Investigation of coolant mixing in pressurized water reactors at the Rossendorf mixing test facility ROCOM

For the analysis of boron dilution transients and main steam line break scenarios the modeling of the coolant mixing inside the reactor vessel is important. The reactivity insertion due to overcooling or deboration depends strongly on the coolant temperature and boron concentra-tion.

The coolant mixing in the downcomer and lower plenum depends significantly on the con-struction of the reactor vessel and flow conditions are different in each reactor type. In the German Konvoi type reactors a complicated flow picture with large vortexes was obtained which is typical probably also for the European Pressurized Water Reactor (EPR). The models and assumptions for the coolant mixing description used in the coupled neutron kinet-ics/thermohydraulic system codes have to be validated on experimental data from the Rossendorf coolant mixing model and detailed computational fluid dynamics (CFD) calcula-tions.

The requirements concerning quality of the experimental results need a non-pressurized test facility operating with water (room temperature). The reactor vessel is made of Plexiglas for flow visualization and LDA measurements. It has to dispose of separately controllable main coolant pumps in each loop. The measurement instrumentation has to be designed for the nec-essary spartial and time resolution.

First experiments at the test facility ROCOM show that the mixing in the downcomer and the lower plenum is incomplete (sector formation) at nominal conditions (all loops are operating). At the moment when the plug is entering the downcomer and also the core inlet, two maxima are clearly to be seen. This phenomenon is caused by a re-circulation area below the inlet noz-zle.

To simulate real accident scenarios and for code validation an extensive test programme is planned. The experimental results will be compared against CFD calculations for analysing the dependencies of the power output of the reactor on the temperature and/or boron distribution.