Comparison of the outer boundary conditions presented here (solid lines) and from Levermore & Pomraning (1981) (dashed lines) for the test case presented in Sect. 5.2. The colour code is the same as in Fig. 3. The emerging fluxes are displayed in semi-log scale to highlight the differences between the BCs. The lower panels show the relative differences profiles with respect to the MC code from Niccolini & Alcolea (2006) in temperature (left) and in emerging flux (right).