Source localization by backward atmospheric transport modelling for radioxenon detections at Mount Schauinsland with six hours sampling duration

Abstract

Part of the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT) is  a network of highly sensitive radionuclide stations sniffing for tiny traces of fission and activation products in the atmosphere.  All IMS radionuclide stations have a high volume sampler for the detection of particulate radionuclides, some are equipped with noble gas systems for the measurement of radioxenon. The specific radioactive xenon isotopes are more likely to escape from underground nuclear explosions and have less complex features in atmospheric transport. There are also radioxenon background emissions from legitimate nuclear facilities. Isotopic ratio analysis allows to a certain extend for source characterization and timing. Atmospheric Transport Modelling (ATM) in backward and forward mode is applied to connect the measurements in space and time with potential releases.   Noble gas systems at IMS radionuclide stations used to operate with 24 or 12 hours sampling time. The next generation noble gas systems utilize shorter sampling periods. At station RN33 on Mount Schauinsland, Germany, a SPALAX system with 24 hours sampling is operated by BfS. In the phase II testing a  “Xenon International” system with six hours sampling time and better sensitivity to Xe-135, Xe-133m and Xe-131m was installed in parallel from July 2021 to April 2022. The main contributing emitter to elevated xenon activity concentrations at RN33 is the medical isotope production facility at Fleurus, Belgium. We investigated how the increase in time resolution in sampling and ATM changes the location capability of backward ATM. For that, the Lagrangian Particle Dispersion model HYSPLIT (NOAA-ARL) is applied driven by GFS meteorological data for all samples of the test phase. Calculation of expected Xe-133 contributions from Fleurus derived by ATM backward sensitivities and emission data show generally good agreement. As the Xenon International system also allows for additional detections particularly of Xe-135 and isomers Xe-133m and Xe-131m, the sensitivity to unknown additional sources is potentially improved and analysed. A coincidence analysis of repeating or in respect to isotopic composition remarkable detections which could not be well explained by emissions from Fleurus show several other potential source regions pointing to several  Nuclear Power Plants and research reactors.