In nuclear transport theory this approximation is generally valid when the scattering cross-section dominates that of absorption and the system is optically thick.  
In nuclear transport theory this approximation is generally valid when the scattering cross-section dominates that of absorption and the system is optically thick.  In that case it allows the transport equation to be simplified to an ordinary differential equation.   More often it is found in diffusive mass transport problems where it states that the diffusion flux is proportional to the concentration gradient of the material which diffusion is occurring in.  FTP name this law often combined with Graham's law to calculate exchange rates of gases across membranes.
In nuclear transport theory this approximation is generally valid when the scattering cross-section dominates that of absorption and the system is optically thick.  
Fick's Law