Technical and scientific activities of the Belgian Data Centre: Annual report 2024

This summary presents the technical activities performed by the Belgian National Data Centre (NDC.be) in 2024 in the context of the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The NDC.be focusses on the follow-up of the radionuclide (RN) measurements performed by the International Monitoring System (IMS) for the verification of the CTBT. Currently, 73 stations for the measurement of radioactive particulates and 25 stations for the measurement of noble gases (radioxenon) are in operation. In addition, 14 IMS radionuclide laboratories are currently in operation. For the other technologies (seismic, infrasound and hydroacoustic measurements), the NDC.be works together in a BeNeLux collaboration. In 2024, the NDC.be reviewed 325 RN particulate spectra to determine the relevance of the detections flagged by the International Data Centre (IDC) of the CTBT Organization. After review, most of these detections were considered as non-relevant for the CTBT verification. For the remaining detections, there was no clear indication that they could be the result of a Nuclear Explosion (NE) event. The follow-up of the noble gas samples in the IMS noble gas stations allowed the NDC.be to identify 10 samples where either an abnormally high xenon isotope detection or an abnormal xenon isotopic ratio occurred. The analysis of these 10 samples suggest that the corresponding detections are most likely resulting from the global civilian radioxenon background as no clear indication would support a NE event. During 2024, the following three specific events were investigated in more detail: i) an increased radioxenon background at JPX38 (Takasaki, Japan), ii) a high number of Cs-137 detections in Europe in September 2024 and iii) the NDC preparedness exercise 2024. The increased radioxenon background at JPX38, which is one of the only systems in operation currently in the East Asian region, stresses again the need to: i) identify and understand background sources (using among others state of the art atmospheric transport modelling capabilities) and ii) minimize their emissions (using among others state of the art radioxenon adsorption technologies). Next to the continuous follow-up of the radionuclide part of the IMS, technical experts of the NDC.be participated in the two Working Group B sessions of the CTBT. Additionally, the NDC.be participated in different CTBT relevant conferences, workshop and meetings. Beside the actual verification activities, the NDC.be is performing research to support the CTBT. One pillar of research involves atmospheric transport modelling. It includes (i) the setup and validation of high-resolution modelling, (ii) inverse modelling using the event analysis and reconstruction code FREAR, (iii) testing the use of deposition measurements for CTBT verification activities and (iv) the use of cosmogenic radionuclides measured by IMS stations to study downward vertical motion in the atmosphere. The second research pillar involves the use of porous materials for adsorption and desorption of noble gases in support of the CTBT verification. This second research pillar aims at: (i) decreasing the radioxenon background by mitigating radioxenon releases at nuclear installations, (ii) enhancing IMS radioxenon monitoring systems to allow a better discrimination between background sources and nuclear weapon tests and (iii) simplify the argon collection and separation process for the measurement of 37Ar for On-Site Inspections (OSI).