4th generation sodium-cooled fast neutron reactors are a lasting solution to the energy crisis and global warming. In the framework of the ASTRID program, the CEA has been involved for about 10 years in the design of an industrial demonstrator. Despite the end of the project in 2019, research on SFR technology continues through innovative reactor concepts studies (Anais, Attrium, etc.).
In the design of this type of reactor, the vessel has a free surface covered with argon to reduce heat transfer to the cover. Under certain conditions, this gas can be entrained through vortices in the lower part of the reactor due to the suction of the pumps The release of a pocket of gas in the fuel assemblies is a source of increased neutron reactivity and raises safety questions. It is then essential to be able to characterize the operating conditions for which vortices can appear on the surface. This task being very difficult to apprehend by computer codes, experiments have been carried out to study the occurrence of gas entrainment in various configurations. However, the tools implemented during these test campaigns do not allow a sufficiently in-depth analysis to understand the phenomenology of vortex formation.
The purpose of this thesis is studying the vortices on the surface of the representative MICAS mock-up at a 1/6 scale of the ASTRID reactor project. Since sodium is complex to handle for experiments, a simulating fluid, water, will be used. The objective of this study is to characterize vortices in terms of occurrence, diameter, depth and velocity in order to provide elements necessary to their modeling and prediction in computer codes. This thesis work will be divided, except the bibliographical part, into 3 parts, approximatively matching the three years od the thesis: development of the characterization system based on imagery, characterization of the vortices according to different configurations (geometric and operating), implementation into a model. A priori, the vortex characterization system will be based on a network of wide-field cameras placed at different angles, but other solutions could be also considered.
The host laboratory already has the mock-up and the means of measurement (fast cameras, velocity measurement system, etc.) in order to carry out the experiments as soon as the PHD student arrives. They may be supplemented by other tools depending on the characterization system developed. Finally, an international opening could be envisaged through a collaboration with the Japanese agency JAEA.
This work opens up career prospects particularly in research centers and R&D departments in industry.