Investigating anomalous radioxenon detections on the International Monitoring System related to a significant source in East Asia

The International Monitoring System (IMS) Radionuclide (RN) network measures airborne fission and activation products for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Recently, the background levels of radioactive isotopes of xenon (radioxenon) at several IMS stations in the East Asian and Pacific regions have been anomalously high. On 21^st December 2023, the International Atomic Energy Agency (IAEA) released a press statement in relation to developments in the Democratic People's Republic of Korea (DPRK) nuclear programme, indicating that a new experimental light water reactor (ELWR) had come online. Here, we discuss several periods of heightened radioxenon activity at IMS stations in the northern hemisphere and begin to associate spatially and temporally distributed detections with emissions from the Yongbyon site using atmospheric transport and dispersion modelling. Source term estimations suggest that a facility operating at Yongbyon is amongst the largest known radioxenon emitters globally, with ¹³³Xe emission rates akin to those of a medical isotope production facility. This source of anthropogenic fission product radioxenon represents a significant signal which must be contended with when monitoring for nuclear explosive testing.