Contribution to the development of atmospheric radioxenon monitoring

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.     

Worldwide measurements of radioxenon background near isotope production facilities, a nuclear power plant and at remote sites: the “EU/JA-II” Project

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) specifies that radioxenon measurements should be performed at 40 or more stations worldwide within the International Monitoring System (IMS). Measuring radioxenon is one of the principle techniques to detect underground nuclear explosions. Specifically, presence and ratios of different radioxenon isotopes allows determining whether a detection event under consideration originated from a nuclear explosion or a civilian source. However, radioxenon monitoring on a global scale is a novel technology and the global civil background must be characterized sufficiently. This paper lays out a study, based on several unique measurement campaigns, of the worldwide concentrations and sources of verification relevant xenon isotopes. It complements the experience already gathered with radioxenon measurements within the CTBT IMS programme and focuses on locations in Belgium, Germany, Kuwait, Thailand and South Africa where very little information was available on ambient xenon levels or interesting sites offered opportunities to learn more about emissions from known sources. The findings corroborate the hypothesis that a few major radioxenon sources contribute in great part to the global radioxenon background. Additionally, the existence of independent sources of ^131mXe (the daughter of ¹³¹I) has been demonstrated, which has some potential to bias the isotopic signature of signals from nuclear explosions.     

Engineering upgrades to the Radionuclide Aerosol Sampler/Analyzer for the CTBT International Monitoring System

The Radionuclide Aerosol Sampler/Analyzer (RASA) is an automated collection and analysis system designed for aerosol radionuclide monitoring for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The advantages of an automated system include minimal need for human intervention and consistent analytical data. However, maintainability and down time issues threaten this utility, even for systems with over 90 % data availability. Engineering upgrades to the RASA are currently being pursued to address these issues, as well as measures relevant to technical lessons learned from the Fukushima nuclear power plant event. Current work includes a new automation control unit and other potential improvements such as alternative detector cooling and sampling options. This paper presents the current state of upgrades and improvements under investigation.     

Detection efficiency calculations using Geant4 for a broad-energy germanium gamma spectrometer

In assistance of radionuclide measurements at Canada’s Comprehensive Nuclear-Test-Ban Treaty (CTBT) laboratory, a Geant4 Monte Carlo application has been developed in simulating a broad-energy germanium detector and calculating detection efficiencies. The detector model was optimized in a reliable and non-biased manner through simultaneous tuning on gap distance and detector dimension, and was validated over various realistic measurement scenarios. All work is based on a series of experiments which covers the typical energy range of gamma radiation in environmental analysis, and considers the variety of the CTBT sample type, dimension and distance-to-detector. In all cases, the predicted efficiencies are consistent with the empirical ones within 5%, with a typical deviation of 3% in majority.     

Field testing of collection and measurement of radioxenon for the Comprehensive Test Ban Treaty

Pacific Northwest National Laboratory, with guidance and support from the U.S. Department of Energy's NN-20 Comprehensive Test Ban Treaty (CTBT) Research and Development program, has developed and demonstrated a fully automatic sampler-analyzer (ARSA) for the collection and quantitative measurement of the four xenon radionuclides,^131mXe (11.9 d),^133mXe (2.19 d),¹³³Xe (5.24 d), and¹³⁵Xe (9.10 h), in the atmosphere. These radionuclides are important signatures in monitoring for compliance to a CTBT, and may have applications in stack monitoring and other areas where xenon radionuclides are present. The activity ratios between certain of these radionuclides permit discrimination between radioxenon originating from nuclear detonations and that from nuclear reactor operations, nuclear fuel reprocessing, or from medical isotope production and usage. With the ARSA system, xenon is continuously and automatically separated from the atmosphere at flow rates of about 100 lpm by sorption-bed techniques. Samples collected in 8 hours are automatically analyzed by electron-photon coincidence spectrometry to provide detection sensitivities as low as 100 μBq/m³ of air. This sensitivity is about 10-fold better than achieved with reported laboratory-based procedures¹ for the short time collection intervals of interest. Gamma-ray energy spectra and gas analysis data are automatically collected.     

Ashing and microwave digestion of aerosol samples with a polypropylene fibrous filter matrix

Radionuclide monitoring is one of the key techniques required by the International Monitoring System (IMS) and On-Site Inspection (OSI) in the Comprehensive Nuclear Test Ban Treaty (CTBT). There shall be a global network of 80 radionuclide monitoring stations. Atmospheric aerosols are collected generally on filters in the stations. A polypropylene (PP) fibrous filter is often used in sampling atmospheric aerosols. There might be much information to be obtained by measuring aerosol samples after digestion rather than nondestructive analysis directly by γ-spectrometry. The present work focused on pretreatment of the filter samples, which includes the influence of different ashing or microwave digestion conditions on the recoveries of analytes. The inductively coupled plasma-atomic emission spectrometric detection results indicated that the recoveries of elements in the PP fibrous filters by ashing were influenced by ashing time, temperature and the properties of the elements. High recoveries of volatile elements and consistent recovery for other elements were obtained by using a closed microwave system to digest the filters. Higher sensitivity was also obtained when the intercomparison sample was measured by a HPGe well detector after pretreatment by the recommended ashing and microwave digestion procedures.     

Compton suppression systems for environmental radiological analysis

Compton suppression (CS) has increased the sensitivity of gamma spectroscopy systems tenfold, and is routinely used in laboratories for environmental analysis and the monitoring of the CTBT. There are several different techniques available, and many more variables to consider when designing or optimising a CS system. An overview and discussion of these is presented here.     

Assessment of element-specific homogeneity in reference materials using microanalytical techniques

The IAEA established in 1994 a co-ordinated research programme (CRP) on "Reference Materials for Microanalytical Nuclear Techniques" as part of its efforts to promote and strengthen the use of nuclear analytical technologies in member states with the specific aim of improving the quality of analysis of nuclear, environmental, and biological materials. The objectives of this initiative were: to identify suitable biological reference materials which could serve the needs for quality control in microanalytical techniques; to evaluate existing CRMs for use in microanalytical investigations; to evaluate appropriate sample pretreatment procedures for materials being used for analysis with microanalytical techniques; to identify analytical techniques which can be used for characterisation of homogeneity determination, and to apply such techniques to the characterization of candidate reference materials for use with microanalytical techniques. The CRP lasted for 4 years and seven laboratories and the Agency's Laboratories in Seibersdorf participated. A number of materials including the candidate reference materials IAEA 338 (lichen) and IAEA 413 (single cell algae, elevated level) were evaluated for the distribution of elements such as Cl, K, Ca, Cr, Mn, Fe, Zn, As, Br, Rb, Cd, Hg, and Pb. The results obtained during this CRP suggest that: each element exhibits its characteristic distribution in a matrix described by the "Ingamels' sampling constant" or the "relative homogeneity factor" of Kurfuerst; both concepts are valid over a large range of sample mass used for analysis (from 0.1 g to around 100 mg); and materials being characterised quantitatively for element homogeneity could be used for the experimental determination of total uncertainty of other analytical techniques. As far as we are aware this is the first time the concept of quantitative characterisation of homogeneity has been applied to potential reference materials and the first demonstration of the feasibility and usefulness of the concept with particular emphasis on enhancing quality control opportunities for microanalytical techniques.     

Cesium-137 concentrations, trends, and sources observed in Kuwait City, Kuwait

As part of the development support for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), the Prototype International Data Center (PIDC) has been processing radionuclide data since 1995. Radionuclide data received from field stations includes gamma-ray spectra, meteorological data, and state of health (SOH) information. To date over 20 radionuclide monitoring stations have transmitted data to the PIDC. The radionuclide monitoring system collects both aerosol and gas samples. Gamma-ray spectral analysis is performed on the samples to determine if they contain anthropogenic radionuclides indicative of nuclear debris. A key radionuclide monitored by this system is ¹³⁷Cs. Due to the half-life of ¹³⁷Cs (30.17 y), amounts of this radionuclide releases are still present in the soil and atmosphere as a result of past nuclear tests and reactor releases. ¹³⁷Cs from these sources are routinely detected in the prototype CTBT radionuclide monitoring system. Out of the multiple stations that contribute data to the PIDC, the highest ¹³⁷Cs activity concentrations and largest range of concentrations are observed at the Kuwait City, Kuwait station. A special study was conducted to investigate the concentrations, trends, and origin of ¹³⁷Cs in the Kuwait aerosol. This study combines over four years worth of aerosol data, meteorological data and soil sample analysis to explore this matter.     

Applied environmental metrology for studying health-pollution interactions

Nuclear and isotopic techniques are valuable tools in assessing the levels of environmental pollution by toxic elements and for studying how these contaminants affect human health. More than 90 counterparts from 55 countries around the world have participated in projects on these topics, supported by the International Atomic Energy Agency (IAEA), during last ten years. With the support of the IAEA, for example, an appropriate metrology for compositional characterization of size fractionated airborne particulate matter was developed, verified and implemented in more than 40 countries. This paper reviews the development and application of environmental metrology tools involving nuclear analytical and isotopic techniques, as seen from the particular perspective of IAEA programmes.     

Innovative concept for a major breakthrough in atmospheric radioactive xenon detection for nuclear explosion monitoring

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.     

Beryllium-7 activity at Comprehensive Nuclear-Test-Ban Treaty stations hosted by the United Kingdom

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a global network of monitoring stations that perform high-resolution gamma-spectrometry on air filter samples for the identification of radionuclides indicative of nuclear weapons tests and reactor incidents. These daily measurements have created an invaluable resource for understanding variations in natural background radioactivity, including the contribution of ⁷Be. Statistical analysis has been performed on ⁷Be data collected by CTBT stations hosted by the United Kingdom including at British Indian Ocean Territory (RN66), St Helena (RN67) and Tristan da Cunha (RN68) during 2005–2013. The results have been found to follow a lognormal distribution which implies that the ⁷Be activity is the multiplicative product of many small independent factors, such as meteorology, elevation, local station conditions, sample acquisition and analysis. This has the potential to identify discrepant measurements not attributable to the intrinsic variability of the distribution and indicative of station malfunction. Variations in ⁷Be activity have been considered on monthly, weekly and daily timescales and characterised using the geometric mean in accordance with the properties of the lognormal probability density function. Seasonal variations have been identified, with summer maxima and winter minima that are attributable to changes in mixing within the stratosphere and troposphere. Such fluctuations have been examined using the Fast Fourier Transform which may indicate variations associated with the 27 day solar cycle.     

Monitoring ground-level air for trace analysis: methods and results

Trace analysis enables the sensitive detection of radionuclide concentrations in ground-level air in the range of microbecquerel per cubic meter (Bq m^–3). Typical sampling intervals of less than one day up to a few days can be used in routine operation. Trace analysis measurements are performed in the framework of the German Integrated Measuring and Information system (IMIS) and the International Monitoring System (IMS) used for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Within the environmental monitoring programmes of the German IMIS the Federal Office for Radiation Protection (BfS) performs measurements of aerosol-bound radionuclides and of radioactive noble gases in the atmosphere. Aerosols are collected on filters with high-volume air samplers and analysed by -spectrometry, -spectrometry, and integral measurements of -activity, with preceding radiochemical separation. Noble gas samples from 15 sites world-wide are analysed to observe the ⁸⁵Kr-release from nuclear fuel reprocessing plants and from seven sites in Germany to monitor the ¹³³Xe emitted from nuclear power plants. As part of the International Monitoring System (IMS) of the CTBT an automatic aerosol sampling and measuring system and an automatic noble gas sampling and measuring system will be operated by the BfS at Mount Schauinsland near Freiburg. Because of its expertise in noble gas measurements the BfS had been chosen to perform an intercomparison experiment in the BfS laboratory in Freiburg with several automatic noble gas sampling and measurement systems before their installation at IMS sites. To establish quality-assurance programmes for trace analysis performed for the German IMIS close collaboration between the involved German institutions has been established. First steps have been taken to expand cooperation to other European laboratories. Informal data exchange already occurs between trace-analysis laboratories in Europe (Ring of Five) and helps in cases of enhanced activity concentrations to get a rapid overview of the radiological situation and to identify possible sources.     

Compton suppressed gamma-spectrometry for Comprehensive Nuclear-Test-Ban Treaty samples

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a network of certified laboratories that perform high-resolution gamma-spectrometry on global air filter samples for the identification of 85 radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), the use of advanced Compton suppressed systems has been investigated to reduce the Compton continuum and improve detection sensitivity. Samples collected from the Philippines and during the Fukushima incident have been measured, demonstrating Compton continuum reductions of 28–59 % with suppression factors of 0.1–147.0. Detection sensitivity has been improved with typically 40 % lower MDAs, including ¹⁴⁰Ba to meet CTBT requirements. True coincidence summing effects have been considered, including the application to remove interferences by the elimination of gamma-rays in cascade. This has been demonstrated for the removal of ¹³⁴Cs allowing improved ¹³¹I measurement.     

Implementation of radionuclide measurements for the Comprehensive Nuclear-Test-Ban Treaty and associated on-site inspections

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) plans the installation of an International Monitoring System (IMS) based upon four global networks. Seismic, hydroacoustic and infrasound waves will help detect underground, underwater and atmospheric nuclear tests and will permit their discrimination from natural events. 80 particulate stations will detect radioactive aerosols, this network being completed with a sub-set of 40 stations which will measure rare gases, typically xenon isotopes. 16 IMS laboratories will perform additional analysis mainly by gamma-spectrometry, using the most sensitive methods such as particulate analysis. In order to have the most effective network, modeling was performed by using an inverse method in which the radioactive tracer is transported back from detectors. Examples will be given, regarding the maps of detection probability, background effects of existing xenon or radon, or the decoupling effects. All these tools and means are anticipated to have a complete process of certification, authentication of the data and discrimination capabilities between nuclear test and releases from civilian nuclear industry (reactors, reprocessing plants,). If a State Party identifies events that it feels could be a nuclear explosion, it can ask for clarification and finally it may send a request for an On-Site Inspection. The rights of the State Party and the constraints for the Inspection Team are defined in the Treaty. That leads to limited time, to limited number of inspectors on the site and to precise methods to be authorized. The means and resulting data have to be blinded in order to make sure the confidentiality is observed. Examples of restricting measurements will be given regarding airborne or vehicle mounted spectrometry as well as laboratory analysis. Cooperation with international organizations (WMO, WHO) will be discussed, depending on confidentiality issues.     

Sensitivity study on modeling radioxenon signals from radiopharmaceutical production facilities

As part of the Comprehensive Nuclear Test-Ban Treaty (CTBT), the International Monitoring System (IMS) was established to monitor the world for nuclear weapon explosions. As part of this network, systems are in place to monitor the atmosphere for radioxenon. The IMS routinely detects radioxenon from sources other than nuclear explosions. One of these radioxenon sources is radiopharmaceutical production facilities. This is a sensitivity study on the nuclear forensic signals possible from such facilities. A fission process model was produced to calculate the activity of ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe in the process utilized to produce ⁹⁹Mo and ¹³¹I for medical applications through high enriched uranium fission. The computer model accounts for fractionation of radionuclides within a decay chain that may result from filtering or chemical procedures. Ratios of the radioxenon isotopes are calculated as a function of decay time after the release. The ratios are then compared to those expected from nuclear explosions. The main conclusion from this work is that the two main factors that affect the nuclear forensic signal from radiopharmaceutical production facilities are the sample irradiation time and the use of emission gas storage tanks.     

Verification of a Fissile Material Cutoff Treaty: The case of enrichment facilities and the role of ultra-trace level isotope ratio analysis

One challenge to a potential verification regime for a Fissile Material Cutoff Treaty (FMCT) would be to assure that enrichment plants are not producing highly enriched uranium (HEU) for weapons purposes. Namely in some older enrichment plants, operated in nuclear weapon states, environmental sampling techniques might detect particles from historic HEU production. Determination of the age of these particles would be the most direct confirmation of treaty-compliance. While methods are available to determine the age of nuclear materials based on the concentrations of decay products, micron-sized uranium particles are particularly difficult to analyze. We will review the sensitivity requirements for age determination of HEU particles in an FMCT, and assess the potential of advanced measurement techniques available for this application.     

Comparison of radionuclide ratios in atmospheric nuclear explosions and nuclear releases from Chernobyl and Fukushima seen in gamma ray spectrometry

The Comprehensive Nuclear Test Ban Treaty has remote radionuclide monitoring followed by an On Site Inspection (OSI) to clarify the nature of a suspect event as part of its verification regime. An important aspect of radionuclide measurements on site is the discrimination of other potential sources of similar radionuclides such as reactor accidents or medical isotope production. The Chernobyl and Fukushima nuclear reactor disasters offer two different reactor source term environmental inputs that can be compared against historical measurements of nuclear explosions. The comparison of whole-sample gamma spectrometry measurements from these three events and the analysis of similarities and differences are presented. This analysis is a step toward confirming what is needed for measurements during an OSI under the auspices of the Comprehensive Test Ban Treaty.     

Further development of a cosmic veto gamma-spectrometer

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a network of certified laboratories that perform high-resolution gamma-spectrometry on global air filter samples for the identification of 85 radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), a novel cosmic veto gamma-spectrometer has been developed to improve the sensitivity of measurements for treaty compliance. The system consists of plastic scintillation plates operated in time-stamp mode to detect coincident cosmic-ray interactions within an HPGe gamma-spectrometer. This provides a mean background reduction of 75.2 % with MDA improvements of 45.6 %. The CTBT requirement for a ¹⁴⁰Ba MDA is achievable after 1.5 days counting compared to 5–7 days using conventional systems. The system does not require dedicated coincidence electronics, and remains easily configurable with dual acquisition of unsuppressed and suppressed spectra. Performance has been significantly improved by complete processing of the cosmic-ray spectrum (0–25 MeV) combined with the Canberra Lynx? multi-channel analyser. The improved sensitivity has been demonstrated for a CTBT air filter sample collected after the Fukushima incident.     

Screening of potential sites for undeclared nuclear facilities in environmental monitoring for nuclear proliferation

The sensitivity of environmental sampling and analysis for the estimated atmospheric concentrations of radionuclides in effluents from clandestine nuclear facilities may require many sampling sites and exorbitant costs to monitor such large areas. An assessment has been made of the key factors necessary to support the operation of a nuclear facility, e.g., the existence of a nearby transportation network, main electrical power supply, and population centers. Screening, the subject of this paper, evaluates how cartographic data and satellite imagery can be used to identify areas most capable of supporting undeclared nuclear operations. As a result, large geographical areas can be eliminated from environmental monitoring and sampling. This leads to reduced costs, a reduction in the number of sampling sites required, and a reduction in the overall level of effort required to accomplish effective environmental monitoring. The screening methodology and techniques are described and examples given.