Dry storage casks are used to store spent nuclear fuel after removal from the reactor spent fuel pool. Even prior to the Fukushima earthquake of March 2011, dry storage of spent fuel was receiving increased attention as many reactor spent fuel pools near their capacity. Many different types of cask designs are used, and one representative design is the TN-24P spent fuel cask, a non-ventilated steel cask with a shielded exterior shell and lid. The cask is typically filled with an inert gas such as helium, argon or nitrogen. In this paper, Computational Fluid Dynamics (CFD) calculation results for the thermal performance of the TN-24P cask using the commercial CFD software STAR-CCM+ are presented. Initial calculations employ a common approach of treating the fuel assemblies as conducting porous media with calibrated volume-averaged properties, and comparison to existing measured temperature data shows good agreement. One of the fuel assemblies is then replaced with a more accurate representation that includes the full geometric detail of the fuel rods, guide tubes, spacer grids and end fittings (flow nozzles), and the results shown are consistent with the initial analysis, but without the need for the assumptions inherent in the porous media approach. This hybrid modeling approach also permits the direct determination of important results, such as the precise location of peak fuel cladding temperatures (PCTs), which is not possible using the more traditional porous media approach.

The Spallation Neutron Source (SNS) is an accelerator-based neutron source at Oak Ridge National Laboratory (ORNL). The nuclear spallation reaction occurs when a proton beam hits liquid mercury. This interaction causes thermal expansion of the liquid mercury which produces high pressure waves. When these pressure waves hit the target vessel wall, cavitation can occur and erode the wall. Research and development efforts at SNS include creation of a vertical protective gas layer between the flowing liquid mercury and target vessel wall to mitigate the cavitation damage erosion and extend the life time of the target. Since mercury is opaque, computational fluid dynamics (CFD) has been used to visualize the general behavior of a protective gas layer arising from various delivery and retention concepts as a guide for design of experimental efforts. Recent advancements in capacity for large scale CFD modeling via the high performance compute systems of ORNL now enable high-fidelity simulation approaching full geometric scale. Accordingly, in this study, CFD simulations of three dimensional, unsteady, turbulent, two-phase flow of helium gas injection in flowing liquid mercury over textured walls are carried out for target design purposes with the commercially available CFD code STARCCM+. The Volume of Fluid (VOF) model is used to track the helium-mercury interface. Different combinations of conical pits and V-shaped straight grooves at different orientations with respect to the gravity vector are simulated at the SNS proton beam window to increase the helium gas holdup. Time integration of predicted helium gas volume fraction over time is done for the design alternatives considered to compare the gas coverage and average thickness of the helium gas layer.