SOURCE ALLOCATION BASED ON ATMOSPHERIC TRANSPORT MODELLING
OF RADIONUCLIDE PLUMES: CHALLENGES AND NEW DEVELOPMENTS

Identifying the source of radioactive detections at the International Monitoring System (IMS) stations is both challenging and central to the CTBTO mandate. Atmospheric Transport Modelling (ATM) calculations provide the link between IMS radionuclide detections and a potential source of emissions, including atmospheric, underground, and underwater nuclear explosions. The additional challenge related to source localization is that both remote and nearby sources might contribute to an elevated level of radionuclide detected at an IMS station. In case of radioxenon, this challenge is magnified by the presence of a radioxenon background from known sources. Radioxenon can be produced either during a nuclear explosion (forming an important tracer to prove the nuclear character of an explosion) or be emitted by nuclear facilities. The regular operational releases of radioxenon from nuclear installations contribute significantly to both the global and regional background. In the Northern Hemisphere radioxenon background at most of the IMS stations is dominated by the contribution of nuclear power plants. For some stations, simulated concentrations above the detection limit may include observable contributions from up to 19 different sources per daily sample (samples are sensitive to 80 or more possible sources of radioxenon). Thus, understanding the radioxenon background at the radionuclide stations of the IMS network is vital for improving the verification capability of the CTBTO. This presentation provides an overview of the new developments aimed at improving the source allocation method e.g., using the ensemble modelling for the backward ATM, more accurate estimation of radioxenon background or a method for identifying samples associated with the same release event.