Simulation of the Flow Rotation and Mixing in the Downcomer of a VVER-1000 Reactor

Flow rotation and mixing in a VVER-1000 reactor is investigated using two system codes with three-dimensional fluid-dynamics modeling capabilities (RELAP5-3D and TRACE) and a computational fluid-dynamics (CFD) code (FLUENT). Coarse-mesh models were developed for the system codes, and their applicability is evaluated using the test data as well as the detailed CFD results obtained. Two different temperature zone mapping schemes for comparison with the measured data are proposed and discussed.

The test is very informative when used to examine the real loop mixing taking place at a nuclear reactor. The results can be used to improve code input data for correct simulation of the phenomenon. Correctly predicting the flow mixing is very important in regard to the prediction of the local three-dimensional feedback effects depending on the vessel mixing in coupled three-dimensional neutron-kinetic/thermal-hydraulic safety analysis of reactivity insertion accidents such as the main steam line break accident.