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Decomposition of Sulfur Hexafluoride by C-4 Explosive, 01-9070

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Principal Investigator
James R. Scott

Inclusive Dates: 01/09/98 - 08/06/98

Background - Sulfur hexafluoride (SF6) is a nontoxic gas that is chemically very stable due to its symmetrical, octahedral molecular structure. Consequently, SF6 is commonly used as a tracer for indoor air-exchange studies, atmospheric dispersion modeling of air pollutants, and as a surrogate to quantify the chemical destruction efficiencies achieved by hazardous waste incinerators. Literature references cite studies showing decomposition losses of less than 1 percent at temperatures up to 1,050 degrees. SwRI had a project to experimentally verify the capture efficiency achieved by a vapor control structure (VCS) designed to contain chemical warfare agents released by the inadvertent detonation of a buried munition during ordnance retrieval operations. SF6 was a logical surrogate to use for these VCS studies; however, mass-balance measurements showed significantly less SF6 being present after the tests compared to the quantities loaded into the simulated munitions. Therefore, the potential degradation of SF6 by the detonation of the high-explosive charge had to be examined prior to conducting further tests on the VCS.

Approach - The simulated chemical munitions consisted of one-inch diameter PVC pipe sections filled with a specific mass of liquefied SF6. The C-4 plastic explosive was placed on the exterior of the munition. The mass ratios between the C-4 explosive, the PVC pipe, and the SF6 in the degradation tests were comparable to those being utilized in the VCS tests. The simulated munitions were placed within a blast chamber capable of total containment of materials generated by the detonation of up to 100 grams of C-4 explosive. The chamber was equipped with high-pressure valves to permit simultaneous venting of the explosion gases and air purging following the detonation of the C-4 charge. A commercially available gas chromatograph (GC) equipped with a thermoconductivity detector (TCD) was selected for analyzing the SF6 vapor concentrations. After detonation of the C-4, the SF6 vapor and explosion gases were purged from chamber and collected in a sample bag. The contents of the sample bag were analyzed using the GCTCD to determine the SF6 concentration. The volume of air collected in the bag was then multiplied by the SF6 x concentration to yield the mass of SF6 that survived the detonation.

Accomplishments - Baseline tests were conducted to verify the capability of the test methodology to effectively recover a known quantity of SF6 released into the blast chamber. The baseline test results showed recovery efficiencies of 100 6 percent. Infrared and gas chromatography analyses of the blast gases created by the detonation of the simulated munitions showed significant concentrations of decomposition products from the C-4, SF6, and PVC pipe. These byproducts included: carbon disulfide, carbonyl sulfide, hydrogen chloride, carbon monoxide, and hydrogen cyanide. At a C-4/SF6 mass ratio of 0.75, the highest mass ratio used in the VCS studies, the GC-TCD analyses failed to detect the presence of any residual SF6 in the gases collected from the blast chamber. As the C-4/SF6 mass ratio decreased, the recovery of SF6 increased. The survivability of SF6 was still only 11 percent at a C-4/SF6 mass ratio of 0.25, recovery rates exceeding 75 percent were obtained at the two lowest C-4/SF6 mass ratios in the test (0.02 and 0.05). Results of these degradation experiments are being currently used for the design of the simulated munitions and the air monitoring protocols for a new series of VCS experiments.

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