In September 2017, numerous measurement stations recorded large surface concentrations of Ru106 in Europe. This event was well recorded by various monitoring stations worldwide and offer a valuable framework to compare the modelling strategies deployed to quickly evaluate where the plume goes and with what concentrations. In general, the source and its intensity are not known and hypotheses have to be done. Models have to be fast and accurate: Lagrangian and Eulerian are often used but rarely compared. In this study, the FLEXPART Lagrangian model and the WRF-CHIMERE Eulerian models are used to simulate the emissions, transport and deposition of this source of Ru106. First, it is shown that the hypothesis of location, timing and intensity of the source is realistic, by comparison to surface measurements. Second, sensitivity analysis performed with the Eulerian model and several transport scheme showed that this model may provide better results than the Lagrangian one. It opens the door to further development, including chemistry and mixing with other pollutants during these specific events.

The French National Data Center (NDC) uses an automated simulation of the 133Xe worldwide atmospheric background as one of the means to categorize the radionuclide measurements of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS). These simulations take into account 133Xe releases from the known or assumed major industrial emitters in the world and global-scale meteorological data. However, a quantification of the simulation uncertainties in this operational set up is yet to be addressed. This work discusses the benefits of meteorological ensemble data as available from National Centers for Environmental Prediction (NCEP) for that purpose. For this study, the daily dispersion of releases from the Institute for Radio Elements (IRE), a medical isotope production facility located in Fleurus (Belgium), was calculated over one year with emissions measured in-site and ensemble meteorological data. The ensemble contains 31 members, which resulted in as many predictions of activity concentration for any given time and place. The resulting distribution statistics (mean, median and spread), and the control run, were confronted to the deterministic run and to measurements at one IMS-like station near Paris (France) and one IMS station in Freiburg (Germany). Overall, the ensemble results have decreased the simulation performance, as expected given the use of meteorological analyses only. However, contrasting patterns were found with a detailed analysis of daily activity concentration over two one-month-and-a-half periods. Noticeably, outlier results were found to carry the best forecast in some significant detections, proving their relevance for the measurement categorization, despite their isolated character. Importantly, the ensemble has allowed the quantification of meteorological uncertainties, which was beneficial in all cases. It either has improved the confidence of IMS data categorization or has pointed to low confidence predictions. A criterion to identify the latter is suggested, based on information provided by the ensemble distributions. In addition, maps of probability of detections and of relative spread are suggested to show additional benefits of ensemble meteorology.

A global-scale simulation of the 133Xe atmospheric background is automated at the French National Data Center (NDC) for the purpose of categorizing the radionuclide measurements of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS). These simulations take into account 133Xe releases from all known major industrial emitters in the world, compiled from the literature and described as constant values. Emission data measured directly at the stack of the Institute for Radio Elements (IRE), a medical isotope production facility located in Fleurus (Belgium), were implemented in the simulations with a time resolution of 15 minutes. This work discusses the contribution of real (measured) emissions to the prediction of the 133Xe atmospheric background at IMS noble gas stations and at a location near Paris, for which IMS-like 133Xe measurements were available. For the purpose of this study, simulations initiated with the IRE measured emissions were run in parallel to those with the a priori emissions used to date. The benefits of including actual emissions in the simulations were found as a function of the distance between the station and the source of the release. At the closest stations, i.e., near Paris (France) and at Schauinsland, Freiburg (Germany), respectively 250 and 400 km from Fleurus, the simulated activity concentrations differed by a factor greater than 2 more than one third of the time, and by a factor of more than 5 about 10% of the time. No significant or detectable differences were found beyond 1500-2000 km. Furthermore, at the Paris station, the timing of the measured peaks was better reproduced with the actual emission data. However, not all peak amplitudes were correctly reproduced even though the real emissions were used, highlighting the remaining uncertainties, primarily in the meteorological data and transport modeling.

The SPALAX-NG is a new-generation system that is designed to detect radioactive xenon at trace levels in the atmosphere following a nuclear explosion or civilian source release. This new system formed part of a validation program led by the Provisional Technical Secretary of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) Organization. In this study, the first SPALAX-NG unit was tested for six months between October 2018 and April 2019 at the CEA/DIF premises near Paris, France. This test period provided an outstanding opportunity to illustrate the high level of detectability and reliability of the system. The data availability obtained over this period was approximately 99%, which was well above the CTBT Data Availability criteria of 95%. The data reliability was demonstrated by a comparison with a collocated SPALAX-1 unit (former version of SPALAX) and by re-measuring several samples at the CTBT-certified French laboratory FRL08. The high sensitivity to the detection of the four relevant radioxenon isotopes was fully demonstrated and enabled the recording of a major dataset for western Europe. A large set of isotopic ratios was measured, which enabled the discrimination criteria between civilian sources and nuclear test signatures to be refined.

Atmospheric transport modeling has been used to interpret the unprecedented number of multi-isotope detections of radioxenons observed during the six months of the qualification process by the Comprehensive Nuclear-Test-Ban Treaty Organization of the new SPALAX-NG system (Système de Prélèvement Automatique en Ligne avec l'Analyse du Xénon - Nouvelle Génération). Highest 133Xe activity concentrations were found to be systematically associated with the concomitant measurement of several other radioxenons at the prevailing wind direction of north/northeast pointing to the Institute for Radio Elements (IRE), a medical isotope production facility located in Fleurus (Belgium). The lowest 133Xe activity concentrations were not associated with a prevailing wind direction or other radioxenons, indicating the contribution of distant sources (global background). The IRE's average source terms for 133mXe and to a lesser extent for 133Xe (slightly overestimated by a factor of 1.7) showed good agreement with the literature values, while corrections by a factor of ~23 and ~53 were proposed for 131mXe and 135Xe since the initial values were underestimated. However, detections of 131mXe alone and some low-activity concentrations of 133Xe associated with only one of the other radioxenons could not be linked to the IRE releases. Analysis of these cases suggests the contribution of local source releases that have been difficult to identify to date. In addition to the global background, releases from such local sources, if not identified, could affect the analysis of the isotopic ratios measured following a nuclear test. The characterization of these local contributions is now possible owing to the capacity of the SPALAX-NG and other new generation measurements systems.

Global simulations of the atmospheric dispersion of worldwide industrial Xe-133 releases have revealed a large spatial and day-to-day variability of the resulting so-called Xe-133 atmospheric background. Most stations of the International Monitoring System (IMS) of the Comprehensive nuclear-Test-Ban Treaty Organization actually detect Xe-133 regularly. Measured levels are explained by a varying combination of local and distant industrial sources and can interfere with discrimination of nuclear test signatures. Therefore, a better understanding of the Xe-133 atmospheric background is needed. In this study, a validated 2year simulation data set and a 4year measurement data set of Xe-133 activity concentrations have been analyzed in order to characterize possible seasonal variations of the Xe-133 atmospheric background due to atmospheric circulation, with a focus on (i) global distributions, (ii) occurrences of detections by the IMS network, and (iii) time series of monthly averages at IMS stations. Results show a larger spatial extent of the atmospheric background during winter months, which translates into a larger number of detections on the IMS network during winter months for both hemispheres. Some IMS stations present a significant seasonal variability in terms of levels, or both in terms of levels and origins. However, not all IMS stations are subject to seasonal variations, given their location with respect to sources and large-scale atmospheric circulation. In addition, a first set of predicted information about expected levels of atmospheric background at IMS stations not yet operational is provided, given the current knowledge of sources.

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.

The International Monitoring System (IMS) is part of the verification regime for the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO). At entry-into-force, half of the 80 radionuclide stations will be able to measure concentrations of several radioactive xenon isotopes produced in nuclear explosions, and then the full network may be populated with xenon monitoring afterward. An understanding of natural and man-made radionuclide backgrounds can be used in accordance with the provisions of the treaty (such as event screening criteria in Annex 2 to the Protocol of the Treaty) for the effective implementation of the verification regime. Fission-based production of (99)Mo for medical purposes also generates nuisance radioxenon isotopes that are usually vented to the atmosphere. One of the ways to account for the effect emissions from medical isotope production has on radionuclide samples from the IMS is to use stack monitoring data, if they are available, and atmospheric transport modeling. Recently, individuals from seven nations participated in a challenge exercise that used atmospheric transport modeling to predict the time-history of (133)Xe concentration measurements at the IMS radionuclide station in Germany using stack monitoring data from a medical isotope production facility in Belgium. Participants received only stack monitoring data and used the atmospheric transport model and meteorological data of their choice. Some of the models predicted the highest measured concentrations quite well. A model comparison rank and ensemble analysis suggests that combining multiple models may provide more accurate predicted concentrations than any single model. None of the submissions based only on the stack monitoring data predicted the small measured concentrations very well. Modeling of sources by other nuclear facilities with smaller releases than medical isotope production facilities may be important in understanding how to discriminate those releases from releases from a nuclear explosion.

The verification regime of the comprehensive test ban treaty (CTBT) is based on a network of three different waveform technologies together with global monitoring of aerosols and noble gas in order to detect, locate and identify a nuclear weapon explosion down to 1 kt TNT equivalent. In case of a low intensity underground or underwater nuclear explosion, it appears that only radioactive gases, especially the noble gas which are difficult to contain, will allow identification of weak yield nuclear tests. Four radioactive xenon isotopes, 131mXe, 133mXe, 133Xe and 135Xe, are sufficiently produced in fission reactions and exhibit suitable half-lives and radiation emissions to be detected in atmosphere at low level far away from the release site. Four different monitoring CTBT systems, ARIX, ARSA, SAUNA, and SPALAX™ have been developed in order to sample and to measure them with high sensitivity. The latest developed by the French Atomic Energy Commission (CEA) is likely to be drastically improved in detection sensitivity (especially for the metastable isotopes) through a higher sampling rate, when equipped with a new conversion electron (CE)/X-ray coincidence spectrometer. This new spectrometer is based on two combined detectors, both exhibiting very low radioactive background: a well-type NaI(Tl) detector for photon detection surrounding a gas cell equipped with two large passivated implanted planar silicon chips for electron detection. It is characterized by a low electron energy threshold and a much better energy resolution for the CE than those usually measured with the existing CTBT equipments. Furthermore, the compact geometry of the spectrometer provides high efficiency for X-ray and for CE associated to the decay modes of the four relevant radioxenons. The paper focus on the design of this new spectrometer and presents spectroscopic performances of a prototype based on recent results achieved from both radioactive xenon standards and air sample measurements. Major improvements in detection sensitivity have been reached and quantified, especially for metastable radioactive isotopes 131mXe and 133mXe with a gain in minimum detectable activity (about 2 × 10−3 Bq) relative to current CTBT SPALAX™ system (air sampling frequency normalized to 8 h) of about 70 and 30 respectively.

Part I of this publication deals with the analysis of fission product releases consecutive to the Fukushima Dai-ichi accident. Reactor core damages are assessed relying on radionuclide detections performed by the CTBTO radionuclide network, especially at the particulate station located at Takasaki, 210 km away from the nuclear power plant. On the basis of a comparison between the reactor core inventory at the time of reactor shutdowns and the fission product activities measured in air at Takasaki, especially 95Nb and 103Ru, it was possible to show that the reactor cores were exposed to high temperature for a prolonged time. This diagnosis was confirmed by the presence of 113Sn in air at Takasaki. The 133Xe assessed release at the time of reactor shutdown (8 × 1018 Bq) turned out to be in the order of 80 % of the amount deduced from the reactor core inventories. This strongly suggests a broad meltdown of reactor cores.

Medical and industrial fadioisotopes are fundamental tools used in science, medicine and industry with an ever expanding usage in medical practice where their availability is vital. Very sensitive environmental radionuclide monitoring networks have been developed for nuclear-security-related monitoring [particularly Comprehensive Test-Ban-Treaty (CTBT) compliance verification] and are now operational.

Activity concentration data from ambient radioxenon measurements in ground level air, which were carried out in Europe in the framework of the International Noble Gas Experiment (INGE) in support of the development and build-up of a radioxenon monitoring network for the Comprehensive Nuclear-Test-Ban Treaty verification regime are presented and discussed. Six measurement stations provided data from 5 years of measurements performed between 2003 and 2008: Longyearbyen (Spitsbergen, Norway), Stockholm (Sweden), Dubna (Russian Federation), Schauinsland Mountain (Germany), Bruyères-le-Châtel and Marseille (both France). The noble gas systems used within the INGE are designed to continuously measure low concentrations of the four radioxenon isotopes which are most relevant for detection of nuclear explosions: 131mXe, 133mXe, 133Xe and 135Xe with a time resolution less than or equal to 24 h and a minimum detectable concentration of 133Xe less than 1 mBq/m3. This European cluster of six stations is particularly interesting because it is highly influenced by a high density of nuclear power reactors and some radiopharmaceutical production facilities. The activity concentrations at the European INGE stations are studied to characterise the influence of civilian releases, to be able to distinguish them from possible nuclear explosions. It was found that the mean activity concentration of the most frequently detected isotope, 133Xe, was 5–20 mBq/m3 within Central Europe where most nuclear installations are situated (Bruyères-le-Châtel and Schauinsland), 1.4–2.4 mBq/m3 just outside that region (Stockholm, Dubna and Marseille) and 0.2 mBq/m3 in the remote polar station of Spitsbergen. No seasonal trends could be observed from the data. Two interesting events have been examined and their source regions have been identified using atmospheric backtracking methods that deploy Lagrangian particle dispersion modelling and inversion techniques. The results are consistent with known releases of a radiopharmaceutical facility.

III. Coupling with dispersion and impact assessment models In most cases, studies of radiological or chemical impact are carried out using average or simplified local weather conditions. This methodology is well appropriate to assess the health impact within a regulatory framework but may be inadequate in case of crisis such as real accidental atmospheric releases. In such situations, the objective is to determine accurately and as quickly as possible a potential danger zone. Taking into account too simplified weather conditions can lead to an underestimation of the consequences. In this context, the operational meteorological forecast system MEDICIS (French acronym for “meteorology at mesoscale dedicated to human health impact assessment”) has been developed to provide realistic meteorological input conditions to radiological and chemical impact assessment models developed by the CEA (French Atomic Energy Commission).

The Radiological and Chemical Impact Labo r t ry (LIRC) of the French Atomic Energy Commissio n (CEA) is in charge of the development of modelling tools to evaluate the cons equences on human health of releases of radionuclid es or toxic chemicals in the environment, for emergency planning and for safety evaluation. In this context, the laboratory has dev eloped the operational meteorological forecast system MEDICIS (French acronym for “meteor ology at mesoscale dedicated to human health impact assessment”) to provide, in case of emergency, meteorological input conditions t radiological and chemical impact assessment mode ls developed by the laboratory.

Radioxenon concentration data collected at International Noble Gas Experiment (INGE) monitoring sites in the Northern Hemisphere are demonstrably impacted by emissions from medical isotope (MI) production facilities at Chalk River (CRL), Ontario and Fleurus, Belgium [Saey, J Environ Radioact 100(5):396–406, 2009]. Temporary cessation at these European MI facilities in the latter half of 2008 allowed an opportunity to assess the relative impact of distant MI facilities at INGE monitoring stations. In particular, the concentration distributions at the measurement sites and calculated sensitivities to the putative emission locations through atmospheric transport modelling (ATM) of meteorological Source Receptor Sensitivity (SRS) computations were used to study MI impacts.

Within the frame of Comprehensive Nuclear-Test Ban Treaty (CTBT), this paper deals with the development of the new techniques necessary for the xenon monitoring requested by the CTBT. An automatic system called SPALAX™, devoted to the on-site sampling and measurement was developed by French atomic energy commission (CEA). Analytical methods and equipments have been studied at our laboratory, using dual X-γ-spectrometry in order to get independent means with better sensitivity within a robust quality assurance program. In the case of a wide number of potential existing sources and depending on meteorological conditions, several solutions can be arrived at.

CONTEXT AND OBJECTIVES The detection and measurement of radionuclides released in the frame of nuclear activities is a major task addressing various issues like the monitoring of the environment for regulatory requirements, or the verification of the compliance with the Comprehensive nuclear Test Ban Treaty (CTBT). In the late 1990s, the French Atomic Energy Commission (CEA) developed a high sensitive technology, capable of performing radioactive noble gas measurements. This apparatus, called SPALAX for Système de Prélèvement d'air Automatique en Ligne avec l'Analyse des radio- Xénons, is described by Fontaine et al. (2004). The SPALAX automatically extracts, purifies and concentrates gaseous xenon from the atmospheric air; it measures the volumetric activity of four radioxenon isotopes 131mXe, 133mXe, 133Xe, 135Xe using high resolution γ spectrometry. The radioxenon isotopes are fission products chronically released from many kinds of nuclear installations such as nuclear power reactors, research reactors, hospitals and medical isotopes production facilities. The magnitudes and the proportions of the four radioactive isotopes may be useful to discern the various nuclear industry xenon emissions. A SPALAX station was set up at the CEA research centre in Bruyères-le-Châtel (located 40 km to the south of Paris) in August 2003. Since then, it has been operated with a 24 hour- sampling time. With the very low detection limit afforded by the SPALAX technology (e.g. less than 1 mBq.m-3 in 133Xe) and the nuclear European environment, a detectable amount of 133 Xe occurs in Bruyères-le-Châtel daily while the other isotopes 135Xe, 133mXe, and 131mXe are detected only from time to time. The four isotopes were seldom observed simultaneously. Between August 2003 and August 2005, ca. eighty peaks with 133Xe atmospheric volumetric activities ranging from ten or so to a few hundreds of mBq.m-3 stand out against a background activity level of some mBq.m-3. These values are far below any levels of health concern. In order to explain the radioxenon detections in Bruyères-le-Châtel, a numerical study has been carried out. The purpose is the determination of the potential xenon sources, which may be local or situated up to some hundreds of kilometres, generating the SPALAX observations. Thus, the atmospheric transport modelling requires a mesoscale approach which is described in the paper. We report successively on the meteorological fields and backward transport simulations performed for each event of xenon detection. The use of the mesoscale modelling is illustrated by the analysis of a quite elevated 133Xe measurement in Bruyères-le-Châtel.

CONTEXT AND OBJECTIVES The exposure rate assessment following a release of radionuclides into the atmosphere from a nuclear facility during an accident or under normal operation conditions is an important step regarding the regulatory requirements and the health risk evaluation of such releases. For computing the exposure rate at a given location, the plume shape and the radionuclides concentration distribution are main parameters. And yet, for the sake of simplicity, an uniform concentration distribution is usually assumed with a plume extending semi-infinitely in all hemispheric directions over a flat terrain. The same principle of modelling is applied to each radionuclide. A ‘global’ coefficient is calculated in advance taking into account both the flux of gamma rays in the idealized semi-infinite geometry with an unit activity concentration in the atmosphere, and factors to convert the activity flux into the irradiation dose. Nevertheless, in case of complex meteorological conditions or non-flat terrain with obstacles, it is worth performing a more precise approach. For example, Ichikawa et al. (1981) & Weng et al. (2003) made use of a Gaussian puff dispersion modelling to evaluate the distribution of the concentration in a radioactive plume and the gamma exposure rate. Raza and Avila (2001) implemented a 3D Lagrangian particle model for direct gamma dose rate computations. Following Raza and Avila (2001), we suggest here to use a Lagrangian particle model to take account of the gamma radiation of 3D continuous or transient radioactive plumes. The spatial distribution of discrete particles is computed with SPRAY, and the dispersion results are postprocessed by a new module called Cloud_Shine, especially developed to evaluate the plume exposure rate. In the following, the paper describes the way to use directly SPRAY results in order to compute the gamma radiation with Cloud_Shine. Some examples of calculations are presented aiming to assess Cloud_Shine results in simple geometrical configurations. At the end, the interest of SPRAY and Cloud_Shine is enlighten in more complicated cases, including a simple relief, a realistic topography, and the presence of buildings with their shadow effects.