Thermal Properties Evaluation of Insulation Overpack Containers for UF6 Transport

1\\vo types of insulation are currently in use in overpacks utilized to protect UF6 during transport. Phenolic foam has been used in the family of UF6 overpacks under U. S. Department of Transportation Specifications 20PF-1, 20PF-2, 20PF-3, 21PF-1, and 21PF-2 and modifications since the mid 1960s. Recently new generation overpacks have been developed which utilize polyurethane foam. A comparative analysis was made of the thermal characteristics of the phenolic foam with that of polyurethane in two different densities. Thermal properties of materials vary with temperature with the relationship being a complex interaction of basic materials properties, processing variables, and environmental conditions. Typically, the thermal conductivity of a material increases with increasing temperature, and adequate thermal models of materials systems or structures require temperature dependent thermal properties such as conductivity. In the event of an overpack container exposed to a fire as a heat source, the thermal properties of the materials of construction will vary with temperature which varies with time. Environmental interactions will result in material properties changes which will be reflected in changes in thermal properties. Thus, analytical models that do not incorporate time, temperature dependent material thermal properties may not adequately evaluate a structural system thermal response.