CFD calculation of a mixing experiment in the VVER-1000 reactor model

A study of the mixing of a deborated slug during start-up of the first coolant pump in VVER-1000 type reactors was performed with the commercial CFD package CFX-5.7 using experimental results of the 1:5 scaled test facility. The numerical grid model was generated with the grid generator IC4C (ICEM-CFD) and the preprocessor CFX-5 and contains 2.5 Mio. tetrahedral elements. The CAD geometry and the grid model include the inlet nozzles, the downcomer incl. spacer elements and the lower plenum with the elliptical sieve plate and internals. Parts of the complicated lower plenum structures were modeled with the porous media approach in CFX-5, which uses directed resistance coefficients.

The output of the Best Practice Guidelines for optimizing the numerical studies of boron dilution transients in different reactor types was used in the numerical simulation. The discretisation scheme in time was set to second order Euler and in space to High Resolution. The SST turbulence model was taken.

The modeled deborated slug first arrives at the core inlet of the reactor pressure vessel in the neighboring areas at the outer case of the starting loop. Later the slug is almost homogenously distributed over the core inlet plane. The minimum averaged deboration value in the experiment and the CFD calculation is in good agreement when using a constant wall temperature (72%). The existing time offset in the numerical calculation could not been clarified.