VVER-1000 Coolant Transient Benchmark - OECD Nuclear Energy ...

CFD RESULTS AND DISCUSSION
and not the conceptual design geometry. It was tested in TRIO_U LES validation runs and in a
sensitivity study (Bieder, 2004, 2005, 2007) which indicated the importance of using the actual vessel
geometry for reliable results.
4.2 Comparative analysis
For the assessment of CFD results, in accordance with the Best Practice Guidelines (Mahaffy, 2008;
Menter, 2002) we consider:
• the code/model ability to reproduce the main features of the flow in the final stabilised state,
such as sector formation and the angular turn of the loop flow;
• choice of target variables and metrics as described in Chapter 3.
The results are visualised in 2-D core maps and graphs representing azimuth, radial and
assembly-by-assembly distributions.
The computed angular turn of loop #1 flow is determined as the angle between the loop axis and
the centreline of the zone of minimal mixing in the disturbed sector. The zone of minimal mixing is
shown in terms of loop to assembly inlet temperature difference. The smaller the difference, the lower
the mixing. In order to fit the original plant data (NEA, 2010a) presented in terms of loop to assembly
mixing coefficients (see Figure 2.3), four temperature intervals were used. The temperature difference
below 1.2 K is presented in red, the range of 1.2-5 K in yellow, 5-8 K in green and over 8 K in blue.
For the analysis of the gradients at the borders of the disturbed sector, the 2-D maps are
supplemented by charts displaying the assembly-by-assembly inlet temperatures in comparison with
the plant estimated data. The core maps, representing the relative difference between computed and
plant estimated assembly inlet temperatures are given in Appendix E.
For precise quantitative and qualitative comparison of the results, the assembly-by-assembly
deviations from the experimental data are shown in separate graphs in Figures 4.23-4.35. These graphs
are supplemented by 2-D core maps in Appendix E representing the differences between computed
and plant estimated assembly inlet temperatures.
Finally, the results of the statistical evaluation are given in Table 4.11.
The CFX results of different participants obtained with the same turbulence models [k-ε or shear
stress transport (SST)] allow a study of the user effects and the impact of specific assumptions. The
verified CFX simulations with minimised numerical errors and with different turbulence models allow
a comparison of the performance of the turbulence models. Finally, the submitted and published
participants’ results give insight into the overall performance of current CFD codes using turbulence
models from k-ε to detached eddy simulation (DES) and LES.
4.3 Initial state results
Figures 4.1 and 4.2 show the FZK CFX 5 computed assembly-by-assembly core inlet and outlet
temperatures in the initial state. Note that the computed assembly outlet temperatures refer to the
end of the heated part of the assemblies and not to the thermocouple tips.
4.4 Final state results
4.4.1 Coolant mixing from the reactor inlet to the upper and lower downcomer
Table 4.2 shows the integral parameters, including the RPV boundary conditions and the measured
hot leg temperatures.
Regarding the requested distributions, several CFX results from different users and with the
same or different turbulence models were submitted. This enables an informative comparison if the
user-provided information about the modelling assumptions is sufficiently detailed.
28 V1000CT-2: SUMMARY RESULTS OF EXERCISE 1 ON VESSEL MIXING SIMULATION – © OECD/NEA 2010