MODELING OF POLYURETHANE FOAM THERMAL DEGRADATION WITHIN AN ANNULAR REGION SUBJECT TO FIRE CONDITIONS

Nuclear materials are placed in shielded, stainless-steel packaging for storage or transport. These drum-type packages often employ a layer of foam, honeycomb, wood, or cement that is sandwiched between thin metal shells to provide impact and thermal protection during the hypothetical accidents, as those prescribed in the Code of Federal Regulations (10 CFR 71.73), to provide impact and thermal protection during hypothetical accidents. This work discusses the modeling of the thermal degradation of a polyurethane foam within an annular region during an 800°C fire. Measurements and analysis by Hobbs and Lemmon [in Polymer Degradation and Stability, Vol. 84, pp. 183–197, 2004] indicate that at elevated temperatures, polyurethane foam exhibits a two-stage, endothermic degradation. The first stage produces a degraded solid and a combustible gas; the second-stage reaction consumes the degraded solid and produces another combustible gas. As a result, during a prolonged fire a gas-filled void develops adjacent to the outer metal shell and grows inward toward the inner shell and the containment vessel. As a result of radial symmetry in the drum geometry, a one-dimensional finite-difference model is constructed for the annular foam region. Heat flux is applied to the inner surface to model the decay heat of the containment vessel contents. Thermal radiation and convection boundary conditions with a specified environmental temperature are applied to the outer surface. The material and reaction rate properties determined by Hobbs and Lemmon are applied to the foam. The annular region temperature and composition are determined as functions of radius and time, after the environmental conditions are changed from room temperature to those of an 800°C fire. The effects of surface-to-surface radiation between the package’s outer skin and the un-degraded foam, the reaction rate reduction due to material damage during the reaction, are evaluated for fires lasting 20 hours. The peak package liner temperature caused by a 30-minute fire is predicted to be 129°C, well below the short-term limit for the containment vessel seal (377°C).