Setting the baseline for estimated background observations at IMS systems of four radioxenon isotopes in 2014

Worldwide monitoring of radionuclides is an essential part of the verification system of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) as it can provide a direct evidence of the nuclear nature of an explosion. In the case of underground nuclear testing, the radioactive noble gases, specifically radioxenon, have the highest probability to escape to the atmosphere. The detection capability of the CTBT noble gas network, which is being built, is weakened due to the presence of a worldwide civilian radioxenon background. Improving the understanding and knowledge of civilian radioxenon sources and their impact on the noble gas systems background is crucial to strengthen their verification capabilities. Two major civilian radioxenon sources have been identified in past research, namely: Medical Isotope Production Facilities (MIPFs) and Nuclear Power Plants (NPPs). In this study, a 2014 baseline radioxenon emission inventory is proposed for all four CTBT relevant radioxenon isotopes (Xe-131m, Xe-133m, Xe-133 and Xe-135) on the basis of a literature review for both the Medical Isotopes Productions Facilities and Nuclear Power Plants. This 2014 baseline radioxenon emission inventory relies on peer-reviewed information on the facility location and corresponding radioxenon emission. The baseline radioxenon emission inventory is used along with Atmospheric Transport Modelling (ATM) to estimate the radioxenon activity concentrations at the noble gas systems. The results reveal the complexity and the geographical dependence of the civilian radioxenon background. The estimations are compared to the observations for CTBT noble gas systems that were operational in 2014. It is demonstrated that the estimated Xe-133 activity concentrations are, for most systems, in the same order of magnitude than observed detections. Non-detections of Xe-131m, Xe-133m, Xe-133 and Xe-135 are, for most samples, well reproduced by the estimation. To our best knowledge, this study is the first attempt to propose, a baseline emission inventory for all four CTBT relevant radioxenon isotopes and compare the estimated Xe-131m, Xe-133m, Xe-133 and Xe-135 activity concentrations with all observations at CTBT noble gas systems during the full 2014 calendar year.