Gas entrainment in the mock-up of the upper plenum of a sodium cooled reactor

Abstract The CEA intends to develop 4th generation sodium fast neutron reactors. From the state of art of the former operating sodium reactors PHENIX and SUPER-PHENIX and the past program EFR (European Fast Reactor) most global and the local thermal–hydraulic issues have been identified. However, the results of those previous projects do not fulfill for the current development requirement and safety standard. New experimental means are needed, especially for both validation of numerical codes and specific studies. In this way, a thermal–hydraulic loop, the PLATEAU facility, was developed and erected at the CEA Cadarache research center in 2012. Since experiments with sodium are complex to carry out, the tests are led 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 engineering design development. It was designed at 1/6 scale from a 600MWe reactor design 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 lead to core reactivity variation. Gas entrainment was investigated at the MICAS free surface using fast imaging. A tendency regarding the geometry and the operating parameters was identified. More 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.