Investigation of coolant mixing in VVER-440/213 RPV with improved turbulence models

B. Kiss, A. Aszódi, Budapest University of Technology and Economics, Hungary

21st Symposium of AER on VVER Reactor Physics and Reactor Safety (2011, Dresden, Germany)
Nuclear applications of three dimensional thermal hydraulics

Abstract

A detailed and complex RPV model of VVER-440/213 type reactor was developed in
Budapest University of Technology and Economics Institute of Nuclear Techniques (BME
NTI) in the previous years 4. This model contains the main structural elements as inlet and
outlet nozzles, guide baffles of hydro-accumulators? coolant, alignment drifts, perforated
plates, brake- and guide tube chamber and simplified core. With the new vessel model a series
of parameter studies were performed considering turbulence models, discretisation schemes,
and modeling methods with ANSYS CFX.
In the course of parameter studies the coolant mixing was investigated in the RPV.
The coolant flow was ?traced? with different scalar concentration at the inlet nozzles and its
distribution was calculated at the core bottom. The simulation results were compared with
Paks NPP measured mixing factors data (available from FLOMIX project 3). Based on the
comparison the SST turbulence model was chosen for the further simulations, which unifies
the advantages of two-equation (k-? and k-?) models 4.
The most widely used turbulence models are Reynolds-averaged Navier?Stokes
(RANS) models that are based on time-averaging of the equations. Time-averaging filters out
all turbulent scales from the simulation, and the effect of turbulence on the mean flow is then
re-introduced through appropriate modeling assumptions.
Because of this characteristic of SST turbulence model a decision was made in year
2011 to investigate the coolant mixing with improved turbulence model as well. The hybrid
SAS-SST turbulence model was chosen, which is capable of resolving large scale turbulent
structures without the time and grid-scale resolution restrictions of LES, often allowing the
use of existing grids created for RANS simulations.
As a first step the coolant mixing was investigated in the downcomer only. Eddies are
occurred after the loop connection because of the steep flow direction change. This turbulent,
vertiginous flow was investigated with the SAS-SST turbulence model. The first results of
this research are described in the paper.

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