Experimental and numerical investigation of the coolant mixing during fast deboration transients

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, because the reactivity insertion strongly depends on boron acid concentration or the coolant temperature distribution.

Calculations for steady state flow conditions for the VVER-440 were performed with a CFD code (CFX-4). For this calculation the RPV from the cold legs inlet through the downcomer, the lower plenum and the lower core support plate was nodalized in detail. The comparison with experimental data and an analytical mixing model which is implemented in the neutron-kinetic code DYN3D showed a good agreement for near-nominal condi-tions (all MCPs are running). The comparison between the CFD-results and the analytical model revealed differences for MSLB conditions [1].

After investigating coolant mixing under steady-state nominal flow conditions, first experiments at the Rossendorf Mixing Test Facility ROCOM were performed simulating the start-up of the first main coolant pump. The reference reactor for the geo-metrically 1:5 scaled Plexiglas model is the German Konvoi type PWR. This transient is impor-tant in the case of the existence of plugs of lower borated water in one of the loops. The travelling of plugs of different size from the inlet nozzle of the started loop to the core inlet and the resulting parameter distribution at the core inlet were investigated. CFD calculations for these experiments show the same qualitative parameter distribution picture with typical maxima, which are located at the opposite of the reactor from the started loop, as it was observed in the experiments. After demonstrating the capability of the CFD code to simulate these complicated flow transients, calculations were performed for the start-up of the first pump in a VVER-440 type reactor. However, no data from transient experiments are available at the moment for this reactor type. Therefore the calculations are a first step of understanding the coolant mixing in the RPV of a VVER-440 type reactor under transient conditions.

The results of the calculation show a very complex flow in the downcomer. The injection is distributed into two main jets, the so called butterfly distribution. In addition several secondary flows are seen in various parts of the downcomer. Especially strong vortices occur in the areas below the non operating loop nozzles and also below the injection loop. The results show that a high downcomer of VVER-440 and the existence of the lower control rod chamber support coolant mixing.