Accidental Release of Chemical Warfare Agents: Risk Assessment Implications Accounting for Dense Vapor Cloud Behavior. Michael A. Lazaro, Heidi M. Hartmann, and Young-Soo Chang, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439
Traditionally, releases of nerve and blister agents in hypothetical accidents have been modeled by assuming passive or neutrally buoyant vapor cloud behavior with a Gaussian horizontal and vertical distribution. However, the formation of a dense vapor cloud can occur in the case of a sufficiently large and rapid release of chemical nerve (e.g., VX and GB) or vesicant (e.g., HD and HT) agent and/or in combination with the relatively high agent molecular weights and associated vapor densities. Vapor densities, for example, range from about 5 (GB) to over 9 (VX) times denser than air at STP. Releases of this type are typical of the bounding accidents considered in most of the environmental assessments conducted under the Army’s chemical demilitarization program. The formed dense vapor cloud will exhibit distinctly different behavior, depending on the degree of gravity slumping governed by the release characteristics and conditions and chemical properties, than that exhibited by a passive release. Significant reduction in vertical in-cloud mixing and the generation of a radial horizontal flow due to gravity-induced spreading are key characteristics of dense gas behavior. The physics generate a much flatter and initially significantly wider cloud than would be formed with a neutrally buoyant release. Heavier-than-air releases may influence exposure times because of relatively slow cloud movement, even under moderate winds. The implications of these phenomena are examined by comparing results generated with a passive versus a dense-gas dispersion model. The significance of the influence on exposure levels and duration and downwind distances of the various toxicity endpoints of interest will be presented. In addition, results from sensitivity analyses with a variety of release scenarios will be shown to illustrate the influences of source strength and agent properties on cloud behavior and exposure consequences.
Work supported under a military interdepartmental purchase request from the U.S. Department of Defense, U.S. Army, through U.S. Department of Energy contract W-31-109-Eng-38.
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