Abstract: There is disclosed an elongated cylindrical rod of heat-conducting and electrically-conducting material having a central bore containing a plurality of thermocouples, the hot and cold junctions of which are placed respectively in separate zones along the rod. In each zone, the hot junction of one thermocouple is fixed generally in position axially at the midway of an annular reduced-section portion of the cylindrical rod, the cold junction is located generally axially between two reduced-diameter portions and an annular insulating chamber is defined annularly opposite to each reduced-diameter portion by an outer shielding tube. The outer tube surrounds the cylindrical rod. The completed unit is to be placed within a reactor core fuel assembly to be monitored.