Numerical modelling of a buoyancy driven flow in a reactor pressure vessel using CFX-5

The influence of density differences between the primary loop inventory and the Emergency Core Cooling water on the mixing in the downcomer of a Pressurised Water Reactor (PWR) was analyzed at the Rossendorf coolant mixing test facility ROCOM. ROCOM is 1:5 scaled model of a German PWR designed for experimental coolant mixing studies. It is equipped with advanced instrumentation, which delivers high-resolution information characterizing either temperature or boron concentration fields. A series of experiments was performed on mixing under the influence of density differences. Water with higher density was injected into the cold leg of the reactor model. Wire mesh sensors measuring the concentration distribution of a tracer added to the injected water were installed in the upper and the lower part of the downcomer. The data were used for CFD code validation. An experiment with 5% of nominal, constant flow rate in one loop (magnitude of natural circulation) and 10% density difference between ECC and loop water was selected for validation of the CFX-5 code. The turbulence was modeled with the BSL Reynolds stress turbulence model and a mesh with two million control volumes was used.

The results of the experiment as well as of the numerical calculations show, that buoyancy effects dominate the mixing. While at higher mass flow rates (close to nominal conditions) the injected slug propagates mainly in circumferential direction around the core barrel, the buoyancy effect partly suppresses this circumferential propagation. The ECC water falls down in an almost straight streamline and reaches the lower downcomer sensor position directly below the affected inlet nozzle. Therefore, the density effects play an important role during natural convection with ECC injection in PWRs. It was important to point out, that CFX-5 is able to cope the specific flow pattern and mixing phenomena.