We present the implementation of two-moment based general-relativistic multi-group radiation transport module in the 𝙶eneral-relativistic 𝚖𝚞ltigrid 𝚗𝚞merical (𝙶𝚖𝚞𝚗𝚞) code.
On top of solving the general-relativistic magnetohydrodynamics and the Einstein equations with conformally flat approximations, the code solves the evolution equations of the zeroth- and first-order moments of the radiations.

Analytic closure relation is used to obtain the higher order moments and close the system. The finite-volume discretisation has been adopted for the radiation moments. The advection in spatial and frequency spaces are handled explicitly.

In addition, the radiation-matter interaction terms, which are very stiff in the optically thick region, are solved implicitly.

Implicit-explicit Runge-Kutta schemes are adopted for time integration.

We test the implementation with a number of numerical benchmarks from frequency-integrated to frequency dependent cases.

Furthermore, we also illustrate the astrophysical applications in hot neutron star and core-collapse supernovae modellings, and compare with other neutrino transport codes.