Analysis of buoyancy-driven flow in the ROCOM test facility

Mixing of coolant with different boron content and/or different temperature in the primary system of pressurized water reactors (PWR) plays an important role during normal operation and under accident conditions [1]. In emergency core cooling (ECC) situations after a loss of coolant accident, cold ECC water is injected into the hot water of the cold leg and downcomer. Temperature distributions near the wall and temperature gradients in time are important to be known for the assessment of thermal stresses (Pressurized Thermal Choc).
Numerous experiments were realized in the test facility ROCOM of the Helmholz Zentrum Dresden-Rossendorf, Dresden, Germany, ([2], [3]) to investigate the effects of density differences between the primary loop inventory and the ECC water on the mixing in the downcomer. The mass flow rate in the hot leg was varied between 0 and 15% of the nominal flow rate, to keep it at the order of magnitude of natural circulation flow. The density differences between ECC and loop water were varied between 0 and 10% in order to simulate cold water injection. In 2005, an experiment with 5% flow rate in loop 1 and 10% density difference between ECC and loop water were compared to CFD calculations with Trio_U [1]. The Froude number is Fr=0.366, which is labelled as density-dominated. The same experiment is analyzed here with the TrioCFD code, taking into account 10 years of code and hardware development.