fast enough, then more and more are produced in exponential growth, which, if unregulated, is a runaway reaction that results in an explosion.
If this neutron economy is negative or positive depends on many factors. Type of material, amount of it, density of it, shape of it, what other materials surround it and are mixed in etc. But critical mass is meant a spherical mass of a pure element in air, sitting on a table or whatnot. How big would a sphere of plutonium have to be to go boom all on its own. Not very big, about 100mm diameter sphere will do.
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