Water experiment on gas entrainment in reactor vessel using 1/1.8th scaled model; Evaluation of onset condition and mechanism

Kimura, Nobuyuki; Ezure, Toshiki ; Nakayama, Okatsu; Tobita, Akira; Ito, Masami*; Kamide, Hideki 

An innovative sodium cooled fast reactor has been investigated in a frame work of the FBR feasibility study. One of the thermal hydraulic issues in this design is gas entrainment at free surface in the reactor vessel. Dipped plates (D/P) are set below the free surface in order to prevent the gas entrainment. We performed an 1/10th scaled model water experiment for the upper plenum of reactor vessel and flow optimization was done to reduce flow velocity near the free surface. However, previous studies showed that the gas entrainment depends on model scale. Then an 1/1.8th scaled model was also planned to confirm the phenomena in an enough large model. As a test section, 90 degree sector and region between the free surface and the D/P was modeled by 1/1.8th scale. Boundary conditions at D/P gaps and radial cross sections of sector ends were obtained by the 1/10th scaled full sector model. The gas entrainment was not observed in the model under the velocity condition of reactor full power operation at water levels higher than 3% of the normal height from the D/P in the case of double D/Ps geometry (current design). As for the case of single D/P geometry, it was found by the visualization and the velocity measurement that the gas entrainment occurred as the circumferential velocity increased at the water level higher than 50% of the normal height condition. It is shown that the gas entrainment in the reactor vessel will be eliminated in the current design approach.