  We study non-linear effects of radiative viscosity of $npe$ matter in neutron
stars for both direct Urca process and modified Urca process, and find that
non-linear effects will decrease the ratio of radiative viscosity to bulk
viscosity from 1.5 to 0.5 (for direct Urca process) and 0.375 (for modified
Urca process). Which means that for small oscillations of neutron star, the
large fraction of oscillation energy is emitted as neutrinos; but for large
enough ones, bulk viscous dissipation dominates.
