The International Monitoring System, the primary means of verification of the Comprehensive Nuclear-Test-Ban Treaty, monitors the planet for any sign of a nuclear explosion. Regarding the International Monitoring System radionuclide stations, it is known that radioxenon released from nuclear facilities such as medical isotope production facilities and nuclear power plants influences the stations. For the purposes of monitoring nuclear explosions, it is important to better understand the radioxenon background based on these nuclear facilities. The probability density functions of background activity concentration at IMS radionuclide stations are estimated using a Monte Carlo method based on emissions from known nuclear facilities and source receptor sensitivity data. This paper describes two case studies of radioxenon detections at radionuclide stations applying the developed approach. This method could be one of several prospective approaches to predict the activity concentrations of isotopes of radioxenon at radionuclide stations in Comprehensive Nuclear Test-Ban Treaty Organization’s (CTBTO’s) prototype xenon background estimation tool software. It can also be used in characterization of CTBT-relevant nuclear events for expert technical analysis. Civil nuclear power stations and medical isotope production facilities release radioisotopes of xenon during their normal operations. These emissions would make it harder to detect xenon produced from any nuclear weapon test that might occur. A method is described that starts with information about civil releases and produces a statistical description of the concentrations of isotopes measured at stations designed to detect nuclear tests. This information makes it possible to enhance nuclear explosion detection performance.

Isotopic activity ratios of the relevant radionuclides, detected at radionuclide stations within the international monitoring system (IMS) for the Comprehensive Nuclear-Test-Ban Treaty, play a crucial role in characterizing release events under assumed scenarios. This analysis involves radioactive gases initially released from an underground nuclear event resulting in radionuclide concentrations in a plume of air passing over an IMS station that are subsequently sampled by that station. Modeling this requires considering the post-shot radionuclide evolution of an assumed underground nuclear explosion, the simulations of atmospheric transport modelling, and, finally, the sample measurement. Activities collected in the samples are determined through spectrum analysis of sample measurements, and the activity concentrations are then estimated by assuming a constant concentration during sampling. While this assumption holds for radionuclides with relatively longer half-lives, that is, exceeding 7 times the sampling duration, challenges arise for the isotopes with short half-lives, such as 135 Xe with half-life of 9.14 hours compared to the 12-hour collection duration for some noble gas systems. Therefore, this study investigates the decay correction during sampling using two approaches, 1) Interval constant concentration: the collection duration is divided into multiple intervals with a constant concentration assumed in each interval; 2) Decaying concentration: the activity collected in the sample is derived based on an analytical solution to the ordinary differential equations of the activity decay and ingrowth. The impact of these approaches on isotopic activity ratios is demonstrated in three cases with short half-lives, 140 Ba/ 140 La, and 133m Xe/ 133 Xe, and 135 Xe/ 133 Xe.

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.

Radioxenon isotopes measured at radionuclide stations of the Comprehensive Nuclear-Test-Ban Treaty’s (CTBT) International Monitoring System (IMS) may indicate releases from underground nuclear explosions (UNEs) but are often caused by emissions from nuclear facilities. Characterization of CTBT-relevant nuclear events may use the evolution of isotopic activity ratios over time, which goes from the release of an assumed UNE, through atmospheric transport, to sample collections and measurements. A mathematical approach is presented to discuss the characterization of the spatial and temporal relationships between a nuclear explosion and radioxenon measurements. On the one hand, activity concentrations at an IMS station are estimated by using the assumed release scenario regarding a UNE and atmospheric transport modelling. On the other hand, the activities collected in the samples are determined by spectral analysis first and the activity concentrations in the air passing over the IMS station are estimated under an assumption of constant concentration during sampling. The isotopic ratios of activities released from the UNE are related to the isotopic ratios of activity concentrations in the plume of air crossing the IMS station, resulting in a function of the isotopic activity ratio over the time from detonation to sample measurement. The latter is used for discrimination of a nuclear test and estimation of the time of detonation, such as a four radioxenon plot of the activity ratio relationship of 135 Xe/ 133 Xe versus 133m Xe/ 131m Xe.

Radionuclide monitoring is one of the verification technologies of the global verification system of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). This global network of sampling stations senses the air 24/7 for suspect noble gases and/or particulates. For noble gases this task is non-trivial due to the ever-present and highly variable background levels of the four radioxenon isotopes that are relevant for CTBT monitoring. An extensive, global effort was initiated to better estimate the civil radioxenon background based on known sources and end up with a more reliable event screening. This challenge, called “1st Nuclear Explosion Signal Screening Open Inter-Comparison Exercise 2021,” provided an assessment of a chain of multilevel, multidisciplinary scientific analyses and built on three previous atmospheric transport modelling (ATM) Challenges. It’s a first since it explored integrating both ATM and radionuclide statistical expertise to characterize the detection, time, location, and source strength of an anomalous event. The exercise ran through 2022 and was a collaboration between participants from around the world who utilized a comprehensive pre-developed test data set based on explosion release scenarios, xenon measurements and emission inventories, and atmospheric transport data provided by the ATM software FLEXPART. The data set was composed of synthetic activity concentrations of the simulated nuclear explosion signals added to the radioxenon measurements at the International Monitoring Station (IMS). Three levels of participation were offered, requiring different areas of expertise: 1) ATM expertise only, where participants simulated radioxenon background time series at the 23 IMS stations to be used as input for screening synthetic radioxenon measurements based on a set of predefined statistical methods; 2) radionuclide expertise, where participants provided their own methods and results for detection, screening, and timing powers; and 3) higher-level ATM and statistical expertise, where, in addition to Level 2, results were provided for location and magnitude estimates for a few selected test cases. This paper gives a general overview of the exercise and provides highlights and discusses the key results.

Detection of radionuclides released from a nuclear explosion is an essential task mandated by the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Atmospheric transport modelling (ATM) identifies either possible source regions for relevant radionuclide observations at anomalous concentrations through the so-called International Monitoring System (IMS) or potential stations for measuring releases from known source locations. This is a well-known methodology for connecting sources and receptors of any substance in the atmosphere. The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) currently investigates the potential advantages of using high-resolution ATM. Past announced underground nuclear tests at the Punggye-ri Nuclear Test Site from the Democratic People’s Republic of Korea (DPRK) are used in this study to scale the CTBTO’s capability to identify IMS stations that might detect a hypothetical release. These events are also used to identify the capability to locate Punggye-ri as the possible source location. A sensitivity study is presented that demonstrates the CTBTO’s capability to identify Punggye-ri as a possible source region for the relevant radionuclide measurements at IMS stations. The aim is to find the best model set-up from varying combinations of meteorological resolution, regional domain set-up, and physical parameterization. Variations in resolution are accomplished by using first the Lagrangian Particle Dispersion Model FLEXPART, which will be driven by meteorological fields from the European Centre for Medium-Range Weather Forecast (ECMWF) with either 0.5° or 0.1° spatial and 1 h temporal resolution; and second, by using a combination of the Weather Research and Forecasting Model (WRF) and FLEXPART-WRF to scale down to 1 km spatial resolution. The potential accuracy increase is evaluated by using metrics from previous ATM challenges.

The operational Atmospheric Transport Modelling (ATM) system deployed and used at CTBTO produces source receptor sensitivity (SRS) fields, which specify the location of the air masses prior to their arrival at any radionuclide station of the International Monitoring System (IMS) network. The ATM computations support the radionuclide technology by providing a link between radionuclide detections and the regions of their possible source. If an IMS station detects an elevated level of radionuclide, the ATM in a backward mode is used to identify the origin of air masses. In the case of a single detection, the FOR (Field of Regard) is computed, which denotes the possible source region for a material detected within one single sample. On some occasions, multiple detections occur at one or more IMS stations. Depending on the nature of these detections and on prevailing meteorological conditions, it is possible that all these detections may come from a unique source. For this case, the PSR (Possible Source Region) is computed for each grid point in space and time by calculating the correlation coefficients between the measured and simulated activity concentration values (SRS fields). Obviously, the result will depend on the algorithms used for that purpose. Currently, in the WEB-connected GRAPhics Engine (WEB-GRAPE) software, designed and developed by the International Data Centre (IDC) to visualize and post-process of the ATM results, three different PSR algorithms are implemented: two based on the Pearson’s correlation coefficient and one based on the Spearman’s rank correlation coefficient. For the quality assessment of these PSR algorithms, subsets of datasets developed in the framework of the 2nd and 3rd ATM Challenge will be used, which satisfy the condition that the agreement between Xe-133 measured and simulated values is very good. In this sense, the selected samples will represent “ground truth” data, where the contribution from all dominated sources (e.g. Isotope Production Facilities or Nuclear Power Plants) is included. For these selected samples, the results produced by the different PSRs algorithms will be assessed, taking into account both spatial and temporal variations.

For enhancement of the International Data Centre (IDC) products such as the Standard Screened Radionuclide Event Bulletin (SSREB), there is a need to associate the detections of CTBT relevant isotopes in samples at International Monitoring System (IMS) radionuclide stations with the same release to characterize its source for the purpose of nuclear explosion monitoring. Episodes of anomalous concentrations at the stations are the best first guess for being related to the same event. For multiple isotope observations, the consistency of their isotopic ratios in subsequent samples with radioactive decay is another plausible hint at coming from the same source. Moreover, atmospheric transport modelling (ATM) will help to get further evidence and gain confidence in sample associations by identifying the air masses that link the release to multiple samples. We focused on the basic approach as well as the criteria for automatic sample association for the SSREB.