Radioactivity of the atmospheric aerosols detected by CTBTO stations in Africa following the accident at the Fukushima Daiichi nuclear power plant.

Radionuclides from the reactor accident Fukushima Daiichi nuclear power plant were observed in the airborne aerosols at CTBT International Monitoring System (IMS) stations (MRP43, CMP13) in Africa. The maximum activity concentrations in the air measured in Mauritania were 186.44 10-6 Bq.m-3, 264.16 10-6 Bq.m-3 and 1269.94 10-6 Bq.m-3 for 134Cs, 137Cs and 131I respectively, and in Cameroon 16.42 10-6 Bq.m-3, 25.53 10-6 and 37.58 10-6 Bq.m-3 respectively for 134Cs, 137Cs and 131I. The activity ratio of 134Cs/137Cs is almost constant throughout the period of time relevant to this study due to their long half-lives of 30.2 years for 137Cs and 2.06 years for 134Cs. Whereas the 131I/137Cs activity ratio varies in time according to the radioactive decay with a half-live of 8.06 days for 131I and different removal rates of both radionuclides from the atmosphere during transport. The EMAC atmospheric chemistry-general circulation was used to simulate the emission and transport of the isotope 137Cs and map the deposition of the 137Cs deposition over Africa.

A calibration procedure for beta-gamma coincidence detector-systems using four radioxenon spikes.

The determination of activity concentrations of the CTBT-relevant radioxenon relies on a robust calibration method. A procedure is outlined using four radioxenon spikes for beta-gamma detector-systems with 4π geometry. Detection efficiencies of beta-gamma coincidences in the net count calculation method, including the interference matrix between radioxenon and radon, are determined by three measurement channels: beta singles, gamma singles and beta-gamma coincidences, without reference activity values.

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.

Global emission inventory of 131mXe, 133Xe, 133mXe, and 135Xe from all kinds of nuclear facilities for the reference year 2014.

Global radioactivity monitoring for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes the four xenon isotopes 131mXe, 133Xe, 133mXe and 135Xe. These four isotopes are serving as important indicators of nuclear explosions. The state-of-the-art radioxenon emission inventory uses generic release estimates for each known nuclear facility. However, the release amount can vary by several orders of magnitude from year to year. The year 2014 was selected for a single year radioxenon emission inventory with minimized uncertainty. Whenever 2014 emissions reported by the facility operator are available these are incorporated into the 2014 emission inventory. This paper summarizes this new emission inventory. The emissions are compared with previous studies. The global radioxenon emission inventory for 2014 can be used for studies to estimate the contribution of this anthropogenic source to the observed ambient concentrations at IMS noble gas sensors to support CTBT monitoring activities, including calibration and performance assessment of the verification system as described in the Treaty as well as developing and validating methods for enhanced detection capabilities of signals that may indicate a nuclear test. One specific application is the 1st Nuclear Explosion Signal Screening Open Inter-Comparison Exercise announced end of 2021.

Overview of temporary radioxenon background measurement campaigns conducted for the CTBTO between 2008 and 2018.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) specifies that an overall network of at least 40 International Monitoring System (IMS) stations should monitor the presence of radioxenon in the atmosphere upon its entry into force. The measurement of radioxenon concentrations in the air is one of the major techniques to detect underground nuclear explosions. It is, together with radionuclide particulate monitoring, the only component of the network able to confirm whether an event originates from a nuclear test, leaving the final proof to on-site inspection. Correct and accurate interpretation of radioxenon detections by State Signatories is a key parameter of the verification regime of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). In this context, the discrimination between the highly variable radioxenon background generated by normal operations of nuclear facilities and CTBT-relevant events is a challenging, but critical, task. To this end, the radioxenon background that can be expected at IMS noble gas systems must be sufficiently characterized and understood. All activities conducted to study the global radioxenon background are focused on the calibration and performance of the verification system as described in the Treaty. The unique CTBTO noble gas system network is designed to optimally covering the globe. By the end of 2019, 31 systems were put in operation, 25 of which being already certified. It took two decades from the first experimental setup of noble gas system in the field to reach this stage of maturity. In the meantime, it was an urgent need to gain empirical evidence of atmospheric radioxenon concentrations with the full spectrum of characteristics that IMS noble gas systems may be observing. This experience was significantly advanced through temporary measurement campaigns. Their objective was to gain the additional necessary knowledge for a correct understanding and categorization of radioxenon detections. The site selection for these campaigns put emphasis on regions with low coverage by the initially few experimental noble gas systems at IMS locations or where potential interferences with normal background might be observed. Short-term measurements were first initiated in 2008. Sites of potential interest were identified, and campaigns up to few weeks were performed. Based on the findings of these short campaigns, transportable systems were procured by the CTBTO. Longer temporary measurement campaigns were started afterwards and operated by local hosts in different regions of the globe. Site selections were based on purely scientific criteria. Objectives of the measurement campaigns were continually reassessed, and projects were designed to meet the scientific needs for radioxenon background understanding as required for nuclear explosion monitoring. As of today, several thousands of samples have been collected and measured. Spectra of temporary measurement campaigns were (and are still) analysed in the International Data Centre (IDC). As they are not part of the CTBT monitoring system, no IDC product is generated. Analysis results are stored in a non-operational database of the CTBTO and made available, together with raw data, to authorized users of States Signatories through a Secure Web Portal (SWP) and to scientific institutions for approved research projects through a virtual Data Exploitation Centre (vDEC) after signing a cost-free confidentiality agreement (https://www.ctbto.org/specials/vdec). This paper aims at providing an overview of the temporary measurement campaigns conducted by the CTBTO since the very first field measurements. It lays out scientific results in a systematic approach. This overview demonstrates the asset of radioxenon background measurement data that have been collected with a wide variety of characteristics that may be observed at IMS stations. It bears a tremendous opportunity for development, enhancement and validation of methodologies for CTBT monitoring. In 2018, a campaign started in Japan with transportable noble gas systems in the vicinity of the IMS station RN38 in Takasaki. It will be described separately once the measurements are completed.

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.

Classification of radioxenon spectra with deep learning algorithm.

In this study, we propose for the first time a model of classification for Beta-Gamma coincidence radioxenon spectra using a deep learning approach through the convolution neural network (CNN) technique. We utilize the entire spectrum of actual data from a noble gas system in Charlottesville (USX75 station) between 2012 and 2019. This study shows that the deep learning categorization can be done as an important pre-screening method without directly involving critical limits and abnormal thresholds. Our results demonstrate that the proposed approach of combining nuclear engineering and deep learning is a promising tool for assisting experts in accelerating and optimizing the review process of clean background and CTBT-relevant samples with high classification average accuracies of 92% and 98%, respectively.

Radioisotopes demonstrate changes in global atmospheric circulation possibly caused by global warming.

In this paper, we present a new method to study global atmospheric processes and their changes during the last decade. A cosmogenic radionuclide measured at ground-level, beryllium-7, is utilized as a proxy to study atmospheric dynamics. Beryllium-7 has two advantages: First, this radionuclide, primarily created in the lower stratosphere, attaches to aerosols that are transported downwards to the troposphere and travel around the globe with the general atmospheric circulation. By monitoring these particles, we can provide a global, simple, and sustainable way to track processes such as multi-annual variation of the troposphere, tropopause heightening, position and speed of atmospheric interface zones, as well as the poleward movement and stalling patterns of jet streams. Second, beryllium-7 is a product of cosmic rays which are themselves directly linked to solar activity and the earth magnetic field. This study shows whether beryllium-7 observed concentration changes are correlated with such natural processes or independent of them. Our work confirms that major changes in the atmospheric circulation are currently ongoing, even though timeseries are too short to make climatological assessments. We provide solid evidence of significant and progressive changes of the global atmospheric circulation as well as modifications of tropopause heights over the past decade. As the last decade happened to be the warmest on record, this analysis also indicates that the observed changes are, at least to some extent, attributable to global warming.

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).

How to predict seasonal weather and monsoons with radionuclide monitoring.

Monsoon in India is of particular importance for the $2 trillion economy, highly dependent on agriculture. Monsoon rains water two-thirds of India's harvest. However, the monsoon season also causes large-scale flooding, resulting in loss of human life and economic damage estimated around $7 billion annually. Beryllium-7 is a tracer that can be used to monitor the intensity of stratosphere-troposphere exchange, which varies in accordance with the annual cycle of the global atmospheric circulation (Hadley, Ferrel and Polar cells). Based on the beryllium-7 data collected globally as part of the monitoring of the Comprehensive Nuclear-Test-Ban Treaty, the presented empirical method demonstrates the possibility to predict the start, withdrawal and intensity of the Indian monsoon season. Onset can be forecasted with an unprecedented accuracy of ±3 days, 2 months in advance compared to 1-3 weeks in advance by traditional methods. Applying this new method will enable better preparation for economic and natural hazard impacts of the monsoon season in India. This method can also be extended to other regions where the movement of Hadley cells governs monsoon onset and withdrawal.

Nuclear event zero time determination using analytical solutions of radioxenon activities under in-growth condition.

Motivated by simplifying the calculation process of radioxenon isotopic activity used by scientist community in nuclear event characterization, the analytical formulas of the numbers of nuclides and isotopic activities of CTBT relevant radioxenon Xe-135, Xe-133m, Xe-133 and Xe-131m proposed in this work can be useful and incorporated in the calculation algorithms for nuclear event studies. The calculated ages using analytical formulas and radioxenon activity data from real observations compare well with the reported ages and show good results of event timing precision.

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.

New evaluated radioxenon decay data and its implications in nuclear explosion monitoring.

This work presents the last updated evaluations of the nuclear and decay data of the four radioxenon isotopes of interest for the Comprehensive Nuclear-Test-Ban Treaty (CTBT): Xe-131 m, Xe-133, Xe-133 m and Xe-135. This includes the most recent measured values on the half-lives, gamma emission probabilities (Pγ) and internal conversion coefficients (ICC). The evaluation procedure has been made within the Decay Data Evaluation Project (DDEP) framework and using the latest available versions of nuclear and atomic data evaluation software tools and compilations. The consistency of the evaluations was confirmed by the very close result between the total available energy calculated with the present evaluated data and the tabulated Q-value. The article also analyzes the implications on the variation of the activity ratio calculations from radioxenon monitoring facilities depending on the nuclear database of reference.

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.

Fast and accurate dating of nuclear events using La-140/Ba-140 isotopic activity ratio.

This study reports on a fast and accurate assessment of zero time of certain nuclear events using La-140/Ba-140 isotopic activity ratio. For a non-steady nuclear fission reaction, the dating is not possible. For the hypothesis of a nuclear explosion and for a release from a steady state nuclear fission reaction the zero-times will differ. This assessment is fast, because we propose some constants that can be used directly for the calculation of zero time and its upper and lower age limits. The assessment is accurate because of the calculation of zero time using a mathematical method, namely the weighted least-squares method, to evaluate an average value of the age of a nuclear event. This was done using two databases that exhibit differences between the values of some nuclear parameters. As an example, the calculation method is applied for the detection of radionuclides La-140 and Ba-140 in May 2010 at the radionuclides station JPP37 (Okinawa Island, Japan).

Update and improvement of the global krypton-85 emission inventory.

Krypton-85 is mainly produced in nuclear reactors by fission of uranium and plutonium and released during chopping and dissolution of spent fuel rods in nuclear reprocessing facilities. As noble gas it is suited as a passive tracer for evaluation of atmospheric transport models. Furthermore, research is ongoing to assess its quality as an indicator for clandestine reprocessing activities. This paper continues previous efforts to compile a comprehensive historic emission inventory for krypton-85. Reprocessing facilities are the by far largest emitters of krypton-85. Information on sources and calculations used to derive the annual krypton-85 emission is provided for all known reprocessing facilities in the world. In addition, the emission characteristics of two plants, Tokai (Japan) and La Hague (France), are analysed in detail using emission data with high temporal resolution. Other types of krypton-85 sources are power reactors, naval reactors and isotope production facilities. These sources contribute only little or negligible amounts of krypton-85 compared to the large reprocessing facilities. Taking the decay of krypton-85 into account, the global atmospheric inventory is estimated to about 5500 PBq at the end of 2009. The correctness if the inventory has been proven by meteorological simulations and its error is assumed to be in the range of a few percent.

A novel vacuum ultra violet lamp for metastable rare gas experiments.

We report on a new design of a vacuum ultra violet (VUV) lamp for direct optical excitation of high laying atomic states, e.g., for excitation of metastable rare gas atoms. The lamp can be directly mounted to ultra-high vacuum vessels (p ≤ 10(-10)mbar). It is driven by a 2.45 GHz microwave source. For optimum operation, it requires powers of ~20 W. The VUV light is transmitted through a magnesium fluoride window, which is known to have a decreasing transmittance for VUV photons with time. In our special setup, after a run-time of the VUV lamp of 550 h the detected signal continuously decreased to 25% of its initial value. This corresponds to a lifetime increase of two orders of magnitude compared to previous setups or commercial lamps.

Characterisation of prompt and delayed atmospheric radioactivity releases from underground nuclear tests at Nevada as a function of release time.

A database with information on about 500 cases of atmospheric radioactivity releases from underground nuclear tests is analysed. The data are statistically evaluated and systematically aggregated in order to characterise prompt uncontrolled as well as delayed operational releases of radioactivity into the atmosphere. The focus is put on the latter. The reported data compare well with theoretically derived xenon activities for reasonable nuclear test scenarios. Conclusions are drawn on the main features of releases that can be expected from underground nuclear tests as a function of release time. These findings are relevant for developing and validating methods to be applied in global monitoring of atmospheric radioactivity with respect to indications of an underground nuclear explosion.

Global radioxenon emission inventory based on nuclear power reactor reports.

Atmospheric radioactivity is monitored for the verification of the Comprehensive Nuclear-Test-Ban Treaty, with xenon isotopes 131mXe, 133Xe, 133mXe and 135Xe serving as important indicators of nuclear explosions. The treaty-relevant interpretation of atmospheric concentrations of radioxenon is enhanced by quantifying radioxenon emissions released from civilian facilities. This paper presents the first global radioxenon emission inventory for nuclear power plants, based on North American and European emission reports for the years 1995-2005. Estimations were made for all power plant sites for which emission data were unavailable. According to this inventory, a total of 1.3PBq of radioxenon isotopes are released by nuclear power plants as continuous or pulsed emissions in a generic year.