The method includes providing the volume of waste in a monitoring space; providing a support, the support being provided with a plurality of detectors for radioactive material; monitoring the volume of waste for radioactive material, in one or more parts, to give a monitoring results; and correcting the monitoring result for geometry and/or attenuation to give a corrected result using a correction factor. The correction factor is obtained by a method that includes providing a simulation of an equivalent volume of waste free of radioactive material in an equivalent monitoring space, with equivalent detectors; providing a known activity radioactive source at one or more positions in the volume of waste; determining the detector response to the source in a position, a comparison of the response and known activity contributing to a correction function for that equal volume, one or more such correction functions providing the correction factor.