Abstract: Prior to the transportation to a discharge site of a casket or flask in which has been placed a container filled with radioactive waste, through a hole formed in the flask is introduced a probe holder (32) connected by electrical connectors (76,80) to an external measuring apparatus. The probe holder (31) is provided with a handle (68) making it possible, through a mechanism (66,56,54,50), to swing a door (42) to which a probe (34) is fixed between a retracted introduction position and a measuring position in which the probe can be oriented at 90.degree. relative to the probe holder axis. After the probe holder has been coupled by a nut (40) to a container carrier in which is located the container containing the waste, a precise, reproducible measurement of the dose rate within the transportation flask can be carried out.