New aspects in modelling buoyancy driven flow at the ROCOM test facility

The influence of density differences on the mixing of the primary loop inventory and the Emergency Core Cooling (ECC) water in the downcomer of a Pressurised Water Reactor (PWR) was analyzed at the ROssendorf COolant Mixing (ROCOM) test facility. ROCOM is a 1:5 scaled model of a German PWR, and has been designed for coolant mixing studies. It is equipped with advanced instrumentation, which delivers high-resolution information for temperature or Boron concentration fields. This paper presents ROCOM experiments in which water with higher density was injected into a cold leg of the reactor model. Wire-mesh sensors measuring the concentration of a tracer in the injected water were installed in the cold leg, in the upper and lower part of the downcomer. An experiment with 15 % of the design flow rate in one loop and 10 % density difference between the ECC and loop water was selected for validation of the CFD software ANSYS CFX. A mesh with two million control volumes was used for the calculations. The effects of turbulence on the mean flow were modelled with a Reynolds stress turbulence model, Shear stress transport model including gravity terms in turbulence production and dissipation. The results of previous numerical studies showed allready that mixing is influenced by buoyancy effects. In this recent study the numerical grid was improved and extendend.
Therefore ANSYS CFX could predict the observed flow patterns and mixing phenomena a lot better than in previous studies.