For 10 points, name this smallest amount of material needed to sustain a fissile nuclear reaction.
The calculation of this quantity for various substances was the original driving force for Nicholas Metropolis and Stanislaw Ulam to develop Monte Carlo models, as the relevant dimensions encountered limit the usefulness of a continuum approach. It can be reduced with the use of a beryllium reflector, while mean free path and radiative capture effects make it inversely proportional to density.  Although some applications require the "prompt" version of this quantity, others use moderation to ensure that delayed neutrons are also needed to reach it. For 10 points, name this smallest amount of material needed to sustain a fissile nuclear reaction.
The calculation of this quantity for various substances was the original driving force for Nicholas Metropolis and Stanislaw Ulam to develop Monte Carlo models, as the relevant dimensions encountered limit the usefulness of a continuum approach.
critical mass