The workbook calculates the temperature distribution due to conduction in a geometry representative of a nuclear fuel rod.   This analytical solution includes volumetric heat generation from nuclear fission in the rod itself and contact resistance at the gap between the fuel pin and its cladding.  Beyond that is conductive resistance of the Zircaloy cladding and convective resistance (=1/hA) at the outer surface, where it is bathed by coolant.

Note the calculated temperature drop of some 1600K over a distance of about half a centimeter.  Certainly, the change in thermal conductivity must be included.