A significant aspect of the economic performance and safety of a nuclear reactor involves maintaining the integrity of the fuel rods, which are susceptible to Turbulence-Induced Vibrations (TIV) resulting from the axial flow of the coolant. TIV can instigate severe repercussions, including structural damage such as fatigue and wear. TIV can be studied numerically, using Fluid-Structure Interaction( FSI) simulations. However, high-resolution approaches are computationally too expensive to use for complex FSI simulations, while Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations severely underpredict the displacement amplitudes of the vibrations as they only resolve the mean flow. Evolving from this shortfall, this paper focuses on a recently developed Anisotropic Pressure Fluctuation Model (AniPFM). This model generates a synthetic velocity fluctuations field, which is used to solve for the pressure fluctuations. Using this model, together with URANS, is a possible way to simulate the excitation mechanisms of TIV of fuel rods in a computationally cheaper way. While previous research has highlighted the potential of this model, there are parameters, definitions, and constants whose impacts on the model are not yet fully understood. Therefore, a comprehensive effort is undertaken to fine-tune the model, optimize its performance, improve understanding of it and further validate it. This is done by applying AniPFM to both pure flow and FSI cases, using high-resolution numerical and experimental data as reference and for comparison. With the optimized model, a substantial decrease in average difference from the experimental data is found for the FSI case under consideration, when compared with the unoptimized version of AniPFM.

The vibration of fuel rods induced by the fast flowing coolant is a known challenge to nuclear power plant designers and operators. If not dealt with adequately, the vibrations could lead to undesirable wear and tear of the cladding, which, in turn, could result in fuel rod failures and costly unplanned outages of the power plant. Hence, knowledge of these vibrations is important in the design phase. Due to the increase in computational power, numerical tools are increasingly often used to assess flow-induced vibrations. As these vibrations are a result of the local axial turbulent flow, scale-resolving methods are typically required for accurate predictions. Such methods, though, are usually computationally too expensive to use for industrial nuclear applications. Medium resolution turbulence models such as URANS generally cannot be used as they only resolve the average flow conditions and not turbulent fluctuations. To overcome this limitation of the URANS models, an Anisotropic Pressure Fluctuation Model (AniPFM) has been developed, which is an improved version of the earlier developed isotropic version. Compared to the isotropic model the generated synthetic turbulence is now anisotropic and is correlated in time based on the transport and decorrelation of turbulence. The current paper gives an overview of the new AniPFM, and presents results for a first fluid–structure interaction test case, demonstrating it can indeed induce and sustain vibrations of a rod, and that results are improved compared to the old model.

In nuclear fuel rod bundles, turbulence-induced pressure fluctuations caused by an axial flow can create small but significant vibrations in the fuel rods, which in turn can cause structural effects such as material fatigue and fretting wear. Fluid-structure interaction simulations can be used to model these vibrations, but for affordable simulations based on the URANS approach, a model for the pressure fluctuations must be utilised. Driven by the goal to improve the current state-of-the-art pressure fluctuation model, AniPFM (Anisotropic Pressure Fluctuation Model) was developed. AniPFM can model velocity fluctuations based on anisotropic Reynolds stress tensors, with temporal correlation through the convection and decorrelation of turbulence. From these velocity fluctuations and the mean flow properties, the pressure fluctuations are calculated. The model was applied to several test cases and shows promising results in terms of reproducing qualitatively similar flow structures, as well as predicting the root-mean-squared pressure fluctuations. While further validation is being performed, the AniPFM has already demonstrated its potential for affordable simulations of turbulence-induced vibrations in industrial nuclear applications.