On Coherent Structures, Flow-Induced Vibrations, and Migratory Flow: In Liquid Metal Nuclear Reactors

Fulvio Bertocchi

Research output: ThesisDissertation (TU Delft)

53 Downloads (Pure)


Flows in rod bundles are common to many industrial applications such as heat exchangers or some types of nuclear reactors. The core of many classes of nuclear reactors can be easily sketched as a bundle of rods, the fuel pins, inmmersed in an axial ow of coolant that removes the heat produced by the fission reaction. Coupling this geometry to an axial ow can trigger periodical vortices, known as large coherent structures or gap vortex streets, that move on both sides of the gaps between the rods. By crossing the gap (cross-ow), these vortices may enhance the heat removal mechanism, thus improving the performance of the reactor. However, coherent structures cause velocity oscillations in the ow that may induce vibrations of the fuel rods, leading to their long term damage. The length (or wavelength) of coherent structures is a key parameter for understanding the interplay between these vortices and the vibrations that may be triggered on the rods. Their wavelength determines the frequency of the velocity oscillations in the uid, hence of the external force imposed on the rods. One of the reactor designs belonging to the next generation (Gen-IV) of nuclear reactors is the Liquid Metal Fast Breeder Reactor (LMFBR). This reactor has the fuel rods in the core arranged in a hexagonal matrix. In this design, a wire is helicoidally wrapped around each fuel rod to keep them separated from each other. The presence of the wire diverts part of the more turbulent ow from the bulk towards the gap between the rods, where the ow would be otherwise less turbulent. This enhances the heat exchange and avoids hot spots on the fuel cladding. A phenomenon known as migratory ow has been observed in rod bundles with wire spacers. In the presence of migratory ow, the uid is diverted from the gap towards the main subchannel and it bends against the helicoid path of the wire, thus leading to a very complex ow, where part of the uid follows the wire direction and part moves against it, away from the gap. Although this behaviour was _rst observed years ago, the governing mechanism is not clear yet. Explaining migratory ow is thus a fundamental step towards a general understanding of the mixing and mass transfer phenomena in rod bundles in the presence of helicoid wires.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Delft University of Technology
  • Kloosterman, J.L., Supervisor
  • Rohde, M., Supervisor
Award date27 Jan 2020
Publication statusPublished - 2020


  • Cross-flow
  • Coherent structures
  • Fluid-Structure Interactions
  • Hexagonal bundle
  • Wire wrapped
  • Laser Doppler Anemometry
  • Particle Image Velocimetry


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