We recently studied the burn behavior of a polyurethane (PU) foam-filled impact limiter — a design for both the Mixed-Oxide Fresh Fuel Package and the Hanford Unirradiated Fuel Package (HUFP) that have been certified for shipment of their authorized contents by the U.S. Nuclear Regulatory Commission and the U.S. Department of Energy, respectively. In this paper, we examined the mechanisms of thermal degradation of PU foam by reaction type and flame temperature. Evidence suggests that, for an air-tight impact limiter, the pyrolysis reaction dominates initially in the limiter enclosure. The pyrolysis generates a large amount of highly flammable gases, creating the conditions necessary for a subsequent combustion reaction near the vent holes where air is abundant. The coupled heat release from the combustion of these flammable gases and oxygen drives the jet flames to burn at a very high temperature. The three-dimensional finite-element analysis code, ANSYS Mechanical, was used to model the HUFP package (with the PU foam-filled impact limiters) and compute the temperatures near the seal region of the containment boundary of the package. We examined the effects of various parameters — including fire duration, chimney flow, foam thickness loss, and jet flame temperature — on the thermal performance of the package. The results indicate that, under the simulated conditions, the O-ring seal temperature near the packaging containment boundary rose to varying degrees, but it did not exceed the seal temperature limit of 400°F (204°C). The remaining foam thickness is critical to maintain package safety.

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