All preceding hydrodynamic computations of plasmas need correction if the thermal conductivity is used because electronic thermal conductivity is decreased on plasma inhomogeneities due to electrostatic double layers. In the worst case, ionic conductivity remains. We compare this with a possible electronic conductivity by the fast tail of the energy distribution. Using the volume ignition for fusion gain computations, we study the increase of gain by spin-polarization of nuclei for the DT reaction especially in nonlinear ranges. Gain can increase by a factor of 3-1.