Speaker
Description
The radiological characterization of activated nuclear reactor components, resulting from neutron-induced reactions, which frequently also exhibit surface contamination, is essential for decision-making regarding their management during decommissioning. Prior to the dismantling and segmentation of reactor components, the selection of an appropriate cutting technology, aimed at minimizing of secondary radioactive waste production and reducing personnel radiation exposure, depends on accurate radiological characterization of radionuclides distributed both within the material volume and on their surfaces as contamination. Following dismantling and segmentation, accurate radiological characterization of radionuclides within the materials and on their surfaces is necessary to assess the effectiveness of decontamination, to select the appropriate decontamination and clearance procedures, as well as to establish appropriate management for the remaining radioactive waste.
In activated reactor components, certain radionuclides are embedded within the material matrix (activation products), others are present as surface contamination (fission products), while some may be present simultaneously both within the volume and on the surface. As an example, ⁹⁴Nb is highly insoluble, in contrast to ⁶⁰Co, which is soluble, although both are activation products. Consequently, ⁹⁴Nb is typically not observed as surface contamination, whereas the presence of ⁶⁰Co on surfaces is significant. In contrast, ¹³⁷Cs, a typical fission product, is predominantly found as surface contamination and only rarely within the material volume.
A non-destructive technique (NDT), based on a single gamma spectrometry measurement and Monte Carlo simulations using MCNP code, for the concurrent quantitative determination of activation within the materials and on their surfaces was developed at NCSRD in the frame of the EU PREDIS project.
For the discrimination and quantification of radioactivity on the surface and within the volume, two alternative and equivalent, accurate methods based on the analysis of the photopeak and the Compton edge were developed. The first method is based on the net counts of the detector in the photopeak and the Compton edge region arising from activities distributed both within the volume and on the surface of steel samples. The second method relies on the correlation between the Compton-to-photopeak ratio (C/P) and the absolute photopeak efficiency, as derived from simulated gamma-ray spectra. This relationship was subsequently utilized for the concurrent determination of both the activities.
Both methodologies produced consistent results, while the agreement between the nominal activities and the experimentally determined values remained within the 1σ to 2σ uncertainty range, confirming the reliability of the proposed approaches for the concurrent determination of surface and volume activities in steel using a single experimental gamma-ray spectrum of an unknown sample.