As part of the development of technical bases which support the extended storage and transportation of used nuclear fuel, Pacific Northwest National Laboratory (PNNL) has implemented detailed thermal analysis methods for the calculation of best estimate temperatures of Dry Cask Storage (DCS) systems. These methods were used to model a NAC International MAGNASTOR® cask located at the Catawba Nuclear Station (Catawba) which is operated by Duke Energy This cask has one of the highest total heat loads of any DCS system loaded in the U.S.. Using best estimate operational data from the utility, fuel assembly and cask design information from the vendors, and decay heat estimates from Oak Ridge National Laboratory (ORNL), PNNL calculated the maximum cladding temperature of 307?C. This is well below the 400?C regulatory guidance limit.