Atmospheric Radionuclide Monitoring: A Swedish Perspective

Abstract

Nuclear weapons tests can be carried out in different environments such as the atmosphere, underground, below the sea surface and space. A lthough there is some overlapping and latitude for synergy, each test environment in a test ban context requires its own typical verification technique. For underground there is seismic, for underwater hydroacoustic, and for the atmosphere radionuclide monitoring, the subject of the present paper. Space and especially deep space is difficult to monitor but the risk of such tests is quite small due to cost limitations. Apart from space it is important that the total verification system is well balanced over the different environments. There is little point in covering, e.g., underground with a dense and effective seismic network if the atmosphere is much less effectively covered. A violator would naturally look for the environment and the point in that environment within his/her reach that minimizes the risk of being disclosed. In reality this means that the boundary layers between the main environments are of special interest as the efficiencies of respective verification techniques wane near these surfaces. The boundary layers should therefore be especially in focus when networks and methods are designed. In radionuclide monitoring this is very clear. It is not the “normal” 20 kt atmospheric nuclear explosion that the system should be built for. Such an explosion is very easy to detect and would require just a few fairly simple control posts around the globe. The design criterion must be instead that in addition a well evaded explosion, e.g., a low-yield test carried out in a rain storm a few meters above the surface of the open sea far away from any land mass, shall be detected with a set level of confidence. Sweden has repeatedly suggested at the Conference on Disarmament (CD) in Geneva that a verification regime for a Comprehensive Test Ban Treaty (CTBT) should include a global network of some 100 high volume sampling stations for radionuclide monitoring. Dubbed ISAR for “International Surveillance of Atmospheric Radioactivity”, it should embrace microbequerel perm3 sensitive sampling and counting techniques for particulate debris and a state of the art noble gas surveillance system. Sweden has focused on a set of xenon isotopes but we are also open to explore argon-37 and krypton-85; the latter perhaps at a later stage when the present quite high background hopefully has decayed away. Our views on radionuclide monitoring are very much drawn from our long experience in chasing radioactive traces from underground nuclear exlosions. Even a very faint signal detected in another country is a violation of the 1963 Partial Test Ban. Along with more general remarks and comments on an international system the present paper describes the Swedish system and it also gives a short account of test ban violations detected by this national system in the past.