Fluctuating pressure calculation induced by axial flow trough mixing grid S. Delafontaine and G. Ricciardi ∗ CEA Cadarache DEN/DTN/STRI/LHC 13108 Saint Paul-Lez-Durance Cedex, France Abstract Significant nuclear fuel assembly design innovations are regularly incorporated to meet the highest expectations in terms of safety, performance and flexibility. Despite the continuous up- grades of fuel designs and materials, the fuel failure rate has not markedly decreased during the last decade, partly because of higher burn-up, longer cycle lengths, mixed cores... Safety aspects give rise to the reduction of contamination and the fuel cladding represents the first con- tainment barrier. It is then necessary to maintain its integrity under all operating conditions. Flow-induced vibrations of fuel rods generating grid-to-rod fretting are the dominant fuel leaker mechanism worldwide. This paper presents a Computational Fluid Dynamics (CFD) simulation of the flow around a fuel rod and the corresponding computed fluid force excitation spectra. 1 Introduction A fuel assembly in PWR core is made of fuel rods hold by spacer grids. The contact between rods and grids is ensured by springs and dimples. Although the primary function of the grids is to maintain the fuel rods, they also increase the heat flux between the cladding of fuel rods and the pressurized water flowing through the core, by the mean of mixing vanes. The flow is highly turbulent in the core, with a Reynolds number about 500 000, the mixing vanes generate strong swirl in the sub- channels. These flow conditions induce vibrations of the rods and relative displacement between rods and grids (Pa¨ ıdoussis, 1982, 2003). The vibration of the fuel rods can induce fretting wear damage of the cladding. The wear is a multi-parameters dependent phenomenon, mechanical properties of the rods and the grids (stiffness, damping, friction coefficient,...), grids cells clearance (Rubiolo and Young, 2009), and flow excitation will affect it (Pettigrew, 1992). Since the cladding provide the contamination primary barrier, insuring the integrity of the cladding is a major safety issue. Engineers need tools to estimate wear damage in core conditions. A large effort has been devoted to develop mechanical modelling of supported beam subjected to axial flow (Kim and Kim, 2005; Choi et * Corresponding author: e-mail: guillaume.ricciardi@cea.fr Accepted Author Manuscript by Nuclear Engineering and Design 2012 1