Wiktionary
supercritical mass
n. (context physics English) A mass of fissile material that is more than sufficient to sustain a nuclear chain reaction
Usage examples of "supercritical mass".
Increasing speed in the vacuum, it crushed the nuclear mass uniformly to one third its original size, further increasing the density of the U-235, which was surrounded by a beryllium shell, and in turn crushing the nemo initiator, which was designed upon being crushed to create a huge influx of electrons, causing nuclear fusion and a supercritical mass.