Combustion Aerosols Formed During Burning of Radioactively Contaminated Materials: Experimental Results (NUREG/CR-4736, PNL-5999)
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Manuscript Completed: February 1987
Date Published: March 1987
M.A. Halverson, M.Y. Ballinger, G.W. Dennis
Pacific Northwest Laboratory
P.O. Box 999
Richland, Washington 99352
Operated by Battelle Memorial Institute
Division of Fuel Cycle and Material Safety
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Safety assessments and environmental impact statements for nuclear fuel cycle facilities require an estimate of potential airborne releases. Radioactive aerosols generated by fires were investigated in experiments in which combustible solids and liquids were contaminated with radioactive materials and burned. Uranium in powder and liquid form was used to contaminate five fuel types: polychloroprene, polystyrene, polymethylmethacrylate, cellulose, and a mixture of 30% tributylphosphate (TBP) in kerosene. Heat flux, oxygen concentration, air flow, contaminant concentration, and type of ignition were varied in the experiments. The highest release (7.1 wt%) came from burning TBP/kerosene over contaminated nitric acid. Burning cellulose contaminated with uranyl nitrate hexahydrate liquid gave the lowest release (0.01 wt%). Rate of release and particle-size distribution of airborne radioactive particles were highly dependent on the type of fuel burned.
Pacific Northwest Laboratory conducted two series of experiments to measure aerosols generated by burning contaminated combustibles. Data from these experiments will replace or expand models currently in FIRIN, a fire compartment computer code that predicts source term releases from fires in nuclear fuel cycle facilities.
Four materials were burned in the combustible solid experiments: polychloroprene (PC), polystyrene (PS), polymethylmethacrylate (PMMA), and cellulose. In two runs a combination of fuels was burned. Contaminant form, configuration, and concentration; heat flux; percent oxygen air flow; and type of ignition were varied in the experiments. The highest releases, up to 4.5 wt% uranium, came from PMMA with powder contaminant. Cellulose burns had the lowest fraction releases, approximately 0.01 wt%. Most of the uranium released from PMMA and PS burns came off during the melting and bubbling preignition, stage before the material burned. Uranium release rates from polychloroprene correlate well with smoke release rates. Cellulose seemed to give off uranium at a steady rate throughout the burn. Uranium release rates correlated well with mass loss rate for that material. One particle size distribution was measured for each of the fuels. For cellulosic material, more than 97% of the particles carrying uranium were less than 10 µm. However, only 16% of the particles less than 10 µm carried uranium for PC. Sizes of radioactive particles from burning contaminated PS and PMMA were between the two extremes.
A mixture of 30% tributyl phosphate in kerosene was burned in the contaminated combustible liquid experiments. The fuel was combined with nitric acid in the following four configurations:
- acid/fuel with uranium
- acid with fission products/fuel with uranium
- acid with fission products and uranium/fuel
- acid with fission products and uranium/fuel with uranium.
Weight percent of uranium airborne ranged from 0.2 to 7.1, with the highest releases coming from a burn of pure organic fuel over acid with fission products and uranium. In all configurations, the mass rate of uranium airborne seemed to be proportional to the mass rate of smoke airborne. Fission product analysis failed to provide accurate data, so it is not included in this report. One particle size distribution was measured. An aerodynamic mass median diameter of 0.6 µm was calculated for airborne particles containing uranium from burning contaminated combustible liquids.