On the usability of event zero time determinations using radioxenon isotopic activity ratios given the real atmospheric background observations.

This work focuses on the usability of event zero time determination using xenon isotopic activity ratios. Two data sets from Nevada underground nuclear test and Fukushima accident debris were used to calculate the age of radioxenon release by considering three kinds of radioactivity release radionuclide sources: nuclear explosion scenarios, nuclear power reactor release and medical isotopes production facilities release. Typical nuclear power reactor releases were characterized and reference values are proposed for six isotopic activity ratios, which data can be considered as reference point of nuclear reactor effluents at the time of their release obtained from real observations. The same reference values of isotopic activity ratio are given for medical isotopes production facilities releases. The purpose of this study is to evaluate the use of zero-time calculation for source characterization under the assumption that a hypothesis about the event time is made. The event time information may come from a seismo-acoustic event of interest or an inverse atmospheric transport simulation or other context information. For both data sets used in this study, the age precisions are calculated and the time precision difference is evaluated and used as a parameter for the characterization of each radionuclide event. Almost all radioxenon isotopic activity ratios are found to correctly identifying the source type of the radionuclide events studied in this work. The results from this radionuclide events characterization study may be helpful for event screening activities of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO).