Methodology for thermal analysis of spent nuclear fuel dry cask using CFD codes

As the capacity in spent fuel pools is depleted, dry storage systems have been increasingly used. The key parameter for fuel cladding behaviour during dry storage is the temperature; that cannot surpass 400 degrees C based on NRC regulation. This paper proposes a new methodology to realistically calculate the thermal behaviour of cask/fuel system. In order to obtain realistic values, the methodology is divided in two steps. In the first step the outside part of the cask is simulated (from the inner surface of the interior cavity to the surrounding air). The second step is a fully fledge simulation of the inner cask (from the clad of the fuel assemblies to the exterior surface of the cask). This two-step approach shows some advantages compared to previous methodologies: the first step calculates a realistic boundary condition to apply in the second step and estimates the impact that the cask has on the near environment while the second step provides the temperature distribution for every single clad in each assembly. To ensure the validity of the methodology, several sensitivity analyses have been conducted, a grid independence study to the discretization error, a turbulence modelling has been applied to the outside and inner model and finally a study of the gaps between the canister and the inner cylinder has been carried out. Of these studies the gaps proved the most important one with changes in PCT of up to 28 degrees C. The changes in the size of the gaps not only change the overall temperature distribution but also the heat transfer mechanisms and the flow paths followed by the helium. (C) 2019 Elsevier Ltd. All rights reserved.