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A high throughput system for processing and detection of low levels of radioxenon in soil gas samples has been developed. Processing and analysis of sub-soil noble gas samples puts high demands on the gas separation part of the system since the samples might contain high levels of Rn, CO2 as well as other gases. The gas process is optimized to remove all CO2, H2O and Rn with a high recovery yield of the xenon in the sample to ensure a high sensitivity even for small samples. The system is designed to handle multiple samples per day with a high level of automation and sample traceability to be suitable for use in an on-site inspection (OSI) an important component in the verification of the Comprehensive Nuclear Test Ban Treaty. To ensure a rapid deployment the system could be pre-installed in a flight container.

Measurement of radioactive gas seepage from an underground nuclear explosion is one of the primary methods to confirm whether an event was nuclear in nature. Radioactive noble gas indicators that are commonly targeted by such measurements (e.g. 133Xe, 37Ar) have half-lives of 35 days or less. Argon-39, an activation product similar to 37Ar, is produced by the interaction between neutrons and potassium in the surrounding geology and has a half-life of 269 years. Measurements taken at three sites near three historic underground nuclear test locations at the Nevada National Security Site have all shown highly elevated levels of 39Ar in soil gas decades after the test events. Elevated levels of 39Ar were also detected in atmospheric air collected near two of these sites, and outside the entrance of the one tunnel site. These measurements demonstrate that 39Ar has the potential to be a long-term signature of an underground nuclear event which can be reliably detected at the surface or in the shallow subsurface. This radionuclide detection of an underground nuclear event decades after the event takes place is in contrast to the commonly held assumption that detecting underground nuclear events via radionuclides at the surface needs to be done in a matter of months. Depending upon what further studies show about the robustness of this signature in a variety of geological settings, it may in fact be easy to detect underground nuclear events at the surface for a very long time post-detonation.

As part of an underground gas migration study, two radioactive noble gases (37Ar and 127Xe) and two stable tracer gases (SF6 and PFDMCH) were injected into a historic nuclear explosion test chimney and allowed to migrate naturally. The purpose of this experiment was to provide a bounding case (natural transport) for the flow of radioactive noble gases following an underground nuclear explosion. To accomplish this, soil gas samples were collected from a series of boreholes and a range of depths from the shallow subsurface (3 m) to deeper levels (~160 m) over a period of eleven months. These samples have provided insights into the development and evolution of the subsurface plume and constrained the relative migration rates of the radioactive and stable gas species in the case when the driving pressure from the cavity is low. Analysis of the samples concluded that the stable tracer SF6 was consistently enriched in the subsurface samples relative to the radiotracer 127Xe, but the ratios of SF6 and 37Ar remained similar throughout the samples.

A Bayesian source-term algorithm recently published by Eslinger et al. (2019) extended previous models by including the ability to discriminate between classes of releases such as nuclear explosions, nuclear power plants, or medical isotope production facilities when multiple isotopes are measured. Using 20 release cases from a synthetic data set previously published by Haas et al. (2017), algorithm performance was demonstrated on the transport scale (400–1000 km) associated with the radionuclide samplers in the International Monitoring System. Inclusion of multiple isotopes improves release location and release time estimates over analyses using only a single isotope. The ability to discriminate between classes of releases does not depend on the accuracy of the location or time of release estimates. For some combinations of isotopes, the ability to confidently discriminate between classes of releases requires only a few samples.

Improving predictive models for noble gas transport through natural materials at the field-scale is an essential component of improving US nuclear monitoring capabilities. Several field-scale experiments with a gas transport component have been conducted at the Nevada National Security Site (Non-Proliferation Experiment, Underground Nuclear Explosion Signatures Experiment). However, the models associated with these experiments have not treated zeolite minerals as gas adsorbing phases. This is significant as zeolites are a common alteration mineral with a high abundance at these field sites and are shown here to significantly fractionate noble gases during field-scale transport. This fractionation and associated retardation can complicate gas transport predictions by reducing the signal-to-noise ratio to the detector (e.g. mass spectrometers or radiation detectors) enough to mask the signal or make the data difficult to interpret. Omitting adsorption-related retardation data of noble gases in predictive gas transport models therefore results in systematic errors in model predictions where zeolites are present.Herein is presented noble gas adsorption data collected on zeolitized and non-zeolitized tuff. Experimental results were obtained using a unique piezometric adsorption system designed and built for this study. Data collected were then related to pure-phase mineral analyses conducted on clinoptilolite, mordenite, and quartz. These results quantify the adsorption capacity of materials present in field-scale systems, enabling the modeling of low-permeability rocks as significant sorption reservoirs vital to bulk transport predictions.

The detection of radioactive noble gases is a primary technology for verifying compliance with the pending Comprehensive Nuclear-Test-Ban Treaty. A fundamental challenge in applying this technology for detecting underground nuclear explosions is estimating the timing and magnitude of the radionuclide signatures. While the primary mechanism for transport is advective transport, either through barometric pumping or thermally driven advection, diffusive transport in the surrounding matrix also plays a secondary role. From the study of primordial noble gas signatures, it is known that xenon has a strong physical adsorption affinity in shale formations. Given the unselective nature of physical adsorption, isotherm measurements reported here show that non-trivial amounts of xenon adsorb on a variety of media, in addition to shale. A dual-porosity model is then discussed demonstrating that sorption amplifies the diffusive uptake of an adsorbing matrix from a fracture. This effect may reduce the radioxenon signature down to approximately one-tenth, similar to primordial xenon isotopic signatures.

A method is presented for estimating 41 Ar, 85,88 Kr and 131m,133 Xe dose rates to terrestrial wildlife without having to resort to comparisons with analogue radionuclides. The approach can be used to calculate the dose rates arising from external exposures to given ambient air concentrations of these isotopes. Dose conversion coefficient (DCC) values for a range of representative organisms are calculated, using a Monte Carlo approach to generate absorbed fractions based on representing animals as reference ellipsoid geometries. Plume immersion is the main component of the total DCC. DCC values calculated for a human-sized organism are compared with human dose conversion factors from ICRP Publication 119, demonstrating the consistency of the biota approach with that for humans. An example of application is provided for hypothetical nuclear power plant atmospheric discharges with associated exposures to birds and insects. In this example, the dose rates appear to be dominated by 133 Xe and 88 Kr, respectively. The biota considered would be protected from the effects of noble gas radiation from a population protection perspective.

Monitoring results of radioactive noble gases after the JCO criticality accident, Tokai, Japan at the MRI, Tsukuba, about 60 km southwest of from the accident site, are presented. Although the experiments to detect radioactive noble gases from the JCO accident proved negative, the possible transport of a radioactive plume from the accident site is described based on the wind data obtained at two meteorological observation towers in Tokai and Tsukuba. Also, the maximum emissions of the noble gases from the accident are evaluated on the basis of the observations.

Measurements of 133Xe (τ12 = 5.2 days) atmospheric concentrations were performed during the fall of 1993 and throughout 1995 on ‘noble gas concentrates’ from the northeastern US. These samples were obtained from a commercial air-reduction plant in Allentown, Pennsylvania. Following Chromatographic purification of the xenon gas, the 133Xe activity was determined using a high-purity germanium gamma-ray spectrometer. The average 133Xe concentrations were in the range 1–3 mBq m−3, which is consistent with nuclear power plant noble gas releases in the region surrounding the sampling point, but approximately 50–100 times lower than those reported in Albany, NY approximately 300 km to the northeast from 1975 through 1984. The lower atmospheric concentrations are also consistent with the 100-fold reduction in radioxenon release from 25 nuclear reactors in that region. Only an upper limit could be established for the 135Xe level (τ12 = 9.1 h), which was about 0.03 of the 133Xe level. These background levels are of concern in monitoring for atmospheric radioxenons to assure compliance with a Comprehensive nuclear Test Ban Treaty (CTBT).

The capability of the International Monitoring System (IMS) to detect xenon from underground nuclear explosions is dependent on the radioactive xenon background. Adding to the background, medical isotope production (MIP) by fission releases several important xenon isotopes including xenon-133 and iodine-133 that decays to xenon-133. The amount of xenon released from these facilities may be equivalent to or exceed that released from an underground nuclear explosion. Thus the release of gaseous fission products within days of irradiation makes it difficult to distinguish MIP emissions from a nuclear explosion. In addition, recent shortages in molybdenum-99 have created interest and investment opportunities to design and build new MIP facilities in the United States and throughout the world. Due to the potential increase in the number of MIP facilities, a discussion of abatement technologies provides insight into how the problem of emission control from MIP facilities can be tackled. A review of practices is provided to delineate methods useful for abatement of medical isotopes.

In order to simulate the impact of mesoscale wind fields and to assess potential capability of atmospheric Lagrangian particle dispersion model (LPDM) as an emergency response model for the decision supports, two different simulations of LPDM with the mesoscale prognostic model MM5 (Mesoscale Model ver. 5) were driven. The first simulation of radioactive noble gas ( 85 Kr exponent) emitted during JCO accident occurred from 30 September to 3 October 1999 at Tokai, Japan showed that the first arriving short pulse was found in Tsukuba located at 60 km away from the accidental area. However, the released radioactive noble gas was transported back to the origin site about 2 days later due to the mesoscale meteorological wind circulation, enhancing the levels of 85 Kr with the secondary peak in Tsukuba. The second simulation of atmospheric dilution factors (the ratio of concentration to the emission rate, χ / Q ), during the underground nuclear test (UNT) performed by North Korea showed that high χ / Q moved to the eastward and extended toward southward in accordance with the mesoscale atmospheric circulations generated by mesoscale prognostic model MM5. In comparison with the measurements, the simulated horizontal distribution patterns of 85 Kr during the JCO are well accord with that of observation in Tsukuba such as the existence of secondary peak which is associated with the mesoscale circulations. However, the simulated level of 85 Kr anomaly was found to be significantly lower than the observations, and some interpretations on these discrepancies were described. Applications of LPDM to two mesoscale emergency response dispersion cases suggest the potential capability of LPDM to be used as a decision support model provided accurate emission rate of accident in case of a large accident.

The Comprehensive Nuclear Test Ban Treaty (CTBT) organisation is implementing a world-wide monitoring network in order to check that the State Signatories comply with the treaty. One of the monitoring facilities consists of an atmospheric noble gas monitoring equipment. According to the requirements annexed in the treaty, the French Atomic Energy Commission (CEA) developed a device, called SPALAXTM, which automatically extracts xenon from ambient air and makes in situ measurements of the activities of four xenon radioisotopes (131mXe, 133mXe, 133Xe, 135Xe). The originality of this device is noticeable essentially in the gas sample processing method: thanks to the coupling of a gas permeator and of a noble gas specific adsorbent, it can selectively extract and concentrate xenon to more than 3×10 E6. This process is carried out continuously without cryogenic cooling, without any regeneration time. The detection of the xenon radioactive isotopes is done automatically by high spectral resolution γ spectrometry, a robust technology well-suited for on-field instrumentation. In the year 2000, a prototype was involved in an international evaluation exercise directed by the CTBT organisation (CTBTO). This exercise demonstrated that the SPALAXTM equipment perfectly met the requirements of the CTBTO for such systems. On the basis of the continuous 24-h resolution record of the atmospheric xenon radioactive isotopes concentrations, the SPALAXTM system also demonstrated that ambient levels of 133Xe can fluctuate quickly from less than the detection limit to over 40×10−3 Bq m−3. In order to build an industrial version of this equipment, the CEA entered into a partnership with a French engineering company (S.F.I., Marseille, France), which is now able to produce an industrial version of SPALAXTM, i.e. more compact and more efficient than the prototypes. The 133Xe minimum detectable concentration is 0.15×10−3 Bq m−3 air per 24 h sampling cycle.