In a BWR, the cooler feedwater is injected in the downcomer of the reactor vessel where it mixes with the recirculation water that is separated from the generated steam at the exit of the core. In order to avoid high temperature fluctuations on the reactor vessel and the structures present in the downcomer, this mixing shall be performed in a controlled manner. In order to improve the design of the feedwater spargers in Olkiluoto Unit 1 and 2 in Finland, the thermal mixing of the feedwater with the reactor water in the vicinity of the spargers has been studied with transient Computational Fluid Dynamics (CFD) calculations. The studies have been performed at three different power levels. Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations have been performed to simulate about 40 s of the water mixing. The time history of the variations of the water temperature on the structures in the downcomer has been analyzed. The calculations have been run for several designs of the replacement spargers. The designs have been assessed using transient statistics of the temperature fluctuations. Minimizing the maximum amplitude of the temperature variations, the so-called peak-to-peak value, has been a design objective.