Radiological Characterization Process in the Decommissioning of the E. Fermi Nuclear Power Plant at Trino (VC)

This Master thesis describes the experience at the nuclear power plant E. Fermi at Trino (VC) during the decommissioning activities managed by SOGIN. Decommissioning represents the final phase in the operational life of a nuclear facility, during which each system is gradually deactivated and dismantled. Every step is planned and carried out in accordance with the regulations governing radioactive waste management. The main objective in decommissioning is to clean and restore the site while minimizing workers exposure during operations. This work presents the activities carried out on the dismantling and shipment of primary circuit tubes for melting. In detail, the operation started with the cleaning of the internal surface of the loops in dedicated closed rooms. These procedures were continuously monitored by health physics personnel to assess worker exposure within the cleaning area. The next step involved the radiological characterization of the components to obtain reliable contamination data for shipment, as required by supervisory authorities. Then each component was loaded into containers for shipment, operations supported by health physics operator to assess the containers dose rate. Regarding radiation protection, different instruments were used to detect radiation and evaluate the dose rate to which workers were exposed. For example, during the cleaning of the primary circuit tubes, an environmental air radiation sampler was used to perform a real-time dose rate measurements in both the cleaning room and surrounding areas. Each worker was checked with a contamination monitor before exiting the controlled area. Floor contamination was assessed using smear tests, which were analysed with a proportional counter capable of detecting low levels of radioisotopes concentration. Instead, concerning radiological characterization, where radioactive waste is analysed to quantify the activity of radionuclides listed by supervisory authorities, it is carried out through gamma spectrometry. This analysis was performed using High-Purity Germanium (HPGe) detectors, which are capable of accurately measuring radionuclides concentrations even in large components. This allows for a quantitative assessment of the activity of each radionuclide present in the sample. These measurements were used, for instance, to characterize the loops after the cleaning procedures. Radiological characterization data from gamma spectrometry collected during the process are presented and discussed, evaluated in relation to the thresholds defined by supervisory authorities. The calibration procedures, focused on the software configuration and energy response of the HPGe crystal, are described in detail. In addition, practical recommendations for the correct use of those detectors in field conditions are provided, aimed at ensuring optimal measurement accuracy and reliability.