Analysis of Structural Materials Exposed to a Severe Fire Environment (NUREG/CR-6987)

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Publication Information

Manuscript Completed: January 2009
Date Published: February 2009

Prepared by:
D.S. Dunn1, R.E. Shewmaker (retired)
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

A.H. Chowdhury
Center for Nuclear Waste Regulatory Analyses
6220 Culebra Road
San Antonio, TX 78228

C. Bajwa, NRC Project Manager

Prepared for:
Office of Nuclear Material Safety and Safeguards
US Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code J5608

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On Sunday morning, April 29, 2007, a gasoline tanker truck carrying 32,500 L [8,600 gal] crashed while heading south along Interstate (I) 880 in Oakland, California. The single-vehicle accident occurred in the MacArthur Maze, which is a network of connector ramps that merges highways I-80, I-580, and I-880. The resulting fire led to the collapse of the I-580 overpass, directly above I-880, approximately 17 minutes after the fire started (based on a review of surveillance camera video of the fire). Early news media reports of the accident and subsequent collapse cited sources at the accident scene estimating that temperatures reached as high as 1,650 °C [3,000 °F] (above the melting point of steel); however, initial inspection of the remaining girders along with photographic documentation revealed that the steel girders did not experience any melting. After the damaged sections were dismantled, removed, and placed in a storage yard, steel girder samples were obtained and analyzed. Samples from the tanker truck were also collected. The main objective of the work reported here was to examine the collected samples and estimate the temperatures reached during the fire. The microstructures of the girders were examined for phase transformations and microstructural alterations that would identify exposure to elevated temperatures. Samples were collected from girders that were known to be close to the fire and from locations that were away from the fire. Weld samples collected near the fire were transformed, whereas no transformation was observed on corresponding samples that were away from the fire. Thermal exposures of selected samples were conducted to determine the effect of temperature on the microstructure of the stiffener welds and the grain size of the girders. Based on a comparison of the samples collected from hot regions and the test samples that were thermally exposed, the girders close to the fire reached a minimum temperature of 850 °C [1,562 °F]. Grain-size measurements suggest that the hottest sections reached temperatures of approximately 1,000 °C [1,832 °F]. Paint degradation was also found to be dependent on the conditions of thermal exposure. At a temperature of 750 °C [1,382 °F], the paint was completely destroyed, which agrees with the material evaluation results of the girders closest to the fire. Near the truck, several alloys did not melt including brass, copper, and cast iron, which have melting points of 930, 1,084, and >1,177 °C [1,706, 1,983, and >2,150 °F], respectively. Melting of multiple truck components manufactured from several aluminum alloys {melting points of 590 to 720 °C [1,094 to 1,328 °F]} was noted. Hardness data and microstructures of hardened steel bolts suggest an exposure temperature between 700 and 750 °C [1,292 and 1,382 °F]. The observed oxide spalling on the steel truck frame suggests a temperature exceeding 500 °C [932 °F]. Based on the information obtained from the materials analyses, the temperatures of the samples collected near the tanker truck on the I-880 road bed were between 590 and 930 °C [1,094 and 1,706 °F]. The I-580 girders, located directly above the fire where hot gases accumulated and where direct flame impingement occurred, reached a temperature between 850 and 1,000 °C [1,562 and 1,832 °F].

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