Impact vibrations of guide thimbles in nuclear fuel assembly

In nuclear fuel assemblies of pressurized water reactors, guide thimbles (GTs) secure a safe control rod drop. The control rod fall into the GT should be as fast as possible to provide prompt reactor shutdown. The GTs are long thin-walled tubes that are dynamically excited by fuel assembly (FA) components motion caused by pressure pulsations of coolant. GTs are embedded in spacer grid cells and sleeve (SL) with radial clearances. Vibration of FA components, caused by the FA support plates motion in the reactor core, possibly generates impact forces between GT and spacer grids. The presented method introduces an original approach to mathematical modelling and simulation analysis of GT nonlinear vibrations respecting impact and friction forces at all the contact points between GT and spacer grids. The dependence of maximal dynamical lateral GT deformations on radial clearances and stiffnesses of spacer grid cells is analysed. The method is applied to GT in the hexagonal-type FA used in the VVER-type reactors.

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