Dynamics of fuel strings in axial flow

Abstract This paper describes a mathematical model for the dynamics of a string of fuel bundles in axial flow; each bundle is composed of fuel elements fixed between end plates, and the bundles are held together by a central support tube running through the assembly—as, for example, in the CANDU-BLW design. This is a one-dimensional model formulated in Lagrangian configuration space; a system of N bundles involves 2 N generalized co-ordinates, namely the total and the shear angular deflections of the bundles. The model takes into account elastic and gravity forces acting on the structure, as well as inviscid and viscous hydrodynamic forces. The equations of motion are obtained both by matrix operator techniques and by FORMAC, an algebraic (computer) processor. Solutions give the eigenvalues and eigenvectors of the system. It was found that fuel strings may be subject to hydroelastic instabilities—buckling and flutter—even under moderate flow conditions. Examples are given of how this model may be used as a design tool, not only to obtain a stable, but also a hydroelastically optimum, system by using flow-induced damping to advantage.