Analysis of the Gas Entrainment by Vortex in the Upper Plenum of the ASTRID Reactor Mock-up

International audienceThe 4th generation sodium fast neutron reactor ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) is developed by the CEA and several industrial partners. The design is in progress especially for the vessel and the equipment. From the state of art of the former sodium reactors Phenix and Super-Phenix and the EFR program (European Fast Reactor) the global and the local thermal-hydraulic issues have been listed. But the results of those previous projects are not sufficient for the current ASTRID development. New needs for experimental means were identified, especially 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 in 2012. Since experiments with sodium are complex to carry out due to the harmfulness of this liquid metal, the tests are lead in models using water as simulant fluid. Different mock-ups can be connected to this loop to study the different issues at various reactor conditions. 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 optical measurements as laser velocimetry and fast imaging.Gas entrainment by vortex is one of the issues identified and can have an impact on the vessel design. Gas occurrence in the core induces neutron effects which may leadto core reactivity variation. Gas entrainment was first investigated at the MICAS free surface using fast imaging. A tendency regarding the geometry and the operating parameters was identified. Further investigations were focused on the velocity field around the vortices in the way to develop a gas entrainment criterion. Two types of vortex occur at the free surface those which induce gas entrainment, and those which do not. By the used of the Burgers vortex model, the downward velocity gradient and the circulation are identified as parameters influencing the gas entrainment. From the tangential velocity measured by particles image velocimetry around various types of vortex, the downward velocity gradient and the circulation were calculated using the Burgers model. Results showed that the Burgers model fit the experimental tangential velocity. Depending on the gas entrainment occurrence among the different studied vortex, a value for the gas entrainment criterion was found