Assessment of emissions from noble gas background sources: what can we learn from atmospheric transport modelling for long term krypton-85 measurements?

The radionuclide network of the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty is in place for the detection of tiny atmospheric traces of radioactive fission and activation products generated by nuclear explosions.  Atmospheric transport modelling supports the assessment of potential source regions and checks for consistency with explosion sites.

All radionuclide station sniff for particulate radionuclides, a part of it is additionally equipped with noble gas systems measuring radioactive xenon isotopes. Those are of particular importance as they are more likely to escape from underground nuclear explosions and the inert character is advantageous for simulating atmospheric transport.

A central challenge of radioxenon monitoring remains to classify radioactive xenon emissions originating from other sources as isotope production facilities and other reactors. This attribution was also crucial for the interpretation of radioxenon detections in the aftermath of the announced North Korean nuclear test explosions.  

Another radioactive noble gas isotope krypton-85. It is not part of the list of CTBT relevant isotopes due to its large background (half-life 10.8 years) and smaller nuclear yield. Large quantities of krypton-85 have been released into the atmosphere by nuclear fuel reprocessing both for military and civilian purposes. This created a significant atmospheric background due to the long krypton-85 half-life. In the context of discussing monitoring possibilities for a future fissile material cut-off krypton-85 is potentially suitable as indicator for the detection of clandestine plutonium separation. The “Bundesamt für Strahlenschutz” (BfS, Federal Office for Radiation Protection) has been operating a network with weekly air sample collection at up to 26 locations in Germany and worldwide since 1973. For the data from 2005-2024 backward ATM is performed for more than 10000 samples from about 10 stations. Taking advantage of the long time series we analyse if backward atmospheric transport modelling allows even in the coarse time resolution of weekly samples for attribution to different emitters on the Northern Hemisphere.  The effect of the shutdown of the Selllafield reprocessing facility on the European network is analysed as example.