Radionuclide Monitoring System
Joachim Schulze
The radionuclide monitoring system has three components, particulate detection at 80 stations, noble gas detection at 40 out of the 80 stations and 16 laboratories for additional analysis of samples.
The objective of the radionuclide network of stations is to detect radionuclides from a nuclear weapons test as a proof that a nuclear explosion has taken place. The combination of particulate and noble gas detection will provide a very high probability to identify an event as nuclear weapons test. Even the differential diagnosis against releases from fresh-fueled nuclear reactors should be possible.
To install and operate a radionuclide station is much more sophisticated than doing gamma spectroscopy in a well established laboratory. State-of-Health Monitoring, continuous power supply, maintenance etc. is a significant part of the whole undertaking in order to assure 95% availability of data meeting all requirements.
The Particulate Radionuclide Stations
Particulate Radionuclide Stations consist basically of an air sampler, a highly sensitive Gamma Ray Detector, control and auxiliary equipment.
Figure 1 A Radionuclide Station (RN46, Chatham Islands, New Zealand)
Six particulate systems are installed and currently sending data through the CTBTO GCI system. Five of them establish the South Pacific Mini-network with two stations in Australia, two in New Zealand and one in Cook Islands. The certification visits have been performed successfully and the stations will probably be certified in November 2000. Spectra and State-of-Health data can be checked in Vienna anytime. The stations monitor Central and South Australia, the Tasman sea and New Zealand.
The 6 stations sending data together with 8 other quite advanced installations form 18% of the particulate network.
The detection limit of 10-30 µBq/m³ for Ba-140 particulates is a quite ambitious requirement and it is not expected that this limit can be significantly improved in the near future.
Noble Gas Systems
The four recently developed noble gas systems undertake currently a demonstration in Freiburg/Germany. The experiment is quite successful. The systems seem to achieve their goals and there is quite good agreement between measurements of the four systems.
Figure 2 The four automatic Xe Detection Systems being tested
ARSA (USA)	SPALAX (France)	ARIX (Russian Federation)	Swedish System (Sweden)
There is a first interesting result from these noble gas measurements with high time resolution in Freiburg. Every few weeks a quite sharp Xenon cloud passes this area, which was not as visible before because of lower time resolution.
Figure 3 ARSA Xe measurement results in the Freiburg experiment
There are no noble gas systems installed at Treaty locations but it is envisaged to have the 4 systems installed at four sites for an experiment to estimate the properties under realistic conditions in different climates.
The required detection limit for Xe-133 is 1mBq/m3. Noble gas systems could be able to reach sensitivities around 100 µBq/m³. For this a Beta-Gamma coincidence counting is needed and probably enhanced Xe purification methods. However, every enhancement makes operation and maintenance of the systems more difficult and needs to be balanced against the advantages of higher sensitivities.
Radionuclide Laboratories
The laboratories are existing and need to be certified. Procedures taking into account national accreditation are under development. Basis for preparation of the PTS document on certification is the draft document of ISO 17025.
Part of the laboratory certification will be the evaluation of spiked sample and intercomparison tests. The first spiked sample test is just under preparation and will be completed by the end of the year 2000. 16 radionuclide laboratories have expressed their willingness to participate in the test.
Outlook
It is expected to have more than 20 particulate stations certified by the end of 2001 and more than 30 stations by the end of 2002 which will then form about 40% of the total particulate network.
To locate an event is the weak point of radionulide monitoring compared to other technologies. Meteorologists estimate an uncertainty of 1000 by 1000 km² to locate an event by backtracking the path of detected radionuclides. The Canadian Meteorological Centre Montreal and other Regional Specialized Meteorological Centers might support backtracking of detected radionuclides in the future and improve this uncertainty somewhat. However, it is not the task of the radionuclide network to locate events. Backtracking could better serve for other technologies to reduce the area to focus on.