LES CFD Simulation of Unsteady Fluid Forces in Tube Bundle

Significant nuclear fuel assembly design innovations are regularly incorporated to meet the highest expectations in terms of safety, performance and flexibility. Despite the continuous upgrades 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 containment 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. The swirls generated by the mixing vanes are reproduced, and the corresponding resulting fluid force excitations are extracted. The computed spectra are compared to envelop spectra obtained by tests on fuel assembly mock-ups in our experimental facilities.