Origin of an elevated radioxenon episode: A comparative study between the Swedish radioxenon array and the CTBT International Monitoring System

Between late November and early December 2022, large airborne activity concentrations of radioxenon were detected by multiple stations in the CTBT International Monitoring System (IMS) and the Swedish radioxenon array, which consisted of four operational radioxenon measurement units at the time. Over 100 samples containing ¹³³Xe were recorded during this period, with ^133mXe found concurrently in about a dozen samples. The consistent ^133mXe to ¹³³Xe activity ratio, and investigations with backward atmospheric transport models (ATM), suggests a sustained release of radioxenon from a single source for about two weeks. A grid of forward ATMs was computed, and model results were matched with observations to identify potential source regions and estimate the source term. We compare locating power of the sparse, globally distributed IMS network with the compact, local Swedish radioxenon array. In this specific case, the use of the array data both significantly improved the source localization area, and helped exclude local sources, when compared to exclusively using IMS observations. Additionally the ^133mXe to ¹³³Xe activity ratio was investigated for source characterization, and was found to be consistent with the start of a nuclear reactor cycle.