Experimental investigation of the effective thermal conductivity in the near-wall region of a packed pebble bed

•A short description of the newly built NWTCTF test facility is provided.•Experimental investigation of effective thermal conductivity in the near-wall region.•Effects of conduction and radiation heat transfer are included.•Results are presented for structured and randomly packed beds of graphite spheres.•Near-wall effects have a notable influence on the effective thermal conductivity.