For a flat surface, the shape factor with respect to itself in the radiative heat transfer is

Concept:

Shape Factor is defined as the fraction of radiation energy leaving a surface that reaches another surface, for example, F12 means the fraction of energy leaving the surface 1 reaches surface 2.

Summation Rule: F11 + F12 = 1

Reciprocity theorem: A1 F12 = A2 F21 (A is the surface area)

So, when the radiation energy leaving from a fat surface, it will not cpme back to the same flat surface. Therefore, for a flat surface, the shape factor with respect to itself in the radiative heat transfer is zero.