Researchers at Polytechnique Montréal (Montreal, QC, Canada) were inspired by shark gills to invent a biomimetic nozzle that reduces the grid-to-rod fretting of nuclear fuel assemblies. This novel safety-enhancing idea holds great potential to minimize wear of critical components, such as the spacer grids supporting fuel rods. To help evaluate the performance of the "shark nozzle", the research team used a Mikrotron Cube4 GigE high-speed camera that documented the dynamic behavior of the rods while they were subjected to jet flow.
Reducing Jet Flow
Jet flow plays an essential role in heat transfer within nuclear pressurized water reactors. In these types of reactors, core periphery baffle plates are machined with loss-of-coolant accident (LOCA) holes and slots that help mitigate the effects of a severe LOCA failure.
Under non-emergency normal operation, however, LOCA jet cross-flow can induce vibrations in fuel rod assemblies near the baffle before being mixed with the surrounding fluid. This phenomenon can cause damage to the fuel assemblies, as reported by the International Atomic Energy Agency. Another complication linked to jet flow is called "baffle jetting". Baffle jetting may induce fuel rod vibrations that leads to fretting and wear of the fuel cladding that contain the fission products. Both phenomena underscore the importance of understanding jet cross-flow induced vibrations and the need for developing a nozzle that will improve the mixing of jet flow with ambient flow.
The Value Of Biomimicry
Biomimicry looks to nature for inspiration. An engineer will observe a trait in nature and copy it to improve human technology and design. Examples of biomimicry include the development of strong adhesives that imitate the toes of geckos or the invention of Velcro fasteners by duplicating the small hooks that allow plant burrs to attach to surfaces.
In the Polytechnique Montréal study, the researchers simulated the secondary lamellae in a shark's gill slits. They conducted a series of tests by circumferentially attaching either ten or fifteen very thin (1 mm) shark-like fins inside a circular nozzle case.
Sharks breathe through a series of five to seven gill slits behind their heads on both sides of their bodies which absorb oxygen from the water as it flows over them and is then flushed out. The lamellae is the mechanism that enables sharks to eject water after oxygen has been removed in ram ventilation.
The Polytechnique Montréal researchers found that by copying the gill slits, they could enhance mixing between the jet flow and the surrounding coolant. As with shark gills, the nozzle decays the jet velocity faster, resulting in fewer vibrations.
Experimental Set-Up
A high-speed imaging system was needed to capture axisymmetric rod bundle vibrations in still water while the rods were excited by circular cross-flows of varying jet velocities. The arrangement called for a total of 36 rods organized in 6x6 bundles to be tested.
At the center of the high-speed imaging system was a Mikrotron Cube4 CMOS 1.3MP digital camera with global shutter, configured at 1280 x 1024 pixel resolution and acquiring images at a speed of 400 frames per second (fps). To obtain full-field rod array vibrations the camera was positioned on the top panel of the test section so the viewing plane was aligned to the rods’ axis. The camera's high frame rate and compact size made its use completely non-intrusive during testing.
To determine the rod bundle vibration at each jet flow velocity, a MATLAB image processing algorithm was generated to detect the center of each rod in the bundle. The developed code detected the edges of rods based on the color difference between the shiny aluminum top rod surface and the dark background test section.
Results
Results obtained by the research team were very promising. They showed that the shark-inspired biomimetic nozzle delayed the critical velocity at which unstable vibration occurs in a rod bundle by 20%. In addition to delaying instability, a vibration amplitude reduction of 85% was observed by using the new nozzle instead of a standard circular nozzle. A better vibration response was obtained with 15 fins than 10 fins which is consistent with the very confined flow channels of a shark’s secondary lamellae. This vibration amplitude reduction can contribute to increased safety for reactors during normal operations.
