Large-eddy simulation, fuel rod vibration and grid-to-rod fretting in pressurized water reactors

The grid-to-rod fretting (GTRF) problem in pressurized water reactors is a flow-induced vibration phenomenon that results in wear and failure of the cladding material on fuel rods. Grid-to-rod fretting is responsible for over 70% of the fuel leaks in pressurized water reactors in the U.S. The GTRFinduced wear process involves turbulent flow, mechanical vibration, tribology, and time-varying irradiated material properties in complex fuel assembly geometries. This paper presents a new approach for predicting GTRF induced fuel rod wear that uses high-resolution large-eddy simulation to drive nonlinear transient dynamics computations. The GTRF fluid-structure problem is separated into the simulation of the turbulent flow field in the complexgeometry fuel-rod bundles, calculation of statistics of the resulting fluctuating structural forces, and the nonlinear transient dynamics analysis of the fuel rod. Implicit large-eddy simulation is used to compute the time-dependent fluid forces downstream of a Westinghouse designed spacer-grid with mixing vanes: The turbulent flow simulation procedure is first validated using a 5×5 rod bundle configuration and comparing simulated and experimentallymeasured velocities immediately downstream of the mixing vanes. Next, ✩LA-UR-14-28497 ∗Corresponding author. Email addresses: [email protected] (Mark A. Christon), [email protected] (Roger Lu), [email protected] (Jozsef Bakosi), [email protected] (Balu Nadiga), [email protected] (Zeses Karoutas), [email protected] (Markus Berndt) Preprint submitted to Journal of Computational Physics April 13, 2016 highly-instrumented large-eddy simulations of the 3 × 3 rod bundle flow using a sequence of four meshes with increasing resolution is performed. The forces on the central fuel rod downstream of the mixing vanes are seen to fall in an asymptotic regime at the finer three resolutions, and this feature of the numerics is exploited to obtain the limiting values of the fuel rod forces using Richardson extrapolation. Furthermore, uncertainty in the fuel rod forces due to turbulent intermittency are estimated using time-series data from the one second turbulent computations. Finally, the simulated fluid dynamic forces on the fuel rod are used in nonlinear transient dynamics analyses of a full length fuel rod with spacer grids to obtain fuel rod acceleration, displacement and fuel rod wear. Robustness of the behavior of both the structural forces computed from the turbulent flow simulations and the results from the transient dynamics analyses highlight the progress made towards achieving a predictive simulation capability for the GTRF problem.