Improvement of Clearance Reliability for Plastic and Metal Mixture Waste Using CT

Clearance is the removal of regulatory control by a regulatory body from radioactive material or radioactive objects. One method of evaluating the radioactivity for clearance is measuring the storage container containing the waste from the outside with a radiation detector. Due to the amendment of clearance regulations of Japan in 2020, plastic and metal mixtures (power cables, switchboards, etc.) can be released under the clearance. However, when the storage container containing these substances is measured as it is, the non-uniform distribution of the atomic number and the density of each substance in the storage container makes it difficult to consider the actual radiation shielding effect. In order to incorporate reliable radiation shielding into radionuclide concentration evaluation, we propose to introduce a non-destructive inspection method, such as CT scan that can give information on the inside of container. Assuming that the elements contained in the storage container were known as prior information, we experimentally verified whether it was possible to identify the elements by the CT scan. In this study, rods of PMMA, aluminum, copper and iron in small container were scanned by CT and images were reconstructed by FPB and ML-EM methods. The results showed that each element could be distinguished from each other successfully when the voltage of X-ray tube was over 100 kV.