Numerical Simulation of Flow and Mixing Characteristics between Borated Coolant and De-borated Slug in Reactor Annular Down-comer

In the emergency core cooling situation, the mixing between external borated coolant and the de-borated slug occurred in the reactor annular down-comer is concerned, as heterogeneous mixing may lead re-critical. In this paper, the flow and mixing characteristics in the reactor annular down-comer was analyzed using Fluent 18.0. At the entrance cross section, velocity decreases as azimuthal angle increases due to the collision against the wall. At lower cross section, the velocity gradient becomes subdued. The three-dimensional streamline indicates that the entrance fluid spreads around the cylinder radially. The mixing between the boric solution and the water was conducted and the concentration distributions at cross sections in the experimental measurement and the CFD (computational fluid dynamics) simulation are consistent. From the time evolution of three-dimensional concentration, the boric acid solution stagnates in the upper space of the annular down-comer when inlet Reynolds number is low. Increasing inlet Reynolds number is favorable to uniform mixing.