Keywords: fluid structure interaction, nonlinear flow effects, squeeze-film, spent fuel storage racks, seismic response, differential-algebraic equations.

Fluid effects induce strong coupling between immersed nuclear fuel racks, when they are subjected to earthquake excitations (see for instance [1,2,3,4]). Therefore, during a seismic event, spent fuel storage racks may bend, slide, twist and uplift. Undoubtedly understanding the complex dynamic behaviour of immersed spent fuel assemblies storage racks under earthquake is of prime importance for the safety of nuclear plant facilities.

In the near-past we introduced a simplified linearized 2-D multirack model for the fluid-coupled vibratory responses of nuclear fuel racks [5]. Time-domain simulations of the system responses to seismic excitations were also produced and, despite the simplifications introduced, the model yielded qualitatively similar predictions when compared with other recently published work. In [6] the above-mentioned model was generalized to account for nonlinear flow effects namely squeeze-film and dissipative effects, connected with very large amplitude responses. This new nonlinear model for fluid-coupled vibrations of spent nuclear racks is based on the main simplifying assumptions: (i) 3-D effects were neglected, (ii) small gaps between the fuel assemblies and between these and the container, when compared with the longitudinal length scales. From these assumptions, a simplified flow inside the channels was postulated such that the gap-averaged velocity and pressure fields were described in terms of a single space- coordinate, for each fluid channel and the flow forces on each rack were obtained by exact integration. Although algebraically involved, the proposed approach can be automatically implemented on a symbolic computer environment, leading to a system of DAE's which is then solved through an adequate time-step integration solver.

In the present paper, our nonlinear model [6] is explored by performing two sets of numerical simulations. In the first set, using a small-storage pool with a single centered rack the significance of the squeeze-film and dissipative effects are exposed. In the second set of numerical simulations, the response to a seismic excitation of a storage pool with racks regularly stored is tested.

Regardless of the fact that neglecting 3-D flow effects can lead to an overestimation of the true flow added mass effects [2,10] this model model can produce realistic predictions of the displacements and squeeze-film forces taking place on immersed spent fuel racks, when excited by a seismic event, contributing to a better understanding of the complex dynamic behaviour of such systems.

1

Broc, D., Queval, J. and T. Chaudat, Fluid-structure interaction for nuclear spent fuel racks, Proceedings of PVP , Emerging Technologies in Fluids, Structures and Fluid/Structure Interactions. Pressure Vessel and Piping Conference, Seattle, USA, PVP 414-2, 171-177, July, 2000.

2

Stabel, J. and Ren, M., Fluid-structure-interaction for the analysis of the dynamics of fuel storage racks in the case of seismic loads, Nuclear Engineering and Design, 206, 167:176, 2001. doi:10.1016/S0029-5493(00)00431-3

3

Zhao, Y., Wilson, P. R. and Stevenson, Nonlinear 3-D dynamic time history in the reracking modifications for a nuclear power plant, Nuclear Engineering and Design, 165, 199-211, 1996. doi:10.1016/0029-5493(96)01197-1

4

Hinderks, M., Ungoreit, H. and Kremer, G., Improved method to demonstrate the structural integrity of high density fuel storage racks, Nuclear Engineering and Design, 206, 177-184, 2001. doi:10.1016/S0029-5493(00)00432-5

5

Moreira, M. and Antunes, J., A simplified Linearized Model for the fluid-coupled vibrations of spent nuclear fuel racks, Journal of Fluids and Structures, 16, (7), 971-987, 2002. doi:10.1006/jfls.2002.0459

6

Moreira, M. and Antunes, J., A nonlinear model for the fluid-coupled vibrations of spent nuclear fuel racks, Proceedings of the FIV2004, 2004.

7

Antunes, J. and Piteau, P., A Nonlinear model for squeeze-film dynamics under axial flow, Proceedings of the ASME Pressure Vessel and Piping Conference, Atlanta, USA, July 2001, 420-2, 53-62, 2001.

8

Brenan, K. E., Campbell, S. L. and Pezold, L. R., Numerical Solution of Initial-Value Problems in Differential-Algebraic Equations, SIAM, 1996.

9

Roberts, A. J., Differential - algebraic equations solver DAE, http://www.mathworks.com/support/ftp/diffeqv5.shtml, 1998.

10

Ren, M. and Stabel, J., Comparison of different analytical formulations for FSI between fuel storage racks, Transactions of the 15th International Conference on Structural Mechanics in Reactor Technology, SMIRT-15, Seoul, Korea, Pp. 15-20, August, 1999.

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