Flow-Induced Vibration of a Candu Fuel String
A comprehensive vibration model is developed in this thesis to simulate the dynamical behaviour of a string of CANDU fuel bundles subjected to unsteady flow of coolant inside a pressure tube. The large-scale dynamical system of interest consists of several hundreds of solid and deformable components interacting with the coolant flow, with each other and with the pressure tube through frictional contact at various interfaces. In the first stage of this thesis, the three-node higher-order mixed beam finite elements and the nine-node thick plate finite elements are employed to model the fuel bundles. The equations of motion of the fuel string system are discretised in the time domain using the Newmark integration scheme.
The CANDU fuel string behaviour is highly nonlinear and the total number of potential frictional contact exceeds thousand sets. In the second stage, a numerical scheme for efficiently handling three-dimensional friction and contact is developed. The incremental displacement is used to relate gaps with contact forces and the problem is formulated to be a linear complementarity problem (LCP). The accuracy and robustness of the presented method is tested against several numerical simulations and experimental results available in the literature.
To find the unsteady fluid forces acting on the fuel string two comprehensive computational fluid dynamic (CFD) models that include endcaps and spacer pads are developed. The models are solved using the large eddy simulation (LES) scheme. The coolant unsteady pressure is integrated over fuel rods surfaces and unsteady fluid forces are found and used as the excitation sources for fuel string vibration. The power spectral density (PSD) of unsteady fluid forces are obtained and peak frequencies are identified. A FORTRAN code consisting of approximately 13000 lines is developed and validated at different stages for use in Canadian nuclear industry to simulate the vibrational behaviour of a 12-bundle fuel string and the material loss during reactor normal operations. Free vibration analyses of a CANDU fuel string are also performed and natural frequencies of the system are obtained.