In the context of safety of Pressurized Water Reactor (PWR), the Primary Coolant Loss Accident (LOCA) is of great importance. The LOCA is a hypothetical accident caused by a breach in the primary circuit. This leads to a pressure decrease in the primary circuit and a loss of water inventory in this circuit. Its resulting in heating of the fuel rods, which must remain limited so that damage to the fuel does not reduce cooling of the reactor core and prevents meltdown.

To remedy this situation, safety injection is activated to inject cold water, in the form of a jet, into the horizontal cold branch, which is totally or partially dewatered by the presence of pressurized steam. A stratified flow appears in the cold branch, with significant condensation phenomena in the vicinity of the jet and at the free surface in stratified flow zones. Numerous experimental and numerical works have been carried out on interfacial transfers at the free surface on rectangular and cylindrical cross-sections. CFD simulations of condensation at the free surface are carried out with the Neptune_CFD code, used by FRAMATOME, EDF and CEA. Currently, three models for heat transfer at the free surface are available in Neptune_CFD. These models have been established from a reduced number of simulations (DNS, LES and RANS) on rectangular configurations that remain far from the configuration of interest. Flows in a rectangular section tend to be parallel, whereas flows in a cylindrical section are three-dimensional.

The aim of this thesis is to improve the modeling of free surface condensation in a cylindrical cross-section configuration. Initially, a bibliographic study will be carried out on the free surface flow map, as well as on experimental works devoted to characterizing of interfacial area, mean interfacial velocity, turbulence terms in the vicinity of the free surface and heat transfer. In parallel, a new model will be developed in relation to the various improvement elements identified, and the associated validation carried out. Work is also planned to upscale two-phase CFD simulations to a macroscopic CATHARE approach. This up-scaling method will be based on Tanguy Herry's thesis work.