The minimum amount of material needed to sustain a nuclear reaction is called the critical mass. The critical mass is the smallest amount of fissile material that can sustain a chain reaction. The size of the critical mass depends on the type of fissile material being used, the density of the material, and the geometry of the material. For example, the critical mass of uranium-235 is about 52 kilograms, while the critical mass of plutonium-239 is about 10 kilograms.