Radiation periodicity is very similar to fluid flow depending on how you treat it numerically. By default, we treat periodic domain interfaces (translational and rotational) as perfect mirrors. That is, emissivity=0, and diffuse fraction=0. If the domain interface is 1:1, you can use the expert parameter radiation periodicity option = 1 to use true periodicity. That is, the photon/ray will leave the domain interface, and return back into the other side of the domain interface. If it is rotational periodicity, it will be rotated accordingly.