Source Location through Radiological Monitoring: System Design and Concept of Operations
Roland R. Draxler
Radionuclide Monitoring Worksho, 15-17 November 1995, ARPA Center for Monitoring Research
Introduction - The accurate determination of the location from which radiological materials may have been released based only upon downwind measurements of airborne material depends upon the material's horizontal transport as well as its dispersion. For instance, no meteorological data or model calculation can be used to determine if a radiological measurement at ground-level was the result of transport in the lower layers of the atmosphere or the upper-layers with subsequent mixing to the ground. Horizontal transport pathways can have large variations with altitude. Because of the irreversible nature of the dispersion processes, any receptor to source calculation, in contrast with a source to receptor calculation, will have greater uncertainty. Uncertainty can be reduced with more information. For instance the time of the release will narrow the number of potential meteorological scenarios that can result in transport from the source to the measurement location. More measurement information, in terms of the number of samples at a site or the number of sites with a detectable concentration, permits greater precision in the source determination through regression or vector triangulation techniques. Note that greater temporal resolution in the sampling data also improve accuracy as well as precision. Sample resolution of six hours or less is essential due to the diurnal cycle in meteorological processes. In the following sections the process of finding a source location using only air concentration measurement data will be demonstrated by example using both trajectory techniques and dispersion model simulations. A subset of the results from a long-range dispersion experiment using inert perfluorocarbon tracers conducted by the European community in 1994 to simulate a nuclear accident will be used for the demonstration. One hundred sixty-eight ground-level samplers collected 3-h samples for three days at sites in France, Germany, Poland, Hungary, and several other European countries. Twenty countries with real-time modeling capability participated by submitting dispersion calculations to the operations center at the conclusion of the experiment. The NOAA model results from those calculations will be used in this example. Although the results have not been officially released, there is sufficient information available to demonstrate source attribution calculations.
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