The core-mantle reaction proceeds on two scales: the short-scale chemical reaction leading to local equilibrium and the large-scale dispersal of reaction products. The second process is connected with a growth of the CMB-radius and may be described with application of the diffusion equation. The departure from a stationary interface is calculated using the gravitational body force as the mechanism controlling the “tension” of the distorted spherical core body. Stability analysis with the help of angular harmonics leads to the result that undulations of CMB are stable for very long wave lengths only.