Analysis of radioxenon and Krypton-85 at the BfS noble gas laboratory

Radionuclide measurements of the international monitoring system

The International Monitoring System (IMS) is a unique global network of sensors, tuned to measure various phenomenology, with the common goal of detecting a nuclear explosion anywhere in the world. One component of this network collects measurements of radioactive particulates and gases (collectively known as radionuclides) present in the atmosphere; through this, compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) can be verified. The radionuclide sub-network consists of 120 sensors across 80 locations, supported by 16 measurement laboratories. All radionuclide stations make use of a form of γ-ray spectroscopy to measure radionuclides from samples; this remains largely unchanged since the network was first established 25 years ago. Advances in sampling and spectroscopy systems can yield improvements to the sensitivity of the network to detect a nuclear explosion. This paper summarises the status of the IMS radionuclide network, the current suite of technology used and reviews new technology that could enhance future iterations, potentially improving the verification power of the IMS.

Trends, events and potential sources of Xe-detections in the German radioxenon network

The measurement of radioxenons (133Xe, 131mXe, 133mXe, 135Xe) in the atmosphere is a keystone for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). At the German Federal Office for Radiation Protection (Bundesamt für Strahlenschutz, BfS) activity concentrations of radioactive noble gases at several sites in Germany have been measured for more than 5 decades, initially to monitor nuclear facilities and since the mid-1990s also to support the development of measurement and monitoring systems and procedures for verification of the CTBT. Average 133Xe activity concentration in air measured daily at station RN33 of the International Monitoring System (IMS) of the CTBTO on Mt Schauinsland has decreased since 2008. Due to the decreasing radioxenon background in the atmosphere, laboratory measurements with less sensitive proportional counters developed in-house are increasingly replaced by an isotope specific β-γ laboratory system for radioxenon analyses. Six years of radioxenon activity concentrations measured with the β-γ laboratory system in weekly samples from monitoring sites in Germany are presented. Activity concentrations of 133Xe in southern Germany are now typically below 1 mBq m-3 and have decreased by an order of magnitude in the past 25 years. Magnitude and variability of 133Xe activity concentrations are generally larger in northern and western Germany compared to the south, most likely due to the prevailing wind directions in the region. Selected, but typical, periods of elevated radioxenon levels at the stations are investigated and the value of stack emission data is demonstrated.

Phase II testing of Xenon International on Mount Schauinsland, Germany

Station RN33 on Mount Schauinsland near Freiburg, Germany, is part of the International Monitoring System monitoring radioxenon in air (131mXe, 133Xe, 133mXe, and 135Xe) for verification of the Comprehensive Nuclear Test Ban Treaty. Here, we present data from phase II testing of a new system, Xenon International at RN33, July 14th, 2021 to Jan 22nd, 2022, together with SPALAX data from the same time period. Radioxenon could be detected in 473 of 719 samples, among them many multiple isotope detections. Activity concentrations of spiked and selected environmental samples were verified by laboratory reanalysis. The sensitivity of Xenon International for radioxenons is up to one order of magnitude better for the metastable isotopes than that of the SPALAX, with a shorter sampling duration of 6 h.

Modelling 85Kr datasets with python for applications in tracer hydrology and to investigate atmospheric circulation

We present a model written in python to evaluate data from comprehensive ⁸⁵Kr collection schemes comprising 11 datasets from different monitoring stations around the globe. The model is designed to (1) calculate atmospheric input functions for the application of ⁸⁵Kr as a dating tracer and (2) to investigate atmospheric circulation based on a two-box model of the atmosphere. Different functions were implemented, to (1) filter the data, (2) fit polynomials and running means, (3) extrapolate fits from the northern to the southern hemisphere, (4) calculate interhemispheric exchange times and ⁸⁵Kr emission rates and (5) export data to a csv file.

Although the model is designed to evaluate atmospheric ⁸⁵Kr datasets, some functionality and basic concepts can be applied to other dating tracers, like tritium and SF₆.•

Standardized method to systematically analyse atmospheric ⁸⁵Kr activity concentration time series for dating water and ice and to investigate atmospheric circulation.

•

Easily modifiable python script to adapt functions for similar data analysis procedures.

Monitoring atmospheric 85Kr by atom counting

Radioactive ⁸⁵Kr is a major gaseous fission product emitted into the air by the nuclear fuel reprocessing industry. Measuring atmospheric ⁸⁵Kr has applications in environmental monitoring, atmospheric transport model validation and dating of environmental water samples, including groundwater, sea water and glacier ice. We present an ultra-sensitive method for fast analysis of atmospheric ⁸⁵Kr at 10^-5 parts per trillion level. This method is based on laser cooling and trapping and is capable of counting individual ⁸⁵Kr atoms. Measurements at the 3% precision level can be made on krypton extracted from 1L STP of air with a turnaround time of 1.5 h. Moreover, we have realized a system for continuous air sampling over days to weeks. Based on this atom-counting technology and a portable air sample integrator we have realized atmospheric ⁸⁵Kr baseline monitoring in Hefei, China, for over 20 months. The technological advances presented in this work lay the ground for a global atmospheric ⁸⁵Kr monitoring network.

Evaluating 5 decades of atmospheric 85Kr measurements in the southern hemisphere to derive an input function for dating water and ice with implications for interhemispheric circulation and the global 85Kr emission inventory

In July 2015, the currently only active monitoring station for atmospheric ⁸⁵Kr measurements in the southern hemisphere went operational at the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Adelaide, Australia. Here, this new data is presented and combined with measurements from historic monitoring stations, to generate a⁸⁵Kr input function for the southern hemisphere which is crucial for the application of ⁸⁵Kr as a dating tracer for water and ice. After a linear increase in atmospheric ⁸⁵Kr concentrations between 1980 and 2005, concentrations stabilized yielding mean ⁸⁵Kr activity concentration during the Adelaide monitoring period of 1.3 ± 0.15 Bq/m³ air with slight variations indicating seasonal effects. Data from three northern hemispheric monitoring stations Schauinsland, Freiburg and Jungfraujoch of the German Federal Office for Radiation Protection (BfS), located in Central Europe are used to calculate an interhemispheric exchange time of 1.25 ± 0.24 years, using a simple box model approach. Furthermore, it is investigated whether a southern hemispheric ⁸⁵Kr input function can be calculated from the baseline of the northern hemispheric data set. A comparison between the calculated and the fitted input function shows that analytical techniques can just resolve the concentration differences, emphasising the need of southern hemispheric monitoring stations for ⁸⁵Kr. Analysing the decay-corrected input function and taking the current detection limit of low-level counting and Atom Trap Trace Analysis of 0.05 Bq/m³ air, a maximum apparent ⁸⁵Kr tracer age of 40 years can be determined in the southern hemisphere. Finally, the ⁸⁵Kr measurements are used to derive global ⁸⁵Kr emission rates which are found to be in good agreement with published emissions from nuclear reprocessing plants.

A study on 85Kr measurement with an internal gas proportional counter

A measurement method of ⁸⁵Kr using an internal gas proportional counter (IGPC) is presented in this study. The operation conditions of the IGPC were determined and optimized, including the operating voltage, pressure, sample volume, interference from other gas components such as nitrogen or air, and mitigation of the memory effect. The IGPC was calibrated using certified standards, and the detection efficiency was approximately 58% for typical samples. A lower limit of detection of approximately 0.11 MBq/m³(Kr) was achieved after counting for 5 h with 1 mL pure Kr, corresponding to the atmospheric activity concentration of 0.18 Bq/m³ (air). It was shown that the IGPC could be used effectively for measuring ⁸⁵Kr.

Half a century of Krypton-85 activity concentration measured in air over Central Europe: Trends and relevance for dating young groundwater

For almost half a century weekly samples for the measurement of krypton-85 (⁸⁵Kr) activity concentrations in surface air have been collected by the Bundesamt für Strahlenschutz (BfS), Germany. Sampling started at Freiburg (230m asl) in 1973, Mt Schauinsland (1205m asl) in 1976 and Mt Jungfraujoch in Switzerland (3454 asl) in 1990. Distinct maxima in the time series of atmospheric ⁸⁵Kr activity concentration are caused by emissions from nuclear reprocessing plants in Europe, mainly the La Hague, France, and Sellafield, UK, reprocessing plants. Between 1970 and 1990 peak activity concentrations measured in winter along the Rhine Rift in Freiburg are often higher than at Mt Schauinsland, due to emissions from the operating pilot reprocessing plant in Karlsruhe - approximately 130 km to the north - and large-scale inversions that inhibit exchange of air masses within the Rhine Rift with those at higher altitudes. From the early 1990s onwards, after the shut-down of the pilot plant, differences between Freiburg and Schauinsland are much smaller. Activity concentrations measured at Jungfraujoch are generally lower and close to baseline levels, due to its location in the free troposphere. Weekly baseline and average ⁸⁵Kr activity concentration in the atmosphere in Central Europe were modelled from almost 12,000 individual measurements at 11 stations. The baseline and average have continuously increased, interrupted by a relatively stable period between 2009 and the end of 2014 with a baseline activity concentration of about 1.39 Bq/m³. Depending on the geographical location and hydrological conditions, the modelled baseline or average ⁸⁵Kr activity concentration time series can be used as input functions for the dating of young groundwater.

Extraction and quantification system for environmental radioxenon sample analysis

A xenon dynamic adsorption setup based on granular activated carbon packed column was developed. The adsorption behavior of xenon under different experimental conditions was studied and the results used to design an appropriate adsorber column for specific conditions. The resulting radioxenon gas extraction and quantification setup was evaluated based on an inter-comparison exercise and standard sample analysis results. The results showed that the quantification setup achieves experimental rules with uncertainty of ±3%.

Simulating the mesoscale transport of krypton-85

Due to its half-life, chemical inertness and low solubility in water, radioactive ⁸⁵Kr is a valuable tracer for testing the performance of atmospheric dispersion models in simulating long-range transport of pollutants. This paper evaluates the capability of simulating the dispersion of radiokrypton emitted by a nuclear fuel reprocessing plant in north-west France. Three time periods during which elevated activity concentrations of ⁸⁵Kr in ground level air were detected in south-west Germany are chosen. Simulations have been performed using the HYSPLIT code and the European Centre for Median-Range Weather Forecasts (ECMWF) data base. Although their results show a slight trend of underestimating the measured ⁸⁵Kr concentrations, there is a significant correlation and moderate scatter between observations and simulations with about 50% of the results being within a factor of two of the measured concentrations. The simulated travel time distributions provided a valuable tool for providing additional insight into the dispersion of the tracer radionuclides and for identifying potential causes of deviations between measured and calculated concentrations.