A Thermodynamic and Kinetic Model of Radionuclide Condensation and Its Effect on the Xenon Isotopes Produced by Underground Nuclear Explosions

Monitoring radioactive xenon isotopes in the atmosphere to detect and discriminate underground nuclear explosions is a major challenge that can be affected by the condensation of the radionuclides in the nuclear cavity. Indeed, this process controls the quantity and the relative abundances of the xenon isotopes. We developed a comprehensive modeling approach in which the proportions of each decay chain member in the gas phase and magma are determined by thermodynamic calculations for specified nuclear test conditions. Thus, the condensation and evaporation of each radionuclide are coupled with the 131, 133, and 135 decay chains for a prescribed cooling history. Additional features include the trapping of volatile radionuclides (e.g., iodine and xenon), which are produced by the decay of more refractory precursors that have already condensed in the magma. Further alteration of the radioxenon inventory in the chimney can also be considered. Our modeling results are consistent with measurements obtained from the International Monitoring System for verification of the Comprehensive Nuclear-Test-Ban Treaty, in relation with the underground nuclear test conducted by North Korea in 2013. This shows that the effect of condensation can no longer be neglected in assessing the radioxenon inventory released from underground nuclear tests. Our model provides more accurate source terms for atmospheric transport modeling and for building better constrained scenarios.