Critical mass

Critical mass refers to the mass of a fissile isotope necessary to cause a sustainable nuclear fission reaction. At the critical mass, the rate of production of neutrons by fission equals the rate at which they are absorbed. The critical mass of an isotope depends on a variety of factors such as its temperature, density and its shape. If critical mass is reached, then the nuclear reaction could run away resulting in a large amount of energy being released and possibly a nuclear explosion.