Climate change as observed through the IMS radionuclide station in Spitzbergen.

The International Monitoring System (IMS), installed and maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) with the support of States Signatories, is a global system of monitoring stations based on four complementary technologies: seismic, hydroacoustic, infrasound and radionuclide. One of the IMS radionuclide stations is located in Spitzbergen, the largest island of the Norwegian Svalbard Archipelago, which borders the Barents Sea and the Northern Atlantic Ocean. It has been demonstrated that signs of climate change are particularly noticeable in that region. As many other radionuclides observed in environmental measurements, 212Pb is always observed at IMS stations, in varying quantities. This is also the case for the IMS station RN49, Spitzbergen, where it can be demonstrated that the average concentration of the measured lead 212Pb increases. This is observable specifically October through December. This paper demonstrates the asset of IMS data to study climate change effects. Our conclusions are supported by global temperature anomaly data from NOAA's Global Surface Temperature Analysis, covering the period 1850 to 2023.

Investigation on atmospheric radioactivity sample association using consistency with isotopic ratio decay over time at IMS radionuclide stations.

For the enhancement of the International Data Centre's products, specifically the Standard Screened Radionuclide Event Bulletin, an important step is to establish methods to associate the detections of the Comprehensive Nuclear-Test-Ban Treaty-relevant nuclides in different atmospheric radioactivity samples with the same radionuclide release to characterize its source for the purpose of nuclear explosion monitoring. Episodes of anomalously high activity concentrations in samples at the International Monitoring System radionuclide stations are used as the primary assumption for being related to the same release. For multiple isotope observations, the consistency of their isotopic ratios in subsequent samples with radioactive decay is another plausible hint for one unique release. The radioxenon observations that are associated with the nuclear test announced by the Democratic People's Republic of Korea in 2013 serve as case study to demonstrate the effectiveness of this basic approach and how the additionally associated samples improve the source location. We use two distinct puff releases, both of short duration, for the atmospheric transport modelling simulations to gain further evidence and confidence in our sample association study by identifying the air masses that link the releases to multiple samples. This basic approach will support the definition of analysis procedures and criteria for automatic sample association to be implemented in the Standard Screened Radionuclide Event Bulletin, which is of relevance for an expert technical analysis.

Characterisation of Xe-133 background at the IMS stations in the East Asian region: Insights based on known sources and atmospheric transport modelling.

Radioxenon can be produced with a high fission yield during a nuclear explosion, making it an important tracer to demonstrate the nuclear origin of an explosion. For this reason, it is continuously monitored by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) as part of the verification regime. Radioxenon is emitted by civil nuclear facilities, like nuclear power plants (NPPs) or isotope production facilities (IPFs), providing significant but variable contribution to the noble gas background. The discrimination between CTBT-relevant radioxenon detections and the background is then a challenging task. This work aims at estimating the radioxenon background at 8 East Asian noble gas stations of the International Monitoring Systems (IMS) (out of 26 certified and 14 others foreseen) based on known sources and atmospheric transport modelling (ATM). For the purpose of this study, the transportable system in Mutsu, Japan, was also included. The results demonstrate a predominant contribution of NPPs to the radioxenon background at most of the East Asian IMS stations, especially during summertime. In autumn, as a result of large-scale atmospheric circulation, the contribution of remote IPFs starts to dominate. In the summertime, up to 80% of the Xe-133 detections at a station may be explained by contributions from NPPs. The detections even rise to 100% in some specific cases. At some stations under investigation in this study, a transition from NPP to IPF domination is observed in September and continues during the autumn season. It has also been shown that, for some stations, simulated concentrations above the detection limit may include observable contributions from up to 19 different sources per daily sample; at the same time the sample being sensitive to 80 or more possible sources of radioxenon. This indicates that the accumulation of many weak sources can lead to a measurable result in a single air sample. This might also explain observations at very remote stations. Another important conclusion is that, despite limited knowledge about release patterns of NPPs, the agreement between simulated and measured values was good in many cases. Availability of IMS measurements allowed for validation of simulations. This comparison revealed that approximately 76% of simulated values were underestimated. Based on the paired t-test, a 95% confidence interval for the true mean difference between measurements and simulations was constructed. It was estimated that for data dominated by NPPs contribution (i.e. NPPs contribution exceeds 70%), the overall uncertainty of simulated results lies between 0.07 and 0.10 mBq/m3. For data dominated by IPFs contribution (i.e. IPFs contribution exceeds 70%), the uncertainty for the simulations is in the range between 0.03 and 0.12 mBq/m3.

International challenge to model the long-range transport of radioxenon released from medical isotope production to six Comprehensive Nuclear-Test-Ban Treaty monitoring stations.

After performing a first multi-model exercise in 2015 a comprehensive and technically more demanding atmospheric transport modelling challenge was organized in 2016. Release data were provided by the Australian Nuclear Science and Technology Organization radiopharmaceutical facility in Sydney (Australia) for a one month period. Measured samples for the same time frame were gathered from six International Monitoring System stations in the Southern Hemisphere with distances to the source ranging between 680 (Melbourne) and about 17,000 km (Tristan da Cunha). Participants were prompted to work with unit emissions in pre-defined emission intervals (daily, half-daily, 3-hourly and hourly emission segment lengths) and in order to perform a blind test actual emission values were not provided to them. Despite the quite different settings of the two atmospheric transport modelling challenges there is common evidence that for long-range atmospheric transport using temporally highly resolved emissions and highly space-resolved meteorological input fields has no significant advantage compared to using lower resolved ones. As well an uncertainty of up to 20% in the daily stack emission data turns out to be acceptable for the purpose of a study like this. Model performance at individual stations is quite diverse depending largely on successfully capturing boundary layer processes. No single model-meteorology combination performs best for all stations. Moreover, the stations statistics do not depend on the distance between the source and the individual stations. Finally, it became more evident how future exercises need to be designed. Set-up parameters like the meteorological driver or the output grid resolution should be pre-scribed in order to enhance diversity as well as comparability among model runs.

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.