Certain fuel element positions of the FRJ-2 research reactor at the Nuclear Research Center, Jülich, exhibited violent flow-rate fluctuations. These were caused by periodic vortices that were formed directly below the inlets of the nozzle seats in the core grid plate at the positions in question. The shallow plenum and multiple inlet pipes through which the water is fed to the plenum were identified as the causes of the vortex formation. To eliminate the vortex formation, flow splitters of a “star”-shaped section were installed in the grid plate nozzles. In this manner, it was possible to prevent the fluctuations and utilize the available flow margin for an increased reactor output.