Coincidence source localization by backward atmospheric transport modelling for a series of radioxenon detections at the IMS station RN 38, Takasaki

A network of radionuclide stations forms one part of the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These radionuclide stations are highly sensitive and continuously monitor the atmosphere for tiny traces of radioactive fission and activation products. All IMS radionuclide stations have a high-volume sampler for detecting particulate radionuclides; some are also equipped with noble gas systems for measuring radioxenon. Specific radioactive xenon isotopes are more likely to escape from underground nuclear explosions and exhibit less complex atmospheric transport characteristics.A central challenge of radioxenon monitoring for the CTBT is attributing and classifying detections originating from reactor sources. As was seen in the aftermath of the announced North Korean nuclear test explosions, atmospheric transport modelling is crucial for interpreting the spatial and temporal relevance of radioxenon detections in the context of potential CTBT non-compliance.In this respect, the IMS noble gas system at RN38 in Takasaki, Japan, is very important due to its location downwind of the Korean Peninsula, particularly during the northern winter months. In summer, the influence of the East Asian monsoon leads to dynamic patterns that extend further north and north-east.Several episodes of considerably high radioxenon activity concentrations in the range of tens of mBq/m³ were observed at RN38 in the years 2024 and 2025. These activity concentrations are ten to twenty times higher than those usually observed at comparable stations, but are still several orders of magnitude below levels of radiological concern. Backward atmospheric transport modelling investigates the potential source region of these detections by identifying areas of coincident atmospheric sensitivity. This enables a clear attribution to a common source region around Yongbyon. In particular, the North Korean nuclear test site can be excluded as the origin of the recurring detections. However, the potential blinding effect for telltale traces from nuclear tests, as well as their impact on other monitoring stations in the region and the IMS, is estimated by evaluation of forward ATM forecasts for hypothetical emissions from the North Korean test-site.