Combination of analytical and numerical methods for the fast thermal evaluation of transport and storage casks

Motivation Transport and storage casks (T/S-casks) for nuclear fuel elements have to fulfil a wide range of require-ments, including the safe dissipation of decay heat. In order not to decrease the strength of the used materials, the maximum temperature within the cask has to be limited. Gas filled gaps can have a big influence on the heat dissipation, as their conductivity is much lower than the conductivity of solid materials. At the same time, as convection and radiation exist as further heat transfer mechanisms next to conduction, fluids are more difficult to consider within Finite-Element-Analysis(FEA). Due to the small geometrical extents of gas filled gaps in comparison to the overall cask dimensions and the three existing heat transfer mechanisms, FE-simulations considering such gaps can bevery time consuming. Therefore, in cooperation with the Swiss Federal Nuclear Safety Inspectorate ENSI, a simulation tool is developed, which considers the thermal effects of gas filled gaps while increasing calculation speed.Combination of analytical and numerical methods for the fast thermal evaluation Therefore, analytical equations are used for a gas filled gap between inner and outer part of T/S-casks. This is done for both upright positioned casks during interim storage (annular gap geometry) and reclining casks during transport (crescent gap geometry). In order to fasten the FE-simulation, a special boundary condition called Thermal Gap Condition (TGC) was developed, which is able to consider all heat transfer mechanisms in the gap without meshing it. This is done by using an adapted Lagrange-Multiplier method, which is able to connect parts by using analytical equations. For an annular gap geometry of upright positioned casks it was already shown in previous work that fast simulations are possible, whereby radiation was neglected. Now, main focus lies on reclining gaps with an alternating gap width by considering radiation at the same time. This means, a novel TGC is necessary, as different heat transfer equations have to be applied.