The CEA and several industrial partners are involved in the development of a 4th generation reactor cooled by sodium. This sodium fast neutron reactor called ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) is a 600 MWe pool reactor type integrating cutting edge technologies. Developments are in progress especially for the vessel and the equipment. From the state of art of the previous mock-ups built in the 90s in the framework of the EFR program (European Fast Reactor), new needs for experimental means were listed for the ASTRID program. Experiments are needed for both validation of numerical codes and specific studies. In this way, a thermal-hydraulic loop, the PLATEAU facility, was developed and built at the CEA Cadarache research center. Different mock-ups, using water as a simulant fluid, can be connected to this loop to study the different issues at various reactor conditions. Currently, 4 mock-ups were identified the hot plenum, the entire pool reactor, the link between the external and the hot plenum, a part of the hot plenum at a higher scale to study specific issues. The first model connected to the PLATEAU facility is MICAS, mock-up of the hot plenum. This one is dedicated to study the flow regime, both for code validation and also engineering design development. It was designed at 1/6 scale and was built in transparent polymer to carry out some optical measurements as laser velocimetry and fast imaging. The main issues identified for the upper plenum and studied on the MICAS mock-up are the thermal interface behavior around the core, the thermohydraulic stability and the flow distribution at the IHX inlet, the thermal and the flow pattern in the UCS and the free surface flow state. The first studies in the MICAS mock-up regard the free surface flow and the gas entrainment in the IHX. The experimental conditions (inlet flow rates, temperatures) are calculated to get the same Froude number as the ASTRID reactor. The vortexes at surface are studied using 3 cameras watching the same area in 3 different plans. A statistical analysis of the vortex is performed by imaging in term of life time, depth and width. The effect of the surface flow level is analyzed. The study is carried out at experimental conditions representative of nominal ASTRID power.