Keywords: axial vibration, CANDU fuel, flexural vibration, torsional vibration.

In a CANDU reactor fuel channel, a string of fuel bundles is placed inside a pressure tube in a horizontal manner. A fuel bundle consists of several ring slender fuel elements held together through two endplates. Bearing pads as appendages are introduce to the outer ring fuel elements at several locations for transfer of weights to the supporting tube. Under operating conditions, coolant flows through the fuel bundles and brings out the heat generated by the fuel elements for steam production. As result of the flow, the fuel bundles vibrate [1], which induces sliding and material loss. To determine the amount of material loss, a dynamic model for fuel bundle vibration is required.

Fuel elements are often modelled as beams [2]. Although the dominating dynamic response of a fuel element is bending, the axial vibration and torsional vibration cannot be ignored as they are coupled with bending vibration through appendages and endplates. In this paper, a comprehensive dynamic model is developed on the basis of a mixed three-node beam finite element scheme for bending, axial and torsional vibration. In developing a dynamic model for a fuel bundle or a string of fuel bundles, it is important to model the appendages. For a fuel element without appendages, there is no coupling among bending, axial and torsional vibrations. Each natural frequency corresponds to a pure mode. However, when appendages are considered, the bending vibration in the bundle radial direction is coupled with the axial vibration; and the bending in the tangential direction is coupled with the torsional vibration. To model the effects of appendages, the tube is first divided into a number of uniform segments. Each segment is meshed using the mixed three node beam elements. The system equations of motion are obtained by direct assembly of the segment equations of motion.

The numerical results obtained for an outer fuel element consisting of fuel sheath with and without bearing pads show that the method is efficient and accurate. The dynamic model of a fuel element is being implemented into a computer code for predicting the flow induced vibration in a CANDU fuel string subjected to large scale dynamic contact.

1

M.J. Pettigrew, "The vibration behavior of nuclear fuel under reactor conditions", Nuclear Science and Engineering, 114(3), 179-189, 1993.

2

J. Veeder, M.H. Schankula, "Bowing of Pelletized Fuel Elements: Theory and In-Reactor Experiments", Nuclear Engineering and Design, 29, 164-179, 1974. doi:10.1016/0029-5493(74)90119-8

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