Daniel Chester

First opened for signatures in the late 90’s, the Comprehensive Nuclear-Test-Ban Treaty (CTBT) prohibits the testing of nuclear weapons in all environments. In order to verify compliance of its member states, the CTBT Organisation operates and maintains a global network of detection systems known as the International Monitoring System (IMS). A range of measurement techniques are used in the verification process, including seismic, hydroacoustic and infrasound, however the detection of radioactive particles in the atmosphere remains the ‘gold standard’ for the attribution of nuclear explosions. Once complete, 80 certified radionuclide stations are to be positioned strategically around the world, 40 of which will be equipped with noble gas (NG) technology.

Radioxenon (radioactive isotopes of the noble gas xenon) is of specific interest to radionuclide analysts at the United Kingdom National Data Centre (UK NDC) due to a variety of factors. Xe-133, Xe-133m, Xe-131m and Xe-135 are all sufficiently long-lived to be transported in the atmosphere and measured before they decay below the detection limits of the detector systems, as well as being volatile and chemically inert, making them the most likely materials to escape from an underground environment. Measurements of multiple isotopes of radioxenon, along with other nuclides present at the time, can be used to deduce information about fission processes during weapons tests.

This is complicated, however, by the presence of a global radioxenon background, mainly produced and added to by civil nuclear sources, such as nuclear power plants (NPPs) and medical isotope production facilities (MIPFs). The vast majority of ‘hits’ at IMS stations are therefore not the result of treaty violations, but instead can be attributed to releases from NPPs and MIPFs during their day-to-day operations. Described here is an automated process for assigning detections of Xe-133 at IMS stations to simulated emissions from known nuclear sources using Atmospheric Transport Modelling (ATM). The UK NDC employs the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to assess the relative contributions that simulated emissions from known emitters would make to IMS locations.

‘Xe-133 events’ - plumes of Xe-133 passing over IMS stations - are periodically assigned by an automated detection algorithm. An event is defined as a time duration during which the activity concentration (mBq of radioactivity per m3 of air) of Xe-133 sits above the minimal detectable concentration (MDC) for at least two collection periods. An ATM simulation is then automatically assigned to the event if an emission from a known nuclear emitter contributes to the IMS station during this time.

Interactive virtual maps (IVMs) are used to visualise a variety of information regarding the event, ATM simulations, wind direction, historical data and so on. IVMs are automatically generated upon the assignment of a new event, rapidly providing specialists a wealth of information when analysing a new occurrence. The software can be used efficiently on past data, and now all events across the global IMS network since 2010 have been mapped.

Shown here is an event assigned to the IMS NG station in Takasaki, Japan. In this case, the station is seen to have been sensitive to emissions from several local emitters: Takahama, a power plant in the south of Japan, along with two locations in the Democratic People’s Republic of Korea (DPRK); the nuclear test site and the nuclear materials production facility at Yongbyon. Immediately, features such as recent nuclear tests, the history of detections at Takasaki, and possible source locations using backward trajectory plots are all available for viewing. It is possible that these detections could be a result of radioxenon releases from DPRK, and therefore can be further interrogated.

Daniel Chester
Ministry of Defence ©Crown Copyright 2020/AWE