Basically, the safe dissipation of heat is among others an important protection objective of dual purpose casks. Gas-filled gaps within such casks can play a major role for the thermal behavior as they act as thermal barriers due to the lower heat conductivity of gaseous fluids in comparison to metallic materials. However, additional heat transmission mechanisms, such as natural convection and radiation can also occur in a gaseous medium. This leads to both an expanded modelling and a prolonged computing time in numerical simulations. Within the scope of a research project in cooperation with Swiss Federal Nuclear Safety Inspectorate ENSI a simulation tool for the fast thermal evaluation of dual purpose casks is developed which combines analytical methods and FEA. The innovation is that the thermal effects of gas-filled gaps are considered by using analytical equations. Main focus lies on the implementation of heat radiation as a non-linear transfer mechanism. Therefore, an iterative calculation process is used and the effects of the iteration number is investigated. Furthermore, the influence of radiation in comparison to pure conduction is examined depending on the gap width.