A global monitoring system for atmospheric xenon radioactivity is being established as part of the International Monitoring System that will verify compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) once the treaty has entered into force. This paper studies isotopic activity ratios to support the interpretation of observed atmospheric concentrations of (135)Xe, (133m)Xe, (133)Xe and (131m)Xe. The goal is to distinguish nuclear explosion sources from civilian releases. Simulations of nuclear explosions and reactors, empirical data for both test and reactor releases as well as observations by measurement stations of the International Noble Gas Experiment (INGE) are used to provide a proof of concept for the isotopic ratio based method for source discrimination

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Between 21–25 October 2006, elevated levels of atmospheric xenon‐133 were observed in Yellowknife (Canada). This station is located in an area where the background level of radioxenon is very low. The few measurements of xenon‐133 above background in the last three years have been traced back to known nuclear facilities. The measurements in late October could not be linked to them. According to backward atmospheric transport models (ATM), the air that contained the measured radioxenon could have originated from the Korean Peninsula. On 9 October 2006, seismic networks world‐wide recorded an event with characteristics of an underground explosion in the Democratic Peoples Republic of Korea. Forward ATM was performed using these coordinates. The results were consistent with the measurements in Yellowknife, more than 7000 km away. The order of magnitude of the amount measured is consistent with simple leak scenarios assumed for a low yield underground nuclear explosion on the Korean peninsula.

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