Extension of the cathare-3 three-field model to large diameter horizontal pipes
Résumé
During hypothetical Loss of coolant accident in a PWR caused by a large break in the primary circuit, the reactor is uncovered and the cladding and fuel materials experience very high temperatures. Emergency core cooling is then activated and the accident enters the reflooding phase. Complex two phase flows and heat transfer phenomena occur during this phase and their analysis request detailed models, included in system codes such as CATHARE. A 3-field model (liquid field split into continuous liquid and droplet field) has been previously implemented in CATHARE 3 and applied to reflooding calculations in the core. This paper aims to describe the extension of this three-field model to the primary circuit, especially for hot legs, in order to improve the simulation of liquid droplet transport from the core to the steam generators.For vertical flows, such as in the core, a comparison of correlations for the entrainment and deposition terms against steam-water adiabatic and heated data series in tubes allowed to select a model for the film-droplets mass exchange. These correlations established for vertical flows are not applicable in horizontal geometries with large diameters such as the hot legs where, during the reflooding phase, the flow regime is dispersed or stratified with entrained droplets. In this case, we use a specific deposition term based on a model developed from air-water REGARD experiments, accounting gravity and turbulent diffusion mechanisms. A dedicated mechanistic entrainment model, also developed from REGARD experiments, completes the set of closure laws. Droplets are assumed to be torn off at wave crest and the model predicts both the wave length and the water volume entrained at the crest. These models are assessed against the REGARD experimental data, and compared with other literature models.