Thermal properties evaluation of insulation in overpack containers for UF6 transport

Two 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 US Department of Transportation Specifications. 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. The need to incorporate temperature dependent thermal properties into analytical finite element codes led to an experimental program to measure thermal properties, principally thermal conductivity, for the 21PF-1 overpack phenolic foam. The thermal conductivity-temperature relationship for this insulator has been measured from room temperature to over 1000F. An alternate UF6 product cylinder overpack container has recently entered service; an overpack design incorporating polyurethane as the primary thermal resistance in place of the phenolic foam in a 2 1/2 ton cylinder overpack. Elevated temperature thermal properties measurements for the polyurethane material system are presented