Wiktionary
subcritical mass
n. (context physics English) A mass of fissile material that is insufficient to sustain a nuclear chain reaction
Usage examples of "subcritical mass".
When they're set down on the counter tops, the platters achieve subcritical mass and begin to heat up!
It looks no more like an atom bomb than I look like Stalin, but it represents-if it were plutonium-what we atom tinkerers call a 'subcritical mass.
Dr Curnow was sharing a joke with his companions - as well as a large bottle that he handled as delicately as a barely subcritical mass of plutonium.