[en] An advanced loop-type sodium-cooled fast reactor has been developed by the Japan Atomic Energy Agency. The upper internal structure (UIS) above the core is a key component where control rod guide tubes are housed. A radial slit is set in the UIS to simplify the fuel-handling system and to reduce the reactor vessel diameter. A high-velocity upward flow is formed in the UIS slit. This slit jet influences thermal hydraulic issues in the reactor vessel. A water experiment was carried out to understand the flow field in the UIS, which is composed of the control rod guide tubes and several horizontal perforated plates with a slit. A refractive index matching method was applied to visualize the flow in such a complex geometry. Velocity measurement using particle image velocimetry showed that the velocity in the UIS slit was accelerated by the multiple slits and kept at a high value at the mid-height of the reactor upper plenum. A numerical simulation was carried out for this complex geometry of the UIS to obtain an adequate simulation method. A comparison between the experimental and analytical velocity profiles showed that the numerical simulation is highly applicable. (author)