What two word phrase means the smallest amount of fissile material needed for a sustained nuclear chain reaction?
Critical mass Critical mass A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. The concept is important in nuclear weapon design. When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a "critical" state in which there is no increase or decrease in power, temperature, or neutron population. A numerical measure of a critical
Fissile material Fissile material In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives. According to the Ronen fissile rule, for a heavy element with 90 ≤ "Z" ≤ 100, its isotopes with 2 × "Z" − "N" = 43 ± 2, with few exceptions, are fissile (where "N" = number