Atmospheric Transport Modeling to Identify the Coincident Source Region for Elevated Radioxenon Measurements at IMS Station JPX38

Abstract Distinguishing radioactive xenon detections that originate from reactor sources remains a key challenge of radioxenon monitoring for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). This was also crucial for the interpretation of radioxenon detections in the aftermath of the announced North Korean nuclear test explosions. Due to its geo-location, the noble gas system JPX38 at the International Monitoring System radionuclide station RN38, Takasaki, played an important role for that. At JPX38 several episodes of recurring high radioxenon activity concentration peaks were observed in 2024/2025. We investigate the common source region of those peaks by stacking sensitivities derived by backward atmospheric transport modeling. The sole maximum of coincidence points to the area of the Yongbyon nuclear complex in North Korea where the Experimental Light Water Reactor started operation in October 2023.