CRITICAL SIZE: The actual size of material which allows the escape of neutrons to such an extent that at least one neutron is positively left behind per fission reaction is known as critical size.
CRITICAL MASS: The mass corresponding to the critical size per fission reaction is called as critical mass of that material. If the mass of the fissionable material is less than the critical mass, nuclear fission would not occur. This situation is called sub critical. If the mass of fissionable material is more than the critical mass, then the situation is called super critical. Critical mass has always a definite value for any specific system.
