Symmetry Condition in Radiation Modeling

How does a symmetry condition in Radiation modeling work?

The net flux crossing the symmetry plane is zero. Radiation should see a symmetric boundary as the same zero-gradient condition that other scalar fields should. "Physically", what should be happening here is that a radiation "ray" should approach this boundary and see another just like it on the other side, so it essentially perfectly "reflects" back into the domain (or you could think of it as it actually goes out, but the other ray that it saw on the other side comes in). The net effect is no net radiative energy gain/loss through the system at these boundaries.