Analysis of Rail Car Components Exposed to a Tunnel Fire Environment (NUREG/CR-6799, CNWRA 2003-04/Revision 1)
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Manuscript Completed: November 2002
Date Published: March 2003
A.S. Garabedian, D.S. Dunn, A.H. Chowdhury
Center for Nuclear Waste Regulatory Analyses
6220 Culebra Road
San Antonio, Texas 78228-5166
C. Bajwa, NRC Project Manager
Spent Fuel Project Office
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code J5434
Rail car components recovered from the train involved in the July 18, 2001, Howard Street Tunnel, Baltimore, Maryland, train derailment and fire were used to estimate the fire duration and temperatures achieved by the components. Steel samples including sections of the box car panels and a bolt from an air brake assembly were analyzed using standard metallurgical methods to determine oxide layer thickness and the amount of metal lost as a result of the elevated temperature exposure. Aluminum alloy air brake valve assemblies, which melted as a consequence of the fire, were analyzed using a heat transfer model.
Analyses of the recovered components suggest the surface temperature of the steel reached 700 to 850 °C [1,292 to 1,562 °F] assuming an exposure time of 4 hours at the elevated temperatures. Independent assessment of fire duration could not be obtained from the steel components because the oxide-scale thickness and metal loss are dependent on both time and temperature. Several limitations to the assessment of temperature were noted including the effects of oxide-scale spalling and post-fire atmospheric exposure for a period of more than 1 year.
Thermal analysis of the aluminum air brake valve body located approximately 10 m [33 ft] from the fire (at the brake end of Car 52) indicated melting occurred early in the fire event, and the temperature achieved by this component was at least 600 °C [1,112 °F]. A similar aluminum cover located at approximately 20 m [66 ft] from the fire (at the mid-point of Car 51) was only partially melted, indicating its temperature may have reached 600 °C [1,112 °F] for a limited time, where an aluminum cover located approximately 30 m [98 ft] away from the spill site (at the brake end of Car 53) did not show signs of melting from the fire exposure. This temperature profile indicating a decrease in exposure temperature with distance away from the spill site was further substantiated by the lack of damage to other components, such as railcar exterior paint.
The analyses conducted suggest the temperatures achieved by materials present in a confined space fire are strongly dependent on the proximity of the component of interest to the fire source. Gas temperatures near the fire source were likely in excess of 800 °C [1,472 °F] for more than 30 minutes, and the reactions of components in this region were likely influenced by the direct radiation from the fire. At a distance of approximately 20 m [66 ft] from the fuel source, where the dominant mode of heat transfer was convection, the exposure was capable of generating surface temperatures as high as 600 °C [1,112 °F], however, only for a much shorter duration.
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