Improving the noble gas detection capability for the verification of the Comprehensive Nuclear-Test-Ban Treaty

Abstract

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans all nuclear weapon tests on Earth. For its verification, a worldwide network of sensors, called the International Monitoring System (IMS), is being deployed (90% completed in 2024) to continuously monitor the Earth. One of the methods used in the IMS to verify compliance with the CTBT is to monitor the atmosphere for radioactive traces released by a nuclear weapon test. For underground nuclear weapon tests, noble gases are the most likely radioactive signal to seep out of the test cavity. Accordingly, the IMS contains sensors to monitor the atmosphere for traces of radioactive xenon (radioxenon) isotopes, which have a high fission yield and well-suited half-lives. Next to continuously monitoring the Earth, the CTBT foresees On-Site Inspection (OSI) as an ultimate verification measure when a nuclear weapon test is suspected. One of the techniques currently developed for OSI is the measurement of 37Ar in atmospheric and soil air at a suspected test site.

The verification of the CTBT by radioxenon monitoring in the IMS is facing a major issue: the presence of a significant background from nuclear installations. Different approaches are considered to minimize the impact of the radioxenon background on CTBT verification, amongst which: i) improve the discrimination capability of the monitoring sensors and ii) reduce the radioxenon emissions of nuclear installations. Concerning the discrimination capability of the sensors, nuclear weapon tests can be discriminated from the civilian background based on the Xe isotopic composition and backward atmospheric transport modelling of the air sample. For backward atmospheric transport modelling, shorter sampling durations would allow to better identify the origin of radioxenon observations and increase the discrimination capability. Regarding emissions, most of the background arises from a few installations, extracting radioactive isotopes from irradiated uranium targets, distributed worldwide. For OSI, the current processes for sampling and measuring 37Ar are complex (multiple adsorbents) and energy intensive (cryogenic temperatures). Research is required on simplified and less energy demanding processes.

In this work, we investigate the use of new porous adsorbents and new process designs to: i) improve the discrimination capability of radioxenon monitoring systems, ii) simplify 37Ar sampling and iii) reduce radioxenon emissions at nuclear installations. We demonstrate that silver-exchanged zeolites or alike (AgZs), in particular Ag-ETS-10 and Ag-ZSM-5, are much more volume-efficient and selective for collecting and separating Xe from dry air than any other porous material currently reported. Our results reveal that AgZs have the potential to simplify significantly radioxenon monitoring systems and could thus reduce the sampling duration for a better discrimination capability. Similarly, we show that both adsorbents are also much more volume-efficient than activated carbon for trapping radioxenon in nuclear installations. They are thus also promising candidates to reduce radioxenon emissions by replacing the current activated carbon-based mitigation systems. In addition, we establish that both adsorbents can withstand radiation levels up to 100 MGy and at least 40 thermal regeneration cycles without significant loss in Xe adsorption performance. Finally, we show that Ag-ETS-10 can be used as a single adsorbent, operating at room temperature, to separate Ar from air. However, to reach a sufficiently high Ar yield for OSI measurement, the separation would need to be performed at a lower temperature. Our results indicate that operating the Ag-ETS-10 at -25°C would improve the Ar separation and could thus be the way forward to simplify, by using a single adsorbent and working well above cryogenic temperatures, the current Ar separation process for 37Ar measurements during OSI.