Analysis of coupled two-phase natural circulation loops by developing HEM and DFM based numerical models

The coupled two-phase natural circulation loop based passive systems find numerous applications, particularly in nuclear reactors where safety is paramount. However, the loops coupling, system conditions (specific to the reactor type), and inherent coupled loops feedback make their dynamics more complex. Hence, the models used should be capable of predicting such intricate coupled dynamics for precise forecasting of passive systems performance. With this objective, the present analyses deliberate the transients of such coupled two-phase loops linked to coupling configurations and operating conditions. Generalized two-phase models based on Homogeneous Equilibrium (HEM) and Drift Flux (DFM) with flashing, wall and fluid conduction, water-steam property formulation, etc., are developed and employed. Validation studies substantiate the prediction capability of these models for various cooler-heater orientations. Subsequently, predicted the start-up transients of vertically and horizontally coupled two-phase loops. The analyses revealed that coupling configurations, heater-cooler orientations and system conditions significantly influence the coupled loops. Further, the influences of various geometrical and operating parameters on the dynamics of coupled loops are investigated. It elucidated the differences in the parameter's effect on single-phase and two-phase coupled loops. The study also identified the vital parameters strongly influencing the coupled loops based on sensitivity analysis. The models and findings will be applied to evaluate the performance of coupled loop-based passive safety systems in nuclear reactors.