For verifying the Comprehensive Nuclear-Test-Ban Treaty (CTBT), different monitoring technologies (seismic, infrasound, hydroacoustic, and radionuclide detection) are combined. The monitoring of radioactive xenon isotopes is one of the principal methods for the determination of the nuclear nature of an explosion. After an underground nuclear detonation the radioxenon isotopes [superscript 131m]Xe, [superscript 133m]Xe, ¹³³Xe, and ¹³⁵Xe, and the radioargon isotope ³⁷Ar have an increased probability of detection. In order to effectively utilize these isotopes as indicators of nuclear testing, an accurate background must be calculated. This work examines the fission products produced by spontaneous fission of ²³⁸U, which is naturally present in the earth's crust, and of ²⁴⁰Pu which is present as a product of nuclear weapons and nuclear reactor accidents. These calculations provide a range of production values for radioxenon in a variety of geologies as well as at various historic locations. The activation of geologic calcium and potassium by cosmic ray neutrons is considered for a variety of properties effecting the neutron flux. These calculations provide a range of radioargon production values across a selection of geologies. The impact of latitude and the solar activity cycle are also examined. In order to examine the transport of the isotopes through soil a model of the transport of xenon and argon through various geologies was developed. This model incorporates both the introduction of xenon from the atmosphere and that produced by spontaneous fission. This is then considered in light of what might be observed in an on-site inspection (OSI). What this work finds is that the radioxenon natural background does exceed detection limits in particular locations and geologies, however, a careful examination of the location and the ideal sampling depths can minimize the impact during an OSI. Radioargon, however, has a much larger natural background at shallow depths which are the realm of OSI sampling. Should radioargon sampling be used in an OSI the sampling time is crucial in distinguishing a nuclear explosion from the natural background. In some scenarios the natural background production of radioargon may be sufficient to interfere with the detection of an underground nuclear weapon test. This information may be beneficial in the development of future OSI noble gas monitoring techniques. / text

On Site Inspection is the final measure for verifying compliance of Member States with the Comprehensive Nuclear-Test-Ban Treaty. In order to enable the use of ³⁷Ar as a radiotracer for On Site Inspection, the sources of radioargon background must be characterized and quantified. A radiation transport model of the University of Texas at Austin Nuclear Engineering Teaching Laboratory (NETL) TRIGA reactor was developed to simulate the neutron flux in various regions of the reactor. An activation and depletion code was written to calculate production of ³⁷Ar in the facility based on the results of the radiation transport model. Results showed ³⁷Ar production rates of (6.567±0.31)×10² Bq·kWh⁻¹ in the re- actor pool and the air-filled irradiation facilities, and (5.811±0.40)×10⁴ Bq·kWh⁻¹ in the biological shield. Although ⁴⁰Ca activation in the biological shield was found to dominate the total radioargon inventory, the contribution to the effluent release rate would be diminished by the immobility of Ar generated in the concrete matrix and the long diffusion path of mobile radioargon. Diffusion of radioargon out of the reactor pool was found to limit the release rate but would not significantly affect the integrated release activity. The integrated ³⁷Ar release for an 8 hour operation at 950 kW was calculated to be (1.05±0.8)×10⁷ Bq, with pool emissions continuing for days and biological shield emissions continuing for tens of days following the operation. Sensitivity analyses showed that estimates for the time-dependent concentrations of ³⁷Ar in the NETL TRIGA could be made with the calculated buildup coefficients or through analytical solution of the activation equations for only (n,[gamma]) reactions in stable argon or (n,[alpha]) reactions in ⁴⁰Ca. Analyses also indicated that, for a generalized system, the integrated thermal flux can be used to calculate the buildup due to air activation and the integrated fast flux can be used to calculate the buildup due to calcium activation. Based on the results of the NETL TRIGA, an estimate of the global research reactor source term for ³⁷Ar and an estimate of ground-level ³⁷Ar concentrations near a facility were produced. / text

Dans le cadre du Traité d'Interdiction Complète des Essais Nucléaires (TICE), le Système de Surveillance International (SSI) a été développé. Ce réseau consiste en plusieurs centaines de stations de mesures dont le rôle est de détecter la signature d'un essai nucléaire clandestin. Quatre types de mesures sont effectués: sismique, hydro-acoustique, infrason et radionucléide. Dans le cas particulier d'un essai nucléaire souterrain, seule la mesure des radionucléides est adéquate pour mettre en évidence le caractère nucléaire de l'explosion. Quatre radionucléides du xénon sont produits en masse lors d'un essai nucléaire: le 131m-Xe, le 133m-Xe, le 133-Xe, et le 135-Xe. Une faible proportion de ces gaz parvient à diffuser à travers les fissures de la cavité et se retrouver dans l'atmosphère. A cause de la dilution atmosphérique, les concentrations de ces quatre radionucléides sont extrêmement faibles et par conséquent difficiles à détecter. De plus, le bruit de fond atmosphérique ne cesse d'augmenter car ces gaz radioactifs sont également produits et relâchés par les usines de production de radio-isotopes médicaux et les centrales nucléaires. Afin de renforcer le réseau de surveillance, le CEA-DAM a proposé de développer un système de détection des radionucléides du xénon ultra-compact et mobile, afin de pouvoir effectuer des mesures de vérification directement sur site suspect, et ainsi contourner la contrainte de dilution atmosphérique. Ces travaux présentent le développement et l'optimisation de ce système. Le système a d'abord été modélisé sur ordinateur et ses performances en terme d'efficacité de détection ont été simulées par méthode de Monte-Carlo avec le logiciel Geant4. Les mesures se font en utilisant la technique de mesure en coïncidences électron/photon. En parallèle, une chaîne d'acquisition ultra-compacte a été développée, ainsi qu'un réseau de communication permettant de synchroniser en temps les différents spectromètres numériques utilisés pour le traitement du signal (protocole IEEE 1588 PTP). Les concentrations minimales détectables de ce système sont inférieures à 5 mBq/m³ pour une acquisition de 12 h, et ce pour les quatre radionucléides du xénon d'intérêt. / As part of the Comprehensive nuclear Test Ban Treaty (CTBT), an International Monitoring System (IMS) has been developed. This network consists of several hundred measuring stations whose role is to detect the signatures of a clandestine nuclear test. Four types of measurements are carried out: seismic, hydroacoustic, infrasound and radionuclide. In the particular case of an underground nuclear test, only the radionuclide measurement is adequate to detect the explosion and quantify its yield. Four xenon radionuclides with long half-lives are produced during a nuclear test: the 131m-Xe, the 133m-Xe, the 133-Xe and the 135-Xe. A small proportion of these gases may diffuse through the cracks in the cavity and end up in the atmosphere. Due to atmospheric dilution, the concentrations of these four radionuclides are extremely low and therefore difficult to detect. In addition, atmospheric background continues to increase as these radioactive gases are also produced by medical radioisotope facilities and nuclear power plants. In order to strengthen the IMS, the CEA-DAM has proposed to develop an ultra-compact and mobile system for detecting xenon radionuclides, in order to be able to carry out verification measurements directly on site, and thus to avoid the problem of atmospheric dilution. This work presents the development and optimization of this system. The system was first modelled on a computer and its performances in terms of detection efficiency were simulated by the Monte-Carlo method using Geant4 software. Measurements are made in electron-photon coincidence mode. In parallel, an ultra-compact acquisition chain has been developed, as well as a communication network to synchronize in time the various digital spectrometers used for signal processing (IEEE 1588 PTP protocol). The minimum detectable concentrations of this system are less than 5 mBq/m³ for a 12 h acquisition, for the four xenon radionuclides of interest.

Underground nuclear explosions (UNEs) produce anthropogenic isotopes that provide the only definitive means by which to determine whether a nuclear explosion has taken place. Verification of a suspected test under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes both on-site and atmospheric sampling of specific noble gas radioisotopes for analysis of origin. It is well-established that the processes of subsurface transport can affect the rate at which such gases will reach the surface. However, the relative abundance of anthropogenic isotopes reaching the surface following transport is currently assumed to rely solely on their direct fission yield, decay rate, and their production from precursor decay, making no account for the influence of transport processes on isotopic ratios. The Underground Transport of Environmental Xenon (UTEX) model has been developed to examine the possible effects of subsurface transport on radioxenon isotopic ratios as well as to consider a number of on-site inspection-related applications. In this work, background on the UTEX model's development, evolution and vetting is presented. This is followed by the characterization and analysis of a number of applications of the model for consideration of CTBT-relevant scenarios. Specifically, the UTEX model's capability to analyze CTBT on-site inspection concept of operations is demonstrated. This is accomplished through an examination of generalized UNE source terms, geological stratigraphy, UNE impact on local geology, natural soil-gas radionuclide backgrounds, atmospheric infiltration, and sampling methodology. It is shown that the processes driving noble gas transport through geological media can significantly skew the ratios of key radioxenon isotopes that are used to help verify whether or not a well-contained underground test has taken place. This result emphasizes the need for a broader understanding of radionuclide signatures used for CTBT verification purposes and the mechanisms that can alter them. / text