Critical mass
The minimum mass of fissile material (e.g., U-235 or Pu-239) required to sustain a chain reaction at multiplication factor k = 1. Critical mass depends on the material, enrichment, temperature, and geometry.
Real World
The Manhattan Project scientists calculating the critical mass of plutonium-239 for the Trinity test used approximately 6 kg — compressed by conventional explosives to increase density and reduce the critical mass threshold.
Exam Focus
Explain critical mass in terms of neutron escape versus neutron production; mention that geometry and density affect it, not just total mass.
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