Currently, nuclear reactors of the 4th generation are being researched.
One of the possible designs of this generation of reactors is a fast ractor, cooled with lead-bismuth eutectic. Both the high density of the cooling fluid and the tight packing of fuels rods in the subassembly increase the impact of the fluid flow on the structural characteristics such as frequency and modal damping. At sufficiently high flow velocities, coupled system instabilities can occur. This contribution aims at providing the tools to numerically compute and analyze this type of instabilities.
The first part of this paper aims at providing a basic understanding of the fluid flow pattern inside a wire-wrapped rod bundle. As it shows that the bulk flow is mainly axially oriented in the inner part of a fuel assembly, the coupled behavior of a cylinder in axial flow is studied in the following parts. First the fluid forces on a rigid, but inclined cylinder are computed, as this provides useful information to understand the interaction of structural movement and fluid flow. It is shown that at sufficiently low angles of attack the fluid force normal to the cylinder depends linearly on the angle of attack and quadratically on the axial flow velocity, in agreement with recent experimental results. At higher angles of attack a stable double vortex develops in the wake of the cylinder and consequently the lift force follows a quadratic dependence on the angle of attack.
In the final part of this contribution a series of coupled structural - fluid mechanics computations are carried out to investigate the onset and development of instabilities of a clamped-clamped flexible cylinder in axial flow. The results of the computations are validated with experimental results of a silicone cylinder in water flow available in open literature.