During a loss of coolant accident in a nuclear power plant, the primary circuit runs out of water. That leads to a total or partial core uncovery. The core consists of bundles of fuel rods protected by metal claddings. During such accidents, the clad cooling is no more ensured regarding the fact that less and less water is available to extract the heat produced in the fuel. Safety systems automatically activate in order to refill the primary circuit during the so-called “reflooding” phasis. When water enters the core, fuel rods are still very hot and prevent the liquid from rewetting the clad. Many droplets appear in the core while the quench front progresses. Theses droplets exchange heat and momentum with vapor and structures in the core: fuels rods and spacer grids. These grids maintain fuel rods and increase the mixing in the core with the help of mixing vanes. Exchanges are very dependent to the droplet size that is quite difficult to estimate, especially as grids have a high influence on it. A dry grid can split a droplet into many little ones, making the mean diameter decrease downstream the grid. In the opposite, it has been experimentally observed that a wet grid (with a stable liquid film on it) is responsible of an increase of the mean diameter.
The objectives of this thesis are then to take into account effect of grids, phase change and most influent phenomena on the mean droplet diameter in the core during the reflooding. As the droplet flow is polydispersed and is not accurately described by a unique diameter, the effect of this polydispersion should be studied.
In order to validate the models developed in the thesis, the work will lean on many experiments especially RBHT (Rod Bundle Heat Transfer) that simulate the reflooding in a core and give many informations on the droplet diameter and wall heat exchanges. The main objectives are then to develop physical models (grids effect, break-up, coalescence, droplet entrainement and deposition) and validate them after implementation in a calculation tool with the help of experimental data available in literature. This work is of a great interest in an academic and industrial point of view. The student will learn to build a physical model, use and develop scientific calculation tools, what will be an investment for his career.