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Presentation to be presented at the International Conference on the Development of Preparedness for National and International Emergency Response (EPR2021), 11–15 October 2021, Vienna, Austria, https://www.iaea.org/events/epr-2021.
Aerosol monitoring for radioactivity is a mature and proven technology. However, by improving key specifications of aerosol monitoring equipment, more samples per day can be collected and analyzed with the same minimum detectable concentrations as current systems. This work models hypothetical releases of 140 Ba and 131 I over a range of magnitudes corresponding to the inventory produced from the fission of about 100 g to 1 kiloton TNT-equivalent of 235 U. The releases occur over an entire year to incorporate the natural variability in atmospheric transport. Sampling equipment located at the 79 locations for radionuclide stations identified in the Comprehensive Nuclear-Test-Ban Treaty (CTBT) for the International Monitoring System are used to determine the detections of the individual releases. Alternative collection schemes in next generation equipment that collect 2, 3, or 4 samples per day, rather than the current 1 sample per day, would result in detections in many more samples at more stations with detections for a given release level. The authors posit that next generation equipment will result in increased network resilience to outages and improved source-location capability for lower yield source releases. The application of dual-detector and coincidence measurements to these systems would significantly boost sensitivity for some isotopes and would further enhance the monitoring capability.
The Comprehensive Nuclear-Test-Ban Treaty mandates the four verification technologies to be used by the International Monitoring System (IMS) to monitor compliance to the Treaty. These technologies are seismic, hydroacoustic, infrasound, and radionuclide. Operational manuals for each of these technologies specify the requirements that equipment installed at IMS stations must meet for data from said stations to be accepted by the International Data Center. Following a model in which a revised set of infrasound sensor specifications and accompanying test procedures were developed in an international collaboration overseen by the Comprehensive Nuclear-Test-Ban Treaty Organization, the same process was recently followed for seismometers potentially of use by the IMS. This document, a product of that collaboration, defines key concepts and recommends test procedures to be followed to characterize broadband seismometers in a consistent and standardized way. Results from these test procedures can be used to verify that a seismometer meets the manufacturers stated specifications and IMS requirements. ACKNOWLEDGEMENTS The writers of this document would like to acknowledge Charles R. Hutt, John R. Evans, Fred Followill, Robert L. Nigbor, and Erhard Wielandt, the authors of the 2009 Guidelines for Standardized Testing of Broadband Seismometers and Accelerometers funded by the U.S. Department of Interior, on which this document is based. Although this document is limited in scope to seismometers and much of the document has been updated, the structure remains essentially the same.
LLNL completed the FY20 workplan entailing application of the Dynamic Correlation Processor (DCP) to International Monitoring System (IMS) primary seismic data in 2019 and 2020. The effective analysis period was curtailed to January 1 through November 12, 2019 due to a data compression change that rendered later data unusable. Fixing the data compression error is a priority for our data source, and we plan to extend the period of data analysis when the fix is implemented. We found that the percentage overlap between detections generated by the International Data Centre (IDC) DFX software and detections generated by DCP varied widely by station. Overlap is 30-40% at some stations, suggesting that a large percentage of DFX detections could be screened (removed) from the detection-event association algorithm. However, detection overlap was much lower, a few percent, at many stations and the percentage overlap was low at stations known to have many mines and other local sources of seismicity. Unexpectedly low detection overlap at many stations prompted us to examine results for the ARCES station in detail. There is considerable mine activity near ARCES, but our initial FY20 analysis found only ~2% DCP/DFX detection overlap. Differences in the pre-processing (e.g. beam recipes) used by DCP and DFX appear to be the cause of low detection overlap at ARCES. DCP uses wideband filters to improve detector robustness and the IDC uses narrow-band pre-filtering to improve sensitivity. After reprocessing ARCES using the IDC beam recipe, DCP/DFX overlap increased to ~74%. This result shows that we must reprocess the IMS network using the IDC station-specific beam recipes if we are to effectively screen DFX detections and improve automatic event building performance at the IDC.
Among the most important problems for the world-wide nuclear explosion monitoring is the interference of naturally occurring and man-made radionuclides. The International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) frequently detects these interferences using sensitive radionuclide measurement equipment. We commonly refer to the presence of radionuclides that are relevant to the CTBT but do not originate from a nuclear explosion as “background”. Backgrounds are highest near the sources but are known to have regional and global effects on the IMS. This review paper summarizes much of the relevant work in the area of background and discusses issues of interest for nuclear explosion detection.
Radionuclide monitoring for nuclear explosions includes measuring radioactive aerosol and noble gas concentrations in the atmosphere. The International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty (CTBT) has made such measurements for decades, revealing much about how atmospheric radioactivity impacts the sensitivity of the network. For example, civilian emissions of radioiodine make a substantial regional impact, but a minor global impact, while civilian radioxenon emissions create major regional and complex global impacts. This example statement is strongly influenced by the minimum signal level anticipated to be interesting. The original design of the IMS anticipated relatively large signal levels, and the accomplished IMS network substantially meets or exceeds the sensitivity needed to detect those signal levels. Much lower signal levels can be motivated, which could require reconsideration of station sensitivity. Using measured and simulated background concentrations, various possible desired signal levels, and an innovative anomaly threshold, maps of sensitivity and a station ranking are developed for current and future stations. These provide a strong motivation for additional experimentation to learn about sources and the potential plusses of new technology.
The International Monitoring System arm of the Comprehensive Nuclear-Test-Ban Treaty Organization utilizes sampling of atmospheric radioxenon to scan the world for nuclear testing events. Current designs utilized in the field involve plastic scintillator cells run in coincidence with Sodium Iodide (NaI) or High Purity Germanium (HPGe) detectors. The use of plastic scintillators as electron detectors exhibit some issues in comparison to other materials. Silicon detectors are analyzed as an alternative to plastic scintillators for beta detection based on their higher energy resolution and lowered memory effect. A radioxenon detection setup was created with the intent of measuring radioxenon samples using coincidence counting between silicon and NaI detectors as well as silicon and HPGe. Finally, the absolute efficiency of both setups is calculated and compared.
The Comprehensive Test Ban Treaty (CTBT) bans nuclear weapons testing and establishes the International Monitoring System (IMS) for the purpose of treaty verification. IMS is a detector network that confirms that global nuclear activity is peaceful in nature. These detector systems use β-γcoincidence measurements to characterize atmospheric samples containing radioxenon isotopes. The goal of this work is to use the system consisting of two sodium iodide detectors and two PIPSbox detectors to characterize isotopically pure samples of 131m Xe, 133m Xe, 133 Xe, and 135 Xe via β, γ, and β-γ coincidence measurements.
Seismic arrays provide useful tools for regional seismic monitoring. Two small-aperture, regionally-oriented arrays, QWAR and HQAR, were deployed in Saudi Arabia and Oman in 2012 and 2016. We utilize a time-domain beampacking method, similar to frequency-wavenumber analysis, to examine the performance of the arrays in terms of slowness and azimuthal bias and event detection capabilities. Additionally, we investigate persistent ambient noise sources recorded by the arrays. We find that the arrays provide slowness vectors with biases comparable with similar-sized arrays of the International Monitoring System in other locations around the world. At QWAR, regional events of magnitude 3.0 and above are detected a majority of the time, and as magnitudes increase to 4.0 and above, the detection rate is greater than 82%. Finally, strong noise generation, primarily with slownesses characteristic of Lg waves, is found in the directions of the northern Arabian/Persian Gulf and southern Red Sea and may be a factor in event detection capabilities.
Global infrasound monitoring provides a persistent means of detecting energetic bolide atmospheric entries, complementing optical observations and extending coverage over remote regions. We present a global assessment of the physical factors governing bolide infrasound detectability by correlating 623 bolide events reported by the Center for Near-Earth Object Studies between 2007 and 2025 with waveform data from the International Monitoring System. We identify 311 events with confirmed infrasound detections, corresponding to a detection rate of approximately 50%, substantially higher than inferred from earlier surveys, reflecting both the maturation of the global infrasound network and advances in automated, multi-frequency array processing. Analysis of flight parameters shows that infrasound detectability is selective rather than uniform across the bolide population. Detected events are preferentially associated with steeper entry angles and lower-altitude energy deposition, while shallow, high-altitude trajectories are less consistently observed. Very high-energy events remain detectable regardless of geometry, but for the more common lower-energy regime, observability depends on specific combinations of entry parameters and propagation conditions. This geometric dependence persists across comparable energy ranges and atmospheric conditions, indicating that entry angle exerts a primary control on detectability, with energy and propagation acting as secondary modulating factors. Furthermore, these results provide new physical constraints on bolide-atmosphere interactions and improve interpretation of global infrasound observations for planetary defense and atmospheric-entry studies.
Radioisotopes of noble gases xenon and argon can be important indicators of underground nuclear explosions. The Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes monitoring capabilities to identify potential nuclear explosions conducted in violation of the CTBT. This monitoring currently focuses on measurement of the xenon isotopes 133Xe, 135Xe, 131mXe and 133mXe. However, it is predicted that within 100 days of an underground nuclear explosion (UNE) 37Ar would be released to the atmosphere at higher concentrations than xenon isotopes (Haas et al. 2010) and with a higher signal to background ratio, depending on the radioxenon background levels. Therefore, inclusion of 37Ar measurement capabilities at atmospheric International Monitoring System (IMS) stations may represent an improvement in the capability to detect a nuclear explosion. At an IMS station, it could be difficult to determine what constitutes an elevated concentration as a result of an UNE without first understanding the expected range of background concentrations. This work describes our analysis of atmospheric samples for 37Ar to evaluate the range of background concentrations. Samples were collected at multiple locations, with approximately half coming from a sampler co-located with an IMS xenon monitoring station (RN75). The range of 37Ar concentrations measured in atmospheric air samples was relatively narrow; for samples considered detectable, the minimum and maximum measured concentrations were 0.56 and 2.3 mBq/m3, respectively. Comparison of 37Ar and 133Xe concentrations measured at the IMS station indicated some correlation between the measured concentrations. The results presented here demonstrate the capability to detect background concentrations of 37Ar in atmospheric air and provide a basis for potential implementation of 37Ar monitoring at IMS stations.
The noble gas collection and measurement stations in the International Monitoring System (IMS) are heavily influenced by releases from medical isotope production facilities. The ability to reliably model the movement of radioxenon from the points of release to these IMS samplers has improved enough that a routine aspect of the analysis of IMS radioxenon data should be the prediction of the effect of releases from civilian nuclear facilities on the sample concentrations. In this work, predicted concentrations at IMS noble gas systems in Germany and Sweden based on measured releases from Institute for Radioelements (IRE) in Belgium and atmospheric transport modeling for a four-month period are presented and discussed.
In 2015 and 2016, atmospheric transport modeling challenges were conducted in the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification, however, with a more limited scope with respect to emission inventories, simulation period and number of relevant samples (i.e., those above the Minimum Detectable Concentration (MDC)) involved. Therefore, a more comprehensive atmospheric transport modeling challenge was organized in 2019. Stack release data of Xe-133 were provided by the Institut National des Radioéléments/IRE (Belgium) and the Canadian Nuclear Laboratories/CNL (Canada) and accounted for in the simulations over a three (mandatory) or six (optional) months period. Best estimate emissions of additional facilities (radiopharmaceutical production and nuclear research facilities, commercial reactors or relevant research reactors) of the Northern Hemisphere were included as well. Model results were compared with observed atmospheric activity concentrations at four International Monitoring System (IMS) stations located in Europe and North America with overall considerable influence of IRE and/or CNL emissions for evaluation of the participants’ runs. Participants were prompted to work with controlled and harmonized model set-ups to make runs more comparable, but also to increase diversity. It was found that using the stack emissions of IRE and CNL with daily resolution does not lead to better results than disaggregating annual emissions of these two facilities taken from the literature if an overall score for all stations covering all valid observed samples is considered. A moderate benefit of roughly 10% is visible in statistical scores for samples influenced by IRE and/or CNL to at least 50% and there can be considerable benefit for individual samples. Effects of transport errors, not properly characterized remaining emitters and long IMS sampling times (12–24 h) undoubtedly are in contrast to and reduce the benefit of high-quality IRE and CNL stack data. Complementary best estimates for remaining emitters push the scores up by 18% compared to just considering IRE and CNL emissions alone. Despite the efforts undertaken the full multi-model ensemble built is highly redundant. An ensemble based on a few arbitrary runs is sufficient to model the Xe-133 background at the stations investigated. The effective ensemble size is below five. An optimized ensemble at each station has on average slightly higher skill compared to the full ensemble. However, the improvement (maximum of 20% and minimum of 3% in RMSE) in skill is likely being too small for being exploited for an independent period.
An understanding of anthropogenic sources of radioactive noble gases in the atmosphere is needed to enhance the discrimination ability of the International Monitoring System's sensors. These sources include commercial and research nuclear reactors and medical isotope production facilities. While abiding by local environmental ordinances these facilities all emit noble gas radioisotopes through normal operation. Here, this research presents measurements and analysis of noble gas isotopes ( 41 Ar, 135 Xe, 135m Xe, 137 Xe, 138 Xe, 87 Kr, 88 Kr, and 89 Kr) made directly at the stack of the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. The Xe and Kr noble gases are concurrently observed with 41 Ar, a neutron activation product, when the reactor is operational. The magnitude of the Xe and Kr noble gases released is not constant over the HFIR cycle, but they temporally match the 41 Ar trend. An isotope activity ratio analysis of these shorter lived isotopes combined with the observation of the cycle's temporal trend helps understand the noble gas production mechanism at the HFIR. Isotopes with short half-lives are not useful for long-range environmental monitoring. However, these measurements could potentially be combined with atmospheric modeling to predict the background source term of the longer-lived Xe ratios at a monitoring station.
Numerous algorithms have been developed to determine the source characteristics for an atmospheric release, e.g., (Bieringer et al., 2017). This study compares three models that have been applied to the data collected by the International Monitoring System operated by the Comprehensive Nuclear-Test-Ban Treaty Preparatory Commission to estimate source event parameters. Each model uses a different approach to estimate the parameters. A deterministic model uses a possible source region (PSR) approach (Ringbom et al., 2014) that is based on the correlation between predicted and measured sample values. An unnamed model developed at Pacific Northwest National Laboratory (herein called BAYEST) uses a Bayesian formulation (Eslinger et al., 2019; Eslinger et al., 2020; Eslinger and Schrom, 2016). The FREAR model uses a different Bayesian formulation (De Meutter and Hoffman, 2020; De Meutter et al., 2021; Pieter De Meutter, 2021). The performance of the three source-location models is evaluated with 100 synthetic release cases for the single xenon isotope, 133 Xe. The release cases resulted in detections in a fictitious network with 120 noble gas samplers. All three source-location models use the same sampling data. The two Bayesian models yield more accurate location estimates than the deterministic PSR model, with FREAR having slightly better location performance than BAYEST. Samplers with collection periods of 3, 6, 8, 12, and 24-h were used. Results from BAYEST show that location accuracy improves with each reduction in sample collection length. The BAYEST model is slightly better for estimating the start time of the release. The PSR model has about the same spread in start times as the FREAR model, but the PSR results have a better average start time. The Bayesian source-location algorithms give more accurate results than the PSR approach, and provide release magnitude estimates, while the base PSR model does not estimate the release magnitude. This investigation demonstrates that a reasonably dense sampling grid will sometimes yield poor location and time estimates regardless of the model. The poor estimates generally coincide with cases where there is a much larger distance between the release point and the first detecting sampler than the average sampler spacing.
A nuclear explosion screening exercise in 2023 (Maurer et al., 2023) found challenges with discerning anomalous radioxenon activity concentrations relative to elevated background concentrations. Research has continued into methods to detect anomalous radioxenon concentrations by comparing samples to estimates of atmospheric radioxenon background concentrations caused by releases at nuclear reactors or medical isotope production facilities. A new approach estimates the sample concentrations using time-varying radioxenon release rates obtained using optimization techniques that constrain the facility release rates to plausible amounts based on historical data or facility knowledge. The purpose of the optimization is to determine whether any combination of plausible release rates from emitting facilities can explain a series of radioxenon measurements at one or more sampling stations. A case study uses radioxenon data collected at three locations in western Europe for a month in 2021 and considers releases from 77 locations. Fewer samples are identified as being anomalous using a simplistic flagging rule than from an application of the current International Monitoring System (IMS) activity concentration-level rule.
Raikoke, a small, unmonitored volcano in the Kuril Islands, erupted in June 2019. We integrate data from satellites (including Sentinel-2, TROPOMI, MODIS, Himawari-8), the International Monitoring System (IMS) infrasound network, and global lightning detection network (GLD360) with information from local authorities and social media to retrospectively characterize the eruptive sequence and improve understanding of the pre-, syn- and post- eruptive behavior. In this work, we observe six infrasound pulses beginning on 21 June at 17:49:55 UTC as well as the main Plinian phase on 21 June at 22:29 UTC. Each pulse is tracked in space and time using lightning and satellite imagery as the plumes drift eastward. Post-eruption visible satellite imagery shows expansion of the island's surface area, an increase in crater size, and a possibly-linked algal bloom south of the island. We use thermal satellite imagery and plume modeling to estimate plume height at 10–12 km asl and 1.5–2 × 10 6 kg/s mass eruption rate. Remote infrasound data provide insight into syn-eruptive changes in eruption intensity. Our analysis illustrates the value of interdisciplinary analyses of remote data to illuminate eruptive processes. However, our inability to identify deformation, pre-eruptive outgassing, and thermal signals, which may reflect the relatively short duration (~12 h) of the eruption and minimal land area around the volcano and/or the character of closed-system eruptions, highlights current limitations in the application of remote sensing for eruption detection and characterization.
The performance of the Radionuclide Aerosol Sampler/Analyzer (RASA) under high-activity conditions has been evaluated to maintain and protect the operations of International Monitoring System (IMS) radionuclide stations. Station measurements following the Fukushima Daichi accident of 11 March 2011 have been combined with laboratory measurements to understand how the quality of the measurements made using the RASA High-Purity Germanium (HPGe) gamma-spectrometer are impacted by increased sample activity and detector dead-time. Detector saturation has been identified to occur at a count rate of 175,000 counts s-1, at which level there are significant impacts to the sample live-time and detector resolution in addition to a system contamination risk. To safeguard against these effects, a dead-time limit of 16.6% is proposed that could be maintained by monitoring the activity collecting on the filter using a Cadmium Zinc Telluride (CZT) detector installed into the particulate collection area of the RASA. This limit would ensure that the IMS technical specifications for measurement time (> 20 h) and detector resolution (2.5 keV at 1332 keV) are met. The CZT would dynamically control the RASA air flow to limit the collection of particulate activity beneath the HPGe dead-time threshold and would also provide measurements useful for CTBT verification purposes and dose assessments.