Hi Benjamin,
Yes, any surface can be set to be reflective including a general plane surface on the diagonal for a 1/8 reactor model. A rotationally-periodic boundary condition is applied to a pair of x-plane and y-plane – it means that when a particle crosses the x-plane, it comes out on the y-plane, and vice-versa. That way, if you have rotational symmetry (but not reflective symmetry), you can still reduce the size of your model. That would normally be applied to 1/4 core models.
Copying the OpenMC user’s group mailing list on this email in case others have this same question in the future.
Best regards,
Paul