Schematic view of the numerical nucleus. Pressure P_i and temperature T_i are defined at the center of each cell i and are represented by the coefficient L_i. The coefficients of heat conduction and gas diffusion G_i are defined at the center and at the edge of each cell i. They are here represented by the coefficients L_i at the center and l_i at the edge of each cell i. The parameters r_i and Δr_i are the distance from the center of the nucleus and the thickness of each cell i.